cal constitution. We may perceive the froren nerve impulse or the act of synaptic transmission. Further, we may develop understanding of life based on ita molecular organization in 8itU.
We will probably not be able to unravel the central nervous system but should be able to construct far better thinking machines. We may indeed discover that to really understand the mechanisms of life is to better appreciate our own machines. LITERATURE CITED
(1) Akashi, K., et al., Proc. Int. Cmgr. Electron Mictoscopy, Grenoble, 1, 143 (1970). (2) Bloom, F., presented at the Image Anal sis Symposium, Washington Electron bicrosco y SOC.,May 1971 (3) Cohen, A. Garner, G. E:, 29th EMSA, Claitors, Baton Rouge, La., 1971, p 450. (4) Chang T. H. P., SEM Symposium, IITRI, bhicago, Ill., 1971, p 417. (.5) Christensen, A. K., 28th EMSA, Claitors, Baton Rouge, La., 1970, p 294. (6) Cosslett, V. E., ibid., p 4. (7) Cowley, J. H., ibid., p 6. ( 8 ) Crewe, A. V etal., ibid., 250. (9) Crewe, A. q., 29th EMgA, Claitors, Baton Rouge, La., 1971, p 22. (10) Crewe, A. V., Beck, V., ibid., p 40. (10A) Crewe, A. V., Saxon, J., 28th EMSA, Claitors, Baton Rouge, La., 1970, 534. (11) Defiosier, D. J. 28th EMSA, Claitors Baton Rouge, La. 1970 p 246, (12) binnis A. R., SBM dymposium, IITRI, Chic 0,Ill., !971, 43. (13) Dupuoy, Perrier, Proc. Int.
E.,
%.,
8,
Congr. Electron Microscopy, Grenoble, 1,
129 (1970).
(14) Echlin, P., et ol. 1970 28th EMSA, Claitors, Baton huge, La., 1970, p 286. (15) Erickson, H. P.,Klug, A,, W., p 248. (16) Fucci, R., McAlear, J. H., Stain Technol.,46,249 (1971). (MA) Germinario, L., McAlear, J. H., 1971, Stuiwtechnol., 46, 249. (17) Gibbard, D. W., et al., 29th EMSA, Claitors, Baton Ro e, La., 1971, ~ p (18) Grieve, G. M., S p r y s , J. W., ., p 132. (19) Kimoto, S., Suganuma, T., 1971 J. Eleetronmirroec.,20,73 (1971). (20) Koike, H., et al., 28th EMSA, Claitors, Baton Rouge, La., 1970 384. (21) Kosuge, T., et al. 28th gMSA, Claitors, Baton Rouge, La., 1970, p 390. (22) Krisch, B., Weichan, C., ibid., p 50. (23) Kubozoe, M., et al., ibid., p 47. (24) Lake, J. A., ibid., p90. (25) Lublin, P., 28th EMSA, Claitors, Baton Rouge, La., 1970, p 388. (26) Lucas, J., ibid., p 374. (27) MacDonald, N. C., SEM Symp. TITRI, Chicago, Ill., 1971, p 91. (28) MacDonald, N. C., Waldrop, J. R., 29th EMSA, Claitors, Baton Rouge, La., 1971, p 86. (29) Masayuki, M., ibid., p 496. (30) Matricardi, V. R., et al., ibid., p 468. (31) McAlear, J. H., et al., ibid., p 446. (32) McAlear, J. H., Veltri, B. J., Amer. Cell Biol. SOC.,Boston, Abstracts, 1970. (33) Moritz, R. C., et al., 29th EMSA, Claitors, Baton Rouge, La., 1971, p 44. (34) Mueller, K. H., Rindfleisch, V., ibid., p 48. (35) Muir, M. D., et al., SEM Symp., IITRI, Chicago, Ill., 1971, p 401. (36) Nathan, R., 28th EMSA, Claitors, Baton Rouge, La., 1971, p 28. (37) Pawle J. B., Hayes, T. L., SEM Symp., fiTR1, Chicago, Ill., 1971, p 105. (38) Ris, H., 28th EMSA, Claitorn. Baton Rouge, La., 1970, p 12.
