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(299) Titova, G. E.; Krishtalik, L. I . Nektrokhimiya 1978, 74, 126-8; Chem. Abstr. 1978, 8 8 , 160627e. (300) Tomkins, R. P. T.; Turner, P. J. J . Chem. Thermodyn. 7977, 9 , 707-10. (301) Trasatti, S. J . Chem. Phys. 1978, 69, 2938-9. (302) Troupel, M.; Folest, J. C.; Chevrot, C.; Perichon, J. Bull. SOC. Chim. Fr. 1978, 45-9, Chem. Abstr. 1978, 8 9 , 118522n. (303) Turek, A.; Riddle, C.: Talerico, F. Chem. Geol. 1978, 2 7 , 351-7. (304) Urisbaev, T. U.; Shchelkunov. A. V.; Petrov. S. I. Zh. Anal. Khim. 1978, 3 3 , 2235-9: Chem. Abstr. 1979, 9 0 , 132354m. (305) Uruska, I . Int. Symp. Specific Interact. Mol. Ions [ P r o c . ] , 3rd 1976, 2 , 542-4. (306) Velinov, G.; Budevski, 0. J . Elecfroanai. Chem. Interfacial Electrochem. 1979, 9 5 , 73-9. (307) Ventura, M.; Rauret, G.; Rubio, R. An. Ouim. 1978, 74, 1321-2; Chem. Abstr. 1979, 9 1 , 32199s. (308) Verhoef, J. J . Nectroanal. Chem. Interfacial Electrochem. 1978, 9 3 , 75-80. (309) Verhoef. J. C.; Kok, W. T.; Barendrecht, E. J . Hectroanal. Chem. Interfacial Electrochem. 1978, 86, 407-15. (310) Verrna, B. C.; Kumar, S. Talanta 1977, 2 4 , 694-5. (311) Verrna, E. C.; Kumar, S. Z h . Anal. Khim. 1977, 3 2 . 2446-9; Chem. Abstr. 1978, 8 9 , 1226671. (312) Verma, K. K.; Ahmed. J.; Sahasrabuddhey, M. P.; Bose, S. J . Indian Chem. SOC. 1977. 54. 699-702. (313) Vucurovic, B. D.';Jovanovic, M. S. Gbs. Hem. Drus. Beograd1978, 43, 621-8; Chem. Abstr. 1979, 9 0 , 1 9 7 9 7 1 ~ . (314) Watts, D. W. Pure Appl. Chem. 1979, 5 7 , 1713-24. (315) Werblan. L.: Suzdorf. A.: Lesinski, J , Rocz. Chem. 1977, 51, 2233-42.
(316) Wiese, G.; Mannaa, A. fresenius' 2 . Anal. Chem. 1978, 293, 118-21; Chem. Abstr. 1979, 9 0 , 4 7 9 9 6 ~ . (317) Wiese, G.; Mannaa, A.; fresenius' Z . Anal. Chem. 1979, 295, 17-20; Chem. Abstr. 1979, 90, 179680e. (316) Wiese, G.; Thieie, E. fresenius' Z . Anal. Chem. 1978, 289, 272-4: Chem. Abstr. 1978. 8 9 , 3 6 1 3 0 ~ . (319) YaD. W. T.: Cumminas. A. L.: Maroolis. S. A,: Schaffer. R. Anal. Chem. 1979,'57, 1595-6 (320) Yoshimura, C Kagaku to Kogyo(0saka) 1978, 5 2 , 296-302; Chem. Abstr. 1979. 90. 66186e. (321) Yoshimura, C.'Bunsek1979, 106-10; Chem. Absb. 1979, 97, 32140r. (322) Yoshimori, T.; Katoh, N. Bunseki Kaqaku 1977, 2 6 , 275-80; Chem. Abstr. 1978, 8 8 , 98752m. (323) Yoshimura, C.; Miyarnoto, K. Bunseki Kagaku 1977, 26, 371-5; Chem. Abstr. 1978. 88. 8 3 0 6 9 ~ . (324) Yoshimura. C.; Miyamoto, K. Bunseki Kagaku 1978, 2 7 , 697-701; Chem. Abstr. 1979, 9 0 , 114568e. (325) Yoshimura, C.; Noda, Y.; Matuoka. M. KlnkiDalgaku Rikogakubu Kenkyu Hokoku 1976, 7 7 , 59-62; Chem. Abstr. 1978, 8 8 , 130297e. (326) Yoshimura, C. Sakamoto, T. BunsekiKagaku 1978, 27. 547-51; Chem. Abstr. 1979, 9 0 , 15822b. (327) Zabusova, S. E.; Fomicheva, M. G.: Alpatova, N. M.; Krishtalik. L. I. Nektrokhimiya 1978, 74, 1619-24; Chem. Abstr. 1979, 9 0 , 6 3 5 3 1 ~ . (328) Zoltai Matolcsy, E.: Posgay Kovacs, E. Acta Pharm. Hung. 1977, 47, 217-26; Chem. Abstr. 1978, 8 8 , 9 4 8 8 9 ~ . (329) Zuman. P.; Wawzonek, S. "The Chemistry of Nonaqueous Solvents", Lagowski. J. J.. Ed.; Academic Press: New York, 1978. Vol. VA, Chapter 3.
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Electron Spectroscopy: Ultraviolet and X-Ray Excitation A.
D. Baker*
Department of Chemistry, City University of New York, Queens College, Flushing, New York 11367
Marion A. Brisk Biomedical Program, City University of New York, City College, New York 1003 1
D. C. Liotta Department of Chemistry, Emory University, Atlanta, Georgia 30322
A. INTRODUCTORY COMMENTS A N D GENERAL ASPECTS The decade just ended saw an explosive growth of interest and activity in photoelectron and other forms of electron spectroscopy. Previous fundamental reviews in ANALYTICAL CHEMISTRYdocumented this growth. The major emphasis for commercial manufacturers of photoelectron spectroscopy equipment is now to cater to those interested in surface analyses and surface phenomena. Photoelectron capabilities are often combined in such instruments with other surface sensitive probes. These surface applications can be quite straightforward, yet of great practical importance; for example, studies on catalysts may show a relationship between an observed photoelectron peak and catalytic activity. This can save a lot of money for industrial operations, and in part this accounts for the increasing polarization toward surface phenomena. On the more academic side, studies of the absorption of molecules onto surfaces hold the promise in the long term of shedding more light on the mechanisms of catalytic processes, corrosion, and related phenomena. Photoelectron spectroscopy applied to solids is of course always a surface technique, so investigators who use the method to probe the electronic structures of solid samples must be ever watchful that no special surface effects are present (e.g., surface oxidation, or coating of the surface with a thin layer of pump oil contaminants) and that X-radiation does not alter the surface composition during an investigation. Such problems are particularly worrisome when one is dealing with sensitive 0 0 0 3 - 2 7 0 0 / 8 0 / 0 3 5 2 - 1 6 1R$01 .OO/O
samples such as biochemical materials, and other compounds that undergo facile photoredox reactions. Later in the review we will mention cases where considerable uncertainty and controversy has surrounded the interpretation of results obtained from some materials owing to the possibility of such surface changes. Conversely, of course, photoelectron spectroscopy is an excellent technique for monitoring attempts to chemically modify a surface, e.g., an electrode surface, and as this kind of work is becoming more and more important in a number of fields, another major application area for the technique seems assured. Frequently, applications of this kind depend on a chemical shift phenomenon, or on the observation of a peak due to an element added to the surface by the chemical modification technique. Although chemical shifts for atoms in different environments are not always as large as perhaps one might hope, these shifts are nevertheless extremely useful. Wagner ( A I ) has pointed out, in this connection, that the joint use of X P S and Auger data is more successful in making chemical state identifications than either technique alone. Vapor phase ultraviolet photoelectron spectroscopy (UPS) has not developed into a routine analytical technique, mainly because bands in UPS are seldom specific for given structural features, e.g., while bands for nitrogen lone-pair electrons always appear within a given region of the spectrum, this region can also be populated by bands due to other lone-pair or x electrons. Nevertheless, UPS does have its value in detecting and analyzing substances under special circum1980 American Chemical Society
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ELECTRON SPECTROSCOPY
stances where other techniques prove less useful. For example, several new pyrolytic reactions have been discovered as a result of U P S investigations. Frequently the products of these reactions have been unstable or metastable species that would be difficult to detect by most other analytical procedures. Photoelectron spectroscopy, in combination perhaps with chromatographic techniques, may be more useful than GC/ mass spectrometry combinations in cases where closely related isomers are involved. Ashmore and Burgess (A53)suggest such an application in the determination of certain combustion end products. For photoelectron/GC combinations, a case can be made for changing to simpler retarding field analyzers and thereby collecting photoelectrons over a wider angular range. The enhanced photoelectron current could then be the basis for a very sensitive GC detector, while the low resolution UPS capability afforded by the grid system retarding analyzer would still be adequate for some selectivity and compound analysis capability. Such an application has yet to be described. T h e main use for vapor phase UPS continues to be in the elucidation of electronic effects within molecules, where it has proved to be the most important experimental technique