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Gas-Phase Reactions of Atomic Lanthanide Cations with Ammonia: Room-Temperature Kinetics and Periodicity in Reactivity Gregory K. Koyanagi, Ping Cheng,† and Diethard K. Bohme* Department of Chemistry, Centre for Research in Mass Spectrometry and Centre for Research in Earth and Space Science, York UniVersity, Toronto, Ontario, Canada, M3J 1P3 ReceiVed: August 17, 2009; ReVised Manuscript ReceiVed: September 29, 2009
Reactions of (14) atomic lanthanide cations (excluding Pm+) with ammonia have been surveyed in the gas phase by using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer to measure rate coefficients and product distributions in He at 0.35 ( 0.01 Torr and 295 ( 2 K. Primary reaction channels were observed corresponding to H2 elimination with formation of the protonated lanthanum nitride and NH3 addition. H2 elimination was seen only in the reactions with La+, Ce+, Gd+, and Tb+ and occurs with these ions exclusively. NH3 addition was seen with Pr+, Nd+, Sm+, Eu+, Dy+, Ho+, Er+, Tm+, Yb+, and Lu+. Higher-order sequential addition of up to five NH3 molecules was observed with the Ln+(NH3) and LnNH+ ions. The reaction efficiency of the primary reactions is seen to decrease as the energy required to promote an electron to make two non-f electrons available for bonding increases. The periodic trend in reaction efficiency along the lanthanide series matches quite closely the periodic trend in the electron-promotion energy required to achieve a d1s1 or d2 excited electronic configuration in the lanthanide cation. With La+, Ce+, Gd+, and Tb+, the electrostatic attraction between the atomic lanthanide cation and ammonia is sufficiently strong to provide enough energy to achieve electron promotion and to overcome any barriers to subsequent NsH bond insertion and H2 loss, but this is not the case with the other lanthanide cations with which collisional stabilization of the intermediate adduct ion, with or without insertion of Ln+, predominates. 1. Introduction Studies of gas-phase reactions of atomic lanthanide cations with small molecules can be very rewarding, because these ions offer a special opportunity to scrutinize fundamental aspects of atomic-ion reactivity in a systematic fashion as a function of the electronic configuration of the ion.1 Gas-phase reactions of isolated lanthanide cations began to be measured in the late 1980s with Fourier-transform mass spectrometry and various ion-beam techniques, together with laser ablation to produce the cations.2 Our own laboratory has contributed a flow tube/ tandem mass spectrometry technique in which atomic cations are produced in an inductively coupled plasma (ICP).3 Investigations over the past 20 years have made available extensive data for gas-phase reactions of atomic lanthanide cations with various organic and inorganic molecules including hydrogen,4 oxygen and nitrous oxide,3 nitric oxide,5 carbon dioxide and carbon disulfide,6 heavy water,7 methane,8 alkanes and cycloalkanes,2b,c,9 alkenes,2a,b,10 alcohols,11 benzene and substituted benzenes,12 phenol,13 orthoformates,12a,14 ferrocene and Fe pentacarbony,15 methyl fluoride16 and methyl chloride,17 and sulfur hexafluoride.18 The results of these studies show that both the reaction efficiencies and the products formed often vary substantially along the 4f series of the Ln+ family. These variations largely arise from the accessibility of excited electronic configurations that make available two unpaired non-f electrons for chemical bonding. Excitation can be achieved by f-to-d electron promotion from 4fn5d06s1 ground states to excited 4fn-15d16s1 states. A recent bonding configuration analysis by Gibson suggests that * Corresponding author. E-mail:
[email protected]. Phone: 416-7362100, ext 66188. Fax: 416-736-5936. † Current address: Elemento-Organic Chemistry Laboratory, Nankai University, 94 Weijin Road, Tianjin, China 3000071.
