Advances in the Production and Chemistry of the Heaviest Elements

Review pubs.acs.org/CR

Advances in the Production and Chemistry of the Heaviest Elements Andreas Türler*,†,‡ and Valeria Pershina§ †

Laboratory of Radiochemistry and Environmental Chemistry, Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland ‡ Laboratory of Radiochemistry and Environmental Chemistry, Department Biology and Chemistry, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland § GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstrasse 1, D-64291 Darmstadt, Germany 8.1.2. Empirical Correlations and Extrapolations 8.2. Complex Formation in Aqueous Solutions 9. Rutherfordium (Z = 104) 9.1. Theoretical Predictions 9.2. Experimental Results 9.2.1. Gas-Phase Chemistry of Rutherfordium 9.2.2. Liquid-Phase Chemistry of Rutherfordium 10. Dubnium (Z = 105) 10.1. Theoretical Predictions 10.2. Experimental Results 10.2.1. Gas-Phase Chemistry 10.2.2. Liquid-Phase Chemistry of Dubnium 11. Seaborgium (Z = 106) 11.1. Theoretical Predictions 11.2. Experimental Results 11.2.1. Gas-Phase Chemistry 11.2.2. Liquid-Phase Chemistry 12. Bohrium (Z = 107) 12.1. Theoretical Predictions 12.2. Experimental Results 12.2.1. Gas-Phase Chemistry of Bohrium 13. Hassium (Z = 108) 13.1. Theoretical Predictions 13.2. Experimental Results 13.2.1. Gas-Phase Chemistry 13.2.2. Liquid-Phase Chemistry 14. Meitnerium (Z = 109), Darmstadtium (Z = 110), Roentgenium (Z = 111) 14.1. Theoretical Predictions 15. Copernicium (Z = 112) 15.1. Theoretical Predictions 15.2. Experimental Results 15.2.1. Gas-Phase Chemistry 16. Element 113 16.1. Theoretical Predictions 16.2. Experimental Results 16.2.1. Gas-Phase Chemistry 17. Flerovium (Z = 114) 17.1. Theoretical Predictions 17.2. Experimental Results 17.2.1. Gas-Phase Chemistry

CONTENTS 1. Introduction 2. History of the Discovery of the Transuranium Elements 2.1. Actinides 2.2. Transactinides 3. Nuclear Properties 4. Nuclear Reactions of Colliding Nuclei 5. Nuclear Experimental Techniques 6. Relativistic Effects on Chemical Properties 6.1. Relativistic Effects on Atomic Electronic Shells 6.2. Current Relativistic Quantum-Chemical Methods 6.3. Relativistic Effects and the Future Periodic Table of the Elements 7. Chemical Techniques To Investigate Transactinides 7.1. Prerequisites for a Chemical Isolation of Heaviest Elements 7.1.1. Synthesis of Heaviest Elements 7.1.2. Rapid Transport 7.1.3. Chemical Isolation, Sample Preparation, and Detection 7.2. Gas-Phase Chemistry: Isothermal Chromatography and Thermochromatography 7.2.1. Isothermal Chromatography 7.2.2. Thermochromatography 7.3. Liquid-Phase Chemistry 7.3.1. Manual Liquid−Liquid Extractions and Column Chromatography 7.3.2. Automated Column Separations 7.3.3. Automated Liquid−Liquid Extractions 8. Methods To Predict Experimentally Measurable Properties of Transactinides 8.1. Volatility 8.1.1. Physisorption and Chemisorption Models

© 2013 American Chemical Society

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Special Issue: 2013 Nuclear Chemistry

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Received: June 15, 2012 Published: February 13, 2013 1237

dx.doi.org/10.1021/cr3002438 | Chem. Rev. 2013, 113, 1237−1312

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Figure 1. Current Periodic Table of the Elements with IUPAC approved numbering of groups and element symbols. The names for elements 114 (Flerovium, Fl) and 116 (Livermorium, Lv) suggested by the team of discoverers have recently been officially accepted by IUPAC4.

18. Element 115, Livermorium (Z = 116), Element 117, and Element 118 18.1. Theoretical Predictions 19. Element 119 and Element 120 19.1. Theoretical Predictions 20. Elements beyond Z = 120 20.1. Theoretical Predictions 21. Conclusions and Outlook 21.1. Summary of Volatility Studies of the Heaviest Elements and Their Compounds 21.2. Summary of Aqueous Chemistry Studies of the Heaviest Elements 21.3. Future Developments Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations and Quantities References

elements will it contain? Will we need to introduce the gorbitals, and will the current principles governing the groups and periods of the Periodic Table still be valid for the heaviest elements? These intricate questions are the topic of current research in the chemistry of the heaviest elements. For increasingly heavy nuclei, the electrostatic repulsions of protons cannot be sufficiently compensated by the attractive nuclear force through an increasing number of mediating neutrons. Therefore, the heaviest stable known nucleus is already reached with 208Pb. All isotopes of heavier elements, including some elements such as Bi, Th, and U that still can be found in nature as remnants of the last nucleosynthesis process, are radioactive and decay preferentially by successive α-particle and β-particle emissions back to the last stable element Pb. The heaviest nuclide, which was reported to be detected in traces in nature, is 244Pu, with a radioactive half-life (t1/2) of 81.2 million years. It is conceivable that 244Pu could still be present on earth as primordial element or be of cosmic origin due to explosive stellar nucleosynthesis, for example from a nearby supernova that occurred after the formation of our solar system. In an attention attracting article, Hoffman et al.5 reported a concentration of about 2350 atoms of 244Pu per gram of Bastnaesite, a mineral highly enriched in rare earth elements. However, a recent search in Bastnaesite from the same mine using accelerator mass spectrometry remained negative with a detection limit of less than 370 atoms per gram (99% upper confidence limit).6 Well established is the presence of small quantities of 237Np and 239Pu in nature due to neutron capture processes on 235U and 238U, respectively.7 Furthermore, recent evidence presented by Marinov et al.8 about the existence of a long-lived superheavy element with atomic number Z = 122 (or neighboring element) and mass number A = 292 with an abundance of about 1 × 10−12 relative to Th in natural Th could not be authenticated.9,10 Also the claimed observation of extremely long-lived, high spin, super- or hyperdeformed isomeric states in neutron deficient heavy nuclei,11 which was used as an argument to explain the observation of long-lived 292 122, could not be observed in independent experiments.12,13 Therefore, all elements heavier than Pu (Z = 94) are manmade. Elements up to Fm (Z = 100) were synthesized in successive irradiations of ever heavier elements with α-particles (4He2+) and neutrons. Weighable amounts of various actinides were then produced by successive neutron capture and β−-

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1. INTRODUCTION After Dmitri Ivanovich Mendeleev and, independently, Julius Lothar Meyer, and others discovered the ordering principles of the elements, Mendeleev proposed the first valid Periodic Table of the Elements nearly 150 years ago.1 We are now at a point where the discovery of the last missing element in the seventh period, namely synthesis of the element with atomic number 117, has recently been announced.2,3 Therefore, the Periodic Table currently contains 118 elements, the lightest being hydrogen and the heaviest yet unnamed eka-radon (or Uuo, ununoctium) as provisional name in IUPAC (International Union of Pure and Applied Chemistry) terminology. However, among heavy element chemists, the IUPAC provisional names are only rarely used. Instead, for elements that have been reported, but not fully authenticated, quite often only the atomic number is being used (i.e., element 113 or E113). A modern Periodic Table of the Elements is shown in Figure 1. But has the far end of the Periodic Table of the Elements now been reached? What is the heaviest element in the Periodic System? Are there still undiscovered ones which might even be found in nature? Is there an eighth period and how many 1238


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only theory that can reveal how relativistic effects influence chemical properties: only a comparison of the observed behavior with that predicted on the basis of relativistic vs nonrelativistic calculations does allow assessing the importance and magnitude of relativistic effects. Theoretical chemical research on the heaviest elements is not less challenging than the experimental one and should be based on the most accurate relativistic electronic structure calculations in order to reliably predict properties and experimental behavior of the new elements and their compounds. It also requires the development of special approaches that bridge calculations with quantities that cannot be so easily predicted from calculations. Due to the recent spectacular developments in relativistic quantum theory, computational algorithms, and techniques, very accurate calculations of properties of the transactinide elements and their compounds are now possible. The experimental verification of these predictions is highly desirable, but also very demanding. The chemical identification of transactinide elements is, up until now, always accomplished by the detection of the characteristic radioactive decay properties of their isotopes. The knowledge about the nuclear decay properties of transactinide nuclei is by no means complete, and a great number of them have not even been discovered. The field of heavy element chemistry is therefore closely related to low energy nuclear physics. Actually, a chemical separation can be regarded as a somewhat slow (in terms of nuclear physics) but powerful Z separator. As will be discussed, the discovery of new elements and the identification of new nuclides were not always straightforward. Especially the correct assignment of mass numbers proved difficult, since nuclear isomeric states are frequent in the heaviest nuclides and the experimental determination of masses has not progressed yet into the region of transactinide nuclides. In the current article, recent advances in the synthesis, chemical characterization, and theoretical studies of the heaviest elements will be reviewed. Recent reviews on the topic were published by Schädel18,19 and in a special edition of Radiochimica Acta edited by Kratz.20−28 Books include The Chemistry of Superheavy Elements29 and book sections by Münzenberg et al.,30 Hoffman et al.,31 and Kratz.32 Previous reviews summarize the theoretical chemistry of the heaviest elements.25,33−41

decay in nuclear reactors. Transactinide elements are synthesized in heavy ion fusion reactions at high power accelerators on a “one atom at a time” level with beams ranging from O to Zn, with 294118 currently being the heaviest observed nucleus.14 Some theoretical studies place the limits of nuclei that can no longer exist as bound entities beyond Z ≅ 300 and A ≅ 960.15 Even though such nuclei could theoretically exist as so-called hyperheavy bubble nuclei, the heaviest elements should exist in the form of atoms. To qualify as a chemical element, the nucleus of the longest lived isotope should exist >10−14 s,16 which is the time needed for the formation of an electron shell. The number of electrons that can be arranged around a nucleus is limited. Modern relativistic electronic structure theory that takes into account quantum electrodynamic effects (QED) predicts this to happen at element Z = 173, where the energy of the 1s electron falls into the negative energy continuum;17 that is, it becomes less than −2mec2. The place an element occupies in the Periodic Table is not only defined by its atomic number, i.e. the number of protons in the nucleus, but also by its electronic configuration, which defines its chemical properties. Strictly speaking, a new element is assigned its proper place only after its chemical properties have been sufficiently investigated. In some cases it has been possible to experimentally investigate chemical properties of transactinide elements and even synthesize simple compounds. Due to the predicted strong influence of relativistic effects, the experimental investigation of superheavy elements is especially fascinating. The study of the chemical properties of the heaviest known elements in the Periodic Table is an extremely challenging task and requires the development of unique methods but also the persistence to continuously improve all the processes and components involved in order to achieve the ultimate goal of chemically isolating one single atom that lives for only a few seconds. At first sight, the study of the chemical properties of the heaviest elements appears to be of purely academic interest. Indeed, as of today, it is not conceivable that weighable quantities of any transactinide element will be produced in the near future, and their immediate practical use appears questionable. Nevertheless, chemical studies of the heaviest elements open up possibilities for a deeper insight into the regularities of the Mendeleev Periodic System. Recent experiments have demonstrated that the chemical properties of the heaviest elements can no longer be predicted from simple extrapolations of the regularities in the groups and periods of the Periodic Table. Due to the low production rates of the heaviest elements, chemical information obtained from experiments is limited to the knowledge of very few properties. It mainly answers the question whether a new element behaves similarly to its lighter congeners in a chemical group, or whether some deviations from the trends occur due to very strong relativistic effects on its valence electron shells. Thus, in this area of the Periodic Table, theory starts to play an extremely important role and is often the only source of useful chemical information. For example, electronic configurations can only be calculated at the moment. Properties such as a chemical composition, stability and geometrical configuration, ionization potential (IP), electron affinity (EA), etc. can also be obtained only theoretically. Theoretical studies are also invaluable in predicting and/or interpreting the outcome of sophisticated and expensive experiments with single atoms. Moreover, it is

2. HISTORY OF THE DISCOVERY OF THE TRANSURANIUM ELEMENTS 2.1. Actinides

It is worthwhile to shed some light on the history of discovery of new elements.42 Until World War II, the heaviest known elements, Th, Pa, and U, were thought to be homologs of Hf, Ta, and W, respectively, and thus members of groups 4, 5, and 6. With the discovery of neptunium (Np, Z = 93)43,44 in 1940 and its chemical resemblance to U, and the subsequent discovery of plutonium (Pu, Z = 94),45,46 serious doubts about the placement of these elements in the Periodic Table surfaced. The new elements were thought to be members of a new “uranide” series. A further significant change to the Periodic Table came in 1944 with the recognition by Seaborg47 that actually all the elements heavier than actinium were misplaced and that this series was the actinide series and not the uranide series, resulting from the filling of the 5f shell. This concept had great predictive value and was instrumental in the discovery of many new actinide and transactinide elements. 1239


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of the nucleus. This effect, generally referred to as lanthanide or actinide contraction, is a consequence of the addition of successive electrons to the inner f shell. The diminishing shielding of the increasing nuclear charge by the f electrons causes a contraction of the valence shells with each additional proton in the nucleus and, hence, a decrease in the atomic or ionic radius. The discovery of the last two actinide elements, nobelium (No, Z = 102) and lawrencium (Lr, Z = 103), was not as straightforward as the discovery of elements Np through Md and involved scientists and institutions outside the United States. The name nobelium for element 102 was suggested by a team of scientists working at the Nobel Institute in Stockholm who claimed to have discovered the new element. This claim, however, has been proven wrong. Nevertheless, the element name has been retained by the IUPAC and will remain in the Periodic Table. The discovery of the last actinide elements No54,55 and Lr56 by the Berkeley Laboratory was not uncontested, and the discovery of new chemical elements became part of the cold war between the United States and the Soviet Union. In this context, the name of G. N. Flerov at the now named Flerov Laboratory of Nuclear Reactions (FLNR) at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, has to be mentioned. A paper by Flerov et al. “History of the transfermium elements Z = 101, 102, 103” presents the view by the involved Dubna scientists about the priority of discovery of elements 101, 102, and 103.57

It is astounding that the rather innocent and misled (due to the wrong placement of Th, Pa, and U as members of groups 4, 5, and 6 in the Periodic Table) search for transuranium elements by Fermi, and Hahn, Meitner, and Strassmann, ultimately started a spiral of monumental discoveries, which had a tremendous impact on humanity. Instead of identifying a new element, Hahn, Meitner, and Strassmann discovered the nuclear fission process late in 1938.48 The spiral took its first turn with an experiment by McMillan, who studied the neutron induced fission process of 238U. Unexpectedly, he observed a radionuclide with t1/2 = 2.3 days which did not recoil from the thin uranium target.43 This radioactivity turned out to be the new element Np.44 Surprisingly, the chemical behavior of Np was very similar to that of U and not at all similar to that of a group-7 element.49 The discovery of Np led to the subsequent discovery of Pu. The β¯ decaying 238Np was produced by the bombardment of 238U with deuterons in 1941.45,46 Only a few years after the synthesis of a few atoms of Pu, kilograms of Pu were produced to build the first nuclear device using Pu as fissile material. The elements curium (Cm, Z = 96) and americium (Am, Z = 95) (in this sequence) were discovered at the Chicago Metallurgical Laboratory by irradiation of 239Pu with α-particles and neutrons, respectively.47 The bombardment with α-particles took place in Berkeley at the 60-in. cyclotron, after which the material was shipped to Chicago for chemical processing. Shortly after the end of World War II, berkelium (Bk, Z = 97)50 and californium (Cf, Z = 98)51 were then discovered in Berkeley by irradiation of Am and Cm with α-particles. For the discovery of the actinide series and the synthesis of all the transuranium elements up to Cf, Seaborg and McMillan were awarded the Nobel price in 1951. Again, very unexpectedly, the discovery of fission, which led to nuclear weapons and to nuclear energy, led to the discovery of rapid multiple neutron capture in the explosion of a thermonuclear device and the isolation of the new elements einsteinium (Es, Z = 99) and fermium (Fm, Z = 100) from the debris in 1952.52 Transplutonium elements are being produced nowadays in weighable quantities, from kilograms of 241Am to a few picograms of 257Fm in high flux reactors through successive neutron capture reactions.42 The synthesis and subsequent identification of mendelevium (Md, Z = 101) in 1955 marked the beginning of a new era,53 since this was the first experiment where a new element was produced on a “one atom at a time” level. Also, the recoil technique was invented, which takes advantage of the fact that, in the fusion process of the heavy ion beam with the target nucleus, enough momentum is transferred to the compound nucleus to eject it from the layer of target atoms. This way, the reaction products are already separated from the target atoms. In the discovery experiment of Md, only 109 target atoms of 253 Es were bombarded with α-particles.53 In present day experiments, targets with thicknesses of typically 1018 target atoms per cm2, such as 244Pu, 248Cm, 249Bk, and 249Cf, are used. The availability of even heavier target nuclides, such as 250Cf, 251 Cf, or 254Es, still is very limited. The high neutron flux from, for example, 252Cf presents enormous technical difficulties in handling milligram quantities of this material. At Md, an era ended where chemical identification of a new element played a dominant role. The position at which a newly created heavy actinide element was eluted from a cation-exchange (CIX) column was indicative of its ionic radius and thus also of its atomic number. Analogous to the lanthanides, the radii of the M3+ and M4+ ions are decreasing with increasing positive charge

2.2. Transactinides

All isotopes of the transactinide elements with atomic number Z ≥ 104 have artificially been synthesized in heavy ion fusion reactions at accelerators and have only been studied in one atom-at-a-time experiments. The discovery experiments employed newly developed methods that relied on physics rather than chemistry to identify a new element. The controversy on the priority of discovery between Berkeley and Dubna also involved the first two transactinide elements, rutherfordium (Rf, Z = 104) and dubnium (Db, Z = 105). A detailed analysis of all the experiments made in Dubna and in Berkeley was presented by Hyde et al.58 The authors concluded that priority of discovery should be awarded to the work of the Berkeley team for both elements. A different view was presented by Flerov and Ter-Akopyan from Dubna.59 Very important points in the chain of evidence of the Russian claim were the chemistry experiments conducted by Zvara et al.60−67 The fierce competition between Berkeley and Dubna, “frequently punctuated by acerbic comments” as noted by Seaborg,42 obscured to a great extent the advances that were made in Berkeley and in Dubna in tackling the enormous difficulties to produce and study few single atoms of very shortlived nuclides. In Berkeley, the development of the gas-jet technique and of sophisticated counting equipment allowed the registration of successive mother−daughter α-particle decay chains originating from the same sample, displaying the correct sequence of decay energies and time differences. Thus, the genetic relationship could be established unequivocally. In Dubna, the development of ultrafast gas chemical separations was instrumental for later chemical studies of transactinide elements in the gas phase. Also the discovery of seaborgium (Sg, Z = 106) was not uncontested. Almost simultaneously, scientists from Berkeley and Dubna announced the discovery of element 106. In Berkeley the reaction 249Cf(18O, 4n)263Sg was used.68 The claim 1240


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from Berkeley was firmly established by the observation of genetically linked α−α correlations from the decay of 263Sg and 259 Rf. In the work of Oganessian et al.,69 the reaction 207Pb54 ( Cr, 2n)259Sg was utilized. The observation of a spontaneousfission (SF) radioactivity with t1/2 ≅ 4−10 ms could later not be confirmed for 259Sg. However, the nuclide 260Sg decays by fission with about 50% branching and t1/2 = 3.6 ms. It is therefore conceivable that the 1n-channel was observed. Oganessian et al.69 realized that the so-called “hot” fusion reactions using light projectiles and heavy actinide targets would at some point reach extremely low cross sections, since the produced compound nucleus was formed with a high excitation energy of the order of 40−50 MeV. The cooling of the compound nucleus by the emission of neutrons always competed with prompt fission. Therefore, the Dubna team favored the so-called “cold” fusion reaction using 208Pb as target material. Due to the heavier projectiles, the fusion cross section was diminished significantly, but due to the almost cold formation of the compound nucleus, only one or two neutrons were evaporated. The consequent exploitation of cold fusion reactions using Pb and Bi targets and the detection of genetically linked decay chains by the group headed by Armbruster, Münzenberg, and Hofmann led to the discovery of 6 new elements70 at the Gesellschaft für Schwerionenforschung, GSI, in Darmstadt, Germany. All new elements were discovered using the velocity filter SHIP (Separator for Heavy Ion Reaction Products), which separates fusion reaction products in flight from the beam and from transfer reaction products. Since all identified new elements decayed rapidly by successive α-particle emissions to lighter already known nuclei, no ambiguities concerning the identification of the new nuclides surfaced. The number of emitted α-particles in the chain was indicative of the atomic number of the new nuclide by adding two units in Z for each emitted α-particle to the last securely identified decay product. The SHIP velocity filter can identify reaction products with production cross sections as low as ≈100 fb (1 fb = 10−39 cm2). With the reported discovery of elements up to Z = 109, the time had come to authenticate the different claims and to come to conclusions about priority of discovery so that official element names could finally be adopted. In 1985 IUPAP and IUPAC decided to establish a Transfermium Working Group (TWG) to consider questions of priority in the discovery of elements with nuclear charge number Z > 100. The TWG issued its report16 in 1992. The contents of the report were accepted, in general, by the Dubna71 and Darmstadt72 laboratories, but heavily criticized by the Berkeley group.73 A concise summary of the developments concerning the discovery of elements 101−111 was published by Greenwood.74 Finally in 1997, a compromise was reached.75 The element names and symbols adopted in August 1997 by the general assembly of IUPAC for elements with atomic numbers 102 through 109 are listed in Table 1. A new joint working party of IUPAP and IUPAC (JWP) took up its work in 1998 to assess the discovery of elements 110−112. In its report,76 the JWP considered the work of Hofmann et al.77 as sufficient to claim discovery of element 110, while confirmation by further results was requested to assign priority of discovery of elements 111 and 112. Priority of discovery of element 11178,79 and element 11279,80 was assigned to Hofmann et al.81,82 in 2003 and 2009, respectively, especially since in the same reactions both nuclides 272 111 and 277112 were independently synthesized by Morita et 83,84 at RIKEN in Japan. The names proposed by the al.

Table 1. Element Names and Symbols Adopted by IUPAC for Transfermium Elements with Atomic Numbers 101−118 atomic no.

element name

symbol

101 102 103 104 105

mendelevium nobelium lawrencium rutherfordium dubnium

Md No Lr Rf Db

106 107 108 109 110 111 112 113 114 115 116 117 118

seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium ununtrium flerovium ununpentium livermorium ununseptium ununoctium

Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo

other names suggested or prev in use flerovium joliotium, Jl kurchatovium, Ku; dubnium, Db hahnium, Ha; nielsbohrium, Ns joliotium, Jl rutherfordium, Rf nielsbohrium, Ns hahnium, Hn

discoverers and accepted by the IUPAC are also included in Table 1. Three decay chains assigned to the nuclide 278113 synthesized in the reaction 209Bi(70Zn, 1n) have been reported by the same laboratory.85−87 The experiment required 553 days of accelerator beam time, yielding a record low production 87 cross section of 22+20 −13 fb. All the isotopes of the newly discovered elements were very short-lived, and t1/2 decreased from ≈1 min for 261Rfa to 1 s) super-heavy nuclei has been credited to different authors.92,98 Myers and Swiatecki99 first predicted an island of super-heavy elements around Z = 126 and N = 184, which was later refined to mostly Z = 114 and N = 184.100−104 Modern theoretical descriptions of superheavy nuclei using a purely microscopic approach have been reviewed by Nazarewicz et al.105 and Cwiok et al.106 These represent selfconsistent Hartree−Fock-type calculations, which use effective density-dependent interactions of both zero (Skyrme) and finite (Gogny) range, and also the relativistic mean field approach. These calculations tend to locate the center of the island at Z = 114, Z = 120, or Z = 126 while confirming the N = 184 neutron shell. The more traditional macroscopic−microscopic methods,21 which include higher orders of nuclear deformation are able to explain the increased stability observed around Z = 108 and N = 162, a region that was much closer to the region of then known nuclei. In Figure 3 the shell

1 pb translates into the synthesis of only 1 atom of a superheavy nuclide every 36 h, on average. Second, of the more than 50 new nuclides produced in these experiments, a number of them have t1/2 > 1 s and, thus, live long enough for chemical investigations. This result is in strong contrast to those of the previously known, more neutron deficient isotopes of Mt, Ds, Rg, and Cn in the range of a few milliseconds. Figure 2 shows the number of discovered transuranium elements as a function of time. The synthesis of new elements

Figure 2. Growth of the number of transuranium elements as a function of time.

was not a continuous process but occurred in phases. A new technical development or a new concept allowed the discovery of several elements before a limit was reached that could only be overcome by substantial improvements or a new concept. Such a period is evident between 1984 and 1994, where a number of improvements at the SHIP separator and the UNILAC accelerator allowed an increase in sensitivity of about 1 order of magnitude, resulting in the subsequent discovery of another three elements without abandoning the concept of cold fusion using Pb or Bi targets. A change of concept using 48Ca beams and actinide targets allowed the Dubna−Livermore collaboration the discovery of six new elements. Since no heavier actinides than Cf are available in sufficient quantities as target materials, this extremely successful path has been exhausted and new ideas are required to push past element 118 and open the eighth period of the Periodic Table.

Figure 3. Contour map of calculated ground-state shell correction energies. Figure adapted from ref 107. Copyright 2001 by The American Physical Society. The locations of the doubly magic nuclei 208 Pb, 270Hs, and 298Fl are indicated. The nucleus in the center of the new subisland, 270Hs, can be reached, for example, in the reaction 248 Cm(26Mg, 4n) and has recently been synthesized.109 48Ca-induced reactions on actinide targets still fall short of reaching the predicted center of spherical shell closure for superheavy elements around Z = 114.

3. NUCLEAR PROPERTIES The synthesis and observation of long-lived heavy nuclides is a triumph of the nuclear shell model that has predicted the existence of such nuclei since the 1950s. These hypothetical nuclei were called “super-heavy”.92 However, the term had appeared already in 1938 in a review by Quill93 on transuranium elements and was later used also for newly discovered elements.94 The limit of existence of heavy elements is determined by the balance between the repulsive Coulomb forces of the many protons and the attractive nuclear forces. Immediately after the discovery of nuclear fission,48 Meitner and Frisch95 located the limit of existence of heavy nuclei at around Z ≈ 100, where the surface tension in the charged nuclear droplet model96 can no longer compensate the repulsive forces. Later, it was realized that at certain “magic

correction energies calculated by Sobiczewski et al.107 are shown. For barrel shaped nuclei (hexadecapole deformation) around Z = 108 and N = 162, the shell-correction energy reached almost the same magnitude as for the traditional, spherical shell closure for superheavy elements around Z = 114. This meant that there exists not an isolated island far removed from the region of fairly well-known nuclides, but a subisland reaching out to the island of superheavies.108 1242


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Figure 4. Cut-out of a chart of nuclides showing all presently known transactinide nuclides (color coding according to the “Karlsruhe” chart of nuclides110).

4. NUCLEAR REACTIONS OF COLLIDING NUCLEI So far, transactinide elements can only be synthesized artificially in complete heavy ion fusion reactions. The fusion of two nuclei is a very complex process. Although there exist numerous articles in the literature treating the collision and fusion of two nuclei, we restrict ourselves here to cite Zagrebaev et al.,111 which gives an in-depth analysis of the individual steps involving the synthesis of a heavy nucleus. First the repulsive Coulomb forces have to be overcome, so that the two colliding nuclei get into contact. When two nuclei are made to collide, several different reaction pathways open, depending on the impact parameter of the approaching nuclei and the kinetic energy of the projectile. This is schematically shown in Figure 5. If the impact parameter is too large or the kinetic energy too low, the projectile is elastically scattered off the target nucleus,

This discovery had a tremendous impact on the synthesis of new elements, since now the region at or beyond Z = 114 could be accessed in a step by step procedure, without risking plunging into the “sea of instability”. This is especially important, since 48Ca-induced reactions on actinide targets, which currently are the only experimentally feasible combinations to synthesize neutron-rich superheavy elements, still fall short by a large margin of reaching the center of the predicted spherical shell closure around Z = 114 and N = 184. Due to the large number of nucleons in superheavy nuclides and the resulting complicated nuclear structures and shapes, nuclear isomerism is a common phenomenon. This means that a nucleus may exist also in excited states, which are observable and have measurable half-lives. For example, the observed decay properties of a nucleus may vary considerably depending on whether it was produced directly in a nuclear reaction, usually with high spin, or as a decay product of a heavier nucleus. Isomeric states of nuclei are denoted with the suffix m (e.g., 211Pom). Often, it is not yet clear which one of the observed states is the ground state and which is the excited one. In this case, the suffix a or b is used to denote these states (e.g., 261 a Rf and 261Rfb). Figure 4 displays an updated transactinide cut-out of the chart of nuclides. The color coding is that of the “Karlsruhe” chart of nuclides.110 One can see that the 48Ca induced reactions on actinide targets have led to the discovery of six new elements and their associated α-decay chains, always ending in SF. This region of >50 new nuclides is detached from the older region of previously known nuclei. The fact that there is no overlap of this new region with the region of previously known nuclei created some problems in unambiguously assigning the atomic number of the synthesized nuclei, since all decay chains ended by SF in a previously uncharted region.

Figure 5. Schematic representation of projectile trajectories depending on impact parameter. Reprinted with permission from ref 112. Copyright 1997 Clarendon Press. 1243


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prompt fission. Production cross sections for transactinide nuclides range from nanobarns (10−33 cm2) to picobarns (10−36 cm2); these very low cross sections have to be compared with the reaction channels a−c with cross sections of 1−100 millibarns (10−27 cm2). In essence, a substantial part of the chart of nuclides (or the Periodic Table) is produced in these collisions. Thus, at typical beam currents of 1012 projectiles/s and target thicknesses of 1018 target atoms/cm2, quickly isolating one single atom from the plethora of reaction products is a formidable task. In Figure 7, experimental production cross sections of heavy and superheavy nuclei for “cold fusion” reactions (1n emission)

leading to Coulomb excitation of the colliding nuclei. At lower impact parameters, the nuclei may get close enough so that the exchange of few nucleons can occur (reaction pathway a). These reactions are called quasi-elastic, since the contact time is very short and the projectile is scattered off the target similarly as in elastic scattering. At impact parameters significantly smaller than the sum of both radii, the contact of both nuclei is deeper and the attractive nuclear forces come into play to the point where the dinuclear system begins to fuse into a single large nucleus. Exchange reactions of many nucleons are called deep inelastic transfer (reaction pathway b). In rare cases the two nuclei fuse (reaction pathway c). In most cases the fused system decays again in two fragments in a fission-like process. Reaction pathways a−c can be observed experimentally. In Figure 6 the total integrated mass yield of the reaction 288

Figure 7. Experimental cross sections for the formation of nuclei with Z ≥ 102 in (■) the 1n evaporation channel of cold fusion reactions, (○) the 5n channel of hot fusion reactions, and (△) the 3−4n channel of warm fusion reactions with 48Ca + actinide targets. The curves are drawn to guide the eye.