for investigating the orbitals structures of atoms and molecules. X P S has found similar applications, mainly for coordination compounds. Such applications, while not of direct interest per se to the analytical community, are important in providing a basic set of data on which analytical expectations and applications can be based. We have decided to organize this review along the same lines as our previous one ( A 2 ) ,although with the field now being so large, it is often difficult to compartmentalize a given application. As already mentioned, many areas of surface science now rely heavily on photoelectron spectroscopy, and our feeling is that a discussion of many of the more applied applications is more appropriate in a review of the surface problem itself, where the role of the photoelectron work can be placed in its proper perspective. Coverage of applied aspects can in fact be found in the “Applications Reviews” of ANALYTICAL CHEMISTRY, published every odd year. T h e “Applications Reviews” for 1979 include discussions of photoelectron spectroscopy in studies of the chemisorption of gases, the oxidation of metal surfaces, the formation of passivation films and similar surface characterization problems ( A 3 ) ,applications to high polymer analysis ( A 4 ) ,studies of inorganic and geological materials ( A 5 ) ,and catalysts (A6). This diverse collection illustrates the versatility of the technique. Among other applications of photoelectron spectroscopy t h a t we will not be able to describe in detail are the following: trace arsenic analysis by volatilization and XPS ( A I 2 ) ,analysis of paper and wood fibers (A13, A14), X P S analysis of several surface cleaning techniques (A15),various archaelogical applications (A16,AI 7 ) ,studies of derivatized electrodes (AIL?),analysis of soil ( A I 9 ) ,analysis of trace elements in water (A20),studies of glass fibers (A21),thin film analysis of photomask coatings (A22),detection of trapped oxygen species in irradiated sodium chlorate (A23),investigation of the surface of platinum paste electrodes (A24), studies of fluorine and tin uptake by tooth enamel (A25),and in the analysis and optimization of gas phase reactions (A26). Books and Review. There seems to have been an unusually large number of books and review during the period 1978-79. Chapters dealing with basic concepts of XPS, with ultraviolet and X-ray photoionization cross-sections and angular distributions, and with final state structure in X P S appeared in Volume 2 of “Electron Spectroscopy: Theory Techniques and Applications” (A7). Volume 3 of the same series also appeared; it includes chapters on the UPS of transient species, on the fate of ions produced in photoelectron spectroscopy, and on analytical applications of X P S ( A 8 ) . Volume IX of “Comprehensive Analytical Chemistry” contains two chapters that deal with aspects of photoelectron spectroscopy ( A 9 ) ;the first of these chapters, Browning discusses physical aspects of UPS, while in the second Hofmann reviews Auger spectroscopy. Ballard (A27) has authored a book dealing with the relationship of photoelectron spectroscopy and M.O. theory. Berkowitz (A53) has written a book on photoionization, including photoelectron work. Another book has t h e intriguing title “A Whiff of Photoelectron Spectroscopy” (AIO). Several aspects of photoelectron spectroscopy have been reviewed in journal articles, Table I 162 R
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Table I. Recent Reviews in Photoelectron Spectroscopy and Related Areas type of review, or topics covered books on electron spectroscopy
photoionization cross-sections, angular distributions deconvolution techniques synchrotron radiation and photoelectron spectroscopy XPS of polymer surfaces XPSiindustrial research interface ana lysis quantitative surface analysis surface analysis spectroscopies high temperature UPS photoelectron spectroscopy and surface adsorption electron spectroscopy and chemical analysis solid state spectroscopies coal a n d photoelectron spectroscopy
A7-Al0,A27,A54 A2 8
A2 9 A30 A3 1 A32 A33 A3.4 A35 A3 6 A37
A38,A41
solid surfaces applications in mineralogy and
A39 A40 A4 2 A4 3 A44
organic chemistry XPS a n d Auger electron
A4 5 A46
XPS
ref.
geochemistry
spectroscopies in surface analysis surfaceithin film analysis PES and surface chemistry synchrotron photoelectron spectroscopy photoelectron spectra of solid samples photoelectron spectroscopy of adsorption layers significant factors in quantitative XPS
A47 A48 A49
A5 0 A5 1 A5 2
gives some of the areas covered. Finally in this section we note an article with the interesting title “Are Ionization Energies Right Handed or Left Handed?”. The article deals with the issue of whether the energy scale in photoelectron spectra should increase from left to right or vice versa. T h e authors recommend strict adherence to IUPAC guidelines, that is that the ionization energy should increase from right to left ( A I I ) .
B. ADVANCES IN INSTRUMENTATION AND INTERPRETATION Photoelectron spectroscopy of liquids and solutions continues to be an experimental challenge. One approach is to quick-freeze a solution; Burger (B5) has described a method for calibrating X P S spectra obtained in this way. In order to work with liquid-phase materials, it is necessary to adopt special techniques, generally involving fairly intricate or nonstandard apparatus (see previous Fundamental Review (A2) for leading references). However, during this review period, Jolly et al. (B3) have reported a relatively simple method for obtaining the X P S spectra of species in liquid solution. These authors point out that since glycerine has a low vapor pressure and yet a t the same time dissolves many ionic substances in high concentration, it is an excellent solvent for XPS. They describe a method for obtaining such spectra which should prove t o be readily adaptable to most commercial instruments. Ballard and co-workers ( B 4 ) have reported t h e He1 photoelectron spectrum of liquid 1,2ethanediol, using the liquid jet method developed previously by Siegbahn et al. I t is noteworthy that Delahay et al. had also obtained such a spectrum using a different technique; as Ballard et al. point out, agreement between the two spectra is somewhat unsatisfactory. In a more recent publication (B52),Delahay et al. discuss these differences. Identification of chemical states through chemical shifts is an important aspect of XPS. Wagner ( A I ,B I ) advocates joint use of photoelectron and X-ray excited Auger lines in identification of chemical states. Typically in such applications, two-dimensional chemical state plots are made in which
ELECTRON SPECTROSCOPY
A. D. Baker holds the rank of Professor of Chemistry at Queens College of the City University of New York. He was born in England, and received his B.Sc. (Special Honors, Chemistry) from Imperial College of the University of London. From 1965 to 1968 he worked as a graduate student of 0.W. Turner, first in London, and then at the University of Oxford, obtaining the Ph.D. degree in 1968. His Ph.D. work involved studies with the original retarding fieid type of photoelectron spectrometer and on the first high resolution focusing type ultraviolet spectrometer. Between 1968 and 1971, Dr. Baker held an appointment as a Research Fellow at the Swansea campus of the University of Wales, working with D. Betteridge on analytical aspects of photoelectron spectroscopy. I n 1971, Dr. Baker moved to the United States to take up a faculty position at Queens College. He has co-authored two books on photoelectron spectroscopy, and numerous research papers and review articles. He is also co-editor of the series "Electron Spectroscopy-Theory, Techniques, and Applications", and a member of the editorial board of the Journal of Electron Spectroscopy and Related Phenomena. I n addition to electron spectroscopy, his research interests lie in other types of instrumentation for chemists, and in aspects of organic nitrogen chemistry. He has published a number of papers in this area, and enjoys the opportunity of participating in a broad spectrum of chemical research.