two unpaired 5d valence electrons rather than a 5d and a 6s electron bring about the bonding between the metal center and the oxygen atom in the formation of LnO+.19 The variations in the promotion energies required to achieve either 5d2 or 5d16s1 excitation across the Ln+ family are qualitatively similar to the observed periodic and Arrhenius-like dependencies of the efficiencies of O-atom transfer on the electron promotion energy. To bring further insight into fundamental aspects of the chemistry of lanthanide cations, we present here systematic experimental measurements of gas-phase reactions of Ln+ with NH3 (excluding Pm+). Surprisingly, there has been only one previous published report on a reaction of a lanthanide cation with ammonia, to the best of our knowledge: Ce+ has been reported to react with ammonia in the dynamic reaction cell of an ICP/MS instrument in a study of an isobaric interference of a product CeNH2+ ion with CeO+.20 Reactions of atomic lanthanide cations with ammonia are also of interest and are used in the elemental analysis of target lanthanides (Ce, Pr, Nd, Sm, Eu, and Gd) in studies of the influence of irradiation by fast neutron bombardment.21 Accurate measurements of the isotopic compositions of the enriched elements are required in these studies before and after irradiation. A cost-effective way to achieve such measurements is with the use of ICP/MS fitted with a collision cell that allows the chemical resolution of the many isobaric interferences that are present in the irradiated samples (e.g., 147,148Nd/147,148Sm, 151,154Sm/151,154Eu, 154Sm/154Gd, 154,155 Eu/154,155Gd) that complicate the direct quantification of the nucleotide. Ammonia is one of the reagents being investigated for use in the collision cell, and results have appeared very recently in a dissertation by G. Favre for the reactions of Nd+, Sm+, Eu+, Gd+, and Dy+ with NH3.21 In the experimental studies reported here, all of the Ln+ cations are formed within the same source, an ICP of argon at
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5500 K. The ions are allowed to react with NH3 after they have been mass-selected and cooled by collisions with He bath gas at room temperature (295 ( 2 K) and a pressure of 0.35 ( 0.1 Torr. The progress of reaction is monitored by using selected ion flow tube (SIFT) tandem mass spectrometry.5,20 Primary reaction rate coefficients and reaction product distributions are measured, and higher-order reactions are followed as well. 2. Experimental Method The experimental results reported here were obtained with the ICP/SIFT tandem mass spectrometer that has been described in detail previously.3 The atomic ions were generated within atmospheric-pressure argon plasma at 5500 K fed with a vaporized solution containing the lanthanide salt. Solutions containing the salt of interest with concentrations of ca. 5 µg l-1 were peristaltically pumped via a nebulizer into the plasma. The plasma gas flow was adjusted to maximize the ion signal detected downstream of the SIFT. The sample solutions were prepared by using atomic spectroscopy standard solutions commercially available from SPEX, Teknolab, J.T. Baker Chemical Co., Fisher Scientific Company, Perkin-Elmer, and Alfa Products. The ions emerging from the ICP were injected through a differentially pumped sampling interface into a quadrupole mass filter and, after mass analysis, introduced through an aspirator-like interface into flowing helium carrier gas at 0.35 ( 0.01 Torr and 295 ( 2 K. After experiencing about 105 collisions with He atoms, the ions were allowed to react with NH3 added into the flow tube. The lanthanide ions emerging from the plasma initially have a Boltzmann internal energy distribution characteristic of the plasma temperature. However, these emerging populations are expected to be down-graded in energy during the approximately 20 ms duration before entry into the reaction region in the flow tube. Energy degradation can occur by radiative decay as well as by collisions with argon atoms and the 105 collisions with He before entry into the reaction region. Electronic states of the lanthanides, because of the presence of f-electrons, are a mixture of states with both positive and negative parity. This means that there is a large number of parity-allowed transitions that will occur quickly (∼10-8 s), changing their original state distribution within the ICP. La+ itself is an exception for lanthanides in that it behaves like a transition-metal ion because it does not have any low-lying states with occupied f-orbitals. The extent to which quenching of any electronically excited states of the lanthanide cations that may be formed within the ICP is complete is uncertain and could be inferred only indirectly from the observed decays of primary ion signals. The observed semilogarithmic decays of the reacting lanthanide cations were invariably linear over as much as three decades of ion depletion and, threfore, were indicative of single-state populations (or multiple-state populations with equal reactivities). The many collisions with Ar and He between the source and the reaction region should ensure that the atomic ions reach a translational temperature equal to the tube temperature of 295 ( 2 K prior to entering the reaction region. Reactant and product ions were sampled at the end of the flow tube with a second quadrupole mass filter, and their signals were measured as a function of added reactant. The resulting profiles provide information about reaction rate coefficients and product-ion distributions. Rate coefficients for primary reactions were determined in the usual manner by using pseudo first-order kinetics with an uncertainty estimated to be less than (30% from the semilogarithmic
Koyanagi et al. decay of the reactant-ion intensity as a function of added reactant. Pure NH3 gas was obtained commercially (Semiconductor grade 99.999%, Matheson/Linde Canada) and introduced into the reaction region of the flow tube as a dilute (15%) mixture in helium. 3. Results and Discussion The reactions of 14 lanthanide cations were investigated with NH3. Both the primary and higher-order chemistries were monitored. All lanthanide cations were observed to exhibit some reactivity toward NH3 mostly resulting in adduct formation. Results obtained for the reactions of La+, Gd+, Pr+, and Lu+ are shown in Figure 1. Table 1 summarizes the measured rate coefficients and derived reaction efficiencies. The reaction efficiency is taken to be equal to the ratio k/kc, where k is the experimentally measured rate coefficient and kc is the capture or collision rate coefficient. kc was computed by using the algorithm of the modified variational transition-state/classical trajectory theory developed by Su and Chesnavich22 with R(NH3) ) 2.145 × 10-24 cm3,23 and µD(NH3) ) 1.47 D.24 The two primary reaction channels that were observed are indicated in reaction 1.