Figure 6. Total integrated mass yield of the reaction 288 MeV 40Ar + 238 U (dash-dotted line) and its decomposition into individual contributions: (a) quasi-elastic processes, (b) deep-inelastic multinucleon transfer, (c) fusion followed by fission, and (d) fission of target-like reaction products.113,114. Reprinted with permission from ref 114. Copyright Wiley VCH Verlag GmbH.

and “hot fusion” reactions (5n emission) are shown as a function of atomic number. The “abnormal” behavior of 48Ca induced production cross sections, which stay rather constant at the level of a few pb, was associated with stabilizing shell effects, i.e. increasing fission barrier heights, when approaching the region of Z = 114.89

MeV 40Ar + 238U (dash-dotted line) and its decomposition into individual contributions is shown.113,114 Quasielastic transfer reactions (a) lead to two narrow distributions with high maximum cross section near the masses of the projectile and the target nucleus. Deep-inelastic transfer reactions lead to wider distributions, where the identity of the projectile or the target is not completely lost (b). The dashed line above mass number 240 indicates that most of these transfer reaction products will disintegrate by fission due to the relatively high excitation energy and angular momentum they are formed with, giving rise to the distribution of fission fragments denoted with (d). Central collisions lead to complete fusion. The Q value of the fusion reaction is positive, meaning that, even if the fusion occurs at the Coulomb barrier, the compound nucleus is formed with an excitation energy between 30 and 50 MeV in so-called “hot fusion” reactions, i.e. reactions with light projectiles (18O, 22Ne, 26Mg, 36S) and actinide targets, and 10−20 MeV in “cold fusion” reactions, i.e. reactions with heavy projectiles and 208Pb or 209Bi targets. In most cases, the formed compound nucleus disintegrates by fission, giving rise to the broad distribution denoted by (c). Only very rarely (≈10−9) is the high excitation energy dissipated by the evaporation of particles, mostly neutrons, and by the emission of γ-rays. Each evaporated neutron removes ≈10 MeV of excitation energy, but each neutron evaporation process is in competition with

5. NUCLEAR EXPERIMENTAL TECHNIQUES The synthesis of transactinide elements requires intense heavy ion beams of mostly neutron-rich, low-abundance isotopes and often exotic, radioactive actinide target materials (see section 7.1.1). The required projectile energies are those close to the Coulomb barrier, i.e. about 5 MeV/nucleon. Accelerators used for transactinide element synthesis are either cyclotrons such as the U400 at FLNR (Russia), the 88-Inch at LBNL (United States), the K-130 at JYFL (Finland), or various cyclotrons at GANIL (France), or linear accelerators such as the UNILAC at GSI (Germany) or the RILAC at RIKEN (Japan). In order to provide long-term stable beams for month-long experiments, highly efficient ion sources are required, which provide intense beams with low material consumption (i.e., about 0.5 mg/h) of expensive enriched stable isotopes. These requirements are best met by modern electron cyclotron resonance (ECR) ion sources, which are installed at all the above-mentioned facilities. Since evaporation residues are recoiling out of the target layer with the momentum of the beam, they can be separated in flight from the projectile beam but also from all other types of reaction products (see section 4). Presently, kinematic 1244


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Figure 8. Schematic view of the velocity filter SHIP. Reproduced with permission from ref 22. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH.

separators such as velocity, mass, or energy filters and gas-filled separators are in use.89,115,116 The flight time through these separators is of the order of microseconds. Extremely successful separators in the discovery of new elements were the velocity filter SHIP at GSI and the Dubna gas-filled recoil separator (DGFRS) at FLNR. SHIP is a vacuum separator and consists of two symmetrically arranged Wien-filters with spatially separated electrostatic and magnetic dipoles. Quadrupole triplets are installed before and after the velocity filter. An additional 7.5° dipole bends the product beam out of a direct line of sight of the target position, which strongly reduces unwanted background. For 48Ca induced reactions, the efficiency of the separator was reported to be about 20%.117 A schematic view of the velocity filter SHIP is shown in Figure 8. SHIP is required to operate in vacuum, due to the high electric fields. In contrast, evaporation residues can spatially be separated in a gas-filled dipole magnet, making use of a charge focusing effect. The recoiling fusion products change their charge state when traveling through a dilute gas (usually H2 or He), which results in a trajectory through a dipole magnet that can be described by an average charge state, thus defining the magnetic rigidity. The magnetic rigidities of the beam and other reaction products are different so that a separation of evaporation residues can be accomplished. Quadrupole magnets are added to the dipole to focus the evaporation residues into a focal plane detector. A schematic of the DGFRS is shown in Figure 9. The transmission through the separator for 48Ca-induced reactions was estimated to be about 40%.89 One of the most important parts of the separator is the focal plane detector, where the decay of a transactinide nucleus is registered. Typically, the rate of particles reaching the focal plane detector is still few hundred hertz. In order to unambiguously detect decay events of transactinide nuclei, this background must be significantly reduced. This is accomplished in several ways. By adding so-called time-offlight (TOF) transmission detectors, each particle transiting the separator leaves a signal in the TOF detectors, which can thus be separated from radioactive decay signals of implanted nuclei, which leave no TOF signal. Furthermore, the Si-detectors are divided into position sensitive strips, so each signal (implantation, α-particle, and SF decay) is associated with a position coordinate in the detector. This way, the observed

decay sequence of a heavy nucleus consists of an implantation signal, followed by one or several α-particle decays. Often the sequence is terminated by SF decay. These events are not only correlated in time but also in position in the Si-detector. This way, the probability that such a sequence is consisting of purely randomly correlated events is greatly diminished. Since the implantation depth is rather shallow, α-particles and fission fragments can escape the detector plane in the backward (upstream) direction. Such escaping events still leave a detectable signal in the focal plane detector, and often the full energy of these escaping events can be recovered by detecting them with side detectors. All these features are shown in the enlarged section of Figure 9. Since all events are registered and stored in an event-by-event mode, month-long experiments create rather large amounts of data that has to be analyzed off-line to reveal the presence of few heavy nuclei. In

Figure 9. Schematic view of the DGFRS. The cut-out shows an enlarged view of the focal plane Si-detector box. Reprinted with permission from ref 89. Copyright 2007 IOP Publishing Ltd. 1245


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the 25% relativistic contraction and 5.8 eV stabilization of the 7s AO of Cn. The relativistic contraction and stabilization of the ns AO reach their maximum in the seventh row of the Periodic Table at Cn (Figure 12).37 The shift of the maximum to Cn in the seventh period in contrast to Au in the sixth period is due to the fact that in Rg and Cn the ground state electronic configuration is d9s2 and d10s2, respectively, while the corresponding electronic configurations in the sixth period are Au(d10s1) and Hg(d10s2). The second (indirect) relativistic effect is the destabilization and expansion of outer d and f orbitals. The relativistic contraction of the s and p1/2 shells results in a more efficient screening of the nuclear charge so that the outer orbitals, which never come close to the core, become more expanded and energetically destabilized. As an example, the expansion and destabilization of the (n−1)d AOs with Z are shown in Figure 11 for group-12 elements. While the direct relativistic effect originates in the immediate vicinity of the nucleus, the indirect relativistic effect is influenced by the outer core orbitals. The third relativistic effect is the well-known spin−orbit (SO) splitting of levels with l > 0 (p, d, f, etc.) into j = l ± 1/2. It also originates from the inner region in the vicinity of the nucleus. The SO splitting for the same l decreases with increasing number of subshells; that is, it is much stronger for inner (core) shells than for outer shells. The SO splitting decreases with increasing l for the same principal quantum number; that is, the np1/2−np3/2 splitting is larger than the nd3/2−nd5/2 and both are larger than the nf5/2−nf7/2 one. This is explained by the decreasing orbital densities in the vicinity of the nucleus with increasing l. In transactinide compounds, the SO coupling becomes similar or even larger in size compared to typical bond energies. All three effects change approximately as Z2 for the valence shells down a column of the Periodic Table. It was suggested that relativistic effects depend even on higher powers of Z, especially for the heaviest elements.123 Breit effects (accounting for magnetostatic interactions and retardation effects to the order of 1/c2) on energies of valence orbitals and IPs are usually small, e.g., 0.02 eV for element 121, but can be as large as 0.1 eV for transition energies between the states including f orbitals.124 They can also reach a few percent for the fine structure level splitting in the 7p elements and are of the order of correlation effects there. QED such as vacuum polarization and electron self-energy are known to be very important for inner-shells,125,126 for example, in accurate calculations of X-ray spectra127,128 for the heaviest elements. For highly charged few electron atoms, they were found to be of similar size as the Breit correction to the electron−electron interaction. It was shown that in the middle range (Z = 30−80) both the Breit and Lamb-shift terms for the valence shells behave similarly to the kinetic relativistic effects, scaling as Z2.129 For the higher Z, the increase is even larger. The nuclear volume effect grows even faster with Z. Consequently, for the superheavy elements, its contribution to the orbital energy will be the second most important one after the relativistic contribution. QED corrections for the valence shells in heavy many-electron atoms of elements Rg through Fl, and 118 through 120 calculated using a perturbation theory are given in Thierfelder et al.130 Thus, for example, QED on the DCB IP of element 120 is −0.013 eV, while it is 0.023 eV for Cn. For element 118, QED effects on the binding energy of the 8s electron cause a 9% reduction

earlier times, the analysis of the data required a computing center. New instruments, such as the gas-filled separator TASCA, which are currently being used in the search for new elements 119 and 120, have a transmission of about 60% for 48Cainduced reactions on actinide targets.118 As focal plane detector, double-sided Si strip detectors are used, which allow for a position resolution of 1 mm2. The focal plane consists thus of more than 6900 pixels. In addition, new and faster electronics is required in order to resolve nuclides with t1/2 ≥ 1 μs.

6. RELATIVISTIC EFFECTS ON CHEMICAL PROPERTIES 6.1. Relativistic Effects on Atomic Electronic Shells

With increasing Z of heavy elements, causing a stronger attraction to the core, an electron is moving faster, so that its mass increase is m = m0 /[1 − (v /c)2 ]1/2

(6.1.1)

where m0 is the rest mass of the electron, v is the velocity of the electron, and c is the speed of light. The Bohr model for a hydrogen-like species gives the following expressions for the velocity, energy, and orbital radius of an electron v = (2πe 2 /nh)Z

(6.1.2)

E = −(2π 2e 4 /n2h2)mZ2

(6.1.3)

r = Ze 2 /mv 2

(6.1.4)

where n is the principal quantum number, e is the charge of the electron, and h is Planck’s constant. With increasing Z along the Periodic Table, the m/m0 ratio gets larger. For H it is 1.000027. From the sixth period onward, this ratio is exceeded by 10%, so that relativistic effects cannot be neglected anymore. For example, for Fl, m/m0 = 1.79, and for element 118, it is 1.95 (see also Pyykkö119 for other examples). The contraction (eq 6.1.4) and stabilization (eq 6.1.3) of the hydrogen-like s and p1/2 electrons is a direct relativistic effect and was shown to originate from the inner K and L shell regions.120 This effect was found to also be large for the valence region due to the direct action of the relativistic perturbation operator on the inner part of the valence density.121 Figure 10 shows the relativistic contraction of the 7s atomic orbital (AO) of element 105, Db, ΔR⟨r⟩ns = ⟨r⟩nr − ⟨r⟩rel/⟨r⟩nr = 21%. Figure 11 shows

Figure 10. Relativistic (solid line) and nonrelativistic (dashed line) radial distribution of the 7s valence electrons in Db. Reprinted with permission from ref 39. Copyright 2003 Kluwer Academic Publishers. 1246


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Figure 11. Relativistic (solid line) and nonrelativistic (dashed line) energies and the maximum of the radial charge density, Rmax, of the valence ns and (n−1)d AOs of group-12 elements25 with data from Desclaux.122 Reprinted with permssion from ref 25. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH.

Bij = −1/2[(αi⃗ αj⃗ )rij−1 + (αi⃗ rij⃗ )(αj⃗ rij⃗ )rij−3]

The operators of the Dirac eq 6.2.1 are 4 × 4 matrix spinor operators, and the corresponding wave function is therefore a four-component (4c) vector function. The Vn includes the effect of the finite nuclear size, while some finer effects, such as QED, can be added to hDCB perturbatively. The DCB Hamiltonian in this form contains all effects through the second order in α, the fine-structure constant. Correlation effects are taken into account by the configuration interaction (CI), many-body perturbation theory (MBPT) and, presently at the highest level of theory, the coupled cluster single double (and perturbative triple) excitations (CCSD(T)) technique. The Fock−Space (FS) DCB CC method137,138 is presently the most powerful method used for atomic calculations. It has an accuracy of a few hundredths of an electronvolt for excitation energies in heavy elements, since it takes into account most of the dynamic correlation (states with high l). Due to the present limitation of the FS CCSD method in treating electronic configurations with no more than two electrons (holes) beyond the closed shell, further developments are underway to remove this limitation.138 Thus, the highsectors FSCC code is under development, which will allow for treating systems with up to six valence electrons/holes in an open shell. The relativistic Hilbert space CC (HSCC) method is also worked on, which could be used for systems with more than a couple of electrons/holes in the active valence shell. The mixed sector (MS) CC method will be a generalization of the previous two (FSCC and HSCC) and will combine their advantages. A further improvement is the introduction of the intermediate Hamiltonian (IH). It is a generalization of the effective Hamiltonian (EH) method and serves as a core of most multiroot multireference approaches. The standard multireference FSCC and HSCC methods (described above) are used in the effective Hamiltonian framework. The most problematic technical problem of the EH method is poor (or no) convergence of iterations due to the presence of so-called intruder states. Recently, many groups developed different forms of “intruder-free” intermediate Hamiltonian formulations of FSCC and HSCC. These formulations substantially extend

Figure 12. Relativistic stabilization of the 6s and 7s orbitals in the 6th and 7th rows of the Periodic Table.25 Redrawn from Schwerdtfeger et al.37 with Dirac−Fock (DF) data from Desclaux.122. Reprinted with permission from ref 25. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH.

(0.006 eV) of EA.131 Thus, the QED effects are not negligible: they are of the order of 1−2% of the kinetic relativistic effects, which means that the existing studies of relativistic effects are up to 99%129 (or 101%17) correct. 6.2. Current Relativistic Quantum-Chemical Methods

The most appropriate quantum chemistry methods for the heaviest elements are those which treat both relativity and correlation at the highest level of theory.132−136 Presently, the highest theoretical level for many-body methods for molecules is the Dirac−Coulomb−Breit (DCB) Hamiltonian hDCB =

∑ hD(i) + ∑ (1/rij + Bij) i

i100,000 USD/mg. From the isolated heavy actinide nuclides, targets suitable for heavy ion irradiations have to be prepared. For nuclear reactions involving the lighter projectiles, target thickness is limiting the compound nucleus recoil ranges (75% and the transfer time only about 10−2 s. In the reaction chamber of section II, vapors of NbCl5 and ZrCl4 were continuously added as chlorinating agents and 1254


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much better. OLGA(III) has very successfully been applied to study volatile halides and/or oxyhalides of Rf,230 Db,231 Sg,232−234 and Bh.235 In all these experiments the separated transactinide nuclides were unambiguously identified via their nuclear decay properties. An improved version of OLGA(I), named HITGAS (high-temperature online gas chromatography apparatus), has been developed at Forschungszentrum Rossendorf, Germany, and successfully applied to study oxide hydroxides of group-6 elements including Sg.236−239 In order to further reduce the background of unwanted αdecaying nuclides, the so-called parent−daughter recoil counting modus had to be implemented at the rotating wheel detection systems. Since the investigated transactinide nuclei decay with characteristic decay sequences involving α-particle decay and/or SF of daughter nuclei, the significance of the observed decay sequence can be enhanced by observing the daughter decays in a nearly background free counting regime. This can be accomplished in the manner shown in Figure 20 on the example of 267Bh. In the parent mode, a 267Bh atom

previously been synthesized and identified by a team of physicists at Dubna. In a number of experiments, Zvara and coworkers identified multiple SF tracks in the mica detectors when they used glass surfaces and temperatures of 300 °C.62 They had shown in preparatory experiments with Hf that indeed the transfer of Hf through the apparatus occurred within less than 0.3 s, and, thus, that the experimental setup was suited to study the short-lived Rf isotope.221 The distribution of SF tracks along the mica detectors appeared consistent with t1/2 = 0.3 s.62 A number of possible sources of SF tracks in the mica detectors other than the SF decay of a Rf isotope were discussed and ruled out. Further experiments with a slightly modified apparatus63 were conducted immediately after the experiments described here. A total of 63 SF events were attributed to the decay of Rf nuclide. The team in Dubna regarded these experiments as further proof of the claim of discovery of element 104. One of the most successful approaches to the study of volatile transactinide compounds is the so-called OLGA (online gas chromatography apparatus) technique. Contrary to the technique in Dubna, reaction products are rapidly transported through a thin capillary to the chromatography setup with the aid of an aerosol gas-jet transport system. Transport times of less than 10 s are easily achieved. This way, the chromatography system and also the detection equipment can be set up in a fully equipped chemistry laboratory accessible during irradiation and close to the shielded irradiation vault. A first version of OLGA (I) was developed and built by Gäggeler and co-workers214 for the search of volatile superheavy elements, and it was tested with 25 s 211Pom. Volatile elements were separated in a stream of He and hydrogen gas at 1000 °C from nonvolatile actinides and other elements. The separated nuclei were condensed on thin metal foils mounted on a rotating wheel (ROMA, rotating multidetector apparatus223,224) and periodically moved in front of solid state detectors, i.e. PIPS (passivated implanted planar silicon) detectors, where α-particles and SF events were registered in an event-by-event mode. For the study of volatile halides and/or oxyhalides of Rf225 and Db225,226 and their lighter homologs,227 OLGA(II) was built.215 Instead of condensing the separated molecules on metal foils, they were attached to new aerosol particles and transported through a thin capillary to the detection system. This so-called reclustering process was very effective and allowed collection of the aerosol particles on thin (≈40 μg/cm2) polypropylene foils in the counting system (ROMA or MG, merry-goround228). Thus, samples could be assayed from both sides in a 4π geometry, which doubled the counting efficiency. At the same time, the PSI tape system was developed,215 which allowed significant reduction of the background of long-lived SF activities, that accumulated on the wheel systems. However, only a 2π counting geometry could be realized. An improved version of OLGA(II) was built at Berkeley and nicknamed HEVI (heavy element volatility instrument).216 With both instruments, the time needed for separation and transport to detection was about 20 s, with the time-consuming process being the reclustering. A schematic of the applied experimental equipment is shown in Figure 19. In order to improve the chromatographic resolution and increase the speed of separation, OLGA(III) was developed.229 Using a commercial gas chromatography oven and a 2 m long quartz column, which ended in a much smaller, redesigned recluster unit, the overall separation time could be reduced by 1 order of magnitude, while the chromatographic resolution was

Figure 20. Parent−daughter mode for rotating wheel systems.187 See text for detailed description. Reprinted with permission from ref 187. Copyright 2003 Kluwer Academic Publishers.

attached to the aerosol transport material is deposited on the surface of a thin foil. The wheel is double-stepped at preset time intervals to position the collected samples successively between pairs of α-particle detectors. When the 267Bh α-decay is detected in the bottom of a detector pair, it is assumed that the 263Db daughter has recoiled into the face of the top detector. The wheel is single-stepped to remove the sources from between the detector pairs, and a search for the 263Db and 259 Lr daughters is made for a second preset time interval, before single-stepping the wheel again to resume the search for decays of 267Bh. The measurement of daughter nuclides at significantly reduced background conditions comes at the expense of a reduced overall efficiency of the apparatus. 7.2.2. Thermochromatography. In TC,240 a carrier gas is flowing through a chromatography column, to which a negative longitudinal temperature gradient has been applied. Open or filled columns can be employed. Species that are volatile at the starting point are transported downstream of the column by the carrier gas flow. Due to the decreasing temperature in the column, the time the species spend in the adsorbed state increases exponentially. Different species form distinct deposition peaks, depending on their ΔH0(T) on the column a surface, and are thus separated from each other. A characteristic quantity is the deposition temperature (Ta), which depends on various experimental parameters. The mixture of species to be separated can be injected continuously into the col1255


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facing each other at a distance between 0.5 and 1.5 mm. A first cryo-thermochromatographic separator (CTS) was constructed at LBNL.201 In the Hs experiment,197 the cryo online detector (COLD) was used, which was constructed at PSI. A schematic of the experimental setup used in the first successful chemical identification of Hs as volatile tetroxide is shown in Figure 21.

umn,62,241−243 or the experiment can be performed discontinuously by inserting the mixture of species through the hot end of the chromatography column and removing the column through the cold end after completion of the separation. The two variants (continuous or discontinuous) result in slightly different peak shapes. The chromatographic resolution is somewhat worse for the continuous variant. Thermochromatographic separations are the method of choice to investigate species containing long-lived nuclides that decay either by γemission, by EC- or β+ decay, or by the emission of highly energetic β− particles.244−249 Thus, the emitted radiation can easily be detected by scanning the length of the column with a detector. The detection of nuclides decaying by α-particle emission or SF decay is more complicated. By inserting SF track detectors into the column, SF decays of short- and longlived nuclides can be registered throughout the duration of the experiment. After completion of the experiment, the track detectors are removed and etched to reveal latent SF tracks. Columns made from fused silica have also been used as SF track detectors.250 However, the temperature range for which SF track detectors can be applied is limited, due to the annealing of tracks with time. It should also be noted that in TC all information about t1/2 of the deposited nuclide is lost, which is a serious disadvantage in experiments with transactinide nuclides, since SF is a nonspecific decay mode of many actinide and transactinide nuclides. However, TC experiments with transactinides decaying by SF have an unsurpassed sensitivity (provided that the chromatographic separation from actinides is sufficient), since all species are eventually adsorbed in the column and the decay of each nuclide is registered. Thus, the position of each single decay in the column contributes chemical information about ΔH0(T) of the a investigated species. TC, as a nonanalytical tool to study the behavior of compounds at a tracer scale, was developed and applied to the gas−solid chromatographic separation of transactinide elements mainly by the group of Zvara and coworkers at FLNR Dubna, Russia. Zvara et al. reported the chemical identification of elements Rf,61−64 Db,65−67 and Sg,250−253 whereas experiments to chemically identify Bh254 and Hs255−257 yielded negative results. However, due to the fact that in all these experiments the separated nuclides were identified by the noncharacteristic SF decay, and no further information such as t1/2 of the investigated nuclide could be measured, most of the experiments fell short of fully convincing the scientific community that indeed a transactinide element was chemically isolated.32,58,258,259 TC experienced a renaissance in the chemical investigations of Hs, as volatile HsO4,197 Cn,192,194 and Fl195,260 in the elemental state, which all are volatile at room temperature. Since it was no longer necessary to introduce highly corrosive halogenating chemicals to synthesize volatile compounds, the simple quartz tubes that served as chromatography columns could be replaced by narrow channels formed by silicon detectors, which are able to record high resolution α-particle spectra and register SF fragments in a time-resolved mode. Along this channel a longitudinal negative temperature gradient is established. Due to the close proximity of the silicon diodes facing each other, the probability to register a complete decay chain of a superheavy nucleus, consisting of a series of α-decays and often terminated by SF correlated in time and position, is rather high. A position resolution of 1−3 cm was sufficient to also extract chemical information. Usually, the channels are formed by silicon detectors of 1 × 1 cm2 dimension, which are

Figure 21. The 26Mg-beam passed through the rotating vacuum window and 248Cm-target assembly. In the fusion reaction, 269,270Hs nuclei were formed which recoiled out of the target into a gas volume and were flushed with a He/O2 mixture to a quartz column containing a quartz wool plug heated to 600 °C by an oven. There, Hs was converted to HsO4, which is volatile at room temperature and transported with the gas flow through a perfluoroalkoxy (PFA) capillary to the thermochromatography detector array registering the nuclear decay (α-decay and SF) of the Hs nuclides. The array consisted of 36 detectors arranged in 12 pairs, with each detector pair consisting of 3 PIN (positive implanted N-type silicon) diode sandwiches. Always, 3 individual PIN diodes (top and bottom) were electrically coupled. A thermostat kept the entrance of the array at −20 °C; the exit was cooled to −170 °C by means of liquid nitrogen. Depending on the volatility of HsO4, the molecules adsorbed at a characteristic temperature.262

The geometrical efficiency for detecting a single α-particle emitted by a species adsorbed inside the detector array was 77%. The detectors of the COLD array were calibrated online with α-decaying 219Rn and its daughters 215Po and 211Bi using a 227 Ac source. A further improved TC detector nicknamed COMPACT (cryo online multidetector for physics and chemistry of transactinides) was constructed at Technical University Munich (TUM). By reducing the gap between the detectors to only 0.5 mm and integrating four detectors on one chip, the active area inside the detector channel could be increased to 93%. The COMPACT detector was used in experiments to discover new Hs isotopes.109,261 Furthermore, the detectors were of the PIPS type, which have a more rugged surface and also allow surface modifications, e.g. the deposition of a thin layer of noble metals such as Au or Pd. This became very important in experiments where the interaction of a single atom of a superheavy element such as Cn or Fl was investigated with a Au surface. 7.3. Liquid-Phase Chemistry

Liquid-phase chemical separations are standard; thus, their utility for separation and isolation of the chemical elements has been demonstrated. Usually, liquid−liquid extractions or column based separations are performed. Adaptations of these well-understood separation techniques have been 1256


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Figure 22. Schematic drawing of AIDA. Reprinted with permission from ref 280. Copyright 2005 Oldenbourg Wissenschaftsverlag GmbH.

The first liquid-phase transactinide chemical separations were manually performed Rf cation exchange separations performed by Silva et al.268 in 1970 using α-hydroxyisobutyrate (α-HIB) as eluent. The then newly discovered 65-s 261Rfa was produced in the 248Cm (18O,5n)261Rfa reaction, and the recoils were stopped on NH4Cl-coated Pt foils which were transported to the chemical separation area with a rabbit system. The 261Rfa and other products from the nuclear reaction, along with the NH4Cl, were collected from the Pt disk in a small volume of αHIB and were run through a small cation exchange resin column. Under these conditions, all cations with charge states of 4+ or higher were complexed with the α-HIB and eluted from the column. These experiments showed that Rf had a charge state of 4+ (or higher) and that its chemical properties are distinctly different from those of the actinides. 7.3.2. Automated Column Separations. With expected detection rates of only a few atoms per day or week, manually performed chemical separations become impractical. With automated liquid-phase chemical separation systems, faster chemical separation and sample preparation times can be achieved and the precision and reproducibility of the chemical separations has been improved over that obtainable via manual separations. An early, already rather sophisticated apparatus for automated extraction chromatographic studies of Rf−chloride complexes was described by Hulet et al. in 1980.269 The experimental apparatus and the data storage were fully automated and computer controlled. A total of six decays attributable to 261Rfa and its daughter 257No were registered. Later, to improve the speed and reduce cross-contamination, the ARCA II (automated rapid chemistry apparatus) was built by the GSI-Mainz collaboration, featuring two magazines of 20 miniaturized chromatography columns.270 With the large number of columns, cross-contamination between samples can be prevented by using each column only once. By miniaturizing the columns, the elution volume, and therefore, the time needed to dry the final sample to produce a source for α-particle spectroscopy, is much reduced. The radioactivity is delivered from the site of production at the accelerator to ARCA by an aerosol gas-jet system. Aerosol particles are collected on a frit or by impaction on a small spot on a slider (seen at the center of Figure 22). At the end of the collection

extensively applied to the transactinide elements. These adaptations have been developed to overcome the singleatom and short t1/2 limitations inherent in the study of transactinide element chemical properties. 7.3.1. Manual Liquid−Liquid Extractions and Column Chromatography. Manually performed liquid−liquid extractions have been used for the study of chemical properties of Rf263−266 and Db.267 The microscale liquid−liquid extraction technique used in these studies allowed minimizing the separation and sample preparation times; phase volumes were kept to ∼20 μL. Usually, a KCl aerosol gas-jet system was used for rapid transport and samples were collected on a special turntable by impaction on a Teflon disk. At the end of collection, the sample was dissolved with a few microliters of liquid. The time from end of collection to the beginning of counting of the transactinide chemical fractions was as short as 50 s, and the collection-separation-counting cycle could be repeated every 60−90 s. Because of interference from the radioactive decay of other nuclides (which are typically formed with much higher yields), extraction systems with relatively high decontamination factors from actinides, Bi, and Po must be chosen. The presence of the transactinide in the selectively extracting organic phase was determined by evaporating the fraction on a hot Ta foil and placing the sample in a α-particle spectroscopy system. With this technique, the measurement of distribution coefficients is somewhat difficult. By comparing the Rf or Db detection rate under a certain set of chemical conditions to the rate observed under chemical control conditions known to give near 100% yield, distribution coefficients between about 0.2 and 5 can be determined. If the control experiments are performed nearly concurrently, many systematic errors, such as gas-jet efficiency and experimenter technique, are canceled out. However, it must be ensured that during the chemical operations the transactinide element is not adsorbed to surfaces of the used equipment. For example, it was observed that Hf did adsorb on the used Teflon surfaces265 and that Pt-foils and polypropylene vials and pipet tips were better suited to conduct the experiments. Additionally, extraction systems which come to equilibrium in the 5−10 s phase contact time must be chosen. 1257


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Figure 23. Schematic of the ion-exchange part in AIDA. Reprinted with permission from ref 281. Copyright 2004 Elsevier.

forward movement of the column magazine to avoid crosscontamination at the collection site. The robotic sample preparation and counting technology, together with mechanical improvements in the chemical separation system, resulted in an automated column chromatography system that runs almost autonomously. Each separation in columns is accomplished within 20 s and the αparticle measurement can be started within 80 s after the collection of the products at the AIDA collection site. To shorten the time for the sample preparation of α sources, the newly developed rapid ion-exchange apparatus AIDA-II was introduced; the apparatus is based on continuous sample collection and evaporation of effluents, and successive αparticle measurement. The ion-exchange part is the same as that of AIDA. The AIDA-II was successfully applied for the chemical experiments with Db.282 The effluent is collected as fraction 1 on a 15 mm × 300 mm tantalum sheet which was continuously moved toward an α-particle detection chamber at 2.0 cm s−1. The sample on the sheet is automatically evaporated to dryness with a halogen heat lamp and then subjected to αparticle spectroscopy in a chamber equipped with an array of 12 silicon PIN photodiode detectors.282 Products remaining on the resin were eluted with a strip solution. The eluate was collected on a second Ta sheet as fraction 2, followed by the same procedures for sample preparation and measurement. The measurements were started 14 and 38 s after the end of product collection. 7.3.3. Automated Liquid−Liquid Extractions. Liquid− liquid extractions allow for very fast, continuously working arrangements, especially if detection of the separated nuclides occurs with highly efficient, continuous liquid scintillation counting (LSC). A system that has successfully been applied to transactinide chemistry is the so-called SISAK (short-lived isotopes studied by the AKUFVE-technique, where AKUFVE is a Swedish acronym for an arrangement of continuous investigations of distribution ratios in liquid extraction) system. This system performs continuous liquid−liquid extractions using small-volume separator centrifuges.283 Nuclear reaction products are delivered to the apparatus by an aerosol gas-jet. The gas-jet is mixed with the aqueous solution to dissolve the radioactivity-bearing aerosol particles, and the carrier gas is removed in a degasser centrifuge. The aqueous solution is then mixed with an organic solution, and the two liquid phases are separated in a separator centrifuge. A scintillation cocktail is then mixed with the organic solution, and this is passed through

time, the slider is moved to position the collection site above one of the miniature chromatography columns. A suitable aqueous solution is used to dissolve the aerosol particles and load the activities onto the column below. Selective elutions of transactinide elements are carried out by passing appropriate solutions through the column. A slider below the ion exchange column is moved at the appropriate time to collect the chemical fraction of interest on a hot Ta disk. A sample suitable for αparticle spectroscopy is prepared by rapid evaporation of the chemical fraction on the Ta disk, which is heated from below by a hot-plate and from above by a flow of hot He gas and a highintensity infrared lamp. The final samples are then manually placed in a detector chamber. The two magazines of chromatography columns can be moved independently. During a chemical separation on the left column, the right column is conditioned by passing an appropriate solution through it. After the separation on the left column is finished, the magazine is moved forward; placing a new column in the left position, the next separation is performed on the right column while the left column is being prepared for the subsequent separation. In this way, up to 40 separations can be carried out, at time intervals of less than 1 min, with each separation performed on a freshly prepared, unused column. Although the column separations are fully microprocessor controlled and performed automatically, still several people were required to operate the ARCA II system for a transactinide chemistry experiment. Transactinide chemical separations with ARCA II have been performed with Rf,271,272 Db,273−277 and Sg.278,279 Building upon the design of ARCA II, an automated column separation apparatus, AIDA (automated ion-exchange separation apparatus coupled with the detection system for αspectroscopy), has been developed at JAERI (now JAEA).280 Even though the apparatus for collection of aerosol particles and performing multiple chemical separations on magazines of miniaturized ion-exchange chromatography columns is very similar to that in ARCA II, AIDA has automated the tasks of sample preparation and placing the samples in the detector chambers. Using robotic technology, the selected fractions are dried on metal Ta disks and are then placed in vacuum chambers containing large-area PIPS α-particle detectors. A schematic drawing of AIDA is shown in Figure 22. In the ion-exchange process as shown in Figure 23, two different paths to supply solutions are available; the first eluent goes through the collection site to the microcolumn, while the second strip solution is directed to the column after one-step 1258


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Figure 24. Schematic of the SISAK liquid−liquid extraction system using the BGS as a preseparator. Reprinted with permission from ref 200. Copyright 2002 The Japan Society of Nuclear and Radiochemical Sciences.

a detector system to perform liquid scintillation α pulse-height spectroscopy on the flowing solution. An electronic suppression of signals originating from β-decays is applied. This modular separation and detection system allows the use of well-understood liquid−liquid extraction separations on time scales of a few seconds, with detection efficiencies near 100%, and it has been used to study the subsecond α-active nuclide 224 Pa.284−286 However, the detection of α-active transactinide isotopes failed despite pulse-height discrimination against interfering β-activities, which are produced with much larger yield. Preseparation with a kinematic recoil separator, as described in section 7.1.2, allowed chemical separation and detection of 4-s 257Rf using the SISAK technique.200 A schematic of the BGS-RTC-SISAK apparatus is presented in Figure 24. These proof-of-principle experiments have paved the way for detailed liquid−liquid extraction experiments with short-lived transactinide element isotopes. By isolating a counting cell from the stream of flowing liquid as soon as a potentially interesting signal occurred, genetically correlated decay chains could be detected.

molecule with its surroundings in order to ensure a statistically significant behavior. Here, chromatographic methods are preferred. 8.1. Volatility

In experiments with single molecules, the behavior of a species in a gas adsorption chromatographic experiment is determined by its molecular properties and the nature of the interaction with the adsorbent. The state of the adsorbent (column material) should be known, if possible. It is assumed that the investigated molecules are rather stable and/or stabilized by the chemical environment, which usually is characterized by extreme surpluses of the reaction partners in the carrier gas. Also, the state of zero surface coverage can be assumed for the stationary phase. As described in section 7, in gas-phase chromatography experiments, a measure of volatility is either the deposition temperature in a TC column, Ta, or the temperature at which 50% of the investigated species emerge from an isothermal column, T50%. From these temperatures, the enthalpy of 287 adsorption, ΔH0(T) a , is deduced using adsorption models, or Monte Carlo simulations are applied using a microscopic model developed by Zvara.288,289 The adsorption enthalpy, ΔH0(T) a , can be empirically related to the sublimation enthalpy, , of the macroamount. However, the usage of a ΔH0(298) S correlation between ΔH0(T) and ΔH0(298) is restricted to some a S groups and types of compounds, while not generally permissible (see below). In macrochemistry, a measure of volatility is an equilibrium vapor pressure over a substance, Pmm. Boiling points, Tb, and enthalpy of evaporation, ΔHevap, basically correlate with Pmm. Besides the low statistics of single events, a difficulty arises with respect to the interpretation of results, since the surface of the chromatographic column is not well-defined. Usually, it is modified by the evaporated aerosol transport particles and/or halogenating agents, so that the mechanisms of adsorption and, associated with it, the nature of chemical or physical interactions can only be assumed.240 Thus, available experimental data are often difficult to interpret and do not correlate with a single property or electronic structure parameter of the adsorbate. 8.1.1. Physisorption and Chemisorption Models. Quantum-mechanical calculations of adsorptionboth physisorption and chemisorptionof atoms and molecules on (poly)crystalline and even amorphous substrates are nowadays performed using modern periodic DFT codes. Within this slab/ supercell-approach, the surface of usually a single crystal is modeled by a slab of finite thickness due to the application of periodic boundary conditions, which introduce the semi-infinite character of the system. While in these codes the relativistic

8. METHODS TO PREDICT EXPERIMENTALLY MEASURABLE PROPERTIES OF TRANSACTINIDES Due to the very small production rates on the “one atom at a time” level, only very few physicochemical quantities of transactinide elements and their compounds can be determined experimentally. A further difficulty arises from the fact that, even in experiments with lighter homolog elements at the tracer scale (i.e., 106 to 109 atoms), there currently exist no analytical instruments with the required sensitivity that would allow determining the speciation of the investigated elements. Therefore, the measured microscopic properties have to be directly predicted by theory. Those theoretical predictions should be based on the best possible relativistic electronic structure calculations, since relativistic effects influence not only properties of volatile atoms and molecules but also the adsorption phenomenon. Some empirical correlation can also be useful. Such correlations between micro- and macroamounts may be valid for selected groups of elements and types of compounds, but they cannot necessarily be extrapolated to transactinide elements. Several constraints have to be fulfilled simultaneously to serve the purpose of a meaningful chemical study of transactinide compounds. First, a class of compounds should be studied, where relativistic effects are expressed in an experimentally detectable parameter. Second, fast, simple, and efficient procedures must be available to isolate the transactinide compound from interfering components. Third, the studied class of compounds should be thermodynamically stable and allow multiple interactions of the transactinide 1259


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effects of the core electrons are treated on the (relativistic) core-potential or pseudopotential level, the valence electrons are treated in nonrelativistic or outermost in a scalar-relativistic manner. Unfortunately, there are no full-relativistic, all-electron periodic implementations available yet. For adsorption of atoms or small molecules on metal surfaces, straightforward fully relativistic calculations of the adsorption energy are possible using relativistic 4c-DFT methods and the cluster approach. In this case, a surface of an adsorbent is modeled by a cluster Mn of a specific number n of metal atoms. The size of the cluster is then steadily enlarged until convergence of the binding energy of the ad-atom-metal cluster is reached with cluster size n. Such direct calculations are now possible up to more than a hundred atoms of the cluster.290a A possibility to treat an even larger number of atoms economically is foreseen via an embedded cluster procedure (see schematic in Figure 25).290b

E (x ) = −

3 ⎛ ε − 1⎞ ⎜ ⎟ 16 ⎝ ε + 2 ⎠

αmol

(

1 IPslab

+

1 IPmol

)x

3

(8.1.1)

where IPslab and IPmol are ionization potentials of the molecule and slab, respectively, ε is the dielectric constant of the surface material, and x is the molecule−surface distance. In a comparative study, x for a lighter element is deduced from the known ΔH0(T) a , while that for a heaviest element is estimated using the difference in their molecular size. Thermodynamic equations to predict Ta of a heaviest element with respect to Ta of a homolog in a comparative study using the knowledge of the electronic structure of the adsorbate are also presented.295 One of those equations is given below for the case of mobile adsorption of molecules with one rotational degree of freedom: e−ΔEA / RT

1 A 1/2 t1/2 rAdAT1/2 A mA

= e−ΔEB / RT

1 B t1/2 rBdBTB1/2mB1/2

(8.1.2)

where t1/2 is the half-life of the central nuclide, r the molecular radius, d the metal−ligand distance, R the gas constant, T the adsorption temperature, m the mass, and ΔE the adsorption energy of a heaviest molecule A and of its lighter homolog B. In the same work, various measures of volatility were critically compared. The most adequate one in a comparative study (in macrochemistry) was shown to be the ratio of adsorption/ desorption constants, Kads/Kdes. For predictions of adsorption of molecules with nonzero dipole moments, equations taking into account long-range interactions, such as molecular dipole−surface charge, dipole− induced dipole, and van der Waals one, were used. Thus, for example, the interaction energy of a molecule with a surface having a charge is as follows:291

Figure 25. Embedded M′−Mn system. Reprinted with permission from ref 25. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH.