Marion A. Brlsk received a B.A. and M.A. from Queens College in 1970 and 1972, respectively. She earned her Ph.D. from the City University of New York in 1975. Her doctoral work involved both the UPS and XPS techniques, and was under the supervision of A. D. Baker. She is currently an Assistant Professor in the Sophie Davis Center for Biomedical Education located at City College.
Dennis Liotta received his undergraduate and graduate training at Queens College of the City University of New York. After two years of postdoctoral study at the Ohio State University, he became an Assistant Professor of Chemistry at Emory University. His research interests include new synthetic methods, singlet oxygen chemistry, and the application of photoelectron spectroscopy to chemical reactivity problems. Dr. Liotta recently received a Sloan Fellowship.
the kinetic energy of the sharpest Auger line is plotted vs. the binding energy of the most intense photoelectron line. Wagner has pointed out that higher energy light sources, e.g. Au M a and Ag M a , are of special value in this kind of endeavor, and has described some applications (B2). Other publications on light sources include the following: a description of how a monochromatized UV source can be fitted to a HP5950A instrument (B43),various new designs for He 11 lamps (B6, B7), a description of a high intensity discharge lamp for monochromatized UV angle resolved PES ( B 8 ) ,a discussion of intermediate X-ray sources for XPS (B44),a description of how Cerenkov continuum light sources may be useful in photoelectron spectroscopy ( B 9 ) ,and two Japanese language reviews (BIO,B I I ) , one dealing in general with light sources for electron spectroscopy, the other concerning X P S with synchrotron radiation. Use of different photoelectron peak intensities obtained with He I and He I1 photons continue to find application in solving assignment problems. A recent example is provided by the work of van Dam and Oskham (B12)directed at resolving a controversy involving the nature of the HOMO in Co(acac)j. These workers showed that a large intensity dependence for the corresponding P E S band was evident in going from He I to He 11, a phenomenon typically seen for metal d orbitals rather than for ligand localized .ir-orbitals. This fits with conclusions
previously reached about the d nature of the HOMO, based on a study of a family of related compounds (B13). Significant improvement in the quality of ultraviolet photoelectron spectra of gaseous species can be obtained by working with supersonic molecular beam sources for the sample. The improvement observed is mainly due to reduction of Doppler and rotational broadening of the sample gas. Experimental details (B14)and applications to molecular species (B15) have been described by Dehmer and Dehmer. The results on the first band of ethylene are quite striking. Although the instrument worked a t ca. 20-meV resolution, results comparable with those obtained from instruments working at less than 10-meV resolution were obtained. Further applications should enable the intricacies of vibrational structure in photoelectron bands to be worked out in greater detail than previously possible. In XPS, a new sampling system for solid samples has been described (B16). The new method involves use of graphite sample holders which afford the dual advantages of very little possibility of instrument contamination and the possibility of obtaining spectra on less than 1 mg of sample. Gonska et al. (B20)have discussed the importance of photoconduction in XPS experiments, and Caillat and co-workers (B21) have dealt with charge effect supression in XPS. Brandt et al. (B22) note that C 1s buildup differs markedly depending on whether one is dealing with conducting or insulating samples, and a possible rationalization of the phenomenon is given. T h e following work on analyzers has been reported: construction of a cylindrical mirror analyzer (CMA) for studies of gases and metal vapors (BI7),use of carbon impregnated epoxy resistive material to eliminate fringe effects in CMAs (B18),and use of a novel time-of-flight electron analyzing system that gives 5% energy resolution and 3% angular resolution (B19). Ebel has documented an approach to the determination of binding energies measured by XPS without retarding field (B23). A discussion has appeared relating to the experimental determination of the transmission factor of a photoelectron spectrometer, and its relevance to quantitative analysis (B24),and Nefedov et al. have described theoretical calculations of relative intensities in XPS (B25). Other papers dealing with relative intensities and photoionization crosssections will be considered in section C of this review. Various papers have dealt with depth selective and depth-profiling analysis by photoelectron spectroscopy (B27-B30). Ohta (B26) has authored a Japanese language paper dealing with on-line control and data handling in photoelectron spectroscopy. Moore et al. have described an e,2e spectrometer which incorporates 10 detectors (B33). Stockbauer et al. have continued their studies of threshold photoelectron spectroscopy, extending their studies to threshold photoelectron-photoion coincidence studies (B32). Baer et al. have also used threshold photoelectron spectroscopy to detect vibrationally excited ions in Franck--Condon gaps (R32). Molecular orbital calculations play an important role in the interpretation of photoelectron spectra. A great variety of approaches is possible ranging from simple semiemperical methods to large ab initio computations. Krause, Taylor, and Fenske (B34) have compared various MO methods with a view to finding which is most satisfactory for predicting ionization potentials. For alkenes, they find that the Fenske-Hall method gives better correlations with experimental 1.P.s than M I N D 0 calculations. Thus, this method, which had previously been used mainly for transition metal complexes and organometallic compounds, may find application to organic systems. Further correlations between 1.P.s and MO calculations should be explored on a systematic basis. The HAM/3 method, mentioned in the last Fundamental review is still somewhat controversial; some aspects are discussed in references (B45)and ( B 4 6 ) . Botter et al. have done calculations on the ionization potentials of oxygen, bromine, and chlorine nonbonding electrons. They find that coulombic factors account for most of the chemical shift effects encountered (B35). Domcke and co-workers have documented cases where an initial state MO description coupled with one-electron processes (the usual approach in interpretation of photoelectron spectra) is inadequate for describing experimental photoionization results. These workers (B36) computed a spectral function for ionization of electrons from H2S and PH, usjng a many bodied Green's function. They found that ionization ANALYTICAL CHEMISTRY, VOL
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from the inner valence 4a, orbital results in a distribution of the spectral intensity over numerous lines. Experimental results were presented which seemed to confirm the main features of the theoretical calculations. On a similar theme Kosugi et al. (B37) have noted the breakdown of Koopmans’ theorem in connection with the presence of strong shake-up bands in the valence shell region of the N, photoelectron spectrum. Wendin and Ohno have discussed strong manyelectron effects in core level photoelectron spectroscopy (B38), and various papers have dealt with correlation effects. Optimization of parameters for an effective Hamiltonian is the subject of a paper by Herndon e t al. (B41); these workers related their results to the photoelectron spectra of alkanes. The importance of Kossel isoelectronic series in photoelectron spectra has been reviewed (B40). Catalan et al. have described a correlation between ring proton affinities and C Is chemical shifts (B47). Factors effecting XPS lineshapes are considered by Mueller e t al. (B42). Berg, Parker, and El-Sayed have used the symmetry properties of Rydberg two photon states to settle the long standing uncertainty of whether the first I.P. of pyridine relates to a nonbonding N orbital or to a x orbital. Their results point to the former. Deconvolution techniques continue to arouse the interest of photoelectron spectroscopists. Allen and Grimm appear to have found a particularly useful way of generating high resolution spectra from spectra taking with a lower resolution instrument. Their computer generated spectra for nitrogen and 0 2 / H 2 0mixtures (B49)compare well with spectra obtained with high resolution instruments, even though they were originally obtained on a spectrometer working a t worse than 20-meV resolution. These authors expect to be able to use their methods to probe the effects of rotation on photoelectron spectra. Also on the topic of deconvolution, Carley and Joyner have described an iterative procedure for dealing with spectra generated by instruments with achromatic light sources (B50). The literature on deconvolution techniques has also been reviewed ( A 2 9 ) . Turner and his group have embarked on an effort to obtain the energy spectra of electrons generated in strong magnetic fields (B51). These studies hold the promise of allowing the determination of the ionization potentials of charged species trapped within a n apparatus by a high magnetic field.