(39) Rues, J. L., SEM Symp., IITRI, Chicsgo, Ill,, 1971. D 65. (40)Rues, J., Kaba o A. 28th EMSA, Chtors, Baton &&e,’ La., 1970, p .win
( 4 5 i u g a t o E., et al., 1970 Proc. Z n t . Conqt. E h r o n MictOecqpy, U r d & , 1,121 (1970). (42)Staehelin, L. A., 28th EMSA, Claitors, Baton Rouge, La., 1970, p
.
.2lW
(4ij%troke G. W., et al. 29th EMSA, Chitors, baton Rouge, La., 1971, p 92. (44)Not mentioned in text. (45) Swann P. R., Swann, G. R., 28th EMSA, klaitors, Baton Rouge, La., 1970,.p 372. (46) Szirmae A., et al., 29th EMSA, Claitors, haton Rouge, La., 1971, p 463. (47) Thomas, L. E. et al., 28th EMSA, Claitors, Baton douge, La., 1970, p 8. (48) Thompson G., ibid., p 500. (49) Thomson, k.G. R., ibid., p 382. (49A) Thon, F., Willasch, A., 29th EMSA, Claitorst Baton Rouge, La., 1971, p 38. (50) Tousimis, A. J., 29th EMSA, Clrutors, Baton Rouee. La.. 1971. D 64. (51) Veltri, B.‘ J.,‘ McAear, J. H., J. Microsc., 93, 191 (1971). (52) Veltri, B. J., McAlear, J. H., Microbwl., 69,in ress. (53) Welter L. Coates; V. J. ’ 1971 29th EdSA, Claitors, Baton kouge, La. 1971, p 32. (54) kelton, T. A., 28th EMSA, Claitors, Baton Rouge, La., 1970 32. (55) Welton, T. A., 29th hbSA, Claitors, Baton Rouge, La., 1971, 94. (56) Yew, N. C., SEM iymp., IITRI, Chicago, Ill., 1971, p 33. (57) Polysciences Inc., Warrington, Pa. (58) E. F. Fullam Inc., Box 444, Schenectady, N.Y. (59) EMventions Inc. Suite 618, 1028 Conn. Ave. NW, Wmhington, D.C., 20036. (60) Commonwealth Scientific Inc., 500 Pendleton St., Arlington, Va.
E.,
Electron Spectroscopy I, Ultraviolet Photoexcitation D. Betteridge, University College Swansea, SA2 8 PP, U.K.
T
HE ECONOMIC DEPRESSION has probably slowed up the rate of development of photoelectron spectroscopy, but the great increase in the rate of publication during the last few months suggests that technique has “taken off .” Since the Report to Analytical Chemists (I), the only published work specifically orientated toward analysis is that of the author’s group (9, S), but significant developments in instrumentation, in the range of samples and in the understanding of the experimentla
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results have taken place. Much of potential interest and importance to the analytical chemist has been published, and this review will attempt to sieve this from the literature on the subject, which is, in the main, written with a bias toward theoretical chemistry. Most of the references are to work published since the completion of the report (I) but in a few instances some earlier work is cited. General, The early results of Turner’s group have been published
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
as a book (4), which is of immense value. Many of the basic inorganic, aliphatic, aromatic, and heterocyclic compounds are presented and discussed. Approximately 300 spectra or spectral details are shown with sufficient clarity to ensure its use as a primary source. A separately available issue of Philosophical Transactions of the .Royal Society of London, Series A , has been devoted to papers on photo electron spectroscopy presented at a meeting of the Society in 1969 (6).