E (x ) = −

ΔH0(T) a

As was mentioned, to determine of a heavy molecule on a complex surface is still a formidable task for quantum chemical calculations. Especially difficult is the prediction of physisorption phenomena caused by weak interactions, where the DFT generally fails. In the past, DS-DV calculations were helpful in establishing some correlations between electronic structure parameters and volatilities of halides, oxyhalides, and oxides known from macrochemistry.35,39 For example, it was established that covalent compounds having high overlap populations (OP) are more volatile than ionic ones, that molecules with dipole moments (μ) interact more strongly with surfaces than those without, and that the sequence in the adsorption energy is defined by the sequence in μ. Lately, predictions of interaction energies of heaviest element molecules with inert surfaces (quartz, silicon nitride, also modified) were made with the use of physisorption models.291−294 These models are based on the principle of intermolecular interactions subdivided into usual types for longrange forces: dipole−dipole, dipole−polarizability, and van der Waals (dispersion) ones. The molecular properties required by those models are then calculated using the most accurate relativistic methods. Thus, for a molecule with zero dipole moment adsorbed on a dielectric surface by van der Waals forces, the following model of the molecule-slab interaction294 is used:

2Qeμmol 2 x

2

−

Q 2e 2αmol 2x

4

−

3 2

αmolαslab

(

1 IPmol

+

1 IPslab

) (8.1.3)

where μmol, IPmol, and αmol belong to the molecule and those with index “slab” to the surface; Q is the charge of the surface atom and x is the molecule−surface distance. With the use of those models (eqs 8.1.1−8.1.3), adsorption of various group-4 through group-8 species, including the heaviest ones, on various (nonmetal) surfaces was predicted (see below). 8.1.2. Empirical Correlations and Extrapolations. The concept that bond energies between identical molecules or atoms in the crystal lattice should be proportional to the adsorptive bond energies between the same single molecules or atoms and a surface was empirically demonstrated by plotting ΔH0(T) versus ΔH0(298) for certain gas chemical systems, for a S example, for chlorides and oxychlorides in Cl2, HCl, and CCl4 (O2) on quartz, and for oxides and oxyhydroxides in O2 (H2O) on quartz.296 In other words, it is assumed that the molar binding energy of an adsorbed single molecule to the surface approximately equals its partial molar adsorption enthalpy at zero surface coverage. As an example, the empirical correlation observed for chlorides and oxychlorides between −ΔH0(T) a measured for single molecules on the chromatographic surface (quartz, statically or dynamically modified by the chlorinating reagents) and the macroscopic ΔH0(298) is shown in Figure 26. S 1260


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enthalpy ΔH0(298) of these elements. Those linear extrapolaS tions should be used with care, as there is no solid theoretical basis for them. 8.2. Complex Formation in Aqueous Solutions

In liquid-phase chemistry, single atoms of transactinide elements (or of their lighter homologs) are transported by an aerosol gas-jet technique from the site of synthesis behind the target or behind a kinematic recoil separator to a filter or impaction device, where samples are collected and dissolved in an aqueous solution after an appropriate collection time. Usually, salt aerosol particles, such as KCl, are used that are easily generated by heating the salt in a gas stream to a temperature slightly below the melting point. The KCl particles readily dissolve in contact with an aqueous solution. The aqueous solution contains suitable ligands for complex formation. The complexes are then chemically investigated using a partition method by studying, for example, their extraction into an organic solvent, or by anion- or cationexchange chromatography, or by reversed-phase extraction chromatography. The ultimate goal of partition experiments is to determine the distribution coefficient Kd as a function of ligand concentration. For a simple complex MLn, the cumulative complex formation constant

Figure 26. Empirical correlation observed for chlorides and oxymeasured for single molecules on the chlorides between −ΔH0(T) a chromatographic surface (quartz, statically or dynamically modified by .234 the chlorinating reagents) and the macroscopic ΔH0(298) S

Unfortunately, there are no generally valid correlations that relate ΔH0(298) to other molecular properties. Attempts were S made to correlate the inverse boiling point (Tb) with the geometric structure of molecules. For the halides of the metals of the type MXn (n = 4, 5, 6; X = F, Cl, Br, I), Zvara empirically derived a formula which, for a given stoichiometry (at maximum symmetry), correlated Tb with the ionic radius of the central metal ion.289,297 For certain classes of compounds, such as the tetrachlorides, an empirical correlation exists between ΔH0(298) and the orbital radii or crystal ionic radii of S the group-4 elements Ti, Zr, and Hf, but also U and Th.298 It should be noted that due to the influence of relativistic effects the extrapolative power of such empirical relationships is limited. A simple and very early extrapolation, which will be referred to later, is a prediction of the standard enthalpies of monatomic gaseous elements, ΔH*298(E(g)), to the heavy transactinides Cn through element 120 by Eichler299 by extrapolating over the atomic number Z (Figure 27). The standard enthalpies of monatomic gases are mostly equal to the standard sublimation

βn = [MLn][M]−1 [L]−n

(8.2.1)

is a measure of its stability. For stepwise processes, consecutive constants Ki are used. If various MLnz−n complexes exist in the aqueous phase, but only one MLip− complex in the organic phase, the distribution coefficient, Kd, for the anion exchange is given by the following equation300 Kd =

p KDM[RB+L−]org βi [L−]i − p N

∑0 βn[L−]n

(8.2.2)

where KDM is the association constant with the organic cation. Thus, a sequence in the Kd values for a studied series, for example, for elements of one group, reflects the sequence in the stability of their complexes. Complex formation is known to increase in the transition element groups. In aqueous solutions, it is, however, competing with hydrolysis. This may change trends in the stabilities of complexes and, finally, in their extraction into an organic phase. One should distinguish between hydrolysis of cations and hydrolysis of complexes.301 The former is described as a process of a successive loss of protons M(H 2O)n z + ⇄ MOH(H 2O)n − 1(z − 1) + + H+

(8.2.3)

In acidic solutions, hydrolysis involves either the cation, anion, or both, and is competing with complex formation described by the following equilibrium x M(H 2O)w ° z + + yOH− + i L− ⇄ MxOu (OH)z − 2u (H 2O)w Li(xz − y − i) + + (xw° + u − w)H 2O

(8.2.4)

In order to predict a sequence in Kd (eq 8.2.4), one should predict a sequence in the formation constants of a series of species of interest. For a reaction such as, for example, eq 8.2.3 or 8.2.4,

Figure 27. Extrapolation of the standard enthalpies of monatomic gaseous elements ΔH*298(E(g)) along the groups of the Periodic Table based on the atomic number Z.299

log K i = − ΔGr /2.3RT 1261

(8.2.5)


Chemical Reviews

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where ΔGr is the free energy change of the complex formation reaction. To obtain it in a straightforward way, binding or total energies of species in the left-hand and right-hand parts of the reaction should be calculated. Such relativistic calculations, with full geometry optimization for the heaviest elements, are extremely computer time intensive. In addition, the obtained accuracy might be insufficient to predict stabilities of similar species of homologs. Therefore, the following economical model was suggested by Pershina. In a fashion analogous to that of Kossiakoff and Harker,302 the following expression for the free energy of formation of the MxOu(OH)v(H2O)w(xz−2u−v)+ species from the elements was adopted

difference to some elements of the sixth row.). The relativistic stabilization of the 7p1/2 electron was explained to be responsible for the unusual ground state in comparison with the (n−1)d2ns2 (3F2) state of the lighter homologs Zr and Hf. More accurate DCB FSCC calculations,312 however, corrected the MCDF result,175,176 giving 7s26d2 (3F2) as ground state configuration. A very high level of correlation with l = 6 was required to reach this accuracy. The IP of Rf of 6.01 eV was calculated at best using the DCB FSCC method.312 It is smaller than the IP of Hf (6.83 eV313) due to the relativistic destabilization of the 6d AOs. The first ionized electron in Rf is the 6d one, like the 5d electron in Hf. MCDF multiple IPs of M through M4+ states were also calculated.176 There is a steady decrease in the IPs(M → M4+) in group 4 due to the same reason, i.e., the destabilization of the (n−1)d AOs with increasing Z (Figure 28).

−ΔG f (u , v , w)/2.3RT =

∑ ai + ∑ aij + log P −

log(u! v ! w! 2w)

+ (2u + v + 1)log 55.5

(8.2.6)

The first term on the right-hand side of eq 8.2.6, ∑ai, is the nonelectrostatic contribution from M, O, OH, and H2O, which is related to the overlap population. For a reaction, Δ ∑ ai = ΔE OP = k ΔOP

(8.2.7)

where k is an empirical coefficient. The next term, ∑aij, is a sum of each pairwise electrostatic (Coulomb) interaction: EC =

∑ aij = −B ∑ Q iQ j/dij ij

(8.2.8)

where dij is the distance between moieties i and j; Qi and Qj are their effective charges, and B = 2.3RTe2/ε, where ε is a dielectric constant. For a reaction, ΔEC is the difference in EC for the species in the left and right parts. P in eq 8.2.6 is the partition function representing the contribution of structural isomers if there are any. The last two terms are statistical: one is a correction for the indistinguishable configurations of the species, and the other is a conversion to the molar scale of concentration for the entropy. ∑aij and ∑ai for each compound are then calculated directly via a Mulliken analysis implemented in most of the quantum-chemical methods (e.g., 4c-DFT163). To predict log Ki or log βi for transactinide complexes, coefficients k and B should then be defined by fitting log Ki to experimental values for the lighter homologs. Using this model, hydrolysis and complex formation constants were predicted for a large number of aqueous compounds of group-4 through group-6 elements303−311 in very good agreement with experimental results. The results of these calculations and a comparison with experimental data reveal that a change in the electrostatic metal−ligand interaction energy (ΔEC) of a complex formation reaction contributes predominantly in the change in ΔGf, i.e., in ΔGr. Thus, only by calculating ΔEC can trends in the complex formation be reliably predicted.

Figure 28. Ionization potentials to the maximum oxidation state (IPmax) and ionic radii (IR) for Rf through Hs obtained from the MCDF calculations.175−179 Reprinted with permission from ref 39. Copyright 2003 Kluwer Academic Publishers.

This results in an increase in the stability of the maximum oxidation state in this group, which was shown by the values of the respective redox potentials.314,315 The atomic radius (AR) defined by the outer 7s AO of Rf is 1.49 Å, as derived from the earlier DF atomic calculations.35 It is smaller than the AR(Hf) of 1.55 Å316 due to the relativistic contraction of the 7s(Rf)AO. Ionic radii (IR) obtained via a correlation with the outer (n− 1)p AOs of the M4+ ions show an increase in the group, so that the IR(Rf4+) is 0.79 Å,176 which is larger than the IR(Hf4+) of 0.71 Å317 (Figure 28). A better value of the IR(Rf4+) derived from molecular calculations is, however, 0.76 Å.318−320 A set of atomic single and triple bond covalent radii (CR) for most of the elements of the Periodic Table, including the heaviest ones till Z = 118 and Cn, respectively, was suggested.319,320 They are deduced from the calculated molecular (equilibrium) bond lengths (Re) of various covalent compounds. The CR of the group-4 to group-8 6d elements are about 0.5−0.8 Å larger than those of the 5d elements. An important finding of these works is a decrease in the R6d − R5d difference starting from group 9, reaching negative values in groups 11 and 12, as a result of the relativistic bond contraction (Figure 29). This is called a “transactinide break”. There are several other calculations of molecular compounds of Rf. The hydrides MH4 (M = Ti through Rf) were calculated using the DF one-center expansion method.321−323 Relativistic effects were shown to decrease the Re in RfH4, so that it is only 0.03 Å larger than Re(HfH4). The relativistic contractions of

9. RUTHERFORDIUM (Z = 104) 9.1. Theoretical Predictions

The chemistry of Rf is expected to be similar to that of Zr and Hf and defined by the four valence electrons, thus favoring the stable 4+ oxidation state. The MCDF calculations175,176 have given 7s27p6d (3D2) as ground state for this element (The stabilization of the 7s2 pair was found for the entire seventh period as a result of the relativistic stabilization of the 7s AO, in 1262


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The binding energy of RfCl4 and homologs was calculated using the 4c-DFT method.318 The compound was shown to be stable with De of 19.5 eV, though less stable than ZrCl4 (21.7 eV) and HfCl4 (21.1 eV). The lower stability was explained via a smaller contribution of the ionic contribution to the binding energy due to a decreasing effective metal charge, QM.315 The RECP calculations327 gave 18.8 eV for De(RfCl4), in overall agreement with ref 318. A decrease in De(RfCl4) in comparison with De(HfCl4) was shown to be due to the larger SO effects on the 6d AOs. Re(RfCl4) was shown to be larger by about 0.05 Å than Re of HfCl4,327 in agreement with other data.319,320 Solid-state calculations were performed on Rf metal.329 The structural and electronic properties were evaluated by first principles DFT in the scalar relativistic formalism with and without SO coupling and were compared with those of its 5d homolog, Hf. It is found that Rf should crystallize in the hexagonal close packed structure, as does Hf. However, under pressure, it should have a different sequence of phase transitions than Hf: hcp → bcc instead of hcp → ω → bcc. An explanation is offered for this difference in terms of the competition between the band structure and the Ewald energy contributions. The aqueous chemistry of Rf has also been studied theoretically.307,310 As do other group-4 elements, Rf undergoes hydrolysis and complex formation in acidic solutions. The following reactions are important: the first hydrolysis step

Figure 29. The difference in the lengths of the single (open circles) and triple (filled triangles) bonds between the 6d and 5d metals.319,320

orbitals and bond lengths were shown to be two parallel but largely independent effects. The calculations showed a decrease in the atomization energy, De, of RfH4 as compared to that of HfH4. Results of ab initio noncorrelated DF calculations were reported for RfCl4.324 DS-DV calculations were performed for MCl4.315,325 The calculations agreed on the fact that the electronic structure of RfCl4 is similar to that of ZrCl4 and HfCl4 and that RfCl4 is a typical d-element compound. Covalency judged by the Mulliken OP was shown to increase down group 4.315 This is due to an increase in an overlap of the relativistic ns and (n−1)d AOs of the central ion with AOs of the ligand. Nonrelativistically, the trend is just opposite. Due to its highest covalency, RfCl4 is therefore expected to be the most volatile among group-4 MCl4.315 This is in contrast to simple extrapolation procedures229,298,326 using a radius−volatility correlation of tetrachlorides that predict the opposite (Figure 30). So experimentally measuring the volatility of RfCl4 in comparison to its lighter homologs HfCl4 and ZrCl4 provides an ideal test case.

M(H 2O)8 4 + ⇄ MOH(H 2O)7 3 +

(9.1.1)

the stepwise fluorination M(H 2O)8 4 + ⇄ MF(H 2O)7 3 + ... ⇄ ... MF3(H 2O)5+ ...⇄ (9.1.2)

MF4 (H 2O)2 ⇄ ... MF5(H 2O)− ⇄ MF6 2 −

(9.1.3)

and the total chlorination M(H 2O)8 4 + + 6HCl ⇄ MCl 6 2 −

(9.1.4)

The free energy change of reactions 9.1.1−9.1.4 was calculated using the model described in section 8.2 and 4cDFT calculations of the electronic structure of the complexes.307 The obtained data suggest the following trend in hydrolysis of the group-4 elements Zr > Hf > Rf. For cation exchange separations (CIX) performed at Rf. In the case of formation of complexes with a lower positive charge from complexes with a higher positive charge, the sequence in the Kd values is opposite to the sequence in complex formation, since complexes with a higher charge are better sorbed on the CIX resin than those with a lower charge. For the formation of anionic complexes sorbed by anionexchange (AIX) resins, the trend becomes more complicated depending on pH, that is, depending on whether the fluorination starts from hydrated or hydrolyzed species. Thus, for experiments conducted in 10−3 to 10−1 M HF (where some hydrolyzed or partially fluorinated species are present), the trend for the formation of MF62− (eq 9.1.3) should be reversed in group 4: Rf ≥ Zr > Hf.

Figure 30. Vapor pressure of group-4 chlorides over their respective solids as a function of temperature. Literature data for ZrCl4 and HfCl4 from Knacke et al.328 The vapor pressure curve labeled “RfCl4 relativistic” resulted from using the QM data from Pershina et al.315 and applying the procedure outlined in section 8.1.1, whereas the shaded area labeled “RfCl4 extrapolated” indicates the predicted vapor pressure of RfCl4 using two different extrapolation procedures.326 1263


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Figure 31. M(SO4)2(H2O)4 and M(SO4)44− complexes of Zr, Hf, and Rf. Reprinted with permission from ref 310. Copyright 2006 Oldenbourg Wissenschaftsverlag GmbH.

For the AIX separations at 4−8 M HCl, where no hydrolysis should occur at such high acidities, the theoretical data suggest that the trend in the complex formation and Kd values should be continued with Rf: Zr > Hf > Rf. Complex formation of group-4 elements in H2SO4 solutions was also studied theoretically.310 In this work, relative values of the free energy change of the M(SO 4 ) 2 (H 2 O) 4 , M(SO4)3(H2O)22−, and M(SO4)44− (M = Zr, Hf, and Rf) formation reactions from hydrated and partially hydrolyzed cations were calculated using the 4c-DFT method. Geometrical configurations of two of the considered complexes are shown in Figure 31. The results indicate the following trend in complex formation, Zr > Hf ≫ Rf. The obtained log Kd values for the extraction of Zr, Hf, and Rf from H2SO4 solutions by amines are shown in Figure 32.

that group-4 elements form rather volatile halides was exploited. A good measure for the volatility of a molecule is its vapor pressure over its respective solid (see Figure 33) (It

Figure 33. Vapor pressure curves for Zr and Hf halides over their respective solids.334. Reprinted with permission from ref 334. Copyright 1994 Springer Science and Business Media.

9.2. Experimental Results

Early on, two different chemical strategies were chosen to demonstrate that the chemical properties of Rf are distinctly different from those of the actinide elements. In Dubna, the fact

was, therefore, originally suggested by Pershina et al.330,331 to use those vapor pressure curves also in the chemical studies on the transactinides in comparison with their lighter homologs). The volatility decreases according to MCl4 > MBr4 > MI4 > MF4 with M = Zr, Hf. While the iodides show poor thermal stability and the fluorides are the least volatile, chlorides and bromides are a good choice for a transactinide chemistry experiment. Clearly, the heavy actinides do not form volatile chlorides or bromides. Therefore, an experiment that isolates Rf in the form of volatile chlorides or bromides also demonstrates that this element does not belong to the actinide series. Immediately after the discovery of a short-lived spontaneously fissioning radioactivity at Dubna in a physics experiment, a series of gas-phase chemistry experiments were conducted by Zvara et al.60−67 to prove that this radioactivity was indeed not due to an actinide element. Later, the volatility of group-4 chlorides and bromides was reinvestigated by Türler,225,230 Kadkhodayan,332 and Sylwester333 using online IC with identification of 261Rfa through α-particle spectroscopy, α−α correlations, and determinations of t1/2 of 261Rfa and its daughter 257No. Enthalpies of adsorption of the investigated volatile compounds on the chromatographic surface (−ΔH0(T) a ) were deduced. In Berkeley, aqueous phase chemistry experiments88 were conducted with the same isotope 261Rfa. Tetravalent Rf (Rf4+)

Figure 32. Predicted log Kd for the extraction of Hf and Rf from H2SO4 solutions by amines with respect to the ones measured for Zr. Reprinted with permission from ref 310. Copyright 2006 Oldenbourg Wissenschaftsverlag GmbH. 1264


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was eluted with the complexing agent ammonium αhydroxyisobutyrate (α-HIB) from a cation exchange column, way ahead of the trivalent actinides or divalent No. By preparing samples suitable for α-particle spectroscopy, Silva et al.268 could unambiguously identify 261Rfa in the eluate containing the tetravalent elements by observing time correlated α−α mother daughter correlations attributed to the decay of 261Rfa followed within 1 min by the α-particle decay of the daughter 257No. 9.2.1. Gas-Phase Chemistry of Rutherfordium. When it became obvious that t1/2(260Rf) < 0.3 s and a newly detected 4.5-s SF radioactivity335 was due to a possible SF (or EC, leading to SF in 259Lr) branch336,337 in 259Rf (which decays primarily by α-emission with t1/2 = 3.2 s338), Zvara et al.64 repeated their experiments with a new apparatus, which combined IC with TC. A schematic of the apparatus and the obtained results is shown in Figure 34.

were interpreted that the SF-tracks observed were due to SF of Rf behaving like the lighter homolog element Hf, and that also in earlier experiments the SF decay of this Rf isotope has been observed. This work and also the earlier experiments by Zvara et al. were criticized by the members of the Berkeley group.258 The three points of criticism concerned the unknown magnitude of the SF-branch in 259Rf, the inability to determine t1/2 of the source of SF, which would have allowed excluding SF from actinide nuclei such as 256Fm, and the absence of many SF tracks in the isothermal section due to the decay of shorterlived 260Rf. Concerning the measurement of the magnitude of the SFbranch of 259Rf not too much progress has been accomplished. The only direct measurement was performed by Bemis et al.,336 who reported a SF branching ratio of 6.3 ± 3.7%. In the work of Bemis et al.,336 a total of 22 SF events were observed. Of these, 8 ± 2 events were ascribed to long-lived 256Md/256Fm. Another source of SF was identified in the presence of 256No. Here, a SF branch of 0.25% was used to estimate an additional contribution of 4.8 ± 1.2 SF events not related to 259Rf, leaving 9.2 ± 5.2 SF events. However, in a recent experiment, the SF/α ratio in 256No was measured339 as 0.0053+0.0006 −0.0003. With this new value, the additional contribution due to 256No increases to 10.2 SF events, leaving now only 3.8 of the observed 22 SF events attributable to 259Rf. Thus, the SF branch in 259Rf reduces to 2.6%. Gates et al.337 have observed an EC branch of 15 ± 4% in 259 Rf, leading to 259Lr, which decays by SF with 25 ± 3% branching. This would result in an apparent SF-branch of 259Rf of 3.8 ± 1.1%. Ascribing some of the observed SF events to 259 Lr after EC decay of 259Rf would not alter the conclusions drawn on the chemical properties of Rf, since, under the conditions of the experiment by Zvara et al.,61,63,64 Lr is expected to form nonvolatile compounds which would deposit essentially at the same position in the column as Rf. In addition, a SF-branch of >2% (or an EC-branch >8%) in 259Rf would be sufficient to explain the number of SF events by Zvara et al.61,63,64 Interestingly, the chemical aspects of the experiment were not discussed. The fact that Rf nuclides with t1/2 = 3 s deposited in the column at the same position as did 170,171Hf (t1/2 = 16.0 and 12.2 h, respectively) does not at all mean that HfCl4 and RfCl4 exhibit the same volatility. With the development of a microscopic description of the chromatographic process,288 the migration of a molecule through the column can be simulated using a Monte Carlo technique. With this new technique, a quantitative analysis of this experiment is possible and will allow a comparison of the obtained ΔH0(T) a (RfCl4) with data of newer, IC experiments. The results of a Monte Carlo simulation with the microscopic model of Zvara288 are shown in Figure 35. In the upper panel, the experimental data are shown together with the simulated deposition zone profiles for 259 RfCl4 and 170,171HfCl4. The only adjustable parameter was ΔH0(T) a ; all other parameters were fixed at their experimental values. In the lower panel the integrated yields are shown in comparison with the simulation. The deposition zone of HfCl4 can be reproduced very accurately. The experimental distribution of SF events of 259Rf is somewhat narrow compared to the simulated zone profile, but in the light of the poor statistics, the two distributions are in reasonable agreement. −1 The determined −ΔH0(T) a (RfCl4) is 110 kJ·mol . For the 0(T) lighter homolog Hf, −ΔHa (HfCl4) = 146 kJ·mol−1 resulted. 259

Figure 34. (a) Schematic of the experimental apparatus to investigate the volatility of 259Rf and 170,171Hf chlorides; (b) temperature profile in the column (isothermal combined with a gradient); (c) distribution of SF tracks (open and closed circles) for 44mSc and 170,171Hf. Reprinted with permission from ref 58. Copyright 1987 Oldenbourg Wissenschaftsverlag GmbH.

As in previous experiments, the nuclides recoiling from the target were stopped in a stream of N2 gas and flushed into the chromatography section. Due to the longer t1/2(259Rf), the flow rate could be reduced by about 1 order of magnitude, allowing higher separation efficiency from short-lived SF-isomers in the actinides and better chromatographic resolution. SF track detectors were inserted into sections II and III. As reactive agents a mixture of SOCl2 and TiCl4 (which also served as carrier) was used. In order to simultaneously produce Hf nuclides, the 242Pu target contained an admixture of Sm. This way, the deposition peak of long-lived 170,171Hf and 259Rf could be measured simultaneously in the same experiment. Indeed, after completion of the experiment, the isothermal section II contained only a few fission tracks at the very beginning and one SF track further downstream, whereas, in the gradient section, 15 SF tracks were observed at about the same position as long-lived Hf was deposited. The results of this experiment 1265


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pure quartz surface but on a surface covered at least partially with KCl. These first experiments were repeated with the significantly improved HEVI setup.216,332,333 Instead of KCl, MoO3 was used as aerosol particle forming material, which is converted to the volatile MoO2Cl2 in the chloride experiments.332 In the bromide experiments, using KBr aerosol particles, significantly smaller deposits inside the chromatography column were observed compared to the cases of the chloride experiments with KCl aerosol particles.333 As in earlier experiments, RfBr4 was more volatile than its lighter homolog HfBr4. The volatility of ZrBr4 was in-between that of HfBr4 and RfBr4. A similar picture emerged for the chlorides. Again RfCl4 was more volatile than HfCl4, while ZrCl4 turned out to be similarly volatile as RfCl4. The volatility of RfCl4 in comparison to its lighter homologs was once again investigated using the OLGA(III) setup.230 In this work, carbon aerosol particles were used, which were converted to CO2 in the hot reaction oven. In these experiments it was shown that the concentration of O2 is an important parameter, which has to be carefully controlled. The addition of O2 led to a significant shift of the volatility of both Hf and Rf to higher temperatures by about 100 to 200 °C, respectively, as shown in Figure 36, but this confirmed the Figure 35. Measured and simulated deposition peaks of 259RfCl4 and 170,171 HfCl4. The modeled deposition peaks were obtained using the microscopic model of Zvara288 and a Monte Carlo simulation technique, with the only adjustable parameter being the adsorption enthalpy (for details see text).326

Further TC experiments with the nuclide 259Rf involving also the study of the tetrabromides were conducted much later (1991) and reported only in the form of a contribution to the Joint Institute for Nuclear Research, Laboratory of Nuclear Reactions annual report.340 A quantitative analysis of these experiments was reported.229 For the adsorption on quartz −1 surfaces, ΔH0(T) and ΔH0(T) a (RfBr4) = −68 kJ·mol a (HfBr4) = −1 −86 kJ·mol were obtained. Again, RfBr4 seemed to be more volatile than the homologous HfBr4. A first experiment using IC of group-4 chlorides and bromides with direct identification of the reaction products with α-particle and SF-spectrometry was published in 1992225 using the OLGA(II) setup in combination with the MG rotating wheel detector. For these studies, the nuclide 261Rfa (t1/2 ≈ 1 m) was used,88 and altogether 14 α−α correlations attributed to the decay sequence 261

α 257

Rf a →

α 253

No →

Figure 36. Yields of 261Rfa and 165Hf as a function of isothermal temperature. Reactive gases were 200−300 mL/min HCl purified from traces of O2 (□, ●) and 150 mL/min Cl2 with SOCl2 and 20 mL/min O2 (△, ◆), respectively.230. Reprinted with permission from ref 230. Copyright 1998 Elsevier.

Fm

higher volatility of RfCl4 compared to HfCl4. In a more recent publication,341 the volatility of Zr, Hf, and Rf chlorides was investigated simultaneously. In this work, no significant differences in the volatility of the three group-4 chlorides were observed. Nevertheless, the T50% temperature of RfCl4 was about 25 °C lower compared to that of the longer-lived Hf at the same isothermal temperature, yielding some indications of a more volatile RfCl4. In the following, the available results on group-4 halides, including Rf, have been analyzed in a meta analysis using the microscopic model of Zvara288 and a standard set of parameters. This was necessary since, in the original publications, differing parameters were used and thus apparent deviations occurred. In Table 2, the published and the evaluated ΔH0(T) values measured for group-4 tetrachlorides a and tetrabromides of Zr, Hf, and Rf in various experiments are summarized. The table shows ΔH0(T) values evaluated with a a consistent characteristic period of oscillation of SiO2 in the

were detected, yielding the correct t1/2 of the mother and the daughter nuclides. So, this experiment yielded unambiguous proof that volatile complexes of Rf were isolated using gasphase chromatography. In addition, the behavior of Rf could be directly compared with that of 162Hf (t1/2 = 41 s), a nuclide with a very similar t1/2 compared to that of 261Rfa. It was shown that group-4 chlorides RfCl4 and HfCl4 were more volatile than their respective bromides. In addition, RfBr4 was more volatile than HfBr4 in the same experiment. Note that in earlier TC experiments the behavior of 3.1-s 259Rf was compared with that of 16-h 170Hf. Although the isothermal temperature profiles were far from being ideal and the column was too short in this first experiment, it was possible to extract thermochemical information326 by applying the microscopic model by Zvara.288 A further drawback was the usage of an aerosol particle gas-jet, which lead to visible deposits of KCl in the chromatography column, and thus the adsorption behavior was not studied on a 1266


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Table 2. Comparison of Published and Evaluated ΔH0(T) Values Measured for Group-4 Tetrachlorides and Tetrabromides in a Various Experiments −ΔH0(T) publ (kJ·mol−1) a technique

ref

year

aerosol or carrier gas

FC TC TC IC (OLGA II) IC (HEVI) IC (OLGA III)

62 64 340 225 332 230

1969 1971 1991 1992 1996 1998

N2 N2 Ar He/KCl He/MoO3 He/C

TC IC (OLGA II) IC (HEVI)

340 225 333

1991 1992 1996

Ar He/KCl He/KBr

ZrCl4 84

74 ± 5

ZrBr4

91 ± 6

−ΔH0(T) eval (kJ·mol−1) a

HfCl4

RfCl4

84 155c 105c ≤70d 96 ± 5 110c

84−96 104c 83c ≤80d 77 ± 5 92c

84

ZrCl4

HfBr4 82c 125d 113 ± 5g

RfBr4 63c 105d 89 ± 5g

ZrBr4

c 79 ± 5 97e

95 ± 6

HfCl4

RfCl4

84 146b c ≤75 103 ± 5 103

100−109a 110b c ≤85 82 ± 5 87

HfBr4 86f 130 117 ± 5

RfBr4 68f 111 93 ± 5

a

Data from ref 343. bSee Figure 35. cData from ref 297; evaluation not possible due to missing experimental details. dData from ref 344. e Unpublished data by Türler et al. fSee also ref 229. gData from ref 345.