C. PHOTOIONIZATION CROSS-SECTIONS A N D ANGULAR DISTRIBUTIONS: ANGLE RESOLVED MEASUREMENTS Methods of calculating photoionization cross-sections and angular distribution have recently been reviewed ( A 7 ) ;we will not cover this area in this present review. Manson, Mintz, and Kupperman have described a rotatable 180’ hemispherical analyzer ( C l ) for experimental angular distribution work. In their paper, these authors also perform a useful service in reviewing much of the early literature on angular distribution studies. In later papers, this group has carried out a number of most useful angular distribution studies. In one study, the asymmetry parameter p was measured for different vibrational states in the spectrum of CO. It was concluded that an autoionizing state must exist at 584 A, and t h a t p is sensitive to this state ( C 2 ) . Other papers on the vibrational state dependence of (I in photoelectron spectra have also appeared (C3-C5). This work suggests that studies of the variation of (I as a function of vibrational state might be a generally useful probe of vibronic coupling. Further demonstrations that angular distributions may be useful in the assignment of photoelectron spectra and in probing substituent effects are to be found in a number of publications that appeared during the review period. Illustrative of such studies is the work of Kobayashi (C6) who found that for benzene derivatives (I values for the outermost x electron ionizations are close to 1.0, similar to the values seen for the Jahn-Teller split el band in benzene itself. Kupperman et al. have also studiea angular distributions for benzene derivatives ((28)as well as for other r systems ( C 7 ) . They find different variations in /3 for the C-C x bands of benzene and ethylene when these molecules are fluorinated, and tentatively suggest that these changes are caused by differing degrees of interaction of the fluorine substituents with the parent x systems. Some discussion is given. Leng and Nyberg ( C 9 ) studied angular distributions of allene for He(1) and Ne(1) irradiation. They interpreted their results as indicative of 164R
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Table 11. Photoelectron Spectroscopic Studies of Atoms and Small Molecules subject of study NH,Cl, HOC1 NHBr, CH,NHCI, CH,NCI, BrF, I F NHCI, , NH,C1 NH,Br N, 0
4
SF,,other fluorosulfuranes silane derivatives Xe, satellite peaks in 304A spectrum Xe2 (CN),, HNC CH,, singlet and triplet H. S S P(OEt)3 P(OMe), > P(NMe2)3> PF3. Deconvolution of the metal d bands provided e-bz splittings (C4”symmetry) which are inversely proportional to .rr-acceptor abilities of the ligands. Consequently, relative acceptor properties were also determined; P R 3 < P(NRz), < P(OR)3 < PF,. MO calculations verified this trend. I t is also interesting to note that deconvolution of W complex spectra gave rise to an essentially constant W spin-orbit coupling parameter regardless of the n acceptability of L. Such a result can be explained by considering the CO ligands as electron reservoirs which can release density into the metal keeping the overall metal d electron delocalization constant. Weiner and Lattman (E2) and English et al. ( E 3 ) also studied the electronic structure of pentacarbonyl complexes through UPS. The former workers studied the electronic effects of RzS and R2P on the Cr(C0)5moiety. Both of these ligands appear to increase the charge in Cr relative to the hexacarbonyl to the same extent. By measuring increases in appropriate ligand I.P.’s upon complexation, stabilization energies were obtained and correlated to the influence of R on the ligating properties of the donor; electron-donating alkyl groups increased the degree of ligand orbital stabilization to a greater extent than more electron withdrawing groups. The Cr d levels were split (e + b2) due to the lowering of symmetry as expected, although English et al. reported that no such splitting was observed in the He(1) spectrum of Cr(CO)5(CSe). In this latter work, assignments were made largely by comarison with the He(1) spectrum of Cr(CO)S(CS)which has PIeen previously elucidated. It is shown that, upon replacement of a CO group in Cr(C0) by CS and then CSe, the bands shift to lower binding energies. Ulman et al. ( E 4 ) interpreted spectra of a number of compounds containing an iron tricarbonyl group and a borane, hydrocarbon, or carbaborane framework using fairly simple MO arguments and observed intensity changes as a function of photon energy. For C4H4Fe(C0)3and B3C2H7Fe(C0)3 however, experimental and theoretical (ab initio calculations) deductions differ in regard to the levels of d orbitals in the Fe(CO), moiety. Instead of ascribing this discrepancy to a failure of Koopmans’ theorean, D. Salahub ( E 5 ) performed MO calculations on the related B4H,Fe(C0)3 using the SCF-Xa-SW method. A simulated He(1) spectrum generated from calculations, agreed remarkedly well with the experimental spectrum. In addition, the interpretation based on experimental deduction is in substantial agreement with calculated results. Hall and Sherwood (E61 obtained the He(1) and He(I1) spectra of Fe2(C0)6B2Hsand Fe2(C0),S2 and expressed a particular interest in the structure and binding energies associated with the first band; recent calculations, on Fez(CO)X, complexes indicated that the highest occupied MO is primarily a metal-metal bonding orbital while the other metal d orbitals and ligand type orbitals are a t least 1-2 eV more stable. Experimental results and semiempirical MO arguments (E7) indicate that the first band contains the seven iron d ionizations in both of these compounds, and therefore the ionization energies associated with the metal-metal bonding electrons and those of the metal-metal nonbonding electrons are about the same. Such a result strongly suggests that the model which describes oxidation of these compounds as removal of an electron from a single metal-metal bonding MO is inappropriate and that the highest occupied MO should be thought to be of metal character and of a nonbonding nature with regard to the cluster skeleton. Green et al. (E81 conducted UPS studies of some metal carbonyl cluster compounds: O S ~ ( C O )R~ U ~ ,~ ( C O )and ~* O S ~ ( C O ) , ~First . IP’s were of particular concern since the authors compared these values with the work functions of the metals; the IP‘s of the metal clusters exceeded the work functions of the corresponding metals. Such a result questions the validity of using metal cluster compounds as models for
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either the surface or the chemisorbed surface state of metals involved in catalytic processes. Hillier et al. (E9) obtained the low energy photoelectron spectra of some dichromium(I1) and dimolybdenum(I1) tetra-p-carboxylates and compared their spectral data with results of Hartree-Fock a b initio calculations on MOZ(OzCH),and Cr2(02CH)4.In these complexes metal-metal 6, n and u ionizations appear below 10 eV while MO’s of largely ligand character arise a t higher IP’s. Results of calculations presented in this work disagree with previous SCF-SW-Xa calculations for Mo(02CH), and Cr(OZCH), particularly in regard to location of the metal-metal u ionizations. Consequently, the SCF-SW-Xa method is considered to yield unreliable results for interpretation of the UPS spectra of dichromium(I1) and dimolybdenum(I1) “quadruple bond” complexes. In a second study by the same group (E10) a close correlation was shown t o exist between bond length and quadruple bond character of the Cr-Cr bond. In this investigation, PE data for Cr2(MHP)4and its molybdenum analogue (H(MHP) = 6-methyl-2-hydroxypyridine) as well as for solid Cr2(02CMe)4and gaseous Cr2(02CCF3)4 are presented. For the latter two complexes, the He(1) spectra exhibited one low energy broad bond which contained ionizations associated with metal type orbitals while for Cr2(MHP)( and its molybdenum analogue the metal-metal bonding orbitals have IP’s 6 < n,0. He(1) PE data were used to acquire information about the electronic structure of some transition metal complexes of the ligands F2PS2-( E l l )and methylaminobis(difluorophosphine) (L) (E12). In the former work chelate ring n and u ionizations as well as metal d IP’s were identified for Cr(II1, Mn(II), Co(II), and Ni(I1) difluorodithiophosphates. The d IP’s reported provided evidence for the trend of decreasing d-orbital energy with increasing atomic number. The second study provided information about the electronic structure of the valence shells for the mononuclear tris-chelate complexes of the d6 