It presents a fair croee-sectional view of the field at that time and includes papers on general aspects by Turner (6). One by Price et aZ. (7), although concerned specifically with Group IV halides, contains much of general interest. Reviews and accounts of recent developments have been given by Baker (8),Berry (9), Brundle (IO), Collin (If), Darmstadt (I$), Lemaire (IS), Price (14), Thompson (16), Turner (16-18), and Worley (19). Betteridge (20) has dealt with the analytical aspects of UV-PES. Herzberg in his Faraday Lecture, brought out the relationships between UV-PES and other forms of molecular spectroscopy (81). Rousseau has made a report of a lively conference held at Oxford in September 1970 whose proceedings are not to be published (89). The Bulletin of Mass Spectrometry includes electron spectroscopy and photoelectron spectroscopy in its search profile and thus helps one to track down relevant papers. This is especially useful since many appear in journals of comparatively small circulation or have titles which fail to betray their relevance to photoelectron spectroscopy. Technique. Improvements in the technique are being made rapidly. Some have been presented at the meeting in Oxford (@), others have been noted briefly in publications where more emphasis was placed upon the results than on the technical development, and others are in the pipeline. It is only possible here, then, to indicate the lines of development that are being pursued. Turner et al. (4) and Kemp (83) have provided more general reviews of the background material and the problems associated with the design of an UV-PES spectrometer, and Ballard and Griffiths have constructed a novel instrument based on the Millikan drop method, which can measure ionization potentials of solutes in nonaqueous media (24). Some attention has been paid to the sampling system. Several attempts have been made to design a viable heated inlet probe, and one will shortly be commercially available (86). With this, the spectra of some steroids, copper phthalocyanine, and other relatively involatile materials have been obtained, so it is clear that this and similar developments will lead to a great increase in the number of samples that can be subjected to spectral analysis. If the sample were introduced as a molecular beam, it would have the practical advantage of allowing involatile materials to be used as samples and the theoretical advantage of an improvement of resolution. There are practical problems relating to the appropriate alignment of the molecular and photon beams and the removal of
the molecular beam after i t has passed through the target zone. Nevertheless, Berkowitz reported at the Oxford conference that he had obtained the spectra of thallium iodides from a molecular beam and Weiss et al. have described a n apparatus with a molecular beam inlet system and shown the spectrum of HC1 (96). The resolution of this spectrum does not seem any better than would have been expected with the usual inlet system and comparable analyzer, but Weisa confirms that it is possible to obtain a n improvement in resolution by using the molecular beam (97). It should, in theory, be possible to use a GLC inlet system, which should serve to extend the range of samples and help in the resolution of mixtures, and experiments are in hand to assess the practicality of this (20). Work is also going on to develop systems which will allow the direct examination of solids, as in X-ray PES, and Bordass and Linnett have already reported spectra obtained from tungsten metal and methanol-coated tungsten (28). The relevant variables . will probably be those described by Yin et al. (89) for Auger and X-ray PES spectrometryviz., sample thickness, sample potentia], surface contamination, and incident angle of source. The helium discharge lamp has proved the most widely used source. The relevant parameters have been studied by Samson, who concluded that the best source would be a dc glow discharge through a 3-cm long capillary which has the minimum possible “dead” volume between the end of its discharge capillary and the sample (SO). Also pumping out the excess helium increases the photon beam intensity (8, 83). There is a need to develop a source of greater energy, and some use has been made of the helium I1 (304 A, 40.8 eV) radiation which is present in the normal helium lamp and can be intensified by a n increase of the current density and a reduction of the helium pressure (4). A certain amount of experimental skill seems to be required to obtain good results, but several instances of its use will be cited below. One great advantage of the helium discharge lamp is that it can be used without dispersion. Difficulties can result if there are impurities in the helium, because these can give rise to spurious peaks in the spectra. Cairns et al. have demonstrated the need to clean up the helium and shown how to do it (31). One of the most important instrumental variables is the electron energy analyzer. This can range from the simple and cheap, e.g., the cylindrical grid (58), parallel plate electrostatic (53), or single focusing magnetic analyzers (34))to the more expensive and sophisticated hemispherical electrostatic analyzer (56) or double-focusing