Monte Carlo model of τ0 = 2 × 10−13 s. Also shown are the ΔH0(T) values originally published in the literature or the a evaluated ΔHa0(T) values of Zvara.297 After analyzing the experiments of Kadkhodayan et al.332 and Türler et al.230 using a consistent τ0, both experiments are in very good values of HfCl4 and RfCl4, agreement concerning the ΔH0(T) a values for ZrCl4. The while there are differences in the ΔH0(T) a (ZrCl ) = 97 kJ·mol−1, which was unpublished value of −ΔH0(T) a 4 evaluated in measurements with the OLGA(III) setup in test experiments with 98Zr under very similar conditions, is in very good agreement with off-line TC experiments, where −ΔH0(T) a (ZrCl4) = 98 kJ·mol−1 was determined.342 The use of MoO3 aerosol particles (Kadkhodayan et al.332) or C aerosols (Türler et al.230) did not influence the adsorption characteristics of group-4 chlorides on the quartz surface. In the first IC experiments with Rf of Türler et al.,225 only upper limits could 0(T) (RfCl4). Due be established for −ΔH0(T) a (HfCl4) and −ΔHa to the very short isothermal length of the chromatography columns, no decrease of the yield at low temperatures was observed. While the upper limit deduced for −ΔH0(T) a (RfCl4) is in agreement with the values obtained in later experiments, HfCl4 was found to be much more volatile. The reason for the observed behavior is not clear, and the experiment was not values for HfCl4 and RfCl4 repeated at the time. The ΔH0(T) a evaluated from a TC experiment of Zvara et al.340 in a quartz chromatography column are in very good agreement with the results of IC experiments. The ΔH0(T) values of RfCl4 evaluated a from the very early experiments at Dubna are in good agreement with each other, but compared to later experiments, Rf-chlorides were about 25 kJ·mol−1 less volatile. However, these experiments were performed on different chromatographic surfaces. The columns were made from glass into which mica sheets were inserted. Also, the ΔH0(T) values for HfCl4 are a quite different in the two experiments, which may point to a varying content of residual O2 in the two experiments. The addition of O2 reduced the volatility of both Hf- and Rfchlorides considerably in isothermal experiments of Türler et al.230 (see Figure 36). The introduction of a consistent characteristic period of oscillation τ0 has reduced the apparent differences in the values significantly. The reason for the disagreeing ΔH0(T) a ΔH0(T) values originally published in the literature is therefore a not mainly due to the varying degree of the chemical modification of the column surface by the different

halogenating agents, as was speculated by Zvara,297 but only due to differences in the analysis procedures. Actually, the agreement between all experiments conducted on quartz surfaces230,332,340 is extraordinary. In all experiments, RfCl4 was found to be more volatile than HfCl4. The volatility of RfCl4 seems to be more similar to that of ZrCl4, so that the sequence in volatility is ZrCl4 ≈ RfCl4 > HfCl4. This result is surprising and was interpreted as a manifestation of relativistic effects.230 Extrapolation procedures that rely on the regularities that govern the trends in the physicochemical properties within the groups and periods of the Periodic Table clearly indicated that RfCl4 should be less volatile than HfCl4. A high volatility of RfCl4 was only discussed after relativistic MO calculations315,325 pointed to a low ionic and a high covalent contribution to the bonding in RfCl4. For group-4 bromides, the agreement of the evaluated values of different experiments is poor (see Table 2). ΔH0(T) a While all experiments consistently found RfBr4 to be more values range from volatile than HfBr4, the evaluated −ΔH0(T) a 68 to 111 kJ·mol−1. The fact that in OLGA(II) experiments about 15 kJ·mol−1 less volatile Hf- and Rf-bromides were observed compared to experiments with HEVI could be attributed to the use of a KCl aerosol gas-jet to transport the reaction products to the chemistry setup. Thus, the quartz column was at least partially covered with KCl. Generally, lower volatilities have been observed on KCl surfaces compared to pure quartz.246 The two experiments using IC found RfBr4 and HfBr4 to be less volatile than the corresponding tetrachlorides, while in the TC experiment RfBr4 and HfBr4 were considerably more volatile. Considering macroscopic properties such as the vapor pressure over the respective solid or the boiling point, ZrBr4 and HfBr4 are expected to be slightly less volatile than their corresponding tetrachlorides, as observed in both IC experiments. In the TC experiment the possibility of two close and thus unresolved deposition peaks was discussed, since the distribution of SF tracks was much wider than that of the Hf deposition peak.340 However, the Monte Carlo analysis of the experiment clearly showed229 that almost all data points are contained in a 3σ error interval if only one deposition peak is assumed. The presence of two deposition zones for Rfbromides is possible but is not corroborated by the measured data due to the low statistics. In conclusion, the amount of data accumulated for RfCl4 and RfBr4 is impressive. In Figure 37 the evaluated adsorption enthalpy values for group-4 tetrachlorides 1267


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appropriate oxycarboxylates, such as α-HIB, which appears to be the most selective. In the experiment by Silva et al.,268 261Rfa was produced by irradiation of only 47 μg of 248Cm with 92 MeV 18O ions. Rf atoms recoiling from the target were swept with He from the recoil chamber through a nozzle and deposited on the surface of a rabbit which was coated with thin layer of NH4Cl. Periodically, the rabbit was transported to the chemistry apparatus, where the surface was washed with 50 μL of α-HIB solution (0.1 M, pH 4.0) on the top of a small, heated (80 °C) Dowex 50-X12 cation exchange column (2 mm i.d., 20 mm long). The solution was forced into the resin, and more eluent was added. The first two drops corresponding to the free column volume contained no or little radioactivity and were discarded. The next four drops (in 2 drop fractions) were collected on Pt disks, evaporated to dryness, and heated to 500 °C. The disks were assayed by α-particle spectroscopy. Group-4 elements Zr, Hf, and Rf were strongly complexed with α-HIB and were thus not retained on the column and eluted far ahead of the actinides in the +3 or +2 oxidation state. A large part of the chemical system was automated so that the average time from beam off to counting was reduced to about 60 s. Several hundred separations were performed, but only 17 α-particles in the energy range from 8.2−8.4 MeV were registered; about half of these events are due to the decay of the 26-s 257No daughter. In two experiments, a correlated α−α pair was registered which was attributed to the decay of 261Rfa and its 257No daughter. Due to the limited 2π detector geometry, this number of correlated events is consistent with the total number of registered α-particles in the energy range 8.2−8.4 MeV. Silva and co-workers concluded268 that “for the particular cation exchange conditions used in these experiments, the behavior of the radioactivity assigned to element 104 with mass 261 is entirely different from trivalent and divalent actinide elements but is similar to Hf and Zr as one would predict for the next member of the Periodic System following the actinide series of elements”. However, the experiment was not yet efficient enough to determine Kd values. 9.2.2.1. Fluoride Complexation of Group-4 Elements Zr, Hf, and Rf. The fluoride complexation of group-4 elements was studied by various authors.266,272,346−350 HF is one of the best media on an anion-exchange resin, because hydrolysis need not

Figure 37. Evaluated adsorption enthalpies (ΔH0(T) a ) of group-4 tetrachlorides and tetrabromides from isothermal gas adsorption chromatographic experiments with HEVI.

and tetrabromides are shown.332,333 It is evident that the adsorption properties of Rf halides do not follow simple linear trends, which seem to be valid for the lighter homologs. Whether the surprisingly high volatility of RfCl4 and RfBr4 is definitely due to relativistic effects requires detailed calculations of the interaction of the molecule with the adsorbent. This is not an easy task as modifications of the surface by the reactive agents occur and the structure of real surfaces is very hard to integrate into the model calculations. Also, it is not clear if the adsorption process on real surfaces is sufficiently approximated by the model of mobile adsorption. Since a detailed discussion of all these aspects is beyond the scope of this review, we refer here to the book by Zvara.240 9.2.2. Liquid-Phase Chemistry of Rutherfordium. The first investigations of Rf in the liquid phase were performed as early as in 1970 by Silva et al.268 The technique employed built on the very successful application of cation-exchange chromatography using the chelating agent α-HIB in discovering actinide elements. The elution position was indicative of the ionic radius of the heavy actinides that were all present in the +3 oxidation state (except for No). The largest hydrated ions are eluted first. This effect can be enhanced by using

Figure 38. (a) Sorption of Zr, Hf, Th, and Rf on the cation exchange resin Aminex A6 in 0.1 M HNO3 at various HF concentrations. Off-line data are shown for Zr, Hf, and Th (open symbols) and online data for Hf and Rf (closed symbols), re-evaluated data from Strub et al.272 Reprinted with permission from ref 26. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH. (b) Sorption of Zr, Hf, Th, and Rf on the anion-exchange resin (Riedel-de Häen) in 0.1 M HNO3 at various HF concentrations. Off-line data are indicated by lines, online data for Hf and Rf by symbols. Reprinted with permission from ref 272. Copyright 2000 Oldenbourg Wissenschaftsverlag GmbH. 1268


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values of Hf and Rf were rising with decreasing NO3− concentration. Apparently, the counterion NO3− is much more effective in competing for binding sites on the anion exchanger to remove Rf compared to Zr and Hf.272 In order to better understand the anion-exchange behavior of group-4 elements in mixed HF/HNO3 solutions, first the behavior in pure HF solutions of varying concentrations of 1.9−13.9 M HF was investigated by Haba et al.348 As it was observed that NO3− can compete as counterion for binding sites for Rf on the anion-exchanger and HF is a weak acid, the equilibrium among HF, H+, F−, and HF2− in aqueous solution has to be considered:

be taken into account due to the high complexing strength of the fluoride ion. In early studies,347,351 Rf nuclides were not directly detected. In the so-called multicolumn technique (MCT), the reaction products, including 261Rfa, were transported to the chemistry setup with the aid of a KCl aerosol gasjet and continuously dissolved in 0.2 M HF. The solution was passed through 3 ion-exchange columns. In the first cationexchange column, heavy actinide elements were adsorbed while 261 a Rf in the form of anionic RfF62− passed through and was adsorbed on an anion-exchange column. A third cationexchange column sorbed the descendants 253Fm and 253Es, which were eluted at the end of each experiment and measured off-line by α-particle spectroscopy. Detection of the descendants was regarded as proof of the formation of anionic fluoride complexes. Pfrepper et al.351,352 refined the method and studied group-4 elements fluoride complexation in mixed HF/HNO3 solutions and determined first Kd values. These were indistinguishable for Hf and Rf. However, these results are at variance with recent results of Toyoshima et al.350 that will be discussed later. The fluoride complexation of group-4 elements Zr, Hf, and Rf, and of the pseudohomolog Th, in mixed 0.1 M HNO3/HF solutions was investigated by Strub et al.272 studying Kd values on both cation exchange resins and anion exchange resins using ARCA. As expected, in cation exchange experiments below 10−3 M HF, the Kd values for Zr, Hf, and Th are >103, indicating the presence of cations. At higher concentrations, between 10−3 M and 10−2 M HF, neutral or anionic fluoride species are formed and the Kd values are decreasing. The behaviors of Zr and Hf were very similar. Also, off-line and online data for Hf were consistent. Later, the Kd values measured for 261Rfa were slightly adjusted26,29,32 compared to the original publication272 by applying a more accurate analysis procedure. The separation of Zr, Hf, Th, and Rf on the cationexchange resin Aminex A6 in 0.1 M HNO3 at various HF concentrations is shown in Figure 38a. Much later, Ishii et al.353,354 reexamined Kd values for Zr, Hf, Th, and Rf on a cation-exchange resin in mixed HF/0.1 M HNO3 solutions as function of the fluoride ion concentration, fully validating the results of Strub et al.272 The observed behavior was theoretically predicted (see section 9.1).310 The Kd values should change in the following way in group-4: Zr ≤ Hf < Rf. This trend was indeed observed in the experiments.272,353 Investigations of the anion-exchange behavior showed some surprises. While the off-line data, as expected, clearly indicated the formation of anionic fluoride complexes for Zr and Hf above 10−3 M HF, the online data for Hf as well as Rf were different. There was almost no sorption of Rf on the resin between 10−2 and 1 M HF, and the sorption of Hf was lower compared to the case of off-line experiments, as displayed in Figure 38b. The unusual result concerning the anion-exchange behavior of Rf-fluorides, which would be in contradiction to earlier results, called for a detailed investigation of the influence of the counterion NO3−. The trend for the formation of MF62− (eq 9.1.3) should be reversed in group 4: Rf ≥ Zr > Hf310 (see also discussion at the end of this section). Such a trend was, indeed, observed in the experiments on the AIX separations of group-4 elements from 0.02 M HF.355 A similar result was obtained later,350 where the formation constant of RfF62− was reported to be at least 1 order of magnitude smaller than those of ZrF62− and HfF62−. When the HNO3 concentration was lowered to 0.01 M and the HF concentration was chosen as 0.05 M, the Kd

H+ + F− ⇄ HF −

(9.2.1)

HF + F ⇄ HF2

−

(9.2.2)

At an initial concentration of 0.4 M [HF] ini , the concentration of the HF2− anion starts to dominate compared to that of F− and becomes by far the dominating anionic species at concentrations >1 M [HF]ini. Indeed, decreasing Kd values are observed with increasing [HF]ini, which can be explained as the displacement of the metal complex from the binding sites of the resin by HF2−. Ignoring the knowledge of the activities of the chemical species involved, the adsorption equilibrium of an anionic complex An− with the charge state n− to the counterion HF2− between the resin phase R and the solution can be represented by the equation nRHF2 + An − ⇄ R nA + nHF2−

(9.2.3)

The equilibrium constant of the exchange reaction Dn can be expressed as follows: Dn =

[R nA] [HF2 ‐]n [An −] [RHF2]n

(9.2.4)

For anionic complexes being present as single chemical species, the Kd value can be written as follows: Kd =

[R nA] [RHF2]n n − = Dn [A ] [HF2−]n

(9.2.5)

Taking the logarithm on both sides yields: log Kd = log Dn − n log

[HF2−] [RHF2]

(9.2.6)

The concentration of the counterion in the resin phase can be regarded as a constant value that is equal to the exchange capacity of the anion-exchange resin. Since log [HF2−] ≈ log [HF]ini − 1.3, the charge of the anionic complex n can thus be evaluated from a plot of log [HF]ini versus log Kd as shown in Figure 39. The adsorption of Rf−fluoride complexes on the anion-exchanger is clearly weaker compared to that of the lighter homologs Zr and Hf. Also, the slope of the curve of −2.0 ± 0.3 for Rf clearly differs from the slope of −3.0 ± 0.1 for Zr and Hf. The associated anionic fluoride complexes are RfF62− and ZrF73− or HfF73−, respectively. For the first time, a significant deviation of the chemical behavior of a transactinide element compared to its lighter homologs was observed. Building on the work of Strub et al.,272 Toyoshima et al.350 have investigated the fluoride complexation of group-4 elements in mixed HF/HNO3 solutions in the concentration ranges of 0.0054−0.74 M HF and of 0.010−0.030 M HNO3. In off-line static distribution experiments, the behavior of Zr and Hf was studied as a function of the equilibrated concentration 1269


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formed.348 In Figure 40b, Kd values of Zr, Hf, and Rf are shown as a function of [F−]eq. The increasing Kd values of Rf, Zr, and Hf with increasing [F−]eq under constant [NO3−]eq indicate that the anionic fluoride complexes are increasingly formed. The formation reactions are represented as follows: MFn − 14 − (n − 1) + F− ⇄ MFn 4 − n

(n = 1 − 6)

(9.2.7)

where M indicates Zr or Hf and n denotes the coordination number in the products. In anion exchange, only anionic fluoride complexes of MF5− and MF62− participate in the adsorption−desorption process, with MF62− being the dominant species, as confirmed by the slope analysis. The sudden drop of the Kd values above [F−]eq > 5 × 10−3 M was associated with the abrupt increase of [HF2−]eq. The solid, dashed, and dotted curves in Figure 40b are results of calculations where Kd is related to the consecutive formation of the fluoride complexes of Zr and Hf in mixed HF/HNO3 solutions and the onset of the additional counterion HF2−. As was mentioned in section 9.1, the trend in the extraction of group-4 species becomes complicated depending on pH and other experimental conditions. For very dilute HF solutions, the trend in the extraction of MF6− was predicted as Rf ≥ Zr > Hf.310 Such a trend was observed in the experiments on the AIX separations of group-4 elements from 0.02 M HF.355 The assumed formation and extraction of the ZrF73− or HfF73 complexes at higher HF concentrations in comparison with RfF6− studied experimentally353 have not yet been considered theoretically. However, the suggested lower coordination number (CN) of Rf equal to 6 in the fluorine complexes in comparison with CN of 7 in complexes of Zr and Hf can hardly be expected, since the Rf ion is larger than those of Hf and Zr: that is, the IR (CN = 6) of Rf4+ (0.76 Å321−323 or 0.79 Å176) is larger than the IR of Zr4+ (0.72 Å) and Hf 4+ (0.71 Å)317 (see section 9.1). Also, the competition between the NO3− and F− ions in the complex formation has not been treated theoretically. Thus, this complex aqueous and extraction behavior of group-4 species at various experimental conditions needs further theoretical considerations. 9.2.2.2. Chloride Complexation of Group-4 Elements Zr, Hf, and Rf. The chloride complexation and hydrolysis of group4 elements including Rf was studied with a variety of methods. The formation of group-4 anionic-chloride species of the form MCl62− (M = Zr, Hf, Rf) was observed on the one hand by the

Figure 39. Variation of the distribution coefficient, Kd, of Rf, Zr, and Hf on the anion-exchange resin CA08Y as a function of the initial HF concentration, [HF]ini. The Kd values of Rf, Zr, and Hf are shown by diamonds, squares, and circles, respectively. Open and closed symbols depict different column dimensions. Linear relationships with slopes of −2.0 ± 0.3 for Rf and −3.0 ± 0.1 for Zr and Hf in the log [HF]ini versus log Kd are indicated by solid (−) and dotted (···) lines, respectively.348

of free F− ([F−]eq) at concentrations of [NO3−]eq of 0.01, 0.03, 0.1, and 0.3 M. In column experiments using AIDA, Kd values for Rf were measured at [NO3−]eq of 0.01 and 0.015 M. It was shown that, at concentrations of [F−]eq < 5 × 10−3 M, the contribution of the additional counterions F− and HF2− present in the HF/HNO3 solutions is negligible. A slope analysis in the log [NO3−]eq versus log Kd plot at a fixed [F−]eq = 3 × 10−3 M consistently resulted in an anionic charge of −2 for Zr, Hf, and Rf, indicating the presence of MF62− complexes (see Figure 40a). However, as already observed in the pure fluoride system, the complexation of Rf was again much weaker compared to that of Zr and Hf, as the Kd values differed by almost 3 orders of magnitude. This result is in contrast to the experiments by Pfrepper et al.351,352 The difference was attributed to incorrect assumptions about the breakthrough volume of trivalent actinides,351,352 which was probably a factor of 10 lower than anticipated,26,32 based on newer experimental data.356 Obviously, at low fluoride concentrations, Zr and Hf form MF62− complexes, whereas, at higher concentrations of HFini, where HF2− becomes the dominant species, MF73− complexes are

Figure 40. (a) Distribution coefficients, Kd, of Zr and Hf under static conditions (open symbols) and those of Zr, Hf, and Rf from column chromatography (closed symbols) as a function of [NO3−]eq and [F−]eq = 3.0 × 10−3 M. Reproduced with permission from ref 350. Copyright 2008 Oldenbourg Wissenschaftsverlag GmbH. (b) Variation of the Kd values of Zr and Hf under static conditions and of Rf in column chromatography as a function of [F−]eq. Values for Zr and Hf are shown for [NO3−] = 0.01, 0.03, 0.1, and 0.3 M. Values for Rf are shown for [NO3−] = 0.01 and 0.015 M. The solid, dashed, and dotted curves are theoretical calculations of Kd values. Reproduced with permission from ref 350. Copyright 2008 Oldenbourg Wissenschaftsverlag GmbH. 1270


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investigated in 4.0−11.5 M HCl. The results of these experiments clearly showed that at concentrations below 8 M HCl the Kd values of Zr and Hf are similar, indicating the formation of cationic or neutral species of the type M(OH)2Cl+, M(OH)2Cl2, and M(OH)Cl3 (M = Zr, Hf). At concentrations above 8 M HCl, the Kd values of both Zr and Hf increase steeply with increasing HCl concentration, indicating that anionic species such as M(OH)Cl52− and MCl62− are formed. The behavior of the pseudohomolog Th is similar to that of Zr and Hf at HCl concentrations below 8 M; at higher concentrations, the Kd values strongly deviate, as Th does not form anionic chloride species. In online experiments at 6 different HCl concentrations between 4.0 and 11.5 M HCl, the extraction behavior of Rf was studied. At each measured concentration, up to 400 separations were performed, resulting in a total of 186 α-particles attributed to 261Rfa and its daughter 257 No including 35 α−α correlations. The resulting adsorption curve as a function of HCl concentration is shown in Figure 41.

adsorption on anion-exchange chromatography columns from >7 M HCl solutions357 and by their extraction into a quaternary amine in reversed phase chromatography269 and into TiOA (triisooctylamine)263 in liquid−liquid extractions. Using a fully automated solvent extraction chromatography apparatus, Hulet et al.269 investigated the chloride complexation of Rf in comparison to Hf, Cm, and Fm. On the basis of their tendency to form strong anionic-chloride complexes in 12 M HCl, group-4 elements were separated from group-1, 2, 3, and actinide elements by extraction chromatography into a quaternary amine (Aliquat-336 on an inert support). Anionicchloride complexes were thus extracted and retained on the column while i.e. actinide elements passed through. Subsequently, group-4 elements were eluted with 6 M HCl. Altogether, in 44 chemistry runs, only 6 α-particle decays of 261 a Rf and its daughter 257No were registered, including one correlated mother−daughter pair. Therefore, Hulet et al.269 concluded that “in 12 M HCI solutions the chloride complexation of element 104 is clearly stronger than that of the trivalent actinides and is quite similar to that of Hf, which is expected to be its homolog in the Periodic Table”. More than 10 years later, liquid-phase chemical separations of Rf were picked up again at LBNL.263,265,358 These were manually performed separations, as described in section 7.3.1. In the beginning only the organic phase was assayed, which turned out to be a source of systematical errors.26,32,265,359 Liquid−liquid extractions from 12 M HCl into TiOA263 corroborated earlier results of Hulet et al.269 Cationic species were extracted into TTA (thenoyltrifluoroacetone).360 The measured distribution coefficient for Rf in comparison to those of the pseudohomologs Th and Pu indicated that Rf is less affected by hydrolysis than Zr, Hf, and Pu.360 The first hydrolysis constant log K11(Rf) ≈ −4 was predicted theoretically on the basis of the 4c-DFT calculations for the group-4 hydrated and hydrolyzed complexes,307 in good agreement with the experimental value of −2.6 ± 0.7.360 The predicted trend Zr > Hf > Rf is also in agreement with the experimental data for Zr and Hf, giving log K11(Zr) = 0.3 and log K11(Hf) = −0.25.301 One should note here that a simple model of hydrolysis301 based on the ratio of a cation charge to its size would give an opposite and, hence, a wrong trend from Zr to Hf, since IR(Zr4+) > IR(Hf4+).317 Bilewicz et al.358 studied the onset of hydrolysis at decreasing HCl concentrations by sorption of Zr, Hf, Th, and Rf on cobalt ferrocyanide surfaces, which are known to be selective sorbents for singly charged cations such as Rb+ or Cs+, but also for tetravalent metal ions such as Zr4+, Hf4+, and Th4+. This result was in contrast to the findings of Czerwinski et al.360 and attributed by Bilewicz et al.358 to relativistic effects which predict that Rf4+ would be more prone to having a CN of 6 rather than 8 in aqueous solutions due to a destabilization of the 6d5/2 shell. The latter supposition can, however, not be supported by the relativistic calculations that show that both the 6d3/2 and 6d5/2 AOs take part in the bond formation of the Rf compounds. The chloride complexation and hydrolysis of group-4 elements using anion-exchange chromatography was studied in much more detail by Haba et al.357 First, in batch experiments Kd values of carrier-free radiotracers of 88Zr and 175 Hf and the pseudohomolog 234Th were determined on the anion-exchange resin CA08Y in 1.0−11.5 M HCl. Second, in online experiments using the fully automated AIDA apparatus, the anion-exchange behavior of 85Zr, 169Hf, and 261Rfa was

Figure 41. Variation of %ads values of carrier-free radionuclides of Zr, Hf, and Rf on the anion-exchanger CA08Y as a function of HCl concentration. Reproduced with permission from ref 357. Copyright 2002 The Japan Society of Nuclear and Radiochemical Sciences.

The %adsorption values of Rf increase rapidly with increasing HCl concentration from 7.0 to 9.5 M, very similar to Zr and Hf, indicating that anionic chloride complexes such as Rf(OH)Cl52− or RfCl62− are formed. Between 7 and 9 M HCl, the extraction sequence is Rf > Zr > Hf, probably indicating a decreasing chloride complexing strength in the same order. 357 This result cannot find its theoretical explanation, as the theory claims that the complex formation at 4−8 M HCl should be continued with Rf, e.g. having the trend Rf: Zr > Hf > Rf.307 In order to elucidate possible differences in the chemistry of Rf compared to its lighter homologs, a series of liquid−liquid extractions into TBP (tributylphosphate) in benzene to study the effect of HCl, Cl−, and H+ concentration between 8 and 12 M on the extraction of Zr4+, Hf4+, and Rf4+, as well as the pseudohomologs Pu4+ and Th4+, was conducted.264 TBP is one of the most important organic extractants and widely used in reprocessing of spent nuclear fuel, and it extracts the metal in the form of a neutral species. The phosphoryl oxygen coordinates with the metal ion by forming an adduct. The TBP extraction process of group-4 elements in HCl can be expressed as follows: 1271


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M(H 2O)8 4 + + 4Cl− ⇄ MCl4(aq) + 8H 2O

(9.2.8)

MCl4(aq) + 2TBP(aq) ⇄ MCl4(TBP)2(org)

(9.2.9)

The extraction of group-4 elements into TBP depends on the strength of the chloride complexation and the stability of the TBP complex. In the experiments by Czerwinski et al.,264 an extraction sequence Zr > Rf > Hf for the group-4 chlorides was established. In additional experiments by Kacher et al.,265 it was observed that significant adsorption of Hf on the Teflon surfaces occurred. Subsequently, all equipment was changed to polypropylene equipment. The extraction sequence from 8 M HCl into TBP/benzene was revised to Zr > Hf > Rf > Ti. A critical evaluation of these experiments is discussed in detail by Kratz.26,32,359 On the one hand, the observed sorption of group-4 elements on Teflon that went unnoticed in the work of Czerwinski et al.264 and the suspected slow kinetics in the work of Bilewicz et al.358 may have contributed to partly conflicting results and warranted improved experiments to determine Kd values of group-4 elements by measuring the distribution in both liquid and organic phases. Günther et al.271 have investigated the Kd values of the liquid−liquid extraction of group-4 elements, including Rf, into undiluted TBP using reversed phase column chromatography and the ARCA II apparatus. Group-4 elements were fed onto undiluted TBP/Voltalef (an inert support) columns in 12 M HCl and quantitatively adsorbed. While 75% Hf and no Zr were eluted in a first fraction with 8 M HCl, the remaining Hf and 93% Zr were stripped in a second elution in 2 M HCl. The distribution of Rf in both fractions was measured. In the Hf fraction, two correlated α−α pairs of 261Rfa were observed, and three were observed in the Zr fraction, indicating an extraction of Rf in-between Hf and Zr. The extraction trend of group-4 elements from undiluted TBP was established as Zr > Rf > Hf at 8 M HCl. The determined Kd values and the extraction sequence were at variance with earlier results and conclusions concerning the extraction of Hf and Rf into TBP.264,265 Such an inversion of the trend is consistent with the theoretical trend for the formation of the MCl4 species.307 Recent, very detailed experiments by Haba et al.361 concerning the extraction behavior of group-4 elements into TBP at different HCl concentrations using the AIDA apparatus largely confirmed the results obtained by Günther et al.271 The %extraction values (%Ext) of Zr, Hf, and Rf are plotted in Figure 42 as a function of HCl concentration. Within the error limits, the extraction sequence between 7.2 and 8.0 M HCl into TBP is Zr > Hf ≈ Rf. Although Günter et al.271 determined at 8 M HCl an extraction sequence Zr > Rf > Hf with Kd values of 1180, 150+64 −46, and 64, respectively, also in this work Rf and Hf are much closer compared to Zr. More interesting than these small differences is the fact that Rf is not extracted ahead of Zr. The fact that Zr is extracted at lower HCl concentrations than Hf is corroborated by the larger complex formation strength of Zr as observed in anion-exchange experiments357 and also EXAFS studies.281 Some further calculations for the MCl4(TBP)2 complexes should be performed to study this case in more detail. 9.2.2.3. Sulfate Complexation of Group-4 Elements Zr, Hf, and Rf. The sulfate ion SO42− is a strong complexing ligand for group-4 elements. Its strength to form complexes with Zr and Hf is intermediate between those of F− and Cl− ions, i.e. F− > SO42− > Cl− ≥ NO3−. Taking into account the experimentally studied complexing behavior of Rf with F−, Cl−, and NO3−

Figure 42. Percent extractions (%Ext) of Rf, Zr, and Hf on the 20 wt % TBP/CHP20Y resin as a function of HCl concentration. Reproduced with permission from ref 361. Copyright 2007 Oldenbourg Wissenschaftsverlag GmbH.

ions, and the sequence of complexing strength, one would expect that also in the sulfate system Rf forms significantly weaker complexes compared to those of Zr and Hf under identical conditions. First, distribution coefficients, Kd, of Zr, Hf, and Th between a cation-exchange resin and a 0.0018 −0.69 M H2SO4/HNO3 mixed solution ([H+] = 1.0 M) by a batch method using the long-lived carrier-free radiotracers 88Zr, 175 Hf, and 234Th were measured. The online chromatographic behavior of Zr and Hf was then studied to obtain the adsorption probability on the resin as well as elution curves of these elements with the simultaneously produced, short-lived isotopes 85Zr and 169Hf. Finally, cation-exchange experiments with 261Rfa were performed together with 169Hf in 0.15−0.69 M H2SO4/HNO3 mixed solutions ([H+] = 1.0 M) using AIDA.362 As can be seen in Figure 43, adsorption of Rf, Zr, and Hf on the cation-exchange resin decreases with an increase of [HSO4−], representing the successive formation of sulfate complexes of these elements. While Rf shows the same functional shape as Hf, the decrease for Rf is shifted to about 0.15−0.20 M higher [HSO4−]. This indicates that the sulfate complex formation of Rf is significantly weaker than that of the lighter homolog

Figure 43. Variation of Kd values of 261Rfa on a cation-exchange resin derived from its adsorption probabilities as a function of [HSO4−] in 0.0018−0.69 M H2SO4/HNO3 mixed solutions ([H+] = 1.0 M), together with those of 88Zr, 175Hf, and 234Th obtained in batch experiments. Reproduced with permission from ref 362. Copyright 2012 Oldenbourg Wissenschaftsverlag GmbH. 1272


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Hf,362 very much similar to the fluoride complexation behavior272,353,354 (see also Figure 38a). The observed trend is in good agreement with the predicted one from the 4c-DFT calculations of various sulfate complexes (see section 9.1 and Figure 32).310 SISAK experiments on liquid−liquid extractions of Zr, Hf, and Rf from H2SO4 solutions by trioctylamines (TOA)363 confirmed the predicted trend in the complex formation, Zr > Hf > Rf, and have given Kd(Rf) values close to the predicted ones.310

reduction of the metal, so that an increase in this energy means an increase in the stability of the 5+ oxidation state in the group. This is also shown by the reduction potentials E°(V− IV) for MCl5 (M = V, Nb, Ta, and Db) estimated using a correlation with the energies of the charge-transfer transitions (Figure 44).38,366 Thus, nonrelativistically, Db5+ would have been even less stable than Nb5+. (Similar correlations can be shown for compounds of group-4 to group-8 elements).

10. DUBNIUM (Z = 105) 10.1. Theoretical Predictions

The predicted ground state electronic configuration of Db is 6d37s2 based on the results of the DF122 and MCDF177 calculations. This suggests that the element will have the maximum oxidation state 5+ as its lighter homologs Nb and Ta. The MCDF calculations have also given the first IP as 7.37 eV (corrected value obtained by an extrapolation procedure177), which is smaller than the IP of Ta (7.89 eV)313 with the 6s ionized electron. However, the energies of the 6d(Db) and 6s(Ta) AO are very similar.122 All multiple IPs(M → MZ+) were also calculated there.177 The IPs(M → M5+) were shown to decrease from V to Db, due to the destabilization of the (n−1)d AOs, so that the stability of the maximum oxidation state should increase toward Db (see Figure 28). The IR(M5+) increase in group 5 with Z,317 as was shown earlier by the MCDF calculations (0.74 Å for Db5+ in comparison with 0.64 Å for Nb5+ and Ta5+), because Rmax values177 of the outer (n−1)p AOs in M5+ increase in this direction (see Figure 28). Later on, molecular calculations confirmed such an increase in IR and gave a more accurate value for IR of Db5+ of 0.69 Å.327,364 Such an increase is also in line with an increase in CR.319,320 On the basis of the MCDF calculated multiple IPs, redox potentials were estimated for Db and its homologs in group 5.177,365 It was indeed shown that the stability of the pentavalent state increases from V to Db, while that of the tetra- and trivalent states should decrease. Thus, for example, the stability of the M3+ should decrease from V3+ to Db3+, because the ground state of the Db3+ ion is not 7s2, as was assumed earlier, but 6d2. MO calculations were performed mostly for chemically interesting stable compounds of Db and its group-5 homologs in the highest oxidation states. Thus, the electronic structures of the following molecular compounds were studied with the use of the relativistic DFT method: MCl5, MOCl3, MBr5, and MOBr3 (M = Nb, Ta, and Db).330,331,364,366,367 RECPs were applied to TaBr5 and DbBr5.327 Various properties, such as optimal geometries, bonding, IPs, polarizabilities, dipole moments, and charge density distribution, were predicted. According to the calculations, Db should be a typical d-element where bonding is defined by the participation of the 6d AO and 7s AO. There is also a slight admixture of the relativistically stabilized 7p1/2 AO. The influence of relativistic effects on the electronic structure of group-5 d-element compounds was studied on the example of MCl5 (M = Nb, Ta, and Db).367 Relativistic effects were shown to increase the HOMO-LUMO gap, ΔE, due to the relativistic destabilization of the (n−1)d AOs. This results in a decrease in the EAs, defined by the LUMO, and an increase in the energies of the electron charge-transfer transitions, E[3p(Cl)→(n−1)d(M)]. The latter is associated with the

Figure 44. Correlation between reduction potentials E°(V−IV) and energies of the lowest charge transfer transitions E[3p(Cl)→ (n−1)d(M)] in MCl5 (M = V, Nb, Ta, and Db). The nonrelativistic value for Db is shown as a filled circle. Reprinted with permission from ref 38. Copyright 1999 World Scientific.

Relativistic effects were also shown to be responsible for a trend to a decrease in ionicity (QM) and an increase in covalency (OP) (Figure 45). A partial OP analysis (Figure 46)

Figure 45. Relativistic (rel) and nonrelativistic (nr) effective charges, QM, and overlap populations, OP, in MCl5 (M = V, Nb, Ta, and Db).367 L denotes the ligand. Reprinted with permission from ref 367. Copyright 1993 American Institute of Physics.

shows that such an increase in covalency (total OP) is due to the increasing contribution of the relativistic ns1/2, np1/2, and (n−1)d AOs. Thus, relativistic effects are responsible for the continuation of trends in IP, EA, and stabilities of oxidation states in the groups in going over to the 6d elements. (The SO effects are, however, responsible for a trend reversal in De; see below.) The nonrelativistic description of these properties would give opposite and, therefore, wrong trends. 1273


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Figure 46. Relativistic (rel) and nonrelativistic (nr) partial overlap populations in MCl5 (M = Nb, Ta, and Db). L denotes valence orbitals of the ligand. Reprinted with permission from ref 367. Copyright 1993 American Institute of Physics.

The De in the Db molecules turned out to be lower than De of the Ta homologs due to a decrease of the ionic contribution to bonding, although the covalent contribution increases with Z from Ta to Db.366 Also, SO effects decrease De, as was shown by the RECP calculations for TaBr5 and DbBr5.327 The bond lengths of the Db molecules are typically about 0.05 Å larger than the Re of Nb and Ta homologs,327,364 in agreement with the IR and CR.319,320 Dipole moments should increase from the Nb to Db oxyhalides due to the increase in the interatomic distances.331 Using the calculated properties of the group-5 molecules, predictions of the volatility of halides and oxyhalides were attempted.315,330,331,364,366,367 Volatility of the oxyhalides, as either a sublimation process or adsorption on a surface, should decrease in group 5 with increasing Z due to an increase in the dipole moments of MOCl3 causing their stronger attraction to the surface.331 Volatility of the pure pentahalides is, however, a more complex process and depends on the definition of this phenomenon and the nature of the interactions. Thus, for example, as a sublimation process, volatility should increase in group 5, as was shown by the calculations of intermolecular interactions.330 It was shown that DbBr5 should have a higher Pmm over the solid than its lighter homologs.330 The same higher volatility of DbBr5 in comparison with the homologs was predicted on the basis of calculations of the molecule−inert surface interaction energies (eq 8.1.1).364 Such a decrease in the molecule−surface binding energy is caused by the increasing molecular size related to the separation distance x with Z (even though polarizabilities are about the same for TaBr5 and DbBr5). This trend is in agreement with ΔHS0(298) of of macroamounts of NbBr5 and TaBr5, so that ΔH0(298) S DbBr5 obtained on the basis of the correlation with E(x) should be smaller than those of the lighter homologs.364 The trend in volatility of MBr5 should, however, be different if adsorption takes place via a chemical bond formation on a surface, for example, on quartz modified with KCl or KBr. The 4c-DFT calculations364 have shown that, in this case, complexes of the type MBr5L− (L = Br and Cl) are formed on the KCl/ KBr surface (Figure 47) and their strength changes as Nb < Ta < Db. This means that volatility should change in an opposite way in group 5, e.g., Nb > Ta > Db. A number of studies were devoted to predict the extraction and ion exchange behavior of Db as well as other group-5 homologs from HF, HCl, and HBr solutions.304,305 Like group4 cations, also group-5 ones undergo hydrolysis according to the reaction

Figure 47. Formation of MBr6− on the KBr surface (M = group-5 elements). Reprinted with permission from ref 364. Copyright 2012 American Institute of Physics.