metals Cr, Mo, and W with L(ML3), and for the polynuclear complexes of Ni and Rh. The d IP’s for the ML3 series occur between 7.7 and 7.9 eV which are relatively low values. In addition because of the very small shift of the np orbitals upon complexation, it is suggested that the methylamine compensates for an increase in positive charge on P perhaps through pN d bond formation. Behan and coworkers (E13)used PE information to interpret trends in the chemical reactivity of methyl (tertiary phosphine) Pt and Au complexes. For example, the reactivity toward oxidative addition of cis-PtMezLzand AuMeL follows the series L = Me3P > Me2PhP > MePh2P > Ph3P. It is not known whether this trend is due to electronic or steric effects of the ligands, L. u (MeAW) and 5d IP’s for (Me3AuL) indicate that the orbital energies follow the series L = MePhzP > Me2PhP > Me3P. A similar result was reported for isostructural Pt(I1) complexes (PtXYLz). Therefore, it is apparent that steric effects are primarily responsible for relative reactivities toward oxidative addition. Also, it was shown that the mechanism for cleavage of alkyl-transition metal bonds by electrophiles could be elucidated in the alkylplatinum(I1) and alkylgold(1) complexes studied, by identification of the highest occupied MO; an electrophile is expected to react by direct attack a t the M-C bond in methyl-gold(1) and -gold(III) complexes since a u (Me-Au) orbital is the highest occupied MO, while the initial attack of methyl-Pt bonds most likely occurs a t the metal, since the highest occupied MO is primarily a 5d orbital. Experimental data confirm these predictions. Brittain, Horozoglu, and Baker (E14) reported the He(1) spectra of some y-substituted Co(II1) acetylacetonate complexes. By measuring the effect of the substients C1, Br, and NO2 on the metal d and the ligand n-orbital energies, new information regarding the nature of the metal-ligand bond was revealed. Fragala et al. (E15) also obtained UPS of some 8-diketonate complexes. Their study promises t o be the first of many involving UPS of actinide complexes since there appears to be a dearth of spectral data on these compounds. In this study, P E of some Q-diketonate uranyl complexes described the bonding within the uranyl moiety, and emphasized the importance of 5f covalency in this species. 5f participation was also shown to be important in the bonding MO’s of the U(1V) and Th(1V) d-diketonate complexes. Thornton and associates (E16)studied the He(1) and He(I1) P E of UCl,, and compared experimental IP‘s with those obtained from quasi-relativistic scattered wave X a calculations;
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good agreement between experimental and theoretical data supported their assignments. XPS Studies.. Chen and Jolly (El7)compared the relative n-acceptor abilities of NO and CO ligands by measuring core binding energies of Ni(C0)4, Co(CO),NO, Fe(C0)2(NO),, Mn(NO),CO, Cr(N0I4,Fe(CO)6,and M~I(CO)~NO. Previous studies indicated that C 1s and 0 1s B.E.’s are well correlated with the C-0 stretching force constants and therefore could be used to measure the degree of back-bonding. In this work, such considerations were applied to the NO ligand so that the N 1s B.E.’s were obtained along with the C 1s and 0 Is B.E.’s. Observed binding energy trends indicated that back-bonding to a CO ligand is influenced by back-bonding to NO ligands coordinated to the same metal atom, but back-bonding to an NO group appears to depend on the nuclear charge of the metal atom. Avanzino et al. (E18) also measured C 1s and 0 1s B.E.’s of Fe(CO)5 in order to distinguish between axial and equatorial carbonyl groups. The C 1s peak could not be deconvoluted while the 0 Is peak gave rise to two signals upon deconvolution. Such a result is in accordance with the valence-bond picture of back-bonding which shows a change in the formal charge of the 0 atom only. The deconvoluted peak of higher intensity is of lower binding energy and corresponds to equatorial 0 atoms which are thought to be more negatively charged than axial 0 atoms because of greater back-bonding. This result is in agreement with the well-established rule that strong n-acceptor ligands preferentially occupy equatorial positions in trigonal bipyramidal dS complexes. X P S has been used to study several mixed valence compounds. J e a n n i n e t al. (E19) reported X P S of [“Me,] [W408C18(H20)4]-2H20 which indicated the presence of W(V) and W(V1) oxidation states. Such a result supported the classification of the tungsten ion complex as a Class I1 trapped-valence system according to the Robin and Day terminology. Bakke and co-workers (E20) determined core B.E.’s of several bis(fulva1ene) dimetal complexes of cobalt and nickel and the corresponding dicyclopentadienylmetal complexes. T h e metal 2p binding energies were used t o determine oxidation states of the metal ions; the mixed-valence compounds showed line broadening presumably due to the two inequivalent metal ions. Intense satellite structure appeared in the nickelocenium salt and the monocationic bis(fulvalene) dinickel complex metal 2p spectra; the authors assign these extra peaks to ligand metal charge-transfer transitions. Sherwood and Hall (E21) analyzed the Fe 2p 3/2 spectra (rz = 1, 2; m of the dimers [CpFe(C0)]2Ph2P(CH2)nPPhzm+ = 0, 1) in order to test Hush’s model (E22); according to this model, a delocalized mixed valence dimer could produce a spectrum like t h a t of a localized dimer. Calculations using the sudden approximation were performed and agreed well with the X P S spectra, but they produced results which were in clear disagreement with predictions of the Hush model. Best and Walton (E23) continued their study of metalmetal bonded dimers and clusters of halides which contain bridging and terminal metal-halide bonds. In their most recent study, they showed that X P S can distinguish between bridging ( X ) and terminal (X,) halogens in a-molybdenum bromide an$ iodide, (Mo6X8)X4,and in the bromide derivatives (Mo6Br8)Br4Lz and (Et4N)2[(M~6Br8)X6], where X = C1 or Br. I t is interesting to note that the relative spread of halogen binding energies (AE= X XJ within the (Mo6X,)X4 series parallels the charge-to-ra$ius ratio; C1 (2.5 eV) > Br (2.1 ev) > I (1.3 eV). Brant et al. (E24)obtained X P S data for the binuclear Pd(1) complexes Pd(dpm)&12 and Pd2(darn),Clz (dpm = bis(dipheny1phosphino)methaneand dam = bis(dipheny1arsino)methane) and compounds produced from insertion of carbon monoxide, methyl isocyanide, sulfur dioxide, and atomic sulfur into the metal-metal bond. The P d 3d5 binding energies indicate that insertion of these small mofecules into the Pd-Pd bond causes little if any decrease in charge density associated with P d . The binding energies of atoms in the inserted ligands relative to the free molecules however decrease, sometimes considerably, indicating a charge flow from metal to ligand. P d 3d BE’s were recorded by Terasawa et al. (E2.5) for aminated polystyrene-bound Pd(I1) catalyst beads. P d 3d BE’s and relative peak intensities for the different polymer complexes studied, appeared to be insensitive to metal loading and the different polymer ligands present. Such information is significant in understanding
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catalysis by polymer-bound complexes. An X P S study of crystalline cis-diammineplatinum a-pyridone blue, [Pt2(NH3),(C5H40N)2]z(N03)5, indicated that Pt 4f BE’s were similar to those of Pt(I1) complexes such as cis-(H,N),PtCl2 (E26).Such a result is in agreement with a previously assigned platinium oxidation state of 2.25 in the blue a-pyridone complex. In addition, the widths of the Pt 4f peaks (fwhm 1.3 eV) imply t h a t the two geometrically different Pt atoms have very close BE’s which indicates that there must be considerable delocalization of charge within the Pt4 unit. XPS of other platinium blues included in this study indicate similar electronic structures to the title compounds. Uchida and co-workers (E27)used X P S to study the bonding between NO and the central metal Fe, Co, or Mn in some nitrosyl metal phthalocyanines. These compounds are particularly interesting because they serve as model systems for NO chemisorption on catalysts. Metal 2p and 3s BE’s for the M-Pc and the nitrosyl complexes indicated that Fez+and Co2+undergo increases in oxidation state upon cool dination to NO. Concomitant peak sharpening also supports this contention since a smaller spin magnetic moment would be associated with a higher oxidation state. Consequently, i t appears that an electron transfer