magnetic analyzer of Siegbahn and Svartholm (36). These and the cylindrical sector electrostatic analyzer (37) and the cylindrical mirror analyzer (38) have been known for mmetime, but recently there has been considerable discussion on their relative merits. Comparison is sometimes difficult, because of the different criteria employed by different authors, but those of Sar El (39), have a wide applicability. The merits of the hemispherical electrostatic analyzer have been shown by several authors (39-41), and it has been incorporated in several commercial instruments. I n principle it allows high resolution and sensitivity, but there are engineering problems in machining it and assembling it within the requisite tolerances and so it is scarcely surprising that considerable interest has been aroused by a re-examination of Blauth’s (38) cylindrical mirror analyzer which is in essence two co-axial cylinders. I n addition i t has the highest luminosity and resolution of any known analyzer (48-44). Bercowitz at the Oxford conference reported that he had used it and several other novel types of analyzer so we may expect developments in this area. The chief difficulty in its application to PES would appear to be related to the volume of interaction of sample and photons which should approximate to a point on the axis of the inner cylinder. Photoemission studies of energy distributions and means of measuring them have a relevance. Those dealing with solids (46),cylindrical electron spectrometers (46),and methods for measuring electrostatic spectrometer performance are representative. Channeltrons are being more widely used as electron multipliers because they are more efficient and can withstand baking out. Their performance as a function of time (48) and the effects of long-term fatigue (49) have been discussed. Lloyd has measured the linearity of the energy scale of his spectrometer and has proposed a series of standards which serve to calibrate the instrument over the whole of the working range of the He(1) lamp (60). SPECTRA A N D THEIR INTERPRETATION
This section reviews work which deals principally, with spectra and their interpretation. It is not intended to be exhaustive. It is organized on the basis of chemical compound (inorganic, organometallic, organic) and progresses broadly from small molecules to large. On the interpretative side, most emphasis is given to those studies which have proved or will prove of most benefit to the understanding of the spectra of large molecules. I n the main, references to work dealing specifically with ionization phenomena, related tech-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
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niques, and theoretical studies of smaD molecules or ions have been omitted. INORGANIC COMPOUNDS
Diatomic and Triatomic Molecules. The book of Turner et aZ. (4) contains the spectra of most of the diatomic and triatomic molecules. More recent work has been concerned with relating the P E spectra with results obtained from other techniques and testing improvements in instrumental design. Brundle, Neumann, Price, Evans, Potts, and Street have made a detailed study of nitrogen dioxide in which the results from mass spectrometry, UV electronic spectroscopy, and PES with both He I (584 A) and He I1 (3041)excitation are used (61). The complementary nature of the several techniques is clearly illustrated and a good correlation was found with SCF molecular orbital energy calculations. Nitric oxide impurity in the “99.9% pure” nitrogen dioxide was easily detected in the PE spectrum. The comments of Natalis et al. on the interpretation of the spectrum of nitrogen dioxide demonstrate the controversial nature of some interpretations (62). Lindholm’s group has embarked on a series of comparative studies with special reference to the Rydberg serifs of small molecules. The paper on nitric oxide, which also contains a He(I1) spectrum, contains a detailed analysis of the fine structure of a PE spectral band and shows how the various excited ionic state contribute to the band shape (63). The Penning and P E spectra of some diatomic molecules have been compared (64). The highest resolution has been obtained by .isbrink who has observed rotational fine structure in the spectrum of hydrogen (66). Jonathan et al. have obtained evidence of transient species of oxygen (66, 67). There have been studies of XeFz (68) halogens (69), halogens, IC1 and IBr (60), ClCN, BrCN, and ICN (61), FzO and Cl2O (62), and NSF (69). Because the molecules are small, the interpretations provided have a sound theoretical basis. They also bring out the extent to which atomic orbitals are mixed into molecular orbitals. Hydrides, Halides, and Oxyhalides. Because of their relative volatility, most of the larger inorganic molecules which have been examined are hydrides, halides, and oxyhalides. They have jointly been the subjects of some of the more complete investigations and so they will be taken together here. Potts et al. have made a complete and important study of the hydrides and halides of several groups and used qualitative arguments based on symmetry to interpret the spectra (7). As might have been expected, the boron hydrides and halides have re102R