M(H 2O)6 5 + ⇄ M(OH)6− + 6H+

(10.1.1)

Hydrolysis of the Nb, Ta, Db, and Pa (for comparison purposes) cations was studied theoretically on the basis of 4cDFT calculations of the components of reaction 10.1.1.303 The calculated relative ΔEC values for this reaction are indicative of the following trend in hydrolysis of group-5 cations: Nb > Ta > Db ≫ Pa. This sequence is in agreement with experiments on hydrolysis of Nb, Ta, and Pa.301 A simple model of hydrolysis301 does not reproduce the difference between Nb and Ta having equal IR. The present model (section 8.2) based on the real (relativistic) distribution of the electronic density correctly describes the experimental observations. In solution, for example HCl, a large variety of complexes, such as M(OH)2Cl4−, MOCl4−, MOCl52−, and MCl6− (M = Nb, Ta, Db, and Pa) can be formed with different degrees of hydrolysis according to the following equilibrium M(OH)6− + i L− ⇄ MOu (OH)6 − 2u Li(6 − i) −

(10.1.2)

Their stability and energy change of the complex formation reactions were predicted theoretically on the basis of the 4cDFT calculations.304,305 The obtained data suggest the following trend in complex formation in group 5: Pa ≫ Nb > Db > Ta. Taking into account the association with an organic cation, the following trend was predicted for the sorption of group-5 complexes by an anion exchanger 1274


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Figure 48. Distribution of SF tracks attributed to the SF decay of a Db isotope. These events were registered in three separate experiments (a, b, and c). The deposition zones of other elements are also indicated. Reprinted with permission from ref 58. Copyright 1987 Oldenbourg Wissenschaftsverlag GmbH. Figure adapted from Zvara et al.65

Pa ≫ Nb ≥ Db > Ta

the temperature gradually decreased to 50 °C. The reaction 22 Ne + 243Am was used to produce 261Db, a nuclide decaying mainly by α-particle emission with a t1/2 = 1.8 s. The branching ratio for SF was determined to be MOCl4− > MCl6−, in agreement with experimental data for Nb, Ta, and Pa. The calculations also reproduced the MF6− > MCl6− > MBr6− sequence as a function of the type of ligand. Theoretical investigations304,305 have shown that the trend in complex formation and extraction (sequence 10.1.3) known for Nb, Ta, and Pa turned out to be reversed in going to Db. This could not have been predicted by any extrapolation of this property within the group, which would have given a continuous and, hence, wrong trend, but this came as a result of the relativistic electronic structure calculations for the real chemical equilibrium. As will be shown below, the first experiments on the AIX separation of the group-5 elements from mixed HF/HCl solutions have given a different trend than theoretically predicted, where Db was found in the Nb/Pa fraction.368 In view of this disagreement, a recommendation was made304 to repeat the AIX separations in either pure HCl or HF solutions. Accordingly, amine separations of the group-5 elements were systematically redone by Paulus et al.277 A reversed extraction sequence Pa > Nb ≥ Db > Ta, as that predicted theoretically (sequence 10.1.3), was then observed (see below). 10.2. Experimental Results

As homolog of the group-5 elements Nb and Ta, also Db is expected to behave chemically like a typical transition element. Early chemical investigations of single atoms of Db were restricted to rapid gas-phase chemical investigations due to the relatively short t1/2(261Db) = 1.8 s that was accessible in the 22 Ne + 243Am reaction. Only when sufficient amounts of the very rare and short-lived target material 249Bk (t1/2 = 320 d) became available was the longer-lived 34-s 262Db synthesized in the reaction 249Bk(18O, 5n).369,370 Therefore, the first aqueous phase chemistry experiments were only conducted in 1988. Indeed, 262Db can also be produced in the reaction 248Cm(19F, 5n), however with lower production rates.371,372 10.2.1. Gas-Phase Chemistry. A similar apparatus as shown in Figure 34 was used as early as 1970 to study volatile chlorides of element 105, Db.65 Isothermal section II was missing: the chromatography column consisted of sections I (30 cm) and a longer temperature gradient section III (120− 130 cm). Section I was heated to 300 °C, whereas in section III 1275


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Table 3. Comparison of −ΔH0(T) Values Evaluated for Group-5 Halides and Oxyhalides in Various Experiments a −ΔH0(T) (kJ·mol−1) a technique

a

ref

year

TC TC IC (HEVI) IC (OLGA III)

378 374 376 231

1973 1991 1993 1996

TC TC IC (OLGA II) IC (OLGA III)

378 374 226 377

1973 1991 1992 2012

aerosol material

NbCl5

NbOCl3

TaCl5

TaOCl3

a

85 95a MoO3 C

75 ± 5b 80 ± 1

KCl KBr

NbBr5 87a 83a 93 ± 4c 89 ± 5

DbCl5

DbOCl3

a

88 118a 157 ± 12b 99 ± 1 NbOBr3

TaBr5

155 ± 5

101 ± 4c 103 ± 5

TaOBr3

76 ± 10b ≤97 DbBr5 82a 108a

117 ± 3 DbOBr3

121 ± 11c 71 ± 5

Evaluated data.297 bReanalyzed data with τ0 = 2 × 10−13 s.376 cData from Gäggeler et al.226 reanalyzed with τ0 = 2 × 10−13 s.231

concluded that while the pure bromides are observed experimentally, also DbBr5 should be formed, since Db showed values for Nbthe lowest affinity toward oxygen.364 The ΔH0(T) a and Db-chlorides evaluated from TC experiments on a quartz values surface374 are in very good agreement with the ΔH0(T) a measured for oxytrichlorides in OLGA experiments.231 Also, ΔH0(T) a (NbCl5) evaluated from HEVI experiments of Kadkhodayan376 is in agreement with the value measured by Türler et al.231 In experiments with Ta, only TaOCl3 was obtained. The nuclide 262Db was positively identified after chemical separation in experiments with HEVI376 at isothermal temperatures as low as 250 °C. A measurement at 100 °C isothermal temperature resulted in a significantly lower yield compared to 250 °C and higher temperatures. If this data point is included into the −1 analysis, −ΔH0(T) (in Kadkhoa (DbCl5) = 76 ± 10 kJ·mol 376 0(T) −1 dayan −ΔHa (DbCl5) = 73 ± 10 kJ·mol ) results. A similar situation as for group-5 chlorides was observed for the bromides. Here, Db-bromides seemed to be less volatile than NbBr5 or TaBr5 in experiments by Gäggeler et al.226 However, since the measured ΔH0(T) value for Db-bromide is a very similar to the one determined for DbOCl3, it is likely that the volatility of DbOBr3 was measured. Interestingly, TaBr5 could only be formed when a mixture of HBr and BBr3 was used as brominating agent, whereas NbBr5 was formed with HBr only. Again, TC experiments on quartz surface374 yielded similar results. Nb- and Db-bromides are about 10 kJ·mol−1 more volatile as compared to the data of Gäggeler et al.226 This could be attributed to the fact that, in experiments of Gäggeler et al.226, a KCl aerosol was used, which may have covered the quartz surface to some extent. Generally, a lower volatility was observed on KCl surfaces compared to a pure quartz surface. In a meta analysis by Zvara,297 Db was listed as DbBr5; however, also DbOBr3 must be considered, as in the experiments of Gäggeler et al.226 More recently, the experimental study of Dbbromide was repeated377 with an improved purification of the He carrier gas. Under these conditions, Db-bromide is more volatile than observed previously. The volatility sequence deduced from this experiment together with independent studies on the behavior of Nb and Ta under identical chemical conditions was Db > Nb > Ta.377 This experimental observation is in conflict with previous studies, but only concerning the behavior of Db (see Figure 49). The deduced adsorption enthalpies for NbBr5 and TaBr5 were in excellent agreement. It was concluded that indeed the previous studies67,226 were performed with DbOBr3 rather than DbBr5. A different explanation is offered by Pershina and Anton,364 where the formation of MBr5L− (L = Br or Cl) complexes is

nuclide, since these experiments were severely hampered by comparably large α-particle emitting Po-activities, which obscured the interesting energy range 8.40−8.60 MeV, where α-decays of 262,263Db and their 258,259Lr daughters were expected. The volatility of group-5 bromides decreased in the sequence Nb ≈ Ta < Db. Later, the HEVI setup and the MGrotating wheel detector were used to investigate the volatility of group-5 chlorides. 376 This part of the PhD-thesis of Kadkhodayan is unpublished and has only appeared as an internal Lawrence Berkeley Laboratory report. In this work the nuclide 262Db was unambiguously identified after chemical separation by observing seven correlated α−α correlations. The α-particle energies as well as the deduced t1/2 for 262Db and its daughter 258Lr were in good agreement with literature values. The observed high volatility of Db-chlorides was comparable to that of Nb under similar conditions. For Ta-chlorides only a significantly less volatile compound was observed, which was attributed to the compound TaOCl3. Group-5 chlorides were reinvestigated using the OLGA(III) setup.231 Again, 262,263Db were identified via α−α correlations and SF decays. This time, two species were observed for Nb- and Db-chlorides, which were attributed to the pentachlorides and the oxytrichlorides. No data was measured for Ta-chlorides. In Table 3 the evaluated ΔH0(T) values measured for group-5 a halides and oxyhalides of Nb, Ta, and Db in various experiments are summarized. In cases where the assignment to either the pure pentahalide or the oxytrihalide was uncertain, the obtained ΔH0(T) value was listed in a separate column a between the two species. In the very first TC experiments with group-5 chlorides on glass and/or mica surfaces,65 similar ΔH0(T) values were measured for Nb- and Db-chlorides, but the a assignment to either the pentachloride or the oxytrichloride cannot be made. In experiments with Nb- and Db-chlorides of Türler et al.,231 two species of different volatility were observed and attributed to MCl5 and MOCl3 (M = Nb, Db). While DbOCl3 is less volatile than NbOCl3, only an upper limit could be established for −ΔHa0(T)(DbCl5), which allowed no conclusions about the relative volatility of NbCl5 and DbCl5. Also, experiments investigating the volatility of TaCl5 are still missing. The strong tendency of group-5 elements to form oxyhalides, even with traces of oxygen, requires experiments with a very low oxygen partial pressure, which is hard to realize in online gas chromatography experiments, where typically 1 L/ min of, for example, He is used as carrier gas. Earlier relativistic calculations predicted an increasing tendency down group 5 to preferentially form the oxyhalide.331 However, in a recent theoretical publication concerning group-5 bromides, it was 1276


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behavior of Db was indicative of the formation of oxo-halide or hydroxohalide complexes such as NbOCl4− and PaOCl4− or Pa(OH)2Cl4− at intermediate HCl concentrations, in contrast to the pure halide complexes of Ta, like TaCl6−. The observation that Db might show some chemical resemblance with Pa prompted experiments where the extraction of Db into 2,6-dimethyl-4-heptanol (diisobutyl carbinol, DIBC), a secondary alcohol, was studied.275 DIBC is a specific extractant for Pa. In online experiments with Db, the KBr aerosol particles delivering the reaction products from the production site were dissolved in concentrated HBr and fed onto a reversed phase chromatography column (DIBC/ Voltalef). The extraction was followed by the elution of an Nb fraction in 6 M HCl/0.0002 M HF and a Pa fraction in 0.5 M HCl. The number of 262Db decays observed in the Nb fraction indicated that Ta above 4 M HCl.304 As can be seen in Figure 50, the experimental extraction sequence is in perfect agreement with predictions concerning the behavior in a pure chloride system. Interestingly, the extraction sequence from 4 M HCl/0.03 M HF into TiOA, also an aliphatic amine, was exactly reversed, i.e. Ta > Db ≥ Nb ≥ Pa, which in retrospect must probably be attributed to the influence of the fluoride ions. This is corroborated by extraction studies from pure HF solutions into Aliquat-336.277 Even in dilute 0.5 M HF solutions, extractable fluoride complexes are formed by group-5 elements and Pa. In 4 M HF, the Kd value of Db is high, of the order of the ones of Nb and Ta, but it differs markedly from that of Pa. In pure HCl and HBr, extractable chloride and bromide complexes are only formed above 3 M HCl and above 6 M HBr, respectively.277 Investigations of the fluoride complexation and the anionexchange behavior of Db have just been initiated. Using the

Figure 49. Evaluated adsorption enthalpies (ΔH0(T) a ) of group-5 halides and oxyhalides from isothermal gas adsorption chromatographic experiments.

discussed on the KCl or KBr covered surface, which would indeed suggest a less volatile Db compared to Nb and Ta. 10.2.2. Liquid-Phase Chemistry of Dubnium. The first studies of aqueous phase properties of Db were performed by Gregorich et al.267 in a manually performed experiment making use of the typical property of group-5 elements to strongly adsorb on glass surfaces from strong nitric acid solutions. The relatively long-lived 34-s 262Db was produced in the reaction 249 Bk(18O, 5n) and transported with the aid of a KCl seeded aerosol gas-jet to the chemistry laboratory. The KCl aerosol particles were collected by impaction on a glass plate. After the end of collection, the glass plate was fumed twice with 15 M HNO3 and then rinsed with 1.5 M HNO3 and acetone. After drying in hot air, the plate was assayed by α-particle spectrometry. In 801 collection and separation cycles, 26 αparticles in the pertinent energy range 8.42−8.70 MeV, of which 5 were time correlated mother−daughter pairs, as well as 26 SF events were observed. Therefore, Db was shown to behave like a typical group-5 element. In a second experiment, the extraction of anionic fluoride species into methyl isobutyl ketone (MIBK) was investigated. The KCl aerosol particles were collected on Pt disks and dissolved in 3.8 M HNO3/1.1 M HF. This solution was transferred to a centrifuge cone containing the MBIK. After mixing and phase separation, the MIBK phase was dried on a Ni foil and subsequently assayed by α-particle spectroscopy. Under the conditions of the experiment, Ta is extracted nearly quantitatively, while Nb is extracted only to a small extent. In 335 extraction experiments, neither α-particles nor SF events of Db were observed, indicating a non Ta-like behavior of Db. Using the ARCA II setup, the extraction of group-5 elements into TiOA from HCl solutions typically containing 0.02 M of HF to prevent hydrolysis was studied in reversed phase extraction chromatography.368 From 12 M HCl/0.02 M HF and from 10 M HCl, 262Db was extracted into TiOA, like the group-5 elements Nb and Ta and the pseudohomolog Pa and separated from actinide elements. In elutions of a Pa−Nb fraction with 4 M HCl/0.02 M HF and a Ta fraction with 6 M HNO3/0.015 M HF, 262Db was found in the Pa−Nb fraction. In separate elutions with 10 M HCl/0.025 M HF (Pa-fraction) and 6 M HNO3/0.015 M HF (Nb-fraction), 262Db was equally distributed among the Pa- and Nb-fractions. Kratz and coworkers368 concluded that the non Ta-like halide complexation 1277


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states decrease. The calculations have shown that the stepwise ionization process results in the 6d2 state of Sg4+ and not in the 7s2 one.380 Since the 6d AOs of the 6d elements are more destabilized than the (n−1)d AOs of the 4d and 5d elements, the Sg4+ will even be less stable than W4+. Nonrelativistically, the trend would be opposite. Using IPs, redox potentials for Sg and its homologs in acidic solutions were estimated.380 They can be used as a guiding tool for reduction experiments on Sg probing the stability of the lower oxidation states. IR(M6+) values of group-6 elements were estimated using a correlation with Rmax of the outer (n−1)p AOs obtained in the MCDF calculations.178 They also show a typical increase of 0.05 Å from W (0.60 Å) to Sg (0.65 Å). This is in agreement with the increase in CR.319,320 Like Mo and W, Sg should form volatile halides, oxyhalides, oxide hydroxides, or carbonyls. In order to predict the stability of these complexes for Sg, electronic structures of MCl6, MOCl4, MO2Cl2, and MO3 (M = Mo, W, and Sg) were calculated using the 4c-DFT and RECP CCSD(T) methods.315,327,381,382 Optimized geometries (Re and bond angles), De, IP, α, and μ were predicted for the Sg compounds. Trends in these properties in group 6 turned out to be similar to those of group-4 and group-5 halides and oxyhalides. Both DFT and RECP calculations predicted an increase in the stability of compounds of the 6d elements with increasing number of oxygen atoms, e.g., from SgCl6 to SgOCl4 and to SgO2Cl2, as is experimentally known for the lighter homologs Mo and W. Thus, SgO2Cl2 was recommended382 as the most stable type of oxychloride for high-temperature gas phase experiments. SgCl6 was shown to be unstable with respect to the loss of Cl transforming into compounds of Sg5+.315,381 RECP CCSD(T) calculations for the group-6 oxyhalides, with and without SO coupling,327 have shown that larger SO effects on the 6d AOs result in a decrease in De of the 6d compounds by 1−1.5 eV in comparison with the 5d ones. The effects are larger for the Sg compounds than for the Rf ones due to an increasing 6d3/2−6d5/2 splitting. As in groups 4 and 5, covalency increases down group 6. The dipole moments of the oxyhalides also increase due to the increase in the metal−ligand separation. Such an increase in μ of the MO2Cl2 molecules (1.04 D for Mo, 1.35 D for W and 1.83 D for Sg) should result in a decrease in the volatility of these compounds, so that the trend is MoO2Cl2 > WO2Cl2 > SgO2Cl2.382 The importance of electron correlation for QM, OP, μ, and De was demonstrated on the example of group-6 MO2Cl2.327 Correlation effects were shown to significantly decrease QM and μ, and increase De, accounting, for example, for about 65% in De(SgO2Cl2). The effects on De were found to be larger in the W compounds than in the Sg ones, and they become more significant as the number of oxygen atoms increases. DS-DV calculations were performed for M(CO)6 (M = Mo, W, Sg, and U).383 Sg(CO)6 was found to be very similar to W(CO)6 and different from U(CO)6. A typical bond length increase was found for the Sg compound in comparison with that of W, as for the other types of species. Group-6 hydrides, MH6 (M = Cr, Mo, W, and Sg) were used to study the influence of relativistic effects on the molecular properties of Sg. The DF one-center expansion calculations321−323 showed relativistic effects to decrease the bond length of SgH6, so that Re(SgH6) is 0.06 Å larger than Re(WH6). The calculations revealed a slight increase in De of SgH6 as compared to that of WH6.

Figure 50. Kd values of Pa, Nb, and Ta in the system Aliquat-336/HCl (left-hand side) and Aliquat-336/HF (right-hand side). The Kd value determined for Db in 6 M HCl (right-hand side) and 4 M HF (lefthand side) is also indicated. Reproduced with permission from ref 277. Copyright 1999 Oldenbourg Wissenschaftsverlag GmbH. 2 4 8

Cm(19F, 5n) Db reaction, Tsukada et al. investigated the anionexchange behavior of group-5 elements Nb and Ta and the pseudohomolog Pa. The Kd value of Db was determined only at one [HF]ini concentration of 13.9 M. The sorption of Db on the resin was significantly different from that of the homologs Nb and Ta. Sorption on the resin decreases in the sequence Ta ≈ Nb > Db > Pa. Theoretical investigations have not been offered for complex formation at such high HF concentrations. It is clear only that the lowest sorption of Pa by the AIX resin found in the experiments379 is due to the PaF72− or PaF83− formation. Further determinations of Kd values at different HF concentrations are, therefore, required to determine the charge of the anionic species. In solutions with more dilute fluoride ion concentration [F−], Nb is known to form fluoro−oxo complexes NbOF4−, whereas Ta forms pure fluoro complexes TaF6−, which was confirmed in anion-exchange experiments in mixed HF/HNO3 solutions.282 The Kd value of Db was measured at 0.31 M HF/ 0.10 M HNO3, corresponding to a concentration of [F−]eq = 0.003 M. The measured Kd value of Db was similar to that of Nb but also to that of Pa. Therefore, it was speculated that Db forms either DbOF4− complexes, as does Nb, or even DbOF52− or DbF72− complexes, as does, Pa. However, theory says that the latter is less probable, as Db5+ has a much smaller IR (0.69 Å) than Pa5+ (0.78 Å) and is not its homolog. Again, further experiments are required to elucidate the speciation of Db. 262

379

11. SEABORGIUM (Z = 106) 11.1. Theoretical Predictions

The ground state of Sg is 6d47s2, and it is a homolog of Mo and W. The first IP is 7.85 eV according to MCDF calculations178 (corrected value obtained by an extrapolation procedure, while the calculated one is 7.03 eV), where the first ionized electron is 6d. This is almost equal to the IP(W) of 7.864 eV, where the first ionized electron is 6s.313 The energies of the 6d(Sg) AO and 6s(W) AO are indeed very close to each other.122 Multiple IPs(M → MZ+) were also calculated within the MCDF approach.178 The IPs(M → M6+) of group-6 elements reveal the same decreasing trend as the IPs in the maximum oxidation state for group-4 and group-5 elements (see Figure 28). Thus, as in groups 4 and 5, the stability of the maximum oxidation state increases in group 6, while those of the lower oxidation 1278


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The aqueous chemistry of Sg and its homologs has also received detailed theoretical consideration. In aqueous solutions, Sg should undergo hydrolysis similarly to its lighter homologs Mo and W. To predict hydrolysis of Sg at various pH of solutions, the free energy change of the following protonation reactions was calculated using the 4c-DFT method:306 MO4 2 − ⇄ MO3(OH)− ⇄ MO2 (OH)2 (H 2O)2 ⇄ MO(OH)3 (H 2O)2+ ⇄ M(OH)4 (H 2O)2 2 + ⇄ ... ⇄ M(H 2O)6 6 +

(11.1.1)

The results indicate that for the first two protonation steps, the trend in group 6 is reversed: Mo < Sg < W. For the further protonation process, the trend is continued with Sg: Mo < W < Sg. Thus, the same reversal of the trend is predicted for the protonation of oxyanions of the group-6 elements as that for the complex formation of the group-4 and group-5 elements. The predicted trends in the complex formation are in agreement with experiments for Mo and W at various pH values.301 log K values were determined for Sg, as given in Table 4.306

Figure 51. Predicted relative values of log Kd of W (triangles) and Sg (squares) with respect to those of Mo (rhomboids) by AIX separations from HF solutions as a function of the acid concentration. Points 1 through 5 correspond to the following extracted complexes: MO3F−, MO2F2(H2O)2, MO2F3(H2O)−, MO2F42‑, and MOF5−. Reproduced with permission from ref 308. Copyright 2004 Oldenbourg Wissenschaftsverlag GmbH.

Table 4. log K Values for the Stepwise Protonation of MO42− (M = Mo, W, and Sg)306

a detailed account of all Sg TC experiments was published by Zvara et al.250 In these experiments the nuclide 263Sg (t1/2 = 0.9 s) was produced in the reaction 249Cf(18O, 4n). A very similar setup, as already shown in Figure 33, was used in these experiments. Reaction products were thermalized behind the target setup in a rapidly flowing stream of Ar gas and flushed to the adjoining TC column. Volatile oxychlorides were synthesized by adding air saturated with SOCl2 as reactive agent. The formed oxychloride species migrated downstream in the fused silica chromatography column, to which a longitudinal, negative temperature gradient was applied, and finally deposited according to their volatility. In contrast to earlier experiments, no mica plates were inserted, but the fused silica column itself served as SF track detector. The deposition of Sg was registered after completion of the experiment by searching for latent SF tracks left by the SF decay of 263Sg. Indeed, in several experiments, a number of SF tracks were found in the column in the temperature region 150−250 °C, which were attributed to the decay of a Sg nuclide. The SF tracks were only found when the quartz wool plug, which was inserted as a filter for aerosol particles, was absent. This was attributed to the increased surface and thus a much longer retention time. In ancillary experiments, the production cross sections of transfer reaction products, notably 256Md/256Fm, were measured. The measured cross sections agreed well with previous values measured at Dubna, but they differed by almost 2 orders of magnitude from a measurement at Berkeley.386 The lower cross sections were attributed to the thick target and the collimation technique employed in Dubna. The adsorption behavior of long-lived 176W was simultaneously studied in the Sg experiments. The results of these experiments are shown in Figure 52. The number of SF tracks has been corrected for losses due to annealing of tracks throughout the experiment. These corrections are substantial. At temperatures above 400 °C, the correction increases the number of observed SF tracks by a factor of 5. No adsorption enthalpies were deduced from the experimental data. Nevertheless, different chemical states are discussed for Sg and W.250

log Kn reaction −

MO4 + H ⇆ MO3(OH) MO3(OH)− + H+ + 2H2O ⇆ MO2(OH)2(H2O)2 MO42− + 2H+ + 2H2O ⇆ MO2(OH)2(H2O)2 MO2(OH)2(H2O)2 + H+ ⇆ MO(OH)3(H2O)2+ 2−

+

Mo

W

Sg

3.7 3.8

3.8 4.3

3.74 4.1 ± 0.2

7.50 0.93

8.1 0.98

8.9 ± 0.1 1.02

Complex formation of Mo, W, and Sg in HF solutions was also studied theoretically on the basis of 4c-DFT calculations308 of the following stepwise fluorination process MO4 2 − [or MO3(OH)− ] + HF ⇄ MO3F− ⇄ MO2 F2(H 2O)2 ⇄ MO2 F3(H 2O)− ⇄ MOF5− (11.1.2)

The obtained ΔE values indicate a very complicated dependence of the complex formation of these elements and trends on pH and HF concentration (Figure 51). Thus, at the lowest HF concentrations (≲0.1 M HF), a reversal of the trends in Kd should occur in the group, while, at higher HF molarities (≲0.1 M HF), the trend should be continued with Sg: Mo < W < Sg. At the range of these HF concentrations, separation between the homologs is the best. The obtained sequences are in agreement with experiments for Mo and W.384,385 Future experiments on the AIX separations of group-6 elements from HF solutions should clarify the position of Sg in this group. C


11.2.1. Gas-Phase Chemistry. The first chemical identification of Sg as volatile oxychloride in TC experiments was reported in a preliminary report by Timokhin et al.251 in 1993 and again in 1996.253 Later, Yakushev et al.252 reported about ancillary experiments with Mo and W nuclides and about a further experiment of the same type with Sg. A full paper giving 1279


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column can be expected, compared to about 64 detected events (after applying the annealing correction). Therefore, contributions other than SF of 263Sga have to be considered. A contribution of about nine SF events can be estimated due to SF in 259Rf (or 259Lr after EC in 259Rf), the α-decay daughter of 263 Sg. A possible EC decay branch in 263Sg leading to SF in 263 Db or in 259Lr after α-decay must also be considered.273,391 Assuming a similar EC-branch as measured for 259Rf (15 ± 3%), a contribution of about 18 SF events must be assumed, which brings the number of expected SF events to roughly 50, not far from the 64 observed events. It is therefore of crucial importance that, under the conditions of the experiment, actinides, as well as the transactinide elements Rf and Db, are much less volatile than the studied element 106 compound. Even though this possibility was dismissed in the work of Zvara et al.,250 it must be noted that Db forms volatile DbOCl3 at temperatures above 250 °C231 (see section 10.2.1), whereas Rf can also be transported in an oxygen containing carrier gas via a transport reaction mechanism230 (see section 9.2.1). The conclusion by Zvara et al.250 that the observed distribution of SF events exclusively reflects the chemical behavior of Sg only can, therefore, not be substantiated. In the upper panel of Figure 52, the registered SF tracks per 5 cm column length (gray histograms) attributed to the SF decay of 263Sga and the corrected number of SF events due to the annealing of tracks (white histograms) are displayed. In the lower panel, the integrated yield along the length of the column is shown. The influence of the vastly different t1/2 values on the deposition temperature was discussed, and the authors came to the conclusion that “this factor can hardly account for so large a difference”.250 The distribution of SF tracks can best be described with ΔH0(T) (Sg-oxychloride) = −110 kJ·mol−1. For a 0(T) W, ΔHa (W-oxychloride) = −102 kJ·mol−1 resulted. Both value deduced for values are very similar. Actually, the ΔH0(T) a W-oxychloride is in perfect agreement with the data of Gärtner et al.392 However, Gärtner et al.392 attributed the observed volatile species to WO 2 Cl 2, based on thermodynamic considerations, while, in the work of Zvara et al.,250 different chemical species are discussed for Sg- and W-oxychlorides. The quantitative analysis performed here shows that the observed differences in the deposition temperature are mainly due to the large differences in t1/2 of the two nuclides and that indeed the values of both compounds are very similar. In TC ΔH0(T) a experiments by Yakushev et al.252 with short- and longer-lived Mo and W isotopes, it was observed that, depending on t1/2, two different chemical species of W with different volatility were formed. The less volatile compound was deposited at about 250 °C, whereas the more volatile one was deposited at about 160 °C. It was concluded that first the less volatile MO2Cl2 (M = Mo, W) was formed, which was then slowly converted to the more volatile MOCl4 (M = Mo, W). Thus, short-lived Sg was deposited in the column as SgO2Cl2, whereas longer-lived W was observed as WOCl4. Again, these experiments were only analyzed qualitatively. Due to the possibility of different chemical states of Sg and W, the TC experiments of Zvara et al.250 allow no conclusions about chemical similarities of Sg to either W or Mo. Furthermore, the uncertainties about the origin of the observed SF tracks, which might be partly due to lighter transactinide elements, render conclusions about the chemical speciation and properties of Sg oxychlorides questionable. The announcement of the synthesis of longer-lived 265,266Sg in the reaction 248Cm(22Ne, 4−5n) by Lazarev et al.393 laid the

Figure 52. Measured250,253 and simulated deposition peaks of 263Sgaand 176W-oxychlorides. The modeled deposition peaks were obtained using the microscopic model of Zvara288 and a Monte Carlo simulation technique, with the only adjustable parameter being the adsorption enthalpy (for details see text).

As with most of the earlier TC experiments with transactinide elements, the Sg experiments have not unanimously been accepted as the first positive identification of Sg after chemical separation. Serious criticism has been voiced by Kratz32,259 concerning mostly the magnitude of a SF-branch in 263 Sg but also the assignment of the volatile species. In the reaction 18O + 249Cf, at a beam energy of 94 MeV, the nuclides 264Sg, 263Sg, and 262Sg could, in principle, be produced in the 3n, 4n, and 5n evaporation channels. The nuclide 264Sg 387 decays by SF with t1/2 = 37+27 However, according to −11 ms. HIVAP calculations, the 3n evaporation channel is expected to have a 1 order of magnitude lower production cross section than the 4n channel at 94 MeV. The 4n deexcitation channel leads to 263Sg, for which two isomeric states are known: 263Sga and 263Sgb with t1/2 = 0.9 and 0.3 s, respectively. The nuclide 263 b Sg is populated by the decay of 271

α 267

Ds →

α 263

Hs →

Sg b

(refs 70 and 388) while 263Sga is predominantly populated in the direct synthesis reaction. For 263Sga a SF branch of 13 ± 8% was measured,387 in contrast to an earlier indirect determination of 70% by Druin et al.389 based on cross section arguments. The 5n deexcitation channel leads to 262Sg, a nuclide decaying by SF and by α-particle emission with a t1/2 = 15+5 −3 ms. The calculated cross section at 94 MeV is again 1 order of magnitude smaller than that for 263Sg. Considering the short t1/2 and low production cross sections of 262,264Sg, the most likely candidate responsible for the observed SF tracks must be 0.9-s 263Sga. Using the efficiencies given by Zvara et al.,250 a SF branch of 13%,387 and a production cross section of 0.3 nb as measured independently by Ghiorso et al.68 and Gregorich et al.,390 detection of about 21 SF events in the 1280


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kJ·mol − 1 was deduced, whereas, for 1 0 4 MoO 2 Cl 2 , −1 ΔH0(T) resulted, in good a (MoO2Cl2) = 90 ± 3 kJ·mol 382 agreement with theoretical predictions. However, it should be noted that, due to the very limited number of observed events, it was not possible to, for example, provide 95% or 99% error intervals, and there remains an about 15% chance that SgO2Cl2 is more volatile than MoO2Cl2 compared to an 85% chance that it is less volatile, as reflected by the deduced −ΔH0(T) value. The observed yield curves for 104MoO2Cl2, a 168 WO2Cl2, and 265SgO2Cl2 are displayed in Figure 53. Thus,

foundation for further chemical experiments with Sg in the liquid- as well as in the gas-phase.232,234,239,279,394 These experiments were all successful; however, due to an erroneous assignment of mass numbers and decay properties in the physics discovery experiment,393 which served as a basis for the search for Sg isotope decays in all subsequent Sg chemistry experiments, it was believed that also in the chemistry experiments two different isotopes of Sg, namely 265Sg and 266 Sg, were observed.232−234,239 Much later, after the discovery of 270Hs (the α-decay mother of 266Sg)109, it became evident that all decay chains observed in the Sg chemistry experiments were due to 265Sg only.395,396 There is now conclusive evidence for two isomeric nuclear states in 265Sg.397 The nuclide 265Sga decays with t1/2 ≈ 9 s preferentially to 261Rfa, which further decays by α-particle emission and t1/2 = 68 s to 257No, whereas 265 b Sg with t1/2 ≈ 14 s decays preferentially to 261Rfb, which undergoes spontaneous fission with t1/2 ≈ 3 s.397,398 Both states are formed in the direct synthesis reaction 248Cm(22Ne, 5n)265Sga,b but also in the α-decay of 269Hs.395 Following the considerations discussed in section 11.1, the obvious choice was to study Sg as the volatile SgO2Cl2 compound. For online gaschromatographic studies of Sg, the OLGA(III) system in conjunction with the ROMA detection system was employed (see section 7.2.1). Preparatory experiments with short-lived Mo-, W-, and U-isotopes were described by Gärtner et al.392 Experimental results with Sg were communicated in a short account by Schädel et al.232 Later, further experiments involving the measurement of a breakthrough curve were published in more detail by Türler et al.233,234 Typically, the reaction products recoiling from the target were transported to the OLGA(III) setup with the aid of a C-aerosol gas-jet. As chlorinating agents, Cl2 saturated with SOCl2 and traces of O2 were introduced to the reaction oven, where the formation of oxychlorides occurred. Volatile species traveled through the colder, isothermal section of the column and were reattached to KCl aerosol particles while exiting into the recluster chamber. This second aerosol gas-jet transported the separated activities to the ROMA detection system. The registered spectra were dominated by α lines originating from isotopes of Po and Bi. Presumably, these nuclides were produced in multinucleon transfer reactions from Pb impurities in the Cm target. These elements were not or only partly retained in the chromatographic column. Except for 211Pom and 212 Pom, all very short-lived Po activities were due to longer-lived Bi and/or Pb precursors. Therefore, the mother−daughter recoil technique had to be implemented in ROMA. In two runs at isothermal temperatures between 300 and 400 °C, ten decay chains attributed to 265Sg were registered, which were summarized in one data point of 350 ± 50 °C. One of the decay chains consisted of a complete chain 265

α 261

Sg →

α 257

Rf a →

α 253

No →

Figure 53. Relative yield of 104 MoO2Cl2, 168WO2Cl2, and 265SgO2Cl2 as a function of isothermal temperature in the chromatography column.234

both relativistic theory and experiment have shown that the volatility of group-6 dioxidichlorides should decrease with increasing Z due tofrom the theoretical point of viewan increase in the molecular dipole moments. Hübener et al.239 investigated the behavior of group-6 oxide hydroxides including Sg in the system O2−H2O(g)/SiO2(s). It is well-known that in an O2/H2O atmosphere the solid MO3 (M = Mo, W) are in equilibrium with gaseous MO2(OH)2. Under the assumption that, in one-atom-at-the-time experiments, the gaseous MO2(OH)2 undergoes a dissociative adsorption process, the process can be described as follows: MO2 (OH)2(g) ⇄ MO3(ads) + H 2O(g) (M = Mo, W, Sg) (11.2.1)

The gas chromatographic investigation of Sg oxide hydroxides in quartz-glass columns with He/O2/H2O as carrier gas must be highly characteristic, since neither actinides nor the lighter transactinides should form volatile species that would obscure the unequivocal identification of Sg. Preparatory experiments involving TC and high-temperature online IC were carried out, which demonstrated that Mo and W oxide hydroxides are not transported by simple reversible adsorption of MO2(OH)2 (M = Mo, W) but can be best described by a microscopic description of the dissociative adsorption process. The relative yields of 104Mo and 168W oxide hydroxides as a function of isothermal temperature are shown in Figure 54. In the actual experiment with Sg, the synthesis reaction 22Ne + 248Cm was employed. Reaction products recoiling from the target were stopped in He seeded with MoO3 aerosol particles and transported to the HITGAS237 setup. At the entrance to the chromatography column, moist O2 was added to the gas-jet. The temperature of the quartz chromatography column was 1325 K in the reaction and 1300 K in the isothermal zone. Loosely packed quartz wool in the reaction zone served as filter

Fm

which unambiguously demonstrated that Sg was chemically isolated. At 250 °C, an additional three decay chains were observed. Due to the complicated detection technique, not only the chromatographic transport through the column but also the decay and detection of Sg nuclei was modeled with a Monte Carlo procedure. This way, the most probable number of initially produced 265Sg nuclei and the chemical yield could be determined, which allowed extracting a probability density +2 distribution of −ΔH0(T) kJ·mol−1 (68% a (SgO2Cl2) = 98−5 168 0(T) error interval). For WO2Cl2, −ΔHa (WO2Cl2) = 96 ± 1 1281