Ln+ + NH3 f LnNH+ + H2 +
f Ln (NH3)
(1a) (1b)
They correspond to bimolecular H2 elimination with formation of the protonated lanthanum nitride, reaction 1a, and NH3 addition, reaction 1b, that is likely to be termolecular with He atoms acting as the third body. The structure of LnNH+ is not established but may well be the imide +LndNsH with the positive charge on the metal center and the lanthanide in its stable trivalent oxidation state. Molecular hydrogen elimination, channel 1a, was seen only in the reactions with La+, Ce+, Gd+, and Tb+ and occurs with these ions exclusively. H-atom elimination to form LnNH2+ + H was not observed. Hatom transfer also was not observed likely because of the low H-atom affinities of the Ln+ cations which are much smaller than HA(NH2) ) 108.6 kcal mol-1,25 as for example HA(La+) ) 57.2 ( 2.1 kcal mol-1 and HA(Lu+) ) 48.6 ( 3.7 kcal mol-1.4 As expected from the much lower first-ionization energy of the lanthanides, IE(Ln), all lower than 6.3 eV, and that for NH3 (10.070 ( 0.020 eV),25 electron transfer was not observed with any of the Ln+ cations. NH3 addition was seen with Pr+, Nd+, Sm+, Eu+, Dy+, Ho+, Er+, Tm+, Yb+, and Lu+. The previous report by Tanner et al. of a bimolecular reaction between Ce+ and NH3 cited the production of both CeNH+ and CeNH2+ by elimination of molecular (80%) and atomic hydrogen (20%), respectively.20 We did not observe the minor channel reported by these authors. The results of Favre et al.21 with Nd+, Sm+, Eu+, Gd+, and Dy+ indicate exclusive addition to the lanthanide cation (in pure ammonia under somewhat uncertain conditions of pressure estimated to be in the milliTorr range) with rate coefficients systematically higher than ours by factors of about 2 to 10, except for the reaction with Gd+ for which we observe H2 elimination with a rate coefficient about five times higher than that reported by Favre et al. for addition.21 Secondary- and higher-order NH3 addition according to reactions 2 and 3 up to n + 1 was observed for Ln+ (n + 1) ) Pr+ (6), Nd+ (5), Sm+ (5), Eu+ (5), Dy+ (6), Ho+ (6), Er+ (6), Tm+ (6), and Yb+ (5) and for LnNH+ (n + 1) ) LaNH+ (5), CeNH+ (5), GdNH+ (5), TbNH+ (5), and LuNH+ (5). All these addition reactions also are expected to proceed in a termolecular
Gas-Phase Reactions of Lanthanide Cations with Ammonia
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Figure 1. Composite of ICP/SIFT results for the reactions of the lanthanide cations La+, Gd+, Pr+, and Lu+ with NH3 in helium buffer gas at 0.35 ( 0.01 Torr and 295 ( 2 K.
fashion under our experimental operating conditions with He buffer-gas atoms acting as the stabilizing third body.
Ln+(NH3)n + NH3 f Ln+(NH3)n+1 +
+
LnNH (NH3)n + NH3 f LnNH (NH3)n+1
(2)
nitrides were also produced, nor with any of the other lanthanide cations. Secondary H2 elimination appeared to occur only with Lu+(NH3) according to reaction 4.
Lu+(NH3) + NH3 f LuNH+(NH3) + H2
(4)
(3)
Fasvre et al.21 observed similar extents of addition for Nd+, Sm+, Eu+, Gd+, and Dy+ with ammonia acting as the stabilizing third body. A small (3.5%) production of NH4+ also was observed in the Gd+ experiment that we attribute to the transfer of a proton from GdNH+ to NH3 in a secondary reaction. Production of small amounts (