ceived considerable attention and the series of papers by Lloyd et al., in particular, provide useful insights into the manner in which to make qualitative deductions from consideration of symmetry (64-71). These and most of the other papers contain the results of ab initio or CNDO calculations so that useful comparisons can be made. The spectra of the trihalides (64, 66, 78), diborane (66, 791, borazine (74, 76), and related compounds, (66, SO), B4Cl4 (70), the addition compounds NHaBHa and NHaCO (71) formed by interaction of the reactants in the vapor phase, trivinyl boron (76), and aminoboranes of the type (R*N)*BX where R is methyl or some bicyclic system and X is H, alkyl, F, C1, or Br (77) have been obtained. The analogies between aiborane and ethane (73) and between borazine and benzene (76) have been drawn; the correspondence between the latter two is quite striking. Bock and Fuss in their study on the a’minoboranes (77) show how simple Huckel MO theory can be applied successfully to the qualitative interpretation of a series of 13 compounds in a manner, which permits the effects on the PE spectrum of substitution, slight changes in symmetry and planarity to be evaluated. Several,studies have been directed to comparing the simple halides and hydrides of carbon, silicon germanium and tin (7, 78-86). There seems to be general agreement that d orbitals are involved in the bonding of all except the carbon compounds and that involvement is greatest in the silicon compounds. This is deduced from the relatively large differences in IP’s of the corresponding carbon and silicon compounds which result from the stabilizing effect of the d-orbitals and which are greater and in the opposite direction to that expected from consideration of the relative electronegativities. Bock and Esslin have convincingly shown the nature of the Si-Si bond in polysilanes (87). By making use of the symmetry of the compounds and the assumption that orbitals which can be considered as localized can be combined, they manage to interpret the main features of the spectra. The spectra of the simplest nitrogen and phosphorus halides and oxyhalides have been obtained and compared, with varying success, against the results of calculations. Branton et al. have compared NH3 and NDa and made a careful analysis of the vibrational fine structure (88). The isotope effect is evident and the problems of determining the adiabatic ionization potential are made manifest. The same group have reported on phosphine and arsine (89). They have also examined NOF3 and POFa and have concluded that nitrogen d-orbitals are involved in an N-0
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
r bond (90). This was not the view of Bassett et al. who made an earlier study of NOFa and NFa (91),although Hillier and Saunders have computed that there is a significant amount of p-d r bonding in the phosphorus compounds PCls, POCls (sa), PH3, PFa, and (CH&P (99) and especially in PFs. The agreement between the ab initio calculations and the spectra is satisfactory. These studies are of considerable value when interpreting spectra of more complex phosphorus and nitrogen compounds. The spectra of the aminoboranes, already referred to, serve to illustrate the point (77). The importance of symmetry is emphasized by PCla and Pocl3 in particular because the peaks arising from the splitting of the nonbonding chlorine 3p orbitals are notable features of the spectra. Eland has obtained and discussed in a straightforward manner the spectra of isocyanic acid and related compounds (94) and the mercury(I1) halides and methyl mercury(I1) halides (96). Ionization from the filled 5d shell is observed in the spectra of the mercury compound and because of spin orbit coupling, gives rise to a “mercury fingerprint” in each of the spectra. The spectra of vc14 and Tic14 have also been obtained and discussed in terms of MO theory (96). Organometallics and Metal Complexes. The spectrum of dimethyl mercury was obtained by Eland as part of the study noted above (96), and the polysilane spectra obtained by Bock and Fuss have also been noted (76). Several of the more volatile transition metal carbonyls-Le., those of Nil Fe (97), Mn (98), and V (99), have yielded interesting results and provided valuable information about the ordering of orbitals in transition metal complexes. The discussion of the compounds related to Mn(C0)s contains spectra of its derivatives and provides a good introduction to the application of UV-PES to the study of organometallics, Ballard and Griffiths have extended their work on the spectrometer based on the Millikan drop method and obtained a spectrum of potassium ferrocyanide (100). The spectra of Ni(PF&, Pt(PF3)4, and PFa were reported almost simultaneously by two groups (101, 108). The results and interpretation are in accord and show how it is possible to deduce the rekcive importance of ligand-metal donation and metal-ligand back donation. This contrasts with comparable examples of X-ray PES spectra where little or no chemical shift was observed because the two different charge transfer operations virtually cancelled each other (103). Organic Compounds. The spectra of many organic compounds are included in the book of Turner el al. (4). Those of the alkanes consist of broad
overlapping bands, which at first sight seem rather featureless, an effect enhanced by the relatively low ionizational cross-sectional area and a falling off in the efficiency of the electrostatic analyzer a t higher IP. Closer examination shows that they are quite distinctive, and the peaks can be interpreted simply as resulting from interactions of C-H pseudo A orbitals (104). These observations are applicable to several of the series of substituted alkanes which have been studied-e.g., the alkyl iodides (3). With different instrumentation, it may be possible to make more of the characteristic features, but at the moment I t must be admitted that alkanes do not represent the molecular ideal for UV-PES. The constriction of a ring gives rise to greater symmetry and the spectra of the cyclic alkanes have more sharp bands than the corresponding chain compound-e.g., cyclopropane, ethylene oxide. and ethylimine (4). Similarly the presence of a A bond gives added character to the spectrum of the alkenes, bicyclic or exocyclic alkenes (105),and cyclic