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with 265Sg, produced in the reaction 248Cm(22Ne, 5n), in 3900 collection and separation cycles with ARCA, three decay chains attributed to the decay chain 261

α 257

Rf a,b →

α 253

No →

Fm

232,278

were observed. On average, the measurement of a sample started 38 s after the end of collection. As the experiment was designed to retain the group-4 element Rf, the decay chains are due to chemically isolated 265Sga,b. Thus, decay of the Sg nuclides occurred after the Rf/Sg separation, which, on average, occurred within 5 s. Since the operation of ARCA still involved also manual labor, the experiment was a tour de force, which also demonstrated that the method had reached its limits. Nevertheless, the experiment signified the first chemical separation of Sg in aqueous solution and demonstrated that, presumably, either SgO3F−, SgO2F3−, or SgO2F42− complexes were formed. Also, a neutral complex such as SgO2F2 cannot be excluded. However, Sg did not behave similarly to the pseudohomolog U, which forms UO22+ under the present conditions (which is natural, in view of 5f contributions to the bonding of uranyl). Due to the low fluoride concentration used, the anionic SgO42− (“seaborgate” in analogy to molybdate, MoO42−, or tungstate, WO42−) could not be excluded.26 A new series of Sg experiments279 was performed to study the possible formation of the seaborgate anion, SgO42−. It was attempted to elute Sg from a cation-exchange column with pure 0.1 M HNO3 (without the addition of HF). If Sg had been eluted from the cation-exchange column, this would have demonstrated the formation of the seaborgate anion SgO42−, in analogy to WO42−. In 4575 experiments, only one candidate α−α correlation was observed, with an expected background of 0.49 correlations. If Sg would have been eluted with the same chemical yield as W, then 4.7+3.7 −2.5 (68% confidence interval) α−α correlations from 261Rfa,b and 257No would have been detected.279 Therefore, the non-W like behavior of Sg was attributed to a weaker tendency to hydrolyze,26 in agreement with the theoretical predictions.306 Thus, in the presence of fluoride ions,278 which have a strong tendency to replace water or OH− ligands, the formation of neutral or anionic fluoride complexes seems to be favored, whereas, in the absence of fluoride ions, the elution of SgO42− anions is rather unlikely.279

Figure 54. Relative yields in isothermal gas chromatography of 104Mo (○) and 168W (●) oxide hydroxides in quartz columns using humid oxygen as reactive carrier gas component. Sg was observed at an isothermal temperature of 1300 K. The solid lines are the result of a Monte Carlo model based on a microscopic description of the dissociative adsorption process236 with ΔH0diss.ads(MoO2(OH)2) = −54 kJ·mol−1 and ΔH0diss.ads(WO2(OH)2) = −56 kJ·mol−1. The dashed line represents a hypothetical yield curve assuming that group6 oxide hydroxides are transported by simple reversible adsorption with ΔHa0 = −220 kJ·mol−1.239 Copyright 2011 Oldenbourg Wissenschafts-verlag GmbH.

for aerosol particles. Retention times of about 8 to 9 s were determined from measurements with short-lived Mo and W nuclides at isothermal temperatures above 1270 K. By condensing the separated volatile species directly on metal foils mounted on the circumference of the rotating wheel of the ROMA detection system, the time-consuming reclustering step could be avoided, however, at the expense of a reduced detection efficiency for α-particles. Due to the thickness of the metal foils, final samples could be assayed only in a 2π geometry. The search for genetically linked decay chains revealed two candidate events, which must be attributed to the sequence 265

α 261

Sg b →

sf

Rf b→

(ref 395). In the original publication, these events were erroneously attributed to the decay of 266Sg. The probability that both of these events were entirely random was only 2%.239 Since Sg appeared to be volatile under the conditions of the experiment, it showed the typical behavior of a group-6 element and was transported presumably as Sg oxide hydroxide. Under the given conditions, this coincides also with the behavior of the pseudohomolog U(VI). U is also known to form a volatile oxide hydroxide. In order to answer the question about the sequence of volatility of oxide hydroxides within group 6, further experiments have to be conducted at lower isothermal temperatures. 11.2.2. Liquid-Phase Chemistry. As described in section 11.1, group-6 elements form MO42−, MO3F−, MO2F2, MO2F42−, MO2F3−, and MOF5− depending on the HF concentration. In test experiments,278 optimum conditions for an isolation of group-6 elements were established. In 0.1 M HNO3/5 × 10−4 M HF, Mo and W are rapidly eluted from a cation-exchange column while di- and trivalent actinides, as well as group-4 elements or UO22+, are retained on the column. The group-6 elements form anions of the type MO42−, MO3F−, or MO2F3− (M = Mo, W). Also, the formation of a neutral compound such as MO2F2 cannot be excluded. In experiments

12. BOHRIUM (Z = 107) 12.1. Theoretical Predictions

The ground state of Bh is 6d57s2, so that it is a homolog of Tc and Re in group 7. MCDF calculations179 have given the first IP of 7.7 eV (corrected value obtained by an extrapolation procedure, while the calculated one is 6.82 eV), which is slightly lower than IP(Re) of 7.83 eV. The first ionized electron both in Bh and Re is the (n−1)d one, which is more bound in Re than in Bh. Multiple IPs(M → MZ+) for group-7 elements179 reveal the same decreasing trend in the group with increasing Z as that found for group-4, group-5, and group-6 elements (see Figure 28). IR values obtained via a linear correlation with Rmax[(n− 1)p] AO in the group are also shown in Figure 28,179 following the established trends for group-4 through group-6 elements. The estimated IR(Bh7+) of 0.58 Å is also typically larger than IR(Re7+) of 0.53 Å. This is also in line with the CR of these elements.319,320 Among gas-phase compounds of Bh in the highest oxidation state, oxyhalides should be most stable. As 4c-DFT calculations 1282


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have shown, there is a trend of decreasing metal−ligand bond strength of the group-4 to group-6 halides with increasing group number. In addition, the metal−ligand bond strength decreases also from the 5d to the 6d compounds within the same group.315 Thus, SgCl6 was shown to be unstable. Consequently, BhCl7 should not exist. This is also connected with a decrease in the relative stability of the maximum oxidation state along the transactinide series (see Figure 5 in Pershina et al.315). Oxyhalides of Bh should be rather stable, like those of the lighter homologs. The 4c-DFT calculations of the spectroscopic properties of MO3Cl (M = Tc, Re, and Bh)291 showed that the electronic structure of BhO3Cl is very similar to that of TcO3Cl and ReO3Cl. The Bh molecule should be stable with De of 22.3 eV, only slightly less stable than ReO3Cl, with De of 24.3 eV. Re(BhO3Cl) values are also typically larger than Re(ReO3Cl). μ and α of MO3Cl should increase in group 7, which is connected with an increase in the molecular size. Increasing dipole moments and electric dipole polarizabilities in the group suggest a decreasing volatility in the sequence TcO3Cl > ReO3Cl > BhO3Cl. Using calculated molecular properties, the volatility of group7 trioxychlorides as adsorption on a surface of a chromatography column was predicted via a physisorption model for longrange interactions.291 Since the surface of the quartz column is obviously modified with chlorine in the gas-phase experiments, the adsorption model takes into account the following types of the molecule−surface interactions: the dipole-effective surface (Cl) charge, the polarizability-surface (Cl) charge, and the dispersion one (see section 8.1.1 and eq 8.1.3). All those contributions were evaluated, and they all were shown to increase in the group with increasing Z. As their sum, the following adsorption energies ΔHa0(T)(BhO3Cl) = −78.5 −1 kJ·mol−1 and ΔH0(T) were then a (TcO3Cl) = −48.2 kJ·mol determined with respect to the experimentally measured −1 399 ΔH0(T) Thus, the sequence in a (ReO3Cl) = −61 kJ·mol . volatility was predicted as TcO3Cl > ReO3Cl > BhO3Cl. This trend is caused by an increasing μ in this group. The aqueous chemistry of Bh has not yet been studied theoretically. Predictions of other chemical properties of Bh based on earlier DF calculations are given in refs 33 and 34.

transport reaction of ReO3: HReO4(g ) ⇄ ReO3(ads) + 1 4 O2(g) + 1 2 H 2O(g)

mobile adsorption of HReO4: HReO4(g) ⇄ HReO4(ads)

(12.2.3)

While formation of volatile 169RemO3 (t1/2 = 16 s) was observed in online isothermal experiments at temperatures above 950 K, the significantly more volatile H169RemO4 could not be synthesized online.413 This is probably the reason why two early attempts to chemically identify Bh in the form of volatile oxides or oxide hydroxides failed.254,414 It is interesting to note that in both attempts the utilized production reactions led to isotopes that were unknown at the time. Two developments were instrumental in leading to the first successful chemical identification of Bh. First, the nuclides 266 Bh and 267Bh were synthesized in the reaction 249Bk(22Ne, 4−5n) and their decay properties and production cross sections determined.415 Especially interesting for chemical investigations is the relatively long-lived 267Bh with t1/2 = 17+14 −6 s. Second, due to the not well suited properties of the oxide-hydroxide system to rapidly isolate group-7 elements, chlorides and oxychlorides were investigated as potential candidate compounds for an online gas chemical isolation of Bh.399 This approach had already been successful in studies of volatile Db and Sg oxychlorides. The first TC experiments with Tc and Re using a mixture of He(g)/O2(g)/HCl(g) revealed only one single deposition zone for each element despite the large number of known chloride and oxychloride compounds of group-7 elements. The formed compound was identified as MO3Cl (M = Tc, Re) as the most likely one and was also formed in online isothermal gaschromatography experiments.399 Interestingly, TcO3Cl was so volatile that it could no longer be reclustered in the OLGA(III) setup with CsCl aerosol particles, while these worked fine for ReO3Cl. Aerosol particles with a reducing surface such as FeCl2 increased the yield of Tc significantly. This property allowed the distinction between a “Tc-like” and a “Re-like” behavior in future experiments with Bh. The first successful chemical isolation and identification of Bh was accomplished in an experiment of four weeks duration at the PSI Philips cyclotron employing the OLGA(III) setup and the ROMA detection system.235 A mixed 249Bk/159Tb target was irradiated with 22Ne ions, producing simultaneously 17-s 267Bh and 5.3-m 176Re. Nuclear reaction products recoiling from the target were attached to carbon aerosol clusters and transported with the He carrier gas through a capillary to the OLGA(III) setup. As reactive gas, a mixture of HCl and O2 was added. After chemical separation, the final products were attached to CsCl aerosol particles and transported to the detection system, where α-particle and SF decays were registered in an event by event mode. The yield of Re and Bh was determined at isothermal temperatures of 180 °C, 150 °C, and 75 °C. Altogether nearly 180,000 samples were collected and measured. A total of six genetically correlated decay chains of 267Bh were observed, four at 180 °C, two at 150 °C, and zero at 75 °C. One of the decay chains at 180 °C was complete and consisted of the sequence


12.2.1. Gas-Phase Chemistry of Bohrium. Similar to group-6 elements, group-7 elements form a number of mononuclear compounds, of which some are appreciably volatile and can thus be utilized for gas chromatographic investigations. Oxides and oxide hydroxides of Tc and Re are typically formed in an O2/H2O containing gas phase. They were extensively studied, mostly using the method of TC.244,400−410 The technique has also been applied to develop Tc and Re generator systems for nuclear medical applications.411,412 Schädel et al.224 and Eichler et al.413 studied the oxide and the oxide hydroxide chemistry of trace amounts of Re in an O2/H2O-containing system with respect to its suitability for a first gas chemical identification of Bh. In TC experiments, the formation of ReO3 and HReO4 was observed. However, three transport processes probably took place simultaneously, depending also on the pretreatment of the quartz columns: mobile adsorption of ReO3: ReO3(g) ⇄ ReO3(ads)

(12.2.2)

267

α 263

Bh →

α 259

Db →

α 255

Lr →

Md

Due to a non-negligible background created by the presence of Po and Bi nuclides, 1.3 of the 4 decay chains at 180 °C and 0.1

(12.2.1) 1283


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of the 2 decay chains at 150 °C had to be subtracted. While, in test experiments, 16-s 169RemO3Cl was still observed with 80% yield at an isothermal temperature of 75 °C, no decay chain of 17-s 267BhO3Cl was observed at this temperature, indicating a less volatile BhO3Cl compared to ReO3Cl. The fact that 267Bh was identified after chemical separation already excludes a “Tclike” behavior of Bh, since CsCl was used as the recluster aerosol material. The relative yields of compounds 108TcO3Cl, 169 RemO3Cl, and (most likely) 267BhO3Cl as a function of isothermal temperature are shown in Figure 55. The deduced

for group-7 oxychlorides by V. Pershina et al.291 The results of these calculations showed that he electronic structure of BhO3Cl is very similar to that of TcO3Cl or ReO3Cl. Increasing dipole moments and electric dipole polarizabilities in the group suggest a decreasing volatility in the sequence TcO3Cl > ReO3Cl > BhO3Cl. Also, the calculated ΔH0(T) a (BhO3Cl) = −78.5 kJ·mol−1 is in perfect agreement with the experimental result. Despite the success of the experiment, it should be noted that the overall efficiency to detect a correlated α−α pair was only about 4%. Significant improvements of the overall efficiency were mandatory to continue chemical investigations of even heavier elements.

13. HASSIUM (Z = 108) 13.1. Theoretical Predictions

The ground state of Hs is 6d67s2, so that it is a homolog of Ru and Os in group 8. MCDF calculations179 have given the first IP of 7.6 eV (corrected value obtained by an extrapolation procedure, while the calculated one is 6.69 eV), which is lower than the IP(Os) of 8.44 eV. The first ionized electron in Hs is the 6d one, while in Os it is the 6s.313 The 6d(Hs) electron is slightly more bound than the 6s(Os),122 so that a larger IP of Hs than that of Os is expected. The opposite trend obtained in MCDF calculations179 should therefore be revisited on the basis of more accurate calculations. All multiple IPs(M → MZ+) are also given in ref 179 and those to the 8+ oxidation state are also shown in Figure 28, revealing the same decreasing trend with Z in the group in IPs(M → MZmax+), as for group-4 through group-7 elements. The IR values obtained via a linear correlation with Rmax of the (n−1)p AOs are also shown in Figure 28,179 following the trend of increasing IR for group-4 through group-7 elements. The IR(Hs8+) of 0.45 Å is also typically larger than the IR(Os8+) of 0.39 Å. This is in agreement with the CR of these elements.319,320 As with other group-8 elements, Hs should form volatile tetroxides, whose volatility was studied experimentally (see below). The group-8 elements Ru and Os are the only elements which can form an 8+ oxidation state (with the exception of Xe, which is known to form tetrahedral XeO4416), the highest oxidation state known for a transition metal.417 The perfect

Figure 55. Relative yields of the compounds 108TcO3Cl (blue circles), 169 RemO3Cl (green circles), and (most likely) 267BhO3Cl (red squares) as a function of isothermal temperature. The error bars indicate a 68% confidence interval. The solid lines indicate the results of simulations with the microscopic model of Zvara288 with the adsorption enthalpies given in the text. The dashed lines represent the calculated relative (BhO3Cl) yield concerning the 68% confidence interval of ΔH0(T) a from −66 to −81 kJ·mol−1.235

enthalpies of adsorption on the column surface were −1 0(T) −ΔH0(T) a (TcO3Cl) = 51 ± 3 kJ·mol , −ΔHa (ReO3Cl) = −1 +6 61 ± 3 kJ·mol−1, and −ΔH0(T) (BhO Cl) = 75 (68% a 3 −9 kJ·mol confidence interval). Therefore, the sequence in volatility is TcO3Cl > ReO3Cl > BhO3Cl. The probability that BhO3Cl is equally or more volatile than ReO3Cl was estimated to be less than 10%. This sequence in volatility agrees well with predictions from fully relativistic density-functional calculations

Figure 56. Relativistic (rel.) and nonrelativistic (nonrel.) bond lengths, Re, ionization potentials, IP, and polarizabilities, α, of MO4 (M = Ru, Os, and Hs). Reproduced with permission from ref 293. Copyright 2008 by The American Physical Society. 1284


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change of the reaction is indicative of the following trend in the complex formation in group 8: Os > Hs ≫ Ru. The predicted lower reactivity of HsO4 with NaOH as compared to that of OsO4 has so far not clearly been revealed experimentally.420

tetrahedral symmetry of the neutral OsO4 makes this molecule nonpolar and highly volatile (Tm = 40.25 °C). This outstanding property distinguishes Os from any other transition metal oxide and allows, thus, a clear differentiation of the chemistry of Hs from that of the actinide and lighter transactinide elements. To predict their properties and volatility, very accurate 4c-DFT calculations with maximum large basis sets were performed.293 The results have shown that group-8 MO4 molecules should all be very similar and stable, with the same trend reversal in De, RuO4 < OsO4 > HsO4, as for the earlier 6d elements with respect to the 5d ones. SO effects on the 6d AO are responsible for such a trend reversal. Re(HsO4) should also be larger than Re of RuO4 and OsO4, as in compounds of group-4 through group-7 6d elements with respect to the 5d ones. The calculations have also revealed an inversion of the trend in α and IPs values beyond Os (Figure 56). This trend reversal is explained by the behavior of the valence (n−1)d AO contributing predominantly to bonding. For the MO4 (M = Ru, Os, and Hs) molecules, the influence of relativistic effects on properties important for gas-phase experimental investigations was studied in detail.292,293 Figure 56 shows the relativistic and nonrelativistic IP, α, and Re values of these molecules. One can see that relativistic effects decrease Re, increase IPs (with the strongest effect on HsO4), and decrease α. However, they do not change trends in these properties in the group, since those for the relativistic and nonrelativistic (n−1)d AOs are the same. There are also ab initio DF418 and infinite-order regular approximation with modified metric method (IORAmm/ HF)419 theoretical studies of the electronic structures of MO4 (M = Os and Hs). These works, however, revealed some deficiency of the calculations that resulted in predicting a wrong trend in properties from Os to Hs, as compared to a more accurate calculation.293 (See a critical analysis in ref 292.) Using a model of dispersion interaction (eq 8.1.1), ΔH0(T) a values of group-8 tetroxides on a silicon nitride surface of the detectors of the chromatography column were predicted.293 The inversion of the trend in α and IPs beyond Os (Figure 56) resulted in a trend reversal in −ΔH0(T) a : RuO4 > OsO4 < HsO4. This prediction turned out to be in agreement with the 197 experimentally observed trend in −ΔH0(T) a : OsO4 < HsO4. −1 0(T) Also, the calculated −ΔHa (HsO4) = 45.1 kJ·mol proved to be in excellent agreement with the measured −ΔH0(T) (HsO4) a = 46 ± 2 kJ·mol−1.197 Relativistic effects were shown to have no influence on the trend in ΔH0(T) a , as they have no influence on the trends in the molecular properties (Figure 56), since relativistic and nonrelativistic (n−1)d AOs change in the same way with increasing Z in group 8.293 In a work of Pershina,295 thermodynamic equations to predict Ta of a heaviest element with respect to Ta of a homolog in a comparative study are given. As an example, the adsorption of group-8 MO4 (M = Os, Hs) was considered. Accordingly, Ta(HsO4) with respect to Ta(OsO4) was predicted. In the same work, various measures of volatility were critically compared. Volatile tetroxides of Hs, like other group-8 elements, should react with a moisturized NaOH surface, forming the sodium hassate (VIII), Na 2 [HsO 4 (OH) 2 ], by analogy with Na2[OsO4(OH)2], according to the reaction: 2NaOH + HsO4 → Na 2[HsO4 (OH)2 ]


13.2.1. Gas-Phase Chemistry. The experimental chemical investigation of the transactinide element hassium (Hs, Z = 108), an expected member of group 8 and homolog of Os and Ru, presented a number of challenges. For obvious reasons described above, from the very beginning, all attempts concentrated only on the isolation of Hs as a very volatile HsO4, probably very similar in volatility to OsO4. The first synthesis of Hs was reported by Münzenberg et al.421 in 1984, which identified the nuclide 265Hs with t1/2 = 1.5 ms, far too short for any currently available chemical separator system. The more neutron-rich nuclide 269Hs with t1/2 ≈ 10 s suitable for chemical investigations was observed much later in the α-particle decay chain originating from 277Cn.80 However, the production cross section of only about 1 pb (10−36 cm2) for the reaction 208Pb(70Zn, 1n)277Cn was discouragingly small. A somewhat larger production cross section of about 7 pb was calculated for the direct production of 269Hs in the reaction 248 Cm(26Mg, 5n).422 This production cross section is 1 order of magnitude smaller than that for the synthesis of 267Bh, the nuclide that was used for chemical investigations of the next lighter transactinide element. Therefore, new techniques had to be introduced for irradiation, separation, and detection in order to accomplish the required sensitivity of chemically investigating the element Hs. Three early attempts to chemically identify Hs as volatile HsO4 demonstrated the high chemical selectivity of the chosen approach; however, the experiments lacked the overall efficiency and the long-term stability to reach the required sensitivity. In experiments conducted at Dubna, Zhuikov et al.255,256 employed the reaction 40Ar + 235U to produce shortlived α-decaying isotopes of element 110 and their Hs daughter nuclides. The technique employed fission track detectors, assuming that the produced Hs nuclides would decay mainly by SF. No SF decays were registered, resulting in a production cross section limit of 10 pb for nuclides with t1/2 > 150 ms. In a second attempt,255,256 the reaction 249Cf(22Ne, 4n)267Hs was used to search also for short-lived α-particle emitting isotopes of Hs. The decontamination from actinides (separation factor >106) as well as that from Po (>103) was excellent. Nevertheless, no α-particles in the energy range above 8.5 MeV and no SF events were registered. An upper limit production cross section of 100 pb for α-decaying nuclides with 50 ms ≤ t1/2 ≤ 12 h and of 50 pb for spontaneously fissioning nuclides was established. A similar experiment was reported by Dougan et al.423 by applying the so-called On-line Separation and Condensation AppaRatus (OSCAR), which was installed at the LBNL 88-Inch Cyclotron. The OSCAR setup was used to search for α-decaying 272Hs, the expected EC decay daughter of 272 Mt (estimated t1/2(EC) ≈ 25 m), produced in the 254 Es(22Ne, 4n) reaction. However, no α-decays between 8.7−11.0 MeV were observed and an upper limit for the production cross section of 1 nb was derived. The first successful Hs chemistry experiment was conducted in the spring of 2001 in the framework of an international collaboration at the GSI, Darmstadt. In order to significantly increase the level of sensitivity, all features of earlier experiments were either improved or replaced by new

(13.1.1)

The reactivity of RuO4, OsO4, and HsO4 with NaOH was studied on the basis of 4c-DFT calculations of the components of the reaction of eq 13.1.1.309 The calculated free energy 1285


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graphic column, was used to evaluate the adsorption enthalpy of HsO4 and OsO4 on the silicon nitride detector surface. The −1 modeled distributions with −ΔH0(T) a (HsO4) = 46 ± 2 kJ·mol 0(T) (68% confidence interval) and −ΔHa (OsO4) = 39 ± 1 kJ·mol−1 are shown as solid lines in Figure 57. The obtained value for HsO4 and the trend from OsO4 to HsO4 are −ΔH0(T) a in excellent agreement with the 4c-DFT theory.293

techniques. The stationary target was replaced by the rotating window and target system ARTESIA196 (see section 7.1.1), which could accept significantly higher beam intensities from the accelerator. The reaction 248Cm(26Mg, 5n)269Hs was chosen as the most promising one, also due to the availability of sufficient quantities of the 248Cm target material for the preparation of three segments of the target wheel and its still manageable radioactive decay properties. With the rotating target wheel, synthesis of about 3 atoms of 269Hs per day could be expected. The transport of reaction products with the aid of an aerosol gas-jet was abandoned. Instead it was observed that Os tetroxides were formed directly in the recoil chamber in test reactions when adding O2 to the He stopping gas.206 The addition of an oven heated to 600 °C at the exit of the recoil chamber helped to complete the oxidation reaction and increased the yield. Volatile tetroxide species could then be transported essentially without loss through Teflon capillaries to the chemistry and detection setup. Instead of IC, TC was applied where the column was replaced with a narrow channel formed by silicon particle detectors;201 see also Figure 21 (section 7.2.2). The efficiency of detecting an α-particle or a SF event was about 80%. In addition, the deposition temperature of each detected single Hs atom also provided chemical information. The required gain in sensitivity of 1 order of magnitude compared to the OLGA setup used in experiments with Bh was thus accomplished. In an experiment conducted at the GSI, valid data was collected during 64.2 h. During this time, 1.0 × 1018 26Mg beam particles passed through the 248Cm target. Only α-lines originating from 211At, 219,220Rn, and their decay products were identified. While 211At and its decay daughter 211Po were deposited mainly in the first two detectors, 219,220 Rn and their decay products accumulated in the last three detectors, where the temperature was sufficiently low to partly adsorb Rn. During the experiment, seven correlated decay chains were observed in detectors 2 through 4 and were assigned to the decay of either 269Hs or the yet unknown 270 Hs.197 This assignment was based on an erroneous assignment of mass numbers and decay properties of Sg isotopes in the physics discovery experiment.393 The excellent separation from unwanted activities resulted in a background count-rate of α-particles with energies between 8.0−9.5 MeV of 0.6 per hour and detector, leading to very low probabilities between 2 × 10−3 and 7 × 10−5 for any of the chains being of random origin. One of the observed decay chains was complete consisting of four consecutive α-particles attributed to the decay sequence 269

α 265

Hs →

α 261

Sg →

α 257

Rf →

α 253

No →

Figure 57. Relative yields of HsO4 and OsO4 for each of the 12 detector pairs. Measured values are represented by bars: 269HsO4, dark blue; 172OsO4, light blue. The dashed line indicates the temperature profile (right-hand scale). The maxima of the deposition distributions were evaluated as −44 ± 6 °C for HsO4 and −82 ± 7 °C for OsO4. Solid lines represent results of a simulation of the adsorption process −1 and −ΔH0(T) with −ΔH0(T) a (HsO4) = 46.0 kJ·mol a (OsO4) = 39.0 kJ·mol−1.

13.2.2. Liquid-Phase Chemistry. In an independent experiment, von Zweidorf et al.420 used the CALLISTO (continuously working arrangement for cluster-less transport of in situ produced volatile oxides) setup to demonstrate that the volatile Hs compound formed in situ with oxygen containing carrier gases reacts readily with a thin layer of hydroxide in the presence of water. This behavior is well-known for OsO4, which behaves as an acid anhydride, forming with aqueous NaOH sodium osmate(VIII) of the stoichiometry Na2[OsO4(OH)2] (see section 13.1). In an experiment similar to the one of Düllmann et al.,197 the reaction 248Cm(26Mg, 5n)269Hs was employed to form volatile 269HsO4 by stopping the fusion reaction products in a mixture of He/O2 and passing the gas stream through a hot quartz wool filter. The addition of water vapors significantly improved the deposition yield of tetroxides on freshly prepared NaOH surfaces. Therefore, the He/O2 gas stream (1 L/min He, 0.1 L/min O2) containing HsO4 or OsO4 was mixed with 0.1 L/min He saturated with water at 30 °C. The detection system consisted of four detection arrays, each containing four silicon PIN-diodes of 10 mm × 8 mm active area facing a stainless steel plate, coated with NaOH at a distance of about 1 mm. The gas stream was passing through three of these arrays, whereas the fourth one was in so-called “service mode”. Every 60 min, one of the stainless steel plates had to be replaced with a freshly coated one, since the reactive surfaces were loosing deposition efficiency with time, probably due to the neutralization of the alkaline surface with CO2, which was probably formed under the influence of the heavy ion beam by a reaction of the carbon beam dump with the oxygen of the jet gas. The flow of the gas stream was controlled by four computer controlled valves. The working principle of the deposition and detection system is illustrated in Figure 58. A disadvantage of the one-sided detection system is the reduced

Fm

The efficiency and selectivity of the system were so superior, that it was also used later on for the search and discovery of the Hs-isotopes 270Hs and 271Hs.109,261 With the correct identification of 270Hs and its α-decay daughter 266Sg, it became obvious that all 7 observed chains in the first Hs experiment were due to 269Hs.109 The thermochromatogram of these 7 events along with the distribution of Os is shown in Figure 54. The maximum of the Hs distribution was found at a temperature of −44 ± 6 °C. The distribution of 172OsO4 (t1/2 = 19.2 s) measured before and after the experiment showed a maximum in detector 6 at a deposition temperature of −82 ± 7 °C. As in experiments with lighter transactinide elements, the Monte Carlo model of Zvara,288 that describes the microscopic migration of a molecule in a gas chromato1286


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Figure 59. Deposition pattern of OsO4 (blue) and 269HsO4 (red) on a NaOH surface from a moist gas stream. The Os α-radioactivity was mostly due to the decay of 19.2-s 172Os and 22.4-s 173Os.424

14. MEITNERIUM (Z = 109), DARMSTADTIUM (Z = 110), ROENTGENIUM (Z = 111) 14.1. Theoretical Predictions

Mt, Ds, and Rg have 6d77s2, 6d87s2, and 6d97s2 ground states and belong to groups 9, 10, and 11 of the Periodic Table, respectively. The ground states for Ds and Rg are different from those of their lighter homologs Pt (5d96s) and Au (5d106s), which is explained by the relativistic stabilization of the 7s AO in the heavier elements. The M+ configurations Ds+(6d77s2) and Rg+(6d87s2) are also different from those of their homologs Pt+(5d9) and Au+(5d10), which is also a relativistic effect. Belonging to groups 9, 10, and 11, Mt, Ds, and Rg are expected to exhibit a behavior similar to that of their lighter homologs in these groups, the noble metals Ir, Pt, and Au. Strong relativistic effects should influence the properties of Mt, Ds, and Rg and their compounds to a larger extent than those of the homologs of the sixth period. The chemistry of these elements predicted on the basis of atomic DF calculations is described elsewhere.33,34 These early DF calculations33,34 have given an IP of 8.7 eV for Mt, which is obviously too low (it is lower than the IP of Ir of 8.967 eV313), while it should probably be larger, because the 6d(Bh) AO is more stabilized than the 6s(Ir) AO.122 For Ds, the DF calculated33,34 IP of 9.6 eV is larger than that of Pt (8.959 eV313), because the 6d(Ds) AO is more stabilized than the 6s(Pt) AO. In any case, more accurate calculations are needed for these two heaviest elements. For Rg, the best DCB FSCC calculated IP is 10.6 eV,425 which is also larger than the IP(Au) of 9.2255 eV313 because the 6d(Rg) is more stabilized than the 6s(Au) AO. According to the higher IPs of the heaviest elements in groups 9 through 11, they should be even more inert and noble than their homologs of the sixth period. For Mt, the Kα1 transition energies for different ionization states were predicted using the DHF theory, taking into account QED and nuclear-size effects. The results were compared with recent experiments in the α-decay of 272Rg.128 The main oxidation states for Mt and Ds are 3+ and 2+, respectively, though some other oxidation states are also foreseen.426 In Rg, the most stable state should be 3+,33,34 while the 1+ state should be very unstable. Due to the relativistic destabilization, that is, the 6d AOs that start to be chemically active at the end of the 6d series, the 5+ state of Rg is also expected. The −1 state is also expected due to the EA of Rg of

Figure 58. Comparison of two different states of the deposition and detection system of CALLISTO. In the upper part of the figure, detection array 4 is in “service mode”; in the lower part of the figure, detection array 1 is in “service mode”, allowing replacement of the steel plate of array 1 with a freshly prepared NaOH surface.424

detection geometry compared to a two-sided geometry in a cryo TC detector, which significantly lowers the probability to detect complete nuclear decay chains. In total, five nuclear decay chains attributed to the decay of Hs isotopes were registered.420 The distribution of the five Hs events in relation to the lighter homolog Os on the 3 times 4 detectors, that is, 12 positions, is depicted in Figure 59. The gas stream always entered the detection setup before detector position 1 and left after passing detector 12. The authors420 concluded that, for the first time, an acid− base reaction was performed with the tetroxide of hassium, leading to the formation of a hassate(VIII). Evidence for a lower reactivity of HsO4 with respect to moisturized NaOH as compared to OsO4, as tentatively suggested by the maximum of the Hs distribution on detector 3 and predicted by the 4c-DFT theory,309 was not judged as significant due to the few detected events. 1287


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stabilizes the molecules in the following order: RgF6− > RgF4− > RgF2−. This order is consistent with the relative involvement of the 6d electrons in bonding for each type of molecule. The aqueous-phase chemistry of Rg received some theoretical attention on the basis of DFT theory. It was shown that Rg(I) is the softest metal ion.438 No chemistry experiments have been conducted with elements Mt, Ds, and Rg so far.