from Fe or Co to NO occurs upon formation of the nitrosyl complex. X P S of the Mn analogue, however, indicates a flow of charge from NO to the metal upon coordination of the NO ligand. Keyes et al. (E28) obtained the Cu 2p electron spectra for a copper macrocyclic complex. The relatively high BE and the absence of shake-up satellite peaks supports the Cu(II1) assignment which was previously inferred from several physiochemical experiments. Furlani et al. (E29,E30) applied ESCA to a study of Ni(I1) complexes of RN2 S2-and of some phosphines. In the former study, broad S 2p and N Is bands were reported indicating the presence of two inequivalent species; deconvolution of these peaks indicated a very small difference between the BE’s of the inequivalent N’s in agreement with calculated charge distributions in the free ligand model employed. The S atoms, however, exhibited a greater difference in BE, a result which is not unexpected since a positive charge (-0.5) is associated with one S while the other is considered to be approximately neutral. Core BE’s were also obtained by this group in their systematic investigation of several complexes of L,NiX2; L = tertiary phosphine, X = C1, Br, I, NCS, and RC=C-. The Ni 2p3,z and P 2p BE’s indicate that these values are nearly insensitive to changes in spin state or coordination geometry. Previous reports on other Ni(I1) complexes agree with this result. Strong n-backdonation (Ni P) coupled with the availability of metal *-donor orbitals in both tetrahedral and planar structures, and the presence of large organic substituent groups at the P atom, account for the similarity of the chemical environments associated with the nickel atoms in these complexes. Several Rh(1) aminophosphine complexes (31) were studied with X P S and IR in an effort to determine the electronic effects of changing the ligands in RhClCOL, complexes with P back donation aminophosphines. The data suggest N in the free aminophosphines (Ph3-,P(N),; n = 1, 2, 3,) but N in the Rh complexes. implies charge transfer from P Some Novel Applications of XPS. Sasaki et al. (E32, E331 and Tsang et al. (E34) used X P S to determine effects of radiation on the surface of several inorganic compounds. The first group ascertained chemical forms of substances produced as a result of in-situ electron irradiation of LiNO, and Li2S04and in addition, estimated the number of molecules decomposed per 100 eV of absorbed radiation (E32). The latter was accomplished from the differential energy losses of the incident electrons. In a second study by the same group (E33)Li2Cr04and Li2W04crystals were irradiated; BE shifts of core-levels indicated the formation of Cr(II1) and W(1V) and W(V) on the surfaces of the individual samples. Another group studied the effects of ion bombardment of the surface of ferrous sulfate, iron sulfide, and pyrite (FeSz)( E 3 4 ) . Examination of Fe2p and S 2p peaks indicate the reduction of iron in both FeS and FeSz to its metallic form upon bombardment although the latter proceeds more rapidly. Ferrous sulfate reacts very differently to argon ion bombardment; the S 2p signal a t -170 eV due to sulfate disappears while the Fe 2p spectra indicates that iron oxide is most likely present on the surface. Oxygen ion bombardment of these same
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compounds yielded some very interesting results; the Fe 2p3 spectra were practically unaffected while the S 2p peak of Fek and FeS2 shifted markedly from 164 to 170 eV indicating the presence of sulfate ions. These results indicate that iron-sulfur compounds may be particularly sensitive to damage while in the spectrometer owing to the ubiquitous presence of oxygen. Garbassi (E35) used XPS to study electron beam effects on hydrated magnesium chloride. Barbaray and co-workers (E36,E31) applied XPS to a study of atmospheric aerosol particles. In one investigation, atmospheric particles were collected and analyzed by XPS and atomic absorption spectroscopy (E36). Quantitative analysis by X P S provided surface composition of the samples while atomic absorption spectroscopy provided a bulk analysis. Discrepancies between the two sets of data revealed heterogeneity in the chemical composition. In a second study by Barbaray (E37),XPS was used to determine the effect of NO, and H 2 0 in the absorption and oxidation of SO, over V,05. In the absence of NOz and H,O a S 2p peak a t 169 eV can be detected above 200 "C indicating the presence of sulfate ions. When NO2 is preabsorbed, sulfate ions appear from ambient temperature upward, and in addition the quantity of S absorbed is greater. In the presence of water, SO, was also found to be adsorbed as sulfate from room temperature upward. This study and others indicate that XPS is a valuable tool for studying the adsorption of pollutants. In fact Brown et al. (E381 used X P S and atomic absorption spectroscopy to show that some naturally occurring sulfides are efficient adsorbers of Hg2+ and Hgo in aqueous solution. Some other interesting studies involve the application of XPS to amorphous transition metal oxide films which exhibit photochromism or electrochromism (E391 and attachment of different groups to electrode surfaces (E40, E 4 1 ) . Shake-up Satellite Structure. Shake-up satellite lines have been primarily reported in the core electron spectra of many first row transition metals in their complexes ( E 4 2 ) . They have also been reported to some extent in the metal core electron spectra of lanthanide and actinide complexes (E43). Recently however, Bancroft et al. (E44) studied the satellite structure which appears in the Mo 3d region of MO(CO)~ and in the W 4f level of W(CO)@At this time these two complexes are the only second and third row transition metal complexes which give rise to shake-up structure in their metal core electron spectra. In addition, the C Is and 0 1s electron spectra for M(CO)6(M = Cr, Mo, W)compounds show satellite lines which rarely appear in the electron spectra of ligands. Such structure has also been reported in the core photoelectron spectra of first row transition metal acetylacetonates (E42). MO calculations and electron spectra data on the carbonyls were used to propose valence electron transitions associated with each satellite; each excitation was M or M L assigned to a charge-transfer of either a L nature. The charge-transfer mechanism appeared to adequately explain the extensive satellite structure in the core electron spectra of these compounds. Another group, however, agreed with application of monopole selection rules to the excitations of the ground-state molecules accompanying metal core ionizations, but believe that carbon and oxygen core ionizations should be interpreted in terms of localized hole states (E45). In fact, the reduction to C4"symmetry is thought to account for the intense satellites accompanying the C 1s and 0 I s main lines. MO calculations on various hole states were performed by this group on Cr(CO)6and assignments were made based on their results. The assignments for the excitations which accompany core electron ejection in Cr agreed with those of Bancroft et al. However, the transitions associated with the ligand core electron satellites differed, although some of these were thought to be of a charge-transfer nature. Thompson et al. ( H I @ applied the charge-transfer model of shakeup to satellite data obtained for some copper chelates. Optical absorption data showed C- T bands in the 3.7-5.0 eV region while the satellite peaks observed appeared a t -8 eV from the main photoline (Cu(I1) 2 ~ ~Consideration ~ ~ ) . of the problems inherent in a comparison of UV and XPS C-T data including relaxation effects does not appear to account for this discrepancy. On this basis the authors suggest the need for further experimental and theoretical work on shake-up structure. I t appears that any comparison between C-T absorption energies and satellite-main peak splittings should