1.56 eV, which is, however, much lower compared to Au (2.31 eV).425 The AR of Rg (1.2 Å) is smaller than the AR of Au (1.35 Å) due to the stronger relativistic contraction of the 7s AO,33,34 as was also mentioned earlier. From group 9 onward, there is a decrease in the difference in the single and triple bond CR between the 6d and 5d compounds, reaching negative values in groups 11 and 12 (Figure 29).319,320 The relativistic bond contraction is caused mainly by the relativistic stabilization of the ns AO. On the MO level of theory, Mt and Ds have received little attention so far. It was suggested that volatile hexafluorides and octafluorides might be produced and used for chemical separation experiments. DS-DV calculations for DsF6 indicate that DsF6 should be very similar to PtF6, with very close values of IPs.427 Relativistic effects were shown to be as large as ligandfield splitting.427 Bond lengths in MtH3, MtC− and DsH2, and DsH3 were calculated using the ADF ZORA method.319,320 Using the same approach, electronic structures of DsC and DsCO were calculated,428 suggesting that these compounds are chemically similar to the corresponding 5d homologs. In contrast, Rg received much attention from theory. A special interest in the chemistry of Rg is explained by the expectation of unusual properties of its compounds due to the maximum of relativistic contraction and stabilization of the 7s AO in this group.122 The electronic structure of the simplest molecule RgH, a sort of a test system such as AuH, was studied at various levels of theory, REPP, DHF, and 4c-DFT.429−434 Because the obtained Re(RgH) between 1.503 Å and 1.546 Å (the DHF CCSD(T) value is 1.523 Å432) is so close to Re(AuH) of 1.5236 Å,435 a very high accuracy is needed to predict the correct trend. At the best present level of accuracy, the bond lengths of these molecules are about the same. A comparison of relativistic (DF or ARPP) with nonrelativistic (HF or NRPP) calculations shows bonding to be considerably increased by relativistic effects doubling the dissociation energy, though the SO splitting diminishes it by 0.7 eV.429 The trend to an increase in De from AgH to AuH should be reversed from AuH to RgH, as was also shown by the BDF calculations.436 The PP and BDF calculations disagree, however, for the trend in Re(MH) in group 11 from Au to Rg. The trend to an increase in force constant, ke, was found to be continued, with RgH having the largest value of all known diatomic molecules.429 The μ was shown to be relativistically decreased from AgH to AuH and to RgH, indicating that RgH is more covalent and element Rg(I) is more electronegative than Au(I).429,436 Results of 4c-BDF436 and 4c-DFT calculations433 for AuX and RgX (X = F, Cl, Br, O, Au, Rg), indicate that relativistic effects follow a similar pattern to that for RgH, except for RgF and RgO, where SO splitting increases De. The singlet state was found to be the ground state for Rg2, in comparison with the triplet state.433 The dissociation energy was found to change in the following order: Au2 > RgAu > Rg2. To study the stability of higher oxidation states, energies of the MF6− → MF4− + F2 and MF4− → MF2− + F2 (M = Cu, Ag, Au, and Rg) decomposition reactions were calculated at the PP, MP2, and CCSD levels of theory.437 Relativistic effects were shown to stabilize higher oxidation states in the highcoordination compounds of Rg due to the destabilization of the 6d AOs and their larger involvement in bonding. RgF6− was shown to be the most stable in this group. SO coupling

15. COPERNICIUM (Z = 112) 15.1. Theoretical Predictions

Heavy group 12 elements all have a closed shell d10s2 ground state and should therefore be rather inert. With increasing relativistic stabilization and contraction of the ns AO in group 12, elements become even more inert. Thus, bulk Hg is known to be a liquid; however, it is very different from the condensed noble gases. In the case of Cn, relativistic effects are expected to be further amplified. Pitzer439 in 1975 found that the very high excitation energy of 8.6 eV from the s2 closed shell into the sp valence state of Cn will not be compensated by the energy gain of the chemical bond formation. Thus, Cn should reveal a noble-gas character. The atomic properties of Cn are very well studied (for older predictions, see refs 33 and 34). The DCB FSCC calculations have given the first IP of Cn of 11.97 eV and the second one of 22.49 eV.440 It is the highest in group 12 (Figure 60) and in the seventh row of the Periodic Table, evidencing a very high inertness of Cn. However, the first ionized electron is the 6d5/2, in difference to Hg (6s) (Figure 11). The DCB FSCC calculations of EA found no bound anion for Cn.440 Excitation energies, IPs, and oscillator strengths for neutral and up to 5+ ionized states of Cn and Zn, Cd, and Hg were calculated using the MCDF method.441 The calculated MCDF IPs441 are, however, less accurate than the DCB FSCC ones.440 IPs of internal conversion electrons, that is, of K-shell (1s) and L-shell (2s), of Cn are predicted to an accuracy of a few tens of electronvolts using DHF theory taking into account QED and nuclear-size effects.127 This data can be used for experimental studies of Cn involving K conversion electron spectroscopy. The main oxidation state of Cn is expected to be 0 due to its closed shell structure and strongest relativistic effects. However, the 2+ state could also be foreseen. As in the case of Rg, the 6d electrons start to be chemically active in Cn. As a consequence, an increase in the stability of the higher oxidation states, 4+, is expected also for this element. Due to the maximum relativistic contraction of the 7s AO in group 12 and in the seventh period (Figure 12), the AR of Cn should be the smallest in group 12 (1.71 Å)442 (see also Figure 60). This also results in the shortest bond lengths of Cn compounds in group 12, with the predominant contribution of the 7s AOs (see also CR in Figure 29319,320). The static dipole polarizability of Cn of 27.64 au was calculated most accurately at the DC CCSD(T) level of theory.442 Because of the relativistic 7s AO contraction, it is the smallest in group 12 (Figure 60) and the smallest in the seventh period. Due to the smallest α, Cn should be very volatile over inert surfaces, which guarantees its transport from the target chamber to the chemistry setup through Teflon capillaries in chemical experiments. Deposition of Cn on ice and quartz at higher temperatures than those for Hg was also predicted.442 The influence of relativistic effects on the atomic properties of group-12 elements, as the most interesting case, was 1288


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Figure 60. Relativistic (solid lines) and nonrelativistic (dashed lines) ionization potentials, IP, polarizabilities, α, and atomic radii, AR, of group-12 elements. Reprinted with permission from ref 294. Copyright 2005 Elsevier.

investigated.294 Figure 60 shows relativistic and nonrelativistic values of IPs, α and AR, which change in an opposite way from Hg to Cn. One can see that, exceptionally due to relativistic effects, Cn should have the largest IP and smallest α and AR and, therefore, be chemically rather inert, much more than the lighter homologs in the group. Taking into account the strong relativistic effects on the AOs of Cn, the following questions were of high interest, especially for gas-phase chemical experiments: Is Cn metallic in the solid state, or is it more like a solid noble gas? How volatile and reactive toward Au is the Cn atom in comparison with Hg and Rn? Since bonding in the solid state in the first approximation is described by bonding in a homonuclear dimer, De(M2) values were calculated to estimate ΔHS0(298) of the Cn metal. Moreover, Hg2 and Cn2 have been of special interest in chemical theory in testing the accuracy of quantum-chemical methods in treating closed-shell interactions. Accordingly, the electronic structures of these dimers were calculated using a variety of methods, such as 4c-BDF, ECP CCSD(T), QP-PP CCSD(T),443 and 4c-DFT.294,433 The calculations have shown that even though bonding in both Hg2 and Cn2 is preferentially of the van der Waals type, a partial overlap occurs. Both the DFT and PP calculations agree on an increase in De of about 0.04 eV from Hg2 to Cn2 with the corresponding bond shortening. Thus, due to the relativistic 7s AO contraction, Cn2 should be more stable than Hg2. This is in agreement with the LDA DFT solid-state calculations for solid Cn.444 A cohesive energy of 1.13 eV was obtained for Cn at the SR-level of theory, which is larger than that of Hg (0.64 eV) and is an order of magnitude larger than those of the solid noble gases. It was also concluded that Cn is not a metal, but rather a semiconductor with a band gap of at least 0.2 eV. In this sense, Cn resembles the group-12 metals more closely than it does the noble gases. Predictions of the interaction of Cn with Au were also very important to compare the volatility of Cn with that of Hg as adsorption on noble metal (Au) surfaces of detectors in gasphase chromatography experiments. With this aim in view, electronic structure calculations were performed for HgM and CnM, where M = Ag, Au, Pt, Pd, and Cu using the 4c-DFT method.445,446 It was demonstrated that Cn forms a chemical bond with Au primarily due to the overlap between the doubly

occupied 7s(Cn) AO and singly occupied 6s(Au) AO, as well as between the 6d5/2(Cn) AO and 5d5/2(Au) AO. Thus, CnAu should be chemically bound, having a σ2σ*1 2Σ+ ground state configuration with two electrons in the bonding and one in the antibonding MOs (Figure 61).

Figure 61. Bond formation (principal MOs) of the CnAu and FlAu molecules. Reproduced with permission from ref 25. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH.

Overall, Cn should be about 0.1−0.2 eV more weakly bound with a transition metal atom M than Hg, due to the stronger relativistic contraction of the 7s(AO) in comparison with the 6s(Hg) AO, while the bonds should be longer, because of the more extended 6d(Cn) than the 5d(Hg). Among the group-11 and group-12 metals, bonding with Ag was found to be the weakest while that with Pt the strongest. The influence of relativistic effects on properties of MAu (M = Hg and Cn) was studied as well.294 Relativity was shown to increase De(HgAu) by 0.13 eV but to decrease it by about the same amount (0.12 eV) in CnAu due to the contraction of the 7s(Cn) AO. This makes trends in nonrelativistic vs relativistic De values opposite from HgAu to CnAu, so that Denr(CnAu) > 1289


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this difference are greater relativistic effects in CnH+ than in RgH. Due to the larger involvement of the 6d AOs in bonding, higher oxidation states should be observed for highcoordination compounds of Cn, as was proven by the PP CCSD(T) calculations.455 No definite conclusion, however, about the existence of CnF4 can be drawn from its decomposition MF4 → MF2 + F2 energy between 100 kJ·mol−1 and 200 kJ·mol−1. Nonrelativistically, CnF4 would be definitely unstable. It was also found that the addition of F− ions to HgF2 and to HgF4 is energetically favorable.459,460 By analogy, it is assumed that, in combination with an appropriate polar solvent, CnF5− and/or CnF3− may be formed.455 The small energy of the decomposition reaction MF2 → M + F2 confirms the prediction that Cn should be more inert than Hg, though the difference to Hg is not that large. A comparison with nonrelativistic results shows that this is a pure relativistic effect.

Denr(HgAu), while Derel(CnAu) < Derel(HgAu). Re is decreased by relativity in both systems, and the trends are the same for both the nonrelativistic and relativistic Re. Further on, 4c-DFT calculations were performed for Hg and Cn interacting with Aun clusters simulating Au (111) and (100) surfaces.290a,446,447 Convergence of Eb with cluster size was reached for n = 95 (top position), n = 94 (bridge), n = 120 (hollow1), and n = 107 (hollow2) on the Au(111) surface. The obtained M−Aun binding energies, Eb, evidence that an Au(111) surface is a good approximation of the real one, which is usually unknown in the experiments. The bridge adsorption position was found to be preferential for Hg, while a hollow2 one was preferential for Cn (Figure 62).


15.2.1. Gas-Phase Chemistry. With the synthesis of partly long-lived isotopes of Cn through element 118 in 48Ca induced reactions on actinide targets,2,89,461−463 the focus of chemists shifted to the chemical exploration of these superheavy elements. Of special interest are chemical investigations of Cn with its 6d107s2 closed shell and strong relativistic stabilization of the 7s AO that should result in a high inertness, higher than that of Hg. Experimental investigations of Cn hold the promise to study this element as the first transactinide in the elemental state. Suitable isotopes of Cn with sufficiently long half-lives are 283 Cn (t1/2 = 3.8 s) and 285Cn (t1/2 = 29 s). The nuclide 283Cn can be synthesized either directly in the reaction 238U(48Ca, 3n) or indirectly as a decay product of 287Fl in the reaction

Figure 62. 4c-DFT calculated binding energies of Pb, Hg, Cn, and Fl of Pb, with the Aun clusters in comparison with experimental −ΔH0(T) a Hg, and Cn on Au.194,448,449 Reproduced with permission from ref 290a. Copyright 2009 American Institute of Physics.

242

(

Pu

48

)

287

Ca, 3n

α 283

Fl →

Cn

with the latter reaction having the higher production cross section. The heavier, but longer-lived 285Cn can only be synthesized indirectly in the

The Eb(Cn-Aun) for the hollow2 position was calculated as 0.46 eV.290a This is significantly lower than −ΔH0(T) a (Hg) of 1.0 eV on Au.449 The reason for that, as in the case of the gold dimers of these elements, is the relativistic stabilization of the 7s(Cn) AO. Thus, a lower volatility as adsorption on a gold surface was expected for Cn in comparison with Hg. Works on RECP and 2c-DFT (SO corrected) calculations for Hg and Cn interacting with small Au clusters (n = 1−4 and 10) arrived at the same conclusion, namely that Eb(Cn-Aun) is about 0.2 eV smaller than Eb(Hg−Aun).450−453 The influence of relativistic effects on the adsorption process of Hg and Cn on metal surfaces was investigated on the example of small M−Aun clusters.294 Relativistic effects were shown to define a decreasing trend in Eb(M−Aun) from Hg to Cn, even though they increase Eb(M−Aun) in these systems, especially at the hollow position due to the involvement of the 6d(Cn) AOs in bonding. This makes the difference in Eb(M− Aun) between Hg and Cn very small. Relativistic effects were shown to decrease Re, the distance of the adatom to the surface, in all the cases. The relativistic contraction of the 7s AO is expected to manifest itself also in properties of other Cn compounds, e.g., in shortening Re in CnH and CnH+.434,454−458 Another interesting point is that, in contrast to the group-11 hydrides, the trend in the dissociation energies from Cd to Hg is continued with Cn, i.e. De(CdH+) < De(HgH+) < De(CnH+), but De(AgH) < De(AuH) > De(RgH).454,456,457 The reasons for

244

(

Pu

48

)

Ca, 3n

289

α 285

Fl →

Cn

reaction. From the viewpoint of identification, the decay chain of 283Cn is preferred since the α-decay (Eα = 9.54 MeV) is followed shortly in time by SF of 279Ds (t1/2 = 0.18 s). This decay chain constitutes quite a unique signature and allows the safe identification of 283Cn after chemical isolation. In this respect, the decay of 285Cn (Eα = 9.16 MeV), which is followed by SF of 281Ds (t1/2 = 13 s), is less favorable. For the ensuing discussions, it is important to note that, in the first attempts to chemically identify Cn, it was believed that 283Cn decays by SF with t1/2 ≈ 3 m.461,462 A first attempt to chemically identify Cn in the elemental state was made by Yakushev et al.464 in Dubna. The isotope 283 Cn was produced by bombarding a U target of natural isotopic composition with 48Ca ions. In test experiments, shortlived Hg isotopes could be isolated in the elemental form from other reaction products, transported in He quantitatively through a 30 m long Teflon capillary, and adsorbed quantitatively on Au, Pt, or Pd coated silicon detectors at room temperature. If Cn behaved chemically like Hg and all efficiencies measured for Hg were also valid for Cn, detection of 3.4+4.3 −22 SF events could be expected assuming the cross section value for the production of 283Cn measured in ref 461. 1290


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is reduced to 5% while for 283Cn it is still 68%. A total of five decay chains was detected in two experiments. They started with the α-decay of 283Cn followed by SF-decays within less than one second.192,194 The positions of the five atoms inside of the detector array are depicted in Figure 63. They represent the

However, no SF events were observed. Therefore, no unambiguous answer as to the chemical and physical properties of Cn was obtained.464 In a next experiment, the question whether Cn remained in the gas phase and passed over the Au and Pd surfaces was addressed.190 Therefore, a special ionization chamber to measure SF fragments of nuclei remaining in the gas was added at the exit of the Au or Pd coated silicon detector array. Again, zero SF events were registered on the Au and Pd coated silicon detectors, confirming the result of the first experiment. However, eight high energy events were detected in the ionization chamber ascribed to SF decays because they were accompanied by neutrons registered in the surrounding neutron counters, while only one background count was expected.190 The majority of the events were attributed to the decay of an isotope of Cn, since there are no other known volatile nuclides decaying by SF. From this experiment it appeared that the interaction of Cn with an Au or Pd surface is much weaker than that for Hg.190 These results were seemingly confirmed in a TC experiment using the COLD detector where one side of the channel was replaced by an Au covered surface, allowing only measurements in a 2π geometry, i.e. no coincident detection of both SF fragments. The temperature in the gradient started at +35 °C and reached down to −185 °C, the adsorption temperature of Rn. Using the same 48Ca + 238U synthesis reaction, reaction products were stopped in He carrier gas and transported through a 25 m long capillary within about 25 s to the detector array. Seven events were detected that were attributed to fission fragments from the SF decay of 283Cn. The position of most of these events was identical to the deposition peak of Rn. However, the fragment energies were lower than expected, which was attributed to a thin layer of ice that has formed on the detectors at such low temperatures. It was concluded that all three chemical studies on Cn yielded a consistent picture, namely that Cn is not interacting with Au and is more similar to Rn.465 However, since later physics studies could not confirm the SF-decay of 3-min 283Cn but rather showed that this isotope decays via α-emission with t1/2 ≈ 4 s to 279Ds, the transport time for nuclei produced at the accelerator to the detector array in all chemistry experiments performed so far was too long for identification of a 4-s isotope. Therefore, without safe identification of the isolated nuclide, all conclusions concerning the chemical properties of Cn were questionable and called for significantly improved experiments. At present it remains unclear what the source of the signals was that were measured in these early experiments. A new attempt to investigate the chemical properties of Cn took advantage of the higher production cross section of the 242

(

Pu

48

)

Ca, 3n

287

Figure 63. Deposition of the five detected atoms (indicated by arrows) assigned to 283Cn in 48Ca + 242Pu experiments. The dotted lines indicate the temperature gradient inside the detector array (right axis in °C). Three different regimes in terms of temperature range inside the detector array and gas flow rates were applied (see text). The solid red lines depict results of a Monte Carlo model prediction (left axis in rel. units), including the given experimental parameters and assuming −1 27,194 the deposited atoms to have always −ΔHAu a (Cn) = 52 kJ·mol . The vertical dashed lines at detectors 17, 19, and 21, respectively, indicate the start of ice layer formation toward lower temperatures corroborated by reduced resolutions in the α-spectra.

outcome of three different experiments with varying values of the temperature range inside the detector array and velocity of the carrier gas, respectively. Also shown are the deposition patterns of α-decaying isotopes of Hg and Rn formed in nuclear reactions in the same experiment with a minor admixture of Nd to the 242Pu target (for Hg) or formed in transfer reactions with the target (for Rn). In the first experiment, the temperature range inside the detector array was −24 to −185 °C. Under this condition, the first decay chain of 283Cn was observed in the second detector, at a position where also major Hg deposition was found. To search for a possible difference in chemical behavior between Hg and Cn, the temperature at the beginning of the detector array was increased to the maximum value at which a semiconductor detector is still operational (+35 °C). Indeed, in the second experiment, one decay chain of 283Cn was observed at −5 °C, which is at the edge of Hg deposition. Therefore, a third experiment was conducted with increased gas flow rate (increase from about 1 L/min to 2 L/min). Under

α 283

Fl →

Cn

synthesis reaction. A prerequisite of this approach is, however, that first an Fl isotope is formed that has a too short t1/2 for chemical study, followed by an isotope of Cn with a sufficiently long t1/2, which is fulfilled in this case as t1/2(287Fl) = 0.48 s. Again the COLD setup was used for this study, now with a 4π detection geometry where one side of the channel was equipped with Au-covered silicon detectors. Moreover, the setup was operated in a closed loop mode to reduce the water vapor content of the carrier gas. This was decisive to reduce formation of ice layers at low temperatures. Still it was impossible to exclude ice formation at temperatures below about −100 °C. With a transport time of 2.2 s, the yield of 287Fl 1291


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α. The extremely small α of element 113, caused by the contraction of the 7p1/2 AO, is the main reason for the very low on inert surfaces. −ΔH0(T) a The main oxidation state of element 113 is expected to be 1+, though the 3+ state should also appear.33,34 There are quite a number of molecular calculations for element 113. The influence of relativistic effects on the 7p compounds was studied on the example of the hydrides MH (M = 113 − 118) using various methods: DFC, RECP, 2c- and 4c-DFT.166,433,434,456,471−479 In 113H, the 6d and 7s AOs of element 113 participate little in bonding and all the effects are defined by the 7p1/2 shell. A large relativistic contraction of the 7p1/2 AO results in a large contraction of the 113-H bond. The SO effects on the bond lengths, ΔRe(SO), are about −0.2 Å. Such a large bond contraction is not found in the other 7p MH. In moist oxygen atmosphere, element 113 should react with OH, forming 113OH by analogy with TlOH. The compound is predicted to be stable, with De of 2.42 eV in comparison with De(TlOH) of 3.68 eV, according to 4c-DFT calculations.480 The lower binding energy is due to the relativistic stabilization of the 7p1/2 AO. If element 113 adsorbs on a Au surface in the form of 113OH, −ΔH0(T) should be smaller than that of a TlOH.480 Element 113 should also form 113F, as other group-13 elements. Results of PP, DCB, RECP, and 4c-DFT calculations433,473,474 reveal increasing Re and μe from TlF to 113F, in contrast to decreasing values from TlH to 113H. These different trends in Re and μe for the MF compounds as compared to MH are explained by a more ionic nature of the MF molecules. DF calculations481 have shown that in the (113)(117) molecule there is a reversal of the dipole direction and a change of the sign of μ in comparison with other group17 homologs. The reason for that is the energetically higherlying 7p3/2 of the element 117 shell that donates into the lowlying 7p1/2 shell of the element 113 atom, while, in lighter homologs, the single electron of the group-13 atom usually completes the valence p shell of the group-17 atom.481 Like in Cn, the relativistic destabilization of the 6d AOs is expected to influence properties of high-coordination compounds of element 113, as was confirmed by PP and RECP calculations for 113X3 (X = H, F, Cl, Br, and I).473,482 As a consequence of the involvement of the 6d AOs, a T-shaped rather than trigonal planar geometric configuration was predicted for these molecules, showing that the valence shell electron pair repulsion (VSEPR) theory is not applicable to the heaviest elements. Vest et al.483 have shown that the decomposition energy of 113H3 into 113H and H2 becomes more favorable in going down group 13. The reason for that is enhanced relativistic effects on the 7p1/2 AO. A stable high-coordination compound of element 113, 113F6−, with the metal in the 5+ oxidation state is also foreseen.474 113F5 will probably be unstable, since the energy of the 113F5 → 113F3 + F2 decomposition reaction is less than −100 kJ·mol−1. The calculated energies of the reaction MX3 → MX + X2 (from M = B through element 113) suggest a decrease in the stability of the 3+ oxidation state in this group. In order to predict ΔH0(298) of the 7p metals and ΔH0(T) of S a the 7p elements on a Au surface that might be measured in future gas-phase experiments, 4c-DFT calculations of M2 and MAu (M = 113 through 118) were performed.484,485,490 Some other calculations were also performed for these species: ab initio DF for (113)2,486 4c-BDF, and 2c-SO ZORA; DC/MP2DFT for the element 113−117 dimers165,166,471,472,487−489 and

this experimental condition, two atoms of 283Cn were detected on the Au covered detector array at −29 and −39 °C, respectively. One further atom was found on the ice covered part at −124 °C. All three atoms were observed where only little if any Hg deposition occurred. The adsorption enthalpy of Cn on Au surfaces was evaluated from the deposition temperature of the four atoms adsorbed on Au as −ΔHAu a (Cn) +4 = 52 −3 kJ·mol −1 (68% confidence interval). For Hg, −1 −ΔHAu was evaluated, in agreement with a (Hg) > 65 kJ·mol 449 −1 literature data, where −ΔHAu was a (Hg) = 98 ± 3 kJ·mol determined. For the noble gas Rn on an ice surface, −1 −ΔHice was measured, in excellent a (Rn) = 19 ± 2 kJ·mol agreement with −ΔHice (Rn) = 20 ± 2 kJ·mol−1 from literature a data.466 In later experiments, one decay of 283Cn and one decay of 285Cn were observed, the latter one in experiments where the online thermochromatographic setup was attached to the exit of a gas-filled separator.467,468 The adsorption temperatures of the additional events were in full agreement with −ΔHAu a (Cn) −1 Au = 52+4 −3 kJ·mol . The theoretically predicted −ΔHa (Cn) of 0.46 eV290a on Au turned out to be in good agreement with the 0.04 experimental −ΔHaAu(Cn) = 0.54−0.03 eV.194 This value, reflecting the fact that Cn was observed in a range of adsorption temperatures that are significantly higher than those of Rn, is indicative of an Eb(Cn−Aun) that is larger than that for pure van der Waals interaction. Thus, in agreement with theory, chemical bond formation obviously takes place in the case of Cn with Au, similar to, but weaker than for Hg with Au. Thus, Cn is a d-metal, and not an inert gas, and its place in group 12 of the Periodic Table is legitimate.

16. ELEMENT 113 16.1. Theoretical Predictions

In elements 113 through 118, filling of the 7p shell takes place. The 7p AOs experience a very strong influence of relativistic effects: a large SO splitting and a strong contraction and stabilization of the 7p1/2 AO, as well as an expansion and destabilization of the 7p3/2 AO, that all increase along the 7p series. The 7s2 stabilization is so large that it practically becomes an inert pair in these elements. Early predictions indicated that these elements should be very volatile.33,34 Extrapolations from lighter homologs in the chemical groups have, indeed, shown that elements 113 through 117 should have smaller ΔH0(298) or formation enthalpies of gaseous atoms, S ΔH*298(E(g)), than their lighter homologs (Figure 27).299 In element 113, the SO splitting is 3.1 eV, and the relativistic stabilization of the 7p1/2 AO is 2.2. eV.37 The DCB FSCC calculations confirmed the 7s27p1/2 ground state of element 113 and have given the first IP of 7.306 eV.469 An EA of 0.68(5) eV was calculated in the same work, which is larger than EA(Tl) due to the relativistic stabilization of the 7p1/2 AO. The polarizability of element 113 was calculated as 29.85 au via the DC FSCC approach.470 A trend reversal is observed in group 13 in IPs (an increase), α (a decrease), and AR (a decrease) from In on. This is connected with a trend reversal in the energies (an increase) and Rmax (a decrease) of the np1/2 AOs, that define those atomic properties, at In. Using the calculated atomic properties, ΔH0(T) of element 113 on Teflon and a polyethylen were estimated via an adsorption model (eq 8.1.1) as 14.0 and 15.8 kJ·mol−1, respectively, which guarantees its transport through Teflon capillaries in chemical experiments. In group 13, ΔH0(T) values were shown to exhibit a trend reversal a beyond In, due to the trend reversal in the atomic IP, AR, and 1292


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RECP ones for Fl- and 118 dimers.456 The obtained 4c-DFT De(M2) and De(MAu) are shown in Figure 64.484,490

17. FLEROVIUM (Z = 114) 17.1. Theoretical Predictions

Element 114, Fl, has a quasi-closed shell 7s27p1/22 ground state caused by the large SO splitting of 4.7 eV122 and relativistic stabilization of the 7p1/2 AO. A high excitation energy into the valence state configuration, 7p1/22 → 7p2, of 6.2 eV calculated by Pitzer439 was a reason for him to believe that Fl, like Cn, would be an inert-gas like element. The most accurate DCB IHFSCC value of the IP(Fl) is 8.626 eV, which is, indeed, indicative of a very high inertness of Fl, though less than that of Cn.493 Other IPs up to the M3+ state for Fl and Pb were also calculated.493 The MCDF IPs and IR of the neutral to the 4+ ionized state of Fl and other group-14 homologs are also given,494 though being less accurate than the DCB values.493 The first IP of Fl and energies of several excited states were also calculated with the use of the relativistic complete active space MC CI method.142,143 The IPs of group14 elements show a decreasing trend from C to Sn and an increasing trend from Sn to Fl, so that the IP of the latter is even higher than the IP of Si. The reason is the relativistic stabilization of the np1/2 AO with Z. Due to the quasi-closed 7p1/22 shell, Fl will have a zero EA, as was shown by DCB FSCC calculations.495 The main oxidation state should be 2+.33,34 IPs of internal conversion electrons, that is, of K-shell (1s) and L-shell (2s), of Fl are predicted to an accuracy of a few 10 eV using DHF theory, taking into account QED and nuclearsize effects.127 This data can be used for experimental studies of Fl involving K conversion electron spectroscopy. A polarizability of Fl of 30.6 au (as well as of Pb of 46.96 au) was calculated using the DC CCSD(T) method.442 Polarizabilities of group-14 elements including Fl were also calculated using the DK and DC methods, though with a slightly smaller basis set496 (without h-functions taken into account in ref 442). The DC CCSD(T) value of 31.49 au is very close to that of ref 442. A Gaunt contribution was estimated in this work as 0.38 au, being rather significant. Finally, the recommended value of 31.0 au is that of ref 442, corrected for the Gaunt term. As in group 13, α shows a reversal of the increasing trend in group 14 at Sn, so that α(Fl) is smaller than that of Ge. This is due to the relativistic contraction of the np1/2 AO, which is documented by a correlation between polarizabilities and the mean radius of the np1/2 AO in group 14.122 The AR of Fl was estimated as 1.75 Å, while the van der Waals radius, RvdW, of 2.08 Å,442 which both are relativistically contracted. The trend in AR and RvdW is also reversed at Sn due to the same reason, the contraction of the np1/2 AO with increasing Z. Using the α and RvdW values, −ΔH0(T) a (Fl) = 10.4 kJ·mol −1 on Teflon was predicted via a model of shows also a physisorption.442 As do α and radii, −ΔH0(T) a reversal of the increasing trend at Sn, so that −ΔH0(T) a (Fl) is smaller than that of Ge. The very low value of −ΔH0(T) a indicates that Fl should be easily delivered to chemical experiments through Teflon capillaries. Due to the very strong stabilization and contraction of the 7p1/22(Fl) shell and, therefore, expected van der Waals nature of the M−M bonding, the homonuclear dimer Fl2 was of particular interest for theory. Also, the knowledge of the Fl−Fl binding energy was important to estimate its sublimation enthalpy. The ECP and 2c- and 4c-DFT calculations166,471,484,485 agree on the fact that Fl2 is more strongly

Figure 64. Calculated atomization energies of MAu and M2 (M are elements Hg/Cn through Rn/118). Filled and open squares are De(MAu) and De(M2) of the 6p elements, respectively, while filled and open rhomboids are De(MAu) and De(M2) of the 7p elements, respectively.484,490. Reprinted with permission from ref 490. Copyright 2010 American Institute of Physics.

According to these results, the (113)2 dimer should be weakly bound because the 7p1/2 electron yields a weak bond having 2/3π bonding and 1/3σ antibonding character (or 2/3π antibonding and 1/3σ bonding).486 ΔH0(298) = ΔH*298(E(g)) values were estimated then for the S 7p metals via a correlation with De(M2) in the respective chemical groups.484 They are given in Table 5 along with ΔH*298(E(g)) predicted via a linear extrapolation in the groups (Figure 27),299 in good agreement with each other. One can see that ΔH*298(E(g)) changes almost linearly with Z in these is lower than that of Tl due groups. For element 113, ΔH0(298) S to the bonding preferentially made by the 7p1/2 AO. To study the interaction of element 113 with Au, 4c-DFT calculations were performed for TlAu and 113Au.480 In 113Au, bonding should be weaker than in TlAu due to the relativistic stabilization of the 7p1/2 AO.490 One can therefore expect that element 113 will adsorb on Au at much lower temperatures −1 on Au was than Tl. An −ΔH0(T) a (113) = 159 ± 5 kJ·mol (Tl) = 240 ± 5 kJ·mol−1 estimated with respect to −ΔH0(T) a using the difference in De(MAu), where M = Tl and element 113.480 Adsorption of an element 113 atom and Tl on Au(111) and Au(100) surfaces was also modeled by M−Aun (n = 20) clusters and 2c-DFT calculations.491 The results show that the difference in binding energy, Eb(M−Aun), between Tl and element 113 stays within ±15 kJ·mol−1 of 82 kJ·mol−1 obtained in ref 480. Thus, the cluster calculations performed on a larger scale confirmed the estimate of Pershina et al.,480 so that −1 −ΔH0(T) a (113) can be given as 159 ± 15 kJ·mol . 16.2. Experimental Results

16.2.1. Gas-Phase Chemistry. The first experiments were conducted by Dmitriev et al. at FLNR Dubna, studying the adsorption of element 113 on Au surfaces. Only preliminary results have been communicated.492 1293


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Review 448 experimental −ΔH0(T) so that a (Pb) = 2.43 eV on Au, Eb(Fl−Aun) (n = 94) was given as 0.71 eV (Figure 62). The obtained −ΔHa0(T)(Fl) = 68.5 kJ·mol−1 is indicative of formation of a chemical bond with Au. A comparison with group-12 Hg (where, however, Hg dissolves into Au) and Cn shows that the trend in −ΔH0(T) should be Cn < Fl < Hg ≪ Pb a (Figure 62).290a Calculations for the C−Aun and Fl-Aun systems using other relativistic DFT methods450−453 came to the same conclusion: Fl should form a rather strong chemical bond with Au, stronger than that with Cn. A comparison of results of various calculations is given in Table 6. Many other, mostly simple, compounds of Fl were treated theoretically using various approaches. In the 7p MH, from Fl on, both the relativistically contracted 7p1/2 and expanded 7p3/2 AOs take part in the bond formation, so that Re(FlH) is longer than Re(PbH). De(MH) (M = 113 through 117) is reduced by large SO effects, with the lowest value at FlH. Trends in the stability of hydrides were predicted as follows: RnH ≪ HgH < PbH and 118H ≪ FlH < CnH. RECP and DC CCSD(T) calculations for PbH+ and FlH+ have also given a 50% weaker bond and a shorter Re in the latter due to the contraction of the 7p1/2 AO.434,456 CAS-SCF/SOCI RECP calculations for FlH2 demonstrated breakdown of the conventional singlet (X1A1) and triplet (3B1) states due to large SO-effects.498 SO-effects are shown to destabilize FlH2 by almost 2.6 eV. Electronic structures of FlX (X = F, Cl, Br, I, O) and FlO2 were calculated using 2c-RECP CCSD(T), 2c-DFT SO ZORA, and 4c-BDF methods.166,471 In contrast to PbO2 (De = 5.60 eV), FlO2 (De = 1.64 eV) was predicted to be thermodynamically unstable with respect to the decomposition into the metal atom and O2. According to results of these calculations, Fl should not react with O2 under typical experimental conditions, as was discussed in ref 442. Ab initio DF and PP calculations481,499 for the decomposition reactions MX4 → MX2 + X2 and MX2 → M + X2 (M = Si, Ge, Sn, Pb, and Fl; X = H, F, and Cl) also predicted a decrease in the stability of the 4+ oxidation state in group 14. The instability was shown to be a relativistic effect. The neutral state was found to be more stable for Fl than for Pb. As a consequence, Fl is expected to be less reactive than Pb, but about as reactive as Hg. The possibility of the existence of FlF62‑ was also suggested in ref 499.

bound than a typical van der Waals system. At the 4c-DFT level of theory, it is slightly more strongly bound than Cn2, but much more weakly bound than Pb2. A Mulliken population analysis indicates that both the 7p1/2 and 7p3/2 AOs of Fl take part in the bond formation.484,485 The participation of the more extended 7p3/2(Fl) AO in bonding in comparison with the 6p3/2(Pb) AO explains an increase in Re from Pb2 to Fl2. SO effects were shown to decrease De, but increase Re in both systems.471 ΔH0(298) of Fl was estimated using a correlation with the S calculated De(Fl2) in group 14 (see Table 5). The obtained Table 5. Standard enthalpies of monotaomic gaseous elements, ΔH*298(E(g)) (in kJ·mol−1), of the 7p Elements method

E113

E114

E115

E116

E117

extrapolation299 correlation484

138.1 144.7

70.3 70.4

146.4 152 ± 12

92.1 101.3

83.7 91.7

value is in very good agreement with that obtained via a linear extrapolation in the group (Figure 27).299 Ecoh of Fl was predicted from the SR and SO-GGA-DFT solid-state calculations.497 The obtained SO-PW91 value of 48.2 kJ·mol−1 is in reasonable agreement with other estimates.299,484 SO effects were shown to lower Ecoh and lead to structural phase transitions for solid Fl (the hcp structure in contrast to the fcc for Pb). In a nonrelativistic world, all group-14 elements would adopt a diamond structure. To estimate the interaction strength of Fl with noble metals, particularly with Au, 4c-DFT calculations were performed for MM′, where M are group-14 elements and M′ are group-10 and 11 metals.485,490 The results have shown that Fl interacts most strongly with Pt, while least strongly with Ag. De(FlAu) is significantly (1.42 eV) smaller than De(PbAu) (Figure 64) due to the very large 7p1/2 AO stabilization. However, the 7p3/2 AO also takes part in the bond formation, which results in an increase in Re from the Pb to Fl dimers. The Fl−Au bond should, however, be stronger than the Cn−Au one.290a,490 This is due to the fact that in FlAueven though both FlAu and CnAu are open shell systems with one antibonding σ* electronelectron density is donated from the 7p1/2(Fl) AO, lying higher in energy, to the 6s(Au) AO, while, in CnAu, some excitation energy is needed to transfer electron density from the closed 7s2(Cn) to the open 6s(Au) shell (Figure 61). Large-scale 4c-DFT calculations were also performed for M = Pb and Fl interacting with large Aun (n > 90) clusters simulating a Au(111) surface.290a Both Pb and Fl were found to prefer the bridge adsorption position. The calculated Eb value for Pb (2.40 eV) is in very good agreement with the


17.2.1. Gas-Phase Chemistry. The heaviest element investigated experimentally to date is Fl, which would be placed as eka-Pb into group 14 of the Periodic Table. Suitable 89 nuclides for chemical experiments are 287Fl (t1/2 = 0.48+0.16 −0.09 s) 500 288 +0.17 289 +0.8 or Fl (t1/2 = 0.69−0.11s) and Fl (t1/2 = 2.1−0.4s), which

Table 6. Cn-Aun and Fl-Aun Binding Energies (in eV) Simulating Interactions of Cn and Fl with Au(100) and Au(111) Surfaces (Bold Values Are for the Preferential Positions) method

n

position

4c-DFT 2c-DFT SO DFT 2c-DFT 2c-DFT 4c-DFT 4c-DFT 4c-DFT −ΔHAu a (exp)

1 1 3 26 37 95 94 107 ∞

top top top, bridge bridge hollow-2 top bridge hollow-2 unknown

surface

Au(100) Au(111) Au(111) Au(111) Au(111) unknown 1294

Cn 0.51 0.47 0.47 0.33 0.30 0.42 0.46 0.540.04 −0.03

Fl

ref

0.73 0.72 0.77 0.55 0.49 0.47 0.71 0.59 0.350.6 −0.1, or ≥ Cn

290a 450−452 450−452 452 453 290a 290a 290a 192, 195, 260


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can be synthesized in the reaction 48Ca + 242Pu or 244Pu, respectively.89 In the experiment where three decay chains of 283 Cn were registered in the COLD setup (see Figure 63),194 also a three member decay chain was registered in detector pair 19 held at −88 °C that was attributed to an atom of 287Fl reaching the detector despite its short t1/2 of only 0.5 s. The transport efficiency was estimated to be only 5%.195 An additional experiment conducted by the PSI group in 2007 at FLNR employing a 244Pu target in combination with the COLD setup revealed two further decay chains that were attributed to 288Fl.195 The transport efficiencies of 288Fl and 289 Fl can be estimated using the currently best t1/2 of 288Fl and 289 Fl to be 11% and 48%, respectively, due to their longer t1/2 compared to that of 287Fl. Indeed, two events attributed to the decay of 288Fl were detected. One α-decay event of 9.95 MeV occurred on the bottom detector of detector pair 18, followed 0.109 s later by one SF fragment in the neighboring top detector 19 held at −90 °C. The deposition of 288Fl occurred most likely on the Au covered detector 19 top, where the SF fragment was observed, whereas the α-particle was emitted across the gap and registered in the detector 18 bottom. The second decay chain started with an α-particle of 9.81 MeV in the top detector of pair 3 and ended 0.104 s later with a single fission fragment observed in bottom detector 6 at −4 °C. The observation that the decay of a daughter atom is displaced from that of the mother atom was explained by the recoil of the daughter atom out of the detector during the α-particle emission, followed by a transport with the carrier gas.195 A background of α-particle events in the region where decays of 289 Fl and its daughter 285Cn were expected prevented the positive identification of the rather long decay chain from 289 195 Fl. The location of the three detected events attributed to deposition of Fl atoms in relation to the elements Cn, Hg, and Rn is shown in Figure 65, panels 1 through 4. The dashed line (right-hand axis) always indicates the temperature gradient established during the experiments. The left-hand axis indicates relative yields per detector pair in percent of a given element.