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be made for transitions involving the same MO's. If the same MO's are not involved because of the different selection rules, dipole for optical absorption and monopole of shakeup, then there must be some evidence to indicate that these MO's are of comparable energies. Currently, a systematic X P S study of transition metal acetylacetonates is being conducted by the authors of this review with emphasis on a comparison between satellite-main peak splittings and C-T absorption energies. Preliminary data indicate a remarkable similarity in the experimental transition energies obtained from the two techniques which may be related to the fact that approximately the same orbitals are involved in the transition. Shake-up satellites were noted by M. Ioffe et al. (E46) in the Cu 2p3 and Co 2p3 photoelectron spectra of some paramagnelic metal compiexes with temperature dependent magnetic moments. Previously a dependence of satellite intensity on the spin state of a metal ion had been established (E471 and therefore it was not surprising that X P S spectra of polynuclear complexes with antiferromagnetic interaction do not change when the temperature is lowered, although magnetic moments decrease significantly. Fe 2p spectra of mononuclear Fe(II1) complexes exhibited temperature-dependent reversible changes of shake-up peak intensity since the spin state of the Fe(II1) ion underwent S = l / , S = j/ 2 transitions. Tatsumi et al. (E48,E49) embarked upon a study of satellite bands which appear in the metal core spectra of some lanthanide hydroxides and of several (diphthalocyaninato) lanthanides and actinides. Origins of these extra-peaks are of particular interest to these workers because satellite bands may provide information on the nature of bonding in these complexes. Although a charge-transfer transition of a L 4f orbital nature was considered to be the source of the satellite structure (E48),several anomalies had to be accounted for; 4f shake-up satellite was not observed in spectra of the L Pr(II1) (P)and Nd(II1) (P)complexes, whereas La(II1) (f") and Ce(II1) (f') complexes give rise to intense satellites. In the actinide analogues, both Th(1V) (f") and U(IV) (C)complexes tend to exhibit satellites. These trends can be explained by determining the effect of adiabatic relaxation upon core electron removal on the overlap of the ligand and metal MO's involved in the shake-up transition, ( $ ~ l $ * , m ( i ) )T. h e presence of half-occupied 4f orbital(s) appears to diminish shake-up intensity. Such a result could be due to these half-occupied f orbitals receiving charge upon metal core ionization instead of empty 4f orbitals. Consequently, the ligand character in the @*h.Iorbital is diminished and the relevant overlap integral will decrease. Because the 4Lorbital has some 5f character in the ground-state of actinide complexes, the electron flow upon ionization is not expected to be as significant and therefore, the presence of half-filled f orbitals will not exert such a dramatic effect in shake-up satellite intensity. At this point a considerable body of information is available on shake-up satellite structure (E42,E43, E50). Most of these studies attribute the origin of these extra peaks to chargetransfer excitations. However, it is apparent that some P E data has not been adequately explained by this mechanism such as the complex satellite structure which appears in the metal core electron spectra of some Ni and Cu complexes. It may be that other factors as yet undetermined play a role in some shake-up transitions. Still more work is required to provide a consistent explanation for shake-up structure in X P S spectra of transition metal complexes.
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F A N D G. P E S T U D I E S OF SOME ORGANOMETALLIC (F) A N D INORGANIC COMPOUNDS (G) Table I11 gives references to leading papers.
H. STUDIES OF BIOMOLECULES X P S and UPS have been used to examine the electronic structures and chemical bonding in some important biomolecules. Such information is needed for understanding the detail of their interactions and reactivities. Macquet et al. ( H I ) reported core binding energies for a series of porphyrins and several platinohematoporphyrin complexes. The N 1s spectra of the free base porphyrins all indicate the presence of two inequivalent N's; the aza type N 1s B.E. is approximately 2 eV lower than the pyrrole type. ANALYTICAL CHEMISTRY, VOL. 52, NO. 5, APRIL 1980
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T h e positions of these peaks appeared to be insensitive to substituents a t the periphery of the porphyrins. The N 1s spectra of cis-Pt2Cl2H2(HernatoIX) and Pt(Hemato IX) indicate not only that X P S can distinguish between N Pt and N-Pt structures, but also that the former metalloprotein has a SAT (“sitting atop”) configuration. The work of Lavallee e t al. ( H 2 ) on N 1s B.E.’s of N-methyltetraphenylporphyrin, its dicationic salt, and some of its metal complexes supports this SAT structure. I n this latter investigation, the N 1 s spectrum of the free base exhibits 2 bands with the higher binding energy signal being due to both the N-H and N-CH, nitrogens. The relative area of this HBE band increases upon formation of the acid due to protonation of the nonmethylated nitrogen atoms. T h e N 1s spectra of the transition metal complexes also exhibited two bands with the nonmethylated N atoms corresponding to the lower B.E. signal. Comparison of spectra indicate that the metal atom perturbs the electron density around the nonmethylated N to a greater extent than t h e N-CH,N atom. I n addition, a correlation was shown to exist between differences in the N Is B.E.’s associated with each complex and metal-nitrogen bond lengths. Lennox and Murray ( H 3 ) also examined N 1s B.E.’s. Analysis of the N 1s spectrum of glassy carbon electrode surfaces with bound tetra(aminopheny1)porphyrin indicated that two amide bonds are responsible for the linkage. Muralidharan and Hayes ( H 4 ) studied the valence band electron spectra of the metalloporphyrins of tetraphenylporphyrin from Co to Cu. They noted an interesting contradiction between theoretical and experimental data concerning the interaction of 3d metal orbitals and the porphine type M.O.’s. The spectra of the metalloporphyrins exhibit an increase in intensity of the bands associated with the metal 3d levels as the metal approaches the end of the series. Although the photoelectric cross-sections for 3d orbitals increase across the series, calculations indicate that coupling between these orbitals and porphine levels do as well. It is shown quite clearly that X P S evidence indicates little coupling between these levels in metalloporphyrins as the metal approaches the end of the first row transition metal series. In addition, the smaller than expected peak intensity of Ag 4d-like levels in the AgTPP valence shell spectrum implies that metal-ligand interaction may be more significant in the second transition series. Berkowitz ( H 5 ) performed a systematic study of the LJPS of phthalocyanine (H,Pc) and some of its metal complexes (MPc; M = Mg, Fe, Co, Ni, Cu, and Zn). In all cases the first I.P. of these molecules was reported to be -6.4 eV and appears to be indifferent to the metal ion. Therefore, the highest energy MO must correspond to one of predominantly ligand character. T h e I.P.’s associated with 3d-like orbitals were determined by reference to photoemission studies on thin films of phthalocyanine samples using several exciting energies. Further elucidation of the spectra was afforded by reference to previously published ab initio SCF calculations on porphine and Xcu multiple scattering calculations on copper porphine. T h e results of these calculations were deemed to be relevant since spectral patterns of phthalocyanine and tetraphenylporphine are remarkably similar. In fact, since the Xcu method appeared to be highly successful in elucidating the electronic structure of the phthalocyanines it is suggested that it be applied t o chlorophyll and hemoglobin. Maroie et al. (H6)obtained core and valence band X P S spectra of monomer, dimer, and polymer iron phthalocyanines, and related their results to the relative catalytic activity of these compounds in the electrochemical reduction of oxygen. T h e Fe 2p and 0 1s lines for these compounds were particularly revealing in regards to the nature of the F-0, bond. T h e relative intensity of the 0 1s line and the Fe 2p main peaks appeared to coincide directly with the strength of the Fe-0, linkage and therefore, also with the catalytic activity of the compound; dimer > polymer > monomer. In addition, Fe 2p B.E.s revealed the oxidation states of Fe in the three compounds and in the case of the dimer was used to estimate the extent of electron donation to the O2ligand. Valence band spectra further elucidated the nature of the Fe-02 bond. In our last review article on photoelectron spectroscopy (H7),analysis of then current data indicated that the high binding energy peak in the S 2p spectrum of bean plastocyanin (H8) and other metalloproteins was due to a photocatalyzed reaction in which the cysteine S bound to Cu(I1) is oxidized.
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Thompson et al. (H9,HIO), Walton and co-workers ( H l l ) ,and LVeser et al. ( H I 2 ) have recently presented data to support this mechanism. Thompson reported the S 2p spectra of 1,8-bis(2’-pyridyl)-3,6-dithiaoctane and its Cu(I1) complex, and of several other cupric complexes for different exposure periods. Exposure of the dithia complex to X-rays for 1 2 h produced a sizeable S 2p high energy band in the region under question (167-169 eV). Argon-ion etching also produced this peak. Since the S 2p spectrum of the uncomplexed ligand was devoid of a signal in this high energy region, a radiation induced redox reaction is strongly implicated. In addition, since the oxidation occurs only upon S coordination, the Cu metal appears to catalyze the reaction. The oxidation S is postulated to be on a sulfone level as a result of reaction with absorbed “spectrometer” oxygen-containing compounds. Weser also attributes the HBE S 2p signal to oxidative damage induced by the X-rays. These workers exposed native parsley plastocyanin to radiation for different periods of time, and then analyzed these samples using gel electrophoresis. T h e degree of degradation of the protein increased with irradiation time. Walton et al. (H11, H13) continued their investigation of metal complexes containing cysteine, methionine, and some‘ related ligands. The absence of the H B E S 2p band in their spectra also argues against the original assignment of Solomen et al. (H8), and adds indirect support to the metal catalyzed oxidation of bound S in the protein. Walton ( H 1 1 ) recently reported that the S 2p3 binding energies of cysteine and penicillamine metal complexes lie between 163.5 and 161.3 eV. Since the relevant free ligand binding energies were at 163.2 eV ligation apparently increases the electron density associated with the S atom. Since a previous study ( H 1 3 ) on metal complexes of methionine indicated a significant positive shift of S 2p B.E.’s with ligation, X P S may provide a method for distinguishing between metal-thiol and metal-thioether linkages. In fact, the S 2p spectra of MO(V1) complexes of (Ch3)2NCH2CH2N(CH2CH2SH)2 and CH3SCH2CH2N(CH2CH2SH), exhibit two peaks as a result of the presence of the two inequivalent S atoms. From the data collected over the past four-year period, it appears that the S 2p signal at 169 eV present in the spectra of several blue proteins is due to a radiation induced oxidation reaction. Furthermore, C 2p spectra (H8-HIO) indicate the copper is being reduced under irradiation. It is apparent that an oxygen-containing species must participate and that the S atom under question be bound to the Cu ion. Although workers have suggested specific mechanisms (HIO, H12) for the radiation induced events, further studies need to be performed. Elucidation of this mechanism may unveil significant information about the samples under study. In addition, X P S may provide a method for monitoring the specific chemical changes incurred by radiation, an application that may grow in importance especially when one considers the deleterious effects of X-rays on many biological systems. XPS may illuminate the exact nature of the effect of radiation on biomolecules and lead to an understanding of these deleterious changes. Already, X P S is being utilized to measure electron and ion beam induced changes in inorganic systems (E32-
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E34).
Domelsmith and Houk (H14) presented results of their third study on the UPS of psychotropic drugs. In this work I.P.‘s of 14 substituted amphetamines and phenethylamines were measured. The electronic structures of these psychoactive drugs are particularly significant since both the amino function and the aromatic ring are believed to interact with specific sites in the receptor located in the central nervous system. Donor abilities of these molecules were ascertained by measurement of I.P.’s, and in addition a b initio STO-3b calculations were performed in order to determine the effects of substituents upon the shapes of high energy orbitals, and upon the conformations of methoxyl groups. T h e results of this study confirmed the usefulness of ionization potentials in predicting the strength of drug-receptor interactions; a good correlation was shown to exist between T I.P.‘s and the tendency of the drug to displace bound d-LSD from rat brain homogenates. Also psychotropic activities of the amphetamines correlated well with I.P.’s only after lipophilicities were considered. Kajfez et al. (H15) studied the electronic structure of 1hydroxy-ethyl-2-methyl-5-nitroimidazole, a strong antitri-
ELECTRON SPECTROSCOPY
chomonal agent using UPS. The He(1) spectrum was elucidated -by comparison with the spectra of 8 simplier compounds and a b initio orbital energies. Padva et al. ( H 1 6 ) and Peeling et al. ( H I 7 ) applied UPS to study the electronic structures of substituted uracils. Padva and co-workers elucidated the valence spectra of 5-halouracils using UPS and showed that as association constants of 5halouracil-adenine complexes increase, the I.P.‘s of the highest occupied -K orbital and the halogen atom lone-pair orbitals decrease. Peeling et al. employed X P S to monitor the electronic effects of methylation, thionation, and ionization on uracil. Measurement of core binding energy shifts, and CNDO/2 calculations of charge distributions for the free molecules indicated several trends; methylation at either N causes a release of charge by that N into the ring, and thionation decreases the charge at the relevant site while the electronic density associated with adjacent atoms increases. Yu and associates (H18) continued their studies on the UPS of pyrimidines by examining the valence electronic structure of cytosine and several of its methyl derivatives. The five highest occupied molecular orbitals were characterized by the spectra and CNDO/S MO calculations; three of the first five bands are associated with TT ionizations while the remaining two arise from predominantly lone-pair electrons. CNDO calculations coupled with observed spectral trends enabled Ajo et al. ( H 1 9 ) to elucidate the He(1) spectra of xanthine and its methyl derivatives theophylline, theobromine, and caffeine. The P E bands were related to the enaminic, 8-dicarbonyl and imidazole fragments of the molecules. In addition, the greater effect of methylation on specific K MO’s was shown to be useful in elucidation of the spectra. Worley and co-workers have applied UPS to several series of molecules containing nitrogen to chloride bonds which behave as antimicrobial agents by donating C1+ to a receptor (H20-H23). In one study ( H 2 2 ) ,and He(1) spectra of Nchloramines and their N,N-dichloramines were measured; low energy bands were readily assigned since each was of primarily N, C1, or 0 lone-pair character. Comparison of the spectra indicates that the addition of a second chlorine destabilizes the ester and the carbonyl oxygen lone-pairs. This unpredicted observation was explained by postulating a throughspace intramolecular interaction between the N-H hydrogen on the N-chloramines with the 0 lone-pairs. This N-H- -0 interaction could also explain the better antimicrobial activity of the N,N-dichloramine analogues; a N-H- -0 interaction increases the electron density on the N thus increasing the polarity of the N-C1 bond. A more polar N-Cl bond is denatured more rapidly through donation of C1+, and therefore antimicrobial activity is reduced. Worley et al. also measured the ionization potentials of an extensive series of oxalolidinones and similar model compounds ( H 2 0 ) , N-halosuccinimides (H21)and some biological piperazines and their N,N-dichloro derivatives (H23) which are effective antimicrobial agents. In this latter study, examination of the low energy region revealed lone-pair ionizations which were identified and then used along with M I N D 0 / 3 SCF lCl0 calculations to predict predominant molecular conformations. Cannington and Ham presented a survey of the UPS of naturally occurring amino acids ( H 2 4 ) . The low energy region (