The vertical dash-dotted line indicates the temperature at which the dew point in the gas was reached. Left of the dash-dotted line, the surface of the COLD detector was either Au (top detectors) or SiO2 (bottom detectors); right of the dash dotted line, the surfaces were covered by a thin layer of ice. In panel 1 the deposition of the nuclide 185Hg (t1/2 = 49 s) (gray bars), produced from an admixture of Nd of natural isotopic composition to the target material, is shown. Single atoms of Hg show the expected diffusion controlled deposition pattern from irreversible adsorption on the Au surface of the top detectors. Due to the high flow rates and since only one side of the detector channel was Au covered, the distribution of Hg extends far into the COLD detector. The nuclide 219Rn (white bars) being produced in transfer reactions was deposited on the ice surface only at very low temperatures close to the exit of COLD, as expected for the noble gas Rn. The deposition patterns of both 185Hg and 219Rn could be satisfactorily described by a microscopic model of the adsorption chromatographic process based on a Monte Carlo approach288 (solid −1 lines) with −ΔH0(T) on a Au surface and a (Hg) ≥ 50 kJ·mol −1 0(T) −ΔHa (Rn) = 19 kJ·mol on an ice surface, respectively, in good agreement with literature data. In panel 2, the location of three decays of 283Cn in COLD (see Figure 63) is depicted that was observed in the same experiment as the decay chain attributed to 287Fl (panel 3). In panels 3 and 4, the locations of deposition of one atom of 287Fl and two atoms of 288Fl, respectively, are shown. From these three events, a most −1 +20 probable adsorption enthalpy of −ΔH0(T) a (Fl) = 34−3 kJ·mol 195 (68% c.i.) on a Au surface was deduced. The corresponding calculated model distributions are truncated at the dash-dotted line indicating the dew point. +20 −1 on An adsorption enthalpy of −ΔH0(T) a (Fl) = 34−3 kJ·mol a Au surface is surprisingly low, since Fl is expected to be more reactive than Cn and should thus deposit at higher temperatures than Cn and not a lower temperatures, as observed. Theoretical calculations290a predicting −ΔH0(T) a (Cn) = 45 kJ·mol−1 on Au, in good agreement with experiment, predicted −1 −ΔH0(T) on Au, corresponding to a a (Fl) = 68 kJ·mol somewhat less volatile Fl compared to Cn. The DFT solidstate calculations give a higher Ecoh of the Cn solid in comparison with the Hg and Fl ones,444,497 (though a direct comparison of the calculations performed in different approximations is not straightforward). Because in group 12 and −ΔH0(T) on Au there is no correlation between ΔH0(298) S a 0(298) 0(298) ΔHS (Hg) < ΔHS (Cn), while, on a Au surface, 0(T) ΔH0(T) a (Hg) > ΔH a (Cn). On the contrary, in group 14 there is a correlation between ΔH0(298) of metals and −ΔH0(T) S a 0(298) on Au, so that ΔH0(298) (Pb) > ΔH (Fl) and −ΔH0(T) S S a (Pb) > −ΔH0(T) a (Fl) on Au. This case shows clearly that there is no on metals and −ΔH0(T) general correlation between ΔH0(298) S a on Au. In groups 15 through 17, this is another obvious case, as will be shown later. The observation that Fl exhibits an unexpected high volatility, together with the fact that a background in the detector array made the positive identification of 289Fl events impossible, caused some skepticism.501 In order to remove the background of undesired reaction products, the chemical techniques developed so far were coupled to a physical preseparator (see section 7.1.2). A first chemical experiment with Fl using a combination of IC and TC on Au surfaces was conducted behind the TASCA separator in the fall of 2009. Two events attributed to the decay of Fl were

Figure 65. Deposition patterns of the elements Hg, Rn, Cn, and Fl in the COLD detector as observed in experiments by Eichler et al.195 Reproduced with permission from ref 195. Copyright 2010 Oldenbourg Wissenschaftsverlag GmbH. For a detailed discussion see text. 1295


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10.8 kJ·mol−1 on Teflon should allow for its transport through Teflon capillaries to the chemistry setup. The obtained almost of element 118 and Rn on various equal values of −ΔH0(T) a surfaces are indicative, however, that experimental distinction between these elements by using these surfaces will be impossible. A possible material could be activated charcoal; however, further studies are needed to test this assumption. To estimate the strength of the M−M bonding in the solid state of elements 115 through 118, 4c-DFT calculations were performed for their homonuclear dimers.484 Results shown in Figure 64 indicate that the M−M bonding is weaker in (115)2 through (117)2 compared to Bi2 through At2, respectively, due to the availability of practically only 7p3/2 AOs for bonding. This means that ΔH*298(E(g)) of elements 115 through 117 should be the smallest in the groups. Their estimates made using the calculated De(M2) values are in good agreement with those obtained via linear extrapolations in groups 15 through 17 (Figure 27), meaning that these elements should be rather volatile. Bonding of (118)2 is, however, stronger than that of Rn2, since it is of van der Waals type and caused by the largest polarizability of the 118 atom in group 18. To predict ΔH0(T) of elements 115 through 118 on a Au a surface, 4c-DFT calculations were performed for the MAu dimers and their 6p homologs.490 Results are shown in Figure 64. One can see that, in groups 15 through 17, De(MAu) values are about the same for the 7p and 6p elements. This is in contrast to the trends in De(M2) in these groups, where De(Bi2) ≫ De[(115)2], De(Po2) ≫ De(Lv2), and De(At2) > De[(117)2]. The relatively strong M−Au bonding of elements 115 through 117 with Au is explained by the relativistic destabilization of the 7p3/2 AOs fitting energetically better to the 6s(Au) AO, thus makingtogether with the 7p1/2 AOa full σ-bond in MAu, in difference to M2, where only the 7p3/2 AOs are involved in bonding.484 In group 18, a reversal of the trend takes place, so that De(118Au) > De(RnAu), in agreement with the trend in De(M2). This is due to the relativistically more destabilized 7p3/2(118) AO than the 6p3/2(Rn) AO, thus better overlapping with the valence AOs of Au. Hence, for elements 115 through 118, temperatures as high as for their 6p homologs will be needed to detect their equilibrium adsorption position on Au. The calculations have also revealed that the M−Au bond strength does not decrease linearly with Z in groups 15, 16, and 17, which means that −ΔH0(T) on Au will not correlate with a in these groups. ΔH0(298) S Formation enthalpies of MX2 and MX4 (X = F, Cl, Br, I, SO42−, CO32−, NO3−, and PO43−) for Po and Lv were estimated on the basis of the MCDF atomic calculations.508 They confirmed the instability of the 4+ oxidation state of Lv. The influence of SO effects on the molecular structure of MX2 (X = F, Cl, Br, I, At, and element 117) of Lv and its lighter homologs was studied with the use of the 2c-HF and DFT ECP methods.509 The results have shown that while the molecules are bent at a scalar relativistic level, SO coupling is so strong that only 7p3/2 AOs of Lv are involved in bonding, which favors linear molecular geometries for MX2 with heavy terminal halogen atoms. The influence of relativistic effects on the electronic structure of 117H was investigated on the basis of the HF and DC CCSD(T) calculations.434 Relativistic effects, including SO ones, were shown to significantly decrease De(117H) and increase Re(117H). Electronic structures of IF, AtF, and 117F were considered at the DC and RECP levels of theory.482 De(117F) was shown to

observed.260 Both α-particle decays of Fl isotopes occurred in the isothermal section at room temperature. A detailed comparison with the experiment of the PSI-FLNR collaboration195 cannot be made, especially since no analysis of the experiment behind TASCA has been published so far. Despite this, it is quite remarkable that two independent experiments were able to separate and detect Fl in a chemistry experiment, demonstrating the power of gas chemical investigations.

18. ELEMENT 115, LIVERMORIUM (Z = 116), ELEMENT 117, AND ELEMENT 118 18.1. Theoretical Predictions

In elements 115 through 118, filling of the 7p3/2 shell takes place that will determine their chemical properties. For element 115, DCB FSCC calculations give an IP of 5.579 eV.502 For Lv and element 117, older DF calculations give IPs of 6.6 and 7.7 eV, respectively,33,34 while the DCF FSCC calculations are in progress.503,504 For element 118, the DC FSCC value of the IP is 8.914 eV.505 Since 7p3/2 AOs are relativistically more destabilized than the np3/2 AOs of the lighter homologs, IPs of elements 115 through 118 are smaller than those of the lighter homologs in groups 15 through 118. This means that the 7p3/2 elements should be chemically more reactive than the 6p3/2 ones. Due to the relativistic destabilization of the np3/2 AOs, EAs of the elements 115 (with a DCB FSCC value of 0.383 eV502), Lv (with a DCB FSCC value of 0.905 eV504), and 117 (with a DC CCSD(T) value of 1.589 eV504) will also be smaller than those of the 6p elements. For element 118, DCB FSCC+QED calculations have given a positive EA of 0.056 eV.131 The reason for that is the relativistic stabilization of the 8s AO. IPs of internal conversion electrons, that is, of K-shell (1s) and L-shell (2s), of Lv and element 118 are predicted to an accuracy of a few 10 eV using DHF theory, taking into account QED and nuclear-size effects.127 This data can be used for experimental studies of Fl involving K conversion electron spectroscopy. For elements 115 through 118, lower oxidation states should be more stable in comparison with those of the lighter homologs due to the inaccessibility of the relativistically stabilized 7p1/2 AO for bonding and the SO destabilized 7p3/2 electrons. Thus, for element 115, the 1+ state should be the most important one. The 3+ state should also be possible, while 5+ should not. For Lv, a decrease in the stability of the 4+ oxidation state is expected, and the 2+ state should be important. For element 117, the 1+ and 3+ oxidation states should be the most important ones, while the 5+ and 7+ states are less important. The 1− state of element 117, having one electron hole on the 7p3/2 AO, should, therefore, be less important (its EA is the smallest in the group). For element 118, the 2+ and 4+ states are possible, while the 6+ one will be less important, since the 7p1/22 pair is very much stabilized. The AR of the 7p3/2 elements should be larger than those of their 6p3/2 homologs due to the spatially more expanded 7p3/2 AOs. Accordingly, the polarizability of these elements should also be larger. For element 118, the polarizability of 167.4 au, a DCB FSCC result, is also the largest in the 18th group.505 Using the calculated atomic properties of element 118, its ΔH0(T) values on noble metals (Au and Ag) and nonmetals a (quartz, ice, Teflon, and graphite) were predicted using a physisorption model.505 A very low value of −ΔH0(T) a (118) of 1296


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Figure 66. Dissociation energies, De of group-1 and group-2 M2 (filled rhomboids are 4c-DFT calculations;518 open squares are experimental (filled triangles are experimental values; open ones are estimates). Reprinted with permission from refs 518 and 519. values), as well as ΔH0(298) S Copyright 2012 Elsevier and 2012 American Institute of Physics, respectively.

be the largest among the group-17 fluorides. For 117F3, the RECP calculations have shown that the D3h geometry is not the proper one, in difference from the sixth period compound of At, thus again indicating that the VSEPR theory is not applicable to the heaviest elements.482,510 117Cl is predicted to be bound by a single π bond and have a relativistically (SO) increased Re.511 Due to the relativistic destabilization of the 7p3/2 AO in element 118, it is predicted to be rather reactive. It should form the 118-Cl bond.512 The RECP calculations for the reactions M + F2 → MF2 and MF2 + F2 → MF4, where M = Xe, Rn, and element 118, confirmed the increasing stability of the fluorides in the group as a result of the increasing polarizability of the central atom.482,513 Also, the following trends in the stability of the fluorides were established: RnF2 < HgF2 < PbF2, while CnF2 < FlF2 < 118F2. The influence of relativistic effects on the electronic structure of 118H+ was investigated on the basis of HF and DC CCSD(T) calculations.434 Relativity (including SO effects) was shown to significantly decrease De(118H+) and increase Re(118H+). The influence of the SO interaction on the geometry of group-18 MF4 was investigated by the RECP-SOCI/CCSD calculations.478,513,514 It was shown that in 118F4, a Td configuration becomes more stable than the D4h one known for the lighter homologs. The reason for this unusual geometry is the availability of only the stereochemically active four 7p3/2 electrons for bonding. This is another example of the inapplicability of the VSEPR theory for the heaviest elements. An important observation was made that the fluorides of element 118 will most probably be ionic rather than covalent, as in the case of Xe. This prediction might be useful for future gas-phase chromatography experiments.

attempts have failed.515,516 The currently best options517 based on the parameters reaction asymmetry and Q value appear to be the reactions 50Ti + 249Bk and 50Ti + 249Cf to synthesize elements 119 and 120, which are currently attempted at TASCA at GSI. However, the t1/2 values of the synthesized nuclei are expected to be of the order of milliseconds or even microseconds. Provided suitable long-lived isotopes of these elements are found, the volatility of their atoms might be studied in the long term using some advanced chromatography (e.g., vacuum) techniques that can cope with extremely short lifetimes of their isotopes. To foresee the outcome of such experiments, as well as to study the influence of relativistic effects on the 8s AOs of these heaviest elements and their compounds, some theoretical studies have been undertaken.506,507,518,519 For earlier predictions of properties of these elements based on the atomic calculations, see refs 33 and 34. Properties that are of interest for gas chromatographic studies, i.e., ΔH0(298) and ΔH0(T) of elements 119 and 120 on S a noble metals were predicted on the basis of 4c-DFT calculations for intermetallic M2 and MAu (M are group-1 and group-2 elements) compounds.518,519 Figure 66 demonstrates the obtained De(M2) and their trends in groups 1 and 2. One can see that, in these groups, there is a reversal of the trends in De(M2) at Cs and Ba, respectively, though in an opposite way. The reason for the different behavior is a different type of M−M bonding in these groups: a covalent one in group 1, while a van der Waals one in group 2, even though both are defined by the behavior of the ns AOs. Thus, (119)2, having a σg2 ground state, should be bound the strongest by covalent forces among the homologs and have a short bond length (about that of Rb2) caused by the contraction of the 8s AO. On the contrary, (120)2 with a σ2gσ*2u ground state should be the most weakly bound, only by van der Waals forces, among the homologs (the number of bonding and antibonding electrons is the same), and the bond should be the longest.

19. ELEMENT 119 AND ELEMENT 120 19.1. Theoretical Predictions

At the time being, the chemistry of elements heavier than Z = 118 rests on a purely theoretical basis. The synthesis of elements beyond Z = 118 appears to be difficult, and several 1297


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Figure 67. 4c-DFT dissociation energies, De, of group-1 and group-2 MAu, as well as adsorption enthalpies −ΔH0(T) (filled symbols are a semiempirical calculations,520 while open ones were obtained via correlations with De(MAu), where M = Au, Pt, and Ag). Reprinted with permission from refs 518 and 519. Copyright 2012 Elsevier and 2012 American Institute of Physics, respectively.

ΔH0(298) (119) = 94 kJ·mol−1 and ΔH0(298) (120) = 150 S S −1 kJ·mol values were obtained via a correlation with the calculated De(M2) in groups 1 and 2, respectively. The ΔH0(298) S values show the same reversal in the groups as the De(M2) values do. According to these results, element 119 metal should be as strongly bound as K, while element 120 metal should be the most weakly bound in group 2, though it is more strongly bound than element 119 (Figure 66). Binding energies of MAu of group-1 and group-2 elements values, estimated using a are shown in Figure 67. ΔH0(T) a correlation with the calculated De(MAu) in these groups, are also depicted there. According to the data, elements 119 and 120 should form stable compounds with gold. The De(MAu) values reveal a reversal of the increasing trend at Cs and Ba in groups 1 and 2, respectively, so that both 119Au and 120Au should be the most weakly bound among the considered homologs in these groups. The trend is defined by the behavior of the ns AOs, whose relativistic stabilization in the groups starts to dominate over the orbital expansion beyond Cs and Ba, respectively. −1 and −ΔH0(T) The −ΔH0(T) a (119) = 106 kJ·mol a (120) = 172 −1 kJ·mol were determined via a correlation with De(MAu) in these groups. Using correlations with −ΔH0(T) a (M) on other of these elements on Ag and Pt were noble metals, −ΔH0(T) a values also predicted (Figure 67). The very moderate −ΔH0(T) a of elements 119 and 120, the lowest in groups 1 and 2, especially on Ag (63 kJ·mol−1 and 50 kJ·mol−1, respectively), would allow adsorption chromatographic measurements of these elements. and −ΔH0(T) values show that there The obtained ΔH0(298) S a is no correlation between these quantities in group 1, as they change in the opposite way with Z. In group 2, there is a and −ΔH0(T) correlation between ΔH0(298) S a . Thermodynamic properties of metals of elements 113 through 120 were also predicted521 using atomic calculations and mathematical models. Hydrides and fluorides of elements 119 and 120 were considered within the PP and ab initio DF approximations.434,522,523 It was shown that bond distances decrease from the seventh to the eighth period for group-1 and group-2

elements due to the relativistic ns AO contraction. The 119F was found to be less ionic than lighter alkaline fluoride homologs, in contrast to expectations based on periodic trends.

20. ELEMENTS BEYOND Z = 120 20.1. Theoretical Predictions

The chemistry of these elements will be defined by many open shells and their mixing.33,34 Due to very strong relativistic effects, things will be much more different than anything known before. However, without relativistic effects, it would also be very different due to the very large orbital effects. Very few molecular calculations exist in this superheavy domain. Properties of elements heavier than 120 predicted on the basis of atomic calculations were discussed.33,34,185,524,525 More recent considerations of their chemistry can be found in refs 526 and 527. A list of possible molecules of elements in the range Z = 121−164 was suggested,527 though their verification should be left to future theoretical studies. Interesting examples are those where the elements are in unusual valence states or coordination, such as, for example, 144F8 (an analog of PuF8) or 148O6 (an analog of UO6). Quasi-relativistic multiple-scattering calculations on 125F6 have found that bonding is defined by the 5g1 electron, with the situation being analogous to NpF6 with the 5f1 electron.528 There are noncorrelated DF calculations for fluorides of element 126.529,530 Accurate predictions of properties of specific compounds will be quite a challenging task in this area. This may need inclusion of QED effects to reach the required accuracy.

21. CONCLUSIONS AND OUTLOOK Despite the fact that only single atoms of transactinide elements can be studied “one-atom-at-the-time” and the short t1/2 are of the order of seconds, the experimental body of data is already substantial and impressive. It was shown that the heaviest elements are basically homologs of their lighter congeners in the chemical groups, though their properties may be rather different due to very large relativistic effects on their electron shells. Relativistic effects were found to be predominant over 1298


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Table 7. Trends in Volatility of the Heaviest Element Compounds and Their Lighter Homologs in the Chemical Groups group

species

theor pred

4 5

MCl4, MBr4 ML5 (L = Cl, Br) MBr5 → MBr6¯

Hf < Rf Nb < Ta < Db DbCl5 > DbOCl3 Nb > Ta > Db

38 331 331 364

MO2Cl2 MO3Cl MO4 M M

Mo > W > Sg Tc > Re > Bh Ru < Os > Hs Hg < Cn Pb ≪ Fl < Cn

382 291 293 290a, 445, 447, 450 290a, 452, 453, 485

6 7 8 12 14

ref

exp obsd

ref

Hf < Rf NbCl5 ≈ DbCl5 DbCl5 > DbOCl3 Nb > Ta > Db Db > Nb > Ta Mo > W > Sg Tc > Re > Bh Os > Hs Hg < Cn Fl ≥ Cn Fl ≤ Cn

332, 333 376 229 226 377 232, 234 235 197 192, 194 195 260

Table 8. Trends in Hydrolysis and Complex Formation of the Heaviest Element Compounds and Their Lighter Homologs in the Chemical Groups group

extracted complex

theor pred

ref

exp obsd

ref

4

hydrolysis of M4+ MFx(H2O)z−x8−x (x ≤ 4) MF62−

Zr > Hf > Rf Zr > Hf > Rf Rf ≥ Zr > Hf

307 307 307

360 272, 354 272, 350, 355

MCl62− MCl4

Zr > Hf > Rf Rf > Hf > Zr

307 307

M(SO4)44− hydrolysis of M5+ MOCl4−, MCl6− MF6−, MBr6− hydrolysis of M6+ hydrolysis of MO2(OH)2 MO2F2(H2O)2 MOF5− MO4(OH)22−

Zr > Hf ≫ Rf Nb > Ta > Db Nb ≥ Db > Ta Nb > Db > Ta Mo > W > Sg Mo > Sg > W Mo > Sg > W Mo < W < Sg Os > Hs ≫ Ru

310 303 304 305 306 306 308 308 309

Zr > Hf > Rf Zr > Hf > Rf Rf ≥ Zr > Hf Zr > Hf ≫ Rf Rf > Zr > Hf Zr > Rf > Hf Zr > Hf ≈ Rf Zr > Hf ≫ Rf Nb > Ta Nb ≥ Db >Ta Nb > Db > Ta Mo > W > Sg Mo > W Mo > W Mo < W Os ≥ Hs

5

6

8

357 271 361 362, 363 360 277 277 279 279 385, 531, 532 385 420

routine application to the heaviest systems lies still in the future. Using the methods described in this review, reliable predictions of properties of the heaviest element and their compounds became available. Theoretical calculations permitted establishment of important trends in spectroscopic properties, chemical bonding, stabilities of oxidation states, ligand-field effects, complexing ability, and others in the groups of the Periodic Table including the heaviest elements, as well as assessment of the role and magnitude of relativistic effects. Detailed studies were offered for elements Rf through 120, as well as for some species of even heavier elements. A high accuracy of total energy calculations allowed for predictions of stability of species, of their geometry and energies of chemical reactions in the gas and aqueous phases, as well as of adsorption on surfaces of metals. However, fully relativistic descriptions of adsorption processes on complicated or inert surfaces are still problematic. Therefore, some models were used in practical applications. Also, physicochemical models were helpful in predicting some other properties that are difficult to handle in a straightforward way, such as, for example, extraction from aqueous solutions or ion exchange separations. Such studies were performed for elements Rf through Hs, Cn, Fl, and element 113. Some estimates of adsorption enthalpies of even heavier elements, up to Z = 120, on noble metals are also available.

the orbital ones in the electronic structures of the elements of the seventh period and heavier. They are responsible for trends in the chemical groups (a continuation, or a reversal) with increasing Z from the elements of the sixth period. Thus, for elements of the seventh period and heavier, the use of relativistic methods is mandatory. Straightforward extrapolations of properties from lighter congeners may, therefore, result in erroneous predictions. Spectacular developments in relativistic quantum theory, computational algorithms, and computer techniques allowed for accurate calculations of properties of the heaviest elements and their compounds. Nowadays, most accurate atomic DC(B) correlated calculations including QED effects are available for the heaviest elements, reaching an accuracy of a few millielectronvolts for electronic transitions and ionization potentials. These calculations allow predictions of electronic configurations of the heaviest elements up to Z = 122. For heavier elements, as well as for the midst of the 6d-element series, MCDF calculations are still the source of useful information. Most of the molecular calculations were performed with the use of relativistic DFT and RECP methods that turned out to be complementary both conceptionally and quantitatively. Their combination is presently the best way to study properties of complex systems of the heaviest elements. DC ab initio molecular methods are in the phase of development, and their 1299


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21.1. Summary of Volatility Studies of the Heaviest Elements and Their Compounds

separation will also need new technological developments to cope with the very low production rates and short t1/2. In this area, theoretical chemistry will have a number of exciting tasks to predict the experimental behavior in chemical separation experiments. Even though some basic properties of these elements have been theoretically outlined, more detailed studies should follow, taking into account experimental details. Some further methodical developments in the relativistic quantum theory, such as, for example, fully relativistic ab initio molecular, cluster, and solid state codes, also with inclusion of QED effects on a SCF basis, will be needed to achieve a required accuracy of the predicted quantities for those very high Z numbers. These future calculations will also need powerful supercomputers. In the long run, new accelerators delivering higher beam intensities and even more exotic target materials, such as 250Cm, 251,252 Cf, and 254Es, will allow production of nuclides closer to the line of β-stability534 in superheavy element factories. Possible electron-capture branches in the members of the αparticle decay chains may lead to the formation of longer-lived nuclides of Mt, Ds, Rg, and Cn, that could be stored and studied in traps. The theoretical and experimental investigation of transactinide elements continues to be a fascinating branch of chemistry that, by its nature, is pure fundamental research.

Predicted trends in volatility of the heaviest elements and their compounds compared to the experimental observations are summarized in Table 7. One can see that almost all the predictions for group 4 through group 8, as well as for group 12 were confirmed by the experiments. In addition, the calculated absolute values of ΔH0(T) were in very good agreement with the a experimental ones, as discussed above. Thus, due to their highest covalency, the pure halides of Rf and Db were expected to be more volatile than their next lighter homologs in their respective groups. This was clearly observed experimentally for Rf in group 4 (see Figure 37). Open questions remain in the interpretation of the volatility of group-5 pure halides, which might need further experimental or/and theoretical considerations. In groups 6 and 7, the trend to a decrease in volatility is clear as defined by decreasing dipole moments of the oxyhalides. In group 8, hassium tetroxide should be less volatile than the Os homolog because of its larger polarizability. Cn is more volatile than Hg due to its high inertness. It should also be more volatile than Fl. While predictions of the adsorption properties of Cn on a Au surface were in line with experimental observations, predictions for Fl are awaiting further experimental verifications. 21.2. Summary of Aqueous Chemistry Studies of the Heaviest Elements

AUTHOR INFORMATION

A summary of the predicted trends in hydrolysis, complex formation, and extraction of the heaviest element complexes and their homologs as compared to the experimental results is given in Table 8. As one can see, most of the predictions were confirmed by experiments for the heaviest elements and their homologs, while some of them are still awaiting verification, as in the case of Sg in HF solutions. The calculations have shown that the theory of hydrolysis301 based on the relation between the cation size and charge did not explain all the experimental behavior, such as, for example, the difference between Nb and Ta, or Mo and W. Only by performing relativistic calculations for real chemical equilibrium in solutions can complex formation and hydrolysis constants, as well as distribution coefficients between aqueous and organic phases (or sorption coefficients), and their order in the chemical groups be correctly predicted. Being often conducted in a close link to the experiment, those theoretical works helped design chemical experiments and interpret their outcome. In turn, experimental results put theoretical predictions (and hence models used for making these predictions) to the test, and in this way, they help to improve the models. The synergism between the theoretical and experimental research in this field led to better understanding of the chemistry of these exotic species.

Corresponding Author

*Electronic address: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Andreas Türler was born in Winterthur, Switzerland. He received his Diploma and Ph.D. in chemistry from University of Bern, Switzerland, having carried out research on nucleon transfer reactions under the supervision of Prof. Hans-Rudolph von Gunten. He then joined the group of Prof. Darleane C. Hoffman at Lawrence Berkeley National Laboratory, Berkeley, United States, as postdoctoral fellow. In 1992, he moved back to Switzerland and in the following years worked as staff scientist at Paul Scherrer Institute, Villigen, with Prof. Heinz Gäggeler. In 1994 he was awarded the “Fritz-Strassmann-Preis” of the Gesellschaft Deutscher Chemiker. The habilitation in 2000 at University of Bern was followed with an appointment as full professor and director of the Institute of Radiochemistry at Technical University of Munich, Germany. In 2009 he returned to Switzerland as head of the Laboratory of Radiochemistry and Environmental Chemistry at

21.3. Future Developments

In the near future, first results concerning the chemistry of element 113 can be expected. Also, element 115 may come within reach with present day technologies, provided a constant supply of 249Bk target material. The knowledge of lighter transactinides will grow, and new classes of compounds, such as volatile carbonyls, open new perspectives,533 especially to first chemical studies of elements such as Mt. For the not yet studied elements, such as Mt, Ds, Rg, and element 115, isotopes with t1/2 suitable for chemical studies have already been identified. For elements 116 through 118, new isotopes suitable for chemical studies must first be discovered. Their 1300


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CI CIX COLD COMPACT

configuration interaction cation exchange cryo online detector cryo online multidetector for physics and chemistry of transactinides CR covalent radius CTS cryo thermochromatographic separator DCB Dirac−Coulomb−Breit De atomization energy DF Dirac−Fock DFT density functional theory DGFRS Dubna gas-filled recoil separator ΔHa0(T) enthalpy of adsorption at standard conditions at Ta or T50% ΔHevap enthalpy of evaporation sublimation enthalpy ΔH0(298) S ΔH*298(E(g)) standard enthalpy of monatomic gaseous elements DHF Dirac−Hartree−Fock DKH Douglas−Kroll−Hess DS Dirac−Slater DS-DV Dirac−Slater discrete variational method E(x) energy of the charge transfer transition EA electron affinity Ecoh cohesive energy ECP effective core potential ECR electron cyclotron resonance EH effective Hamiltonian fb femtobarn (10−39 cm2) FC frontal chromatography FLNR Flerov laboratory of nuclear reactions FS Fock-space GANIL grand accélérateur national d’ions lourds GARIS gas-filled recoil isotope separator GGA generalized gradient approximation GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt IC isothermal chromatography HEVI heavy element volatility instrument HITGAS high-temperature online gas chromatography apparatus IH intermediate Hamiltonian IP ionization potential IR ionic radius HSCC Hilbert space coupled cluster IUPAC International Union of Pure and Applied Chemistry IUPAP International Union of Pure and Applied Physics IVO in situ volatilization JAEA Japan atomic energy agency JYFL Jyväskylän Yliopiston Fysiikan Laitos Kads adsorption constant Kdes desorption constant LBNL Lawrence Berkeley National Laboratory LDA local density approximation LSC liquid scintillation counting mb millibarn (10−27 cm2) MBPT many-body perturbation theory MCDF multiconfiguration Dirac−Fock MCSCF multiconfiguration self-consistent field MCT multicolumn technique MeV mega electronvolt MG merry-go-round

Paul Scherrer Institute and University of Bern. His scientific interests are nuclear and radiochemistry in general, with one focus on the physics and chemistry of transactinide elements.

Valeria G. Pershina was born in Cheliabinsk, Russia. She received her Diploma from Mendeleev University of Chemistry and Technology (Moscow) and her Ph.D. degree (with Professors G. Ionova and V. Spitsyn) from Institute of Physical Chemistry, USSR Academy of Sciences (Moscow). In the following years she worked as a researcher, group leader, and deputy head of the quantum chemistry lab at the same Institute of Physical Chemistry. Since 1990, she has been at the University of Kassel (with Prof. B. Fricke), and from 1996 on, at the GSI, Darmstadt, as a senior researcher. In 1994 she habilitated at the Institute of Physical Chemistry, Russian Academy of Sciences, Moscow, and received a degree of a professor of the Russian Academy of Sciences. Her scientific interests are in general relativistic quantum chemistry, inorganic chemistry, and physical chemistry, with specialization in the chemistry of the heavy and heaviest elements.

ACKNOWLEDGMENTS One of the authors gratefully acknowledges the financial support of this work through the Swiss National Science Foundation Grant No. 200020_126639, Paul Scherrer Institute, and University of Bern. ABBREVIATIONS AND QUANTITIES 4c four component vector function A mass number ADF Amsterdam DFT AIDA automated ion-exchange separation apparatus coupled with the detection system for αspectroscopy AIMP ab initio model potential AL average level AO atomic orbital AR atomic radius ARCA automated rapid chemistry apparatus ARTESIA a rotating target wheel for experiments with superheavy-element isotopes at GSI using actinides as target material BGS Berkeley gas-filled separator CALLISTO continuously working arrangement for clusterless transport of in situ produced volatile oxides CASMCSCF complete active space multiconfiguration selfconsistent field CC coupled cluster CCSD(T) coupled cluster single double (and perturbative triple) excitations 1301


Chemical Reviews MIBK MP MSCC N nb OL OLGA OSCAR pb PBE PIPS PP PSI QED QM Re RECP RGGA RIKEN RILAC Rmax ROMA RTC RvdW SCF SHIP SISAK SO SO ZORA SRAFAP T50% Ta TASCA Tb TC TiOA Tm TUM TWG UNILAC XIH Z

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methyl isobutyl ketone model potential mixed sector coupled cluster neutron number nanobarn optimal level online gas chromatography apparatus online separation and condensation apparatus picobarn (10−36 cm2) Perdew−Burke−Ernzerhof passivated implanted planar silicon pseudopotential Paul Scherrer Institute Quantum Electro Dynamics effective charge on the central metal atom molecular bond length relativistic effective core potential relativistic generalized gradient approximation rikagaku kenkyusho RIKEN linear accelerator maximum of the radial charge density rotating multidetector apparatus recoil transfer chamber van der Waals radius self-consistent field separator for heavy ion products short-lived isotopes studied by the AKUFVEtechnique spin−orbit spin−orbit zeroth-order regular approximation students running as fast as possible temperature of 50% relative yield in IC adsorption temperature in TC transactinide separator and chemistry apparatus boiling point thermochromatography triisooctylamine melting point Technical University Munich transfermium working group universal linear accelerator extrapolated intermediate Hamiltonian atomic number

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