alkenes and alkadienes (106). The peak due to ionization from the highest occupied n orbital usually occurs a t about 9 eV and has fine structure, and is thus a distinctive feature of the spectrum. The interaction between p lone-pair and A orbitals was discussed earlier (1) and is shown very clearly in the spectra of the monohalo and dihalo acetylenes (107, 108). I n these molecules there are no nonbonding lone-pair orbitals to serve as markers, because both “lonepair” p orbitals on the halogen have appropriate symmetry to interact with the two C-C n orbitals of acetylene. Because the molecule is so simple it is a straightforward matter to make the proper linear combinations of atomic orbitals and to forsake the simplified terminology used above. Heilbronner et al. have compared the experimental IP’s with those predicted by ZDO molecular orbital theory. The shape of the spectral bands reflects the extent of p . - ~ orbital mixing. Similar interactions are observed in the spectra of the chlorinated ethylenes which have been discussed in qualitative terms (109, 110). I n these compounds interactions between the chlorine nonbonding 3p orbitals show up as patterns in the spectra which serve to distinguish between isomers. The cyanide group also offers possibilities of interaction with A bonds (111) or pseudo A or Walsh type orbitals of methyl (112) and these have been investigated. Dewar, Worley, et al. have made a n investigation of a large number of organic compounds (113-118) and supported their interpretations with MIND0 calculations. The spectra were recorded with a retarding grid in-
strument, and thus provide an interesting contrast with those obtained at higher resolution. The use of a grid has the advantages of cheapness and sensitivity, since the electrons are collected over a very much wider angle, but the resolution is poor and the grid is subject to contamination which leads to spectral distortions. This work suggests that the practical problems are not insuperable, even though the sample molecules may be quite large. Aromatic compounds often give spectra with distinct or sharp bands which reflect the presence of substituents. The interaction of benzene A orbitals with substituent groupings was commented on earlier (1) and has been summarized by Bock, and extended by considering the effect of boron, silicon, and phosphorus (119). A reasonable number of heterocyclic compounds have been examined, and shown to give rise to distinctive spectra (2, 120). The presence of the heteroatom cannot always be distinguished since its characteristic “lone pair” p orbitals may be involved in the A bonds around the ring. If the p orbital is not thus involved, then the lone pair peak may be detected-e.g., the nitrogen lone pair is not evident in the spectrum of pyrrole but it is in pyrazine (2). The presence of the heteroatom in the ring does, of course, alter the spectrum relative to the parent hydrocarbon and relative to other heterocycles. The spectra of several &membered ring heterocycles have been presented in the form of a correlation diagram and contrasted with I R and mass spectra. The spectra of isomers differ significantly. Lindberg et al. have extended their studies on di- and triatomic molecules to furan (121) thiophene (122), pyrrole (123), and cyclopentadiene (124). They are full studies and make use of other spectroscopic information in the interpretation of band structure. They also contain spectra obtained witb the He 304 A as well as the He 584 A and this results, for example, in extra peaks in the spectrum of cyclopentadiene in the region 2 1 4 0 eV but a loss of resolution in the A bands a t lowest IP’s. The relative intensities of the bands change as well. Pyrimidine, substituted pyridines, and diazines have also been examined and discussed a t length by Baker and Turner (126). Pyridine has also been examined by several techniques by Jonsson et al. (126).
Interactions of Nonbonding Orbitals. UV-PES provides clear-cut evidence of the interactions of nonbonding orbitals. The interactions depend upon molecular geometry and may thus provide structural evidence, although, of course a t this stage the knowledge of the molecular structure is essential in deducing
the existence of an interaction. The interactions are between two orbitals of the same symmetry and comparable or identical energy and may take place directly through space or through bond. I n the former, the process is rather like the combination of two atomic orbitals to give a bonding and anti-bonding, or, better, a symmetric, S, and anti-symmetric, A, orbitals, with S lying below A. Examples are the splitting of chlorine lone pair orbitals of dichloroalkanes by about 0.2 eV (3) and the interactions of the A orbitals in norbordiene which results in two bands separated by 0.85 eV (127). The through bond interactlon is less obvious. The combinations of the non-bonding (lone pair) orbitals are S and A as before and they may interact A) of orbiwith a suitable pair of (S tals within the molecule. (The A orbital may be unoccupied and hence not be evident in the PE spectrum,) Then the interaction between the 4 orbitals whose energies before interaction are in the sequence S (bond), S (nb) = A (nb), A (bond), where (nb) indicates the nonbonding orbitals, and (bond) the interacting molecular orbitals. By interaction the S (nb) will be raised and the A (nb) will be lowered so that a splitting occurs. This is evident in the spectrum of tram-azomethane for which the split is 3.3 eV, and whose geometry would seem to preclude a through space interaction (128, 129). The prime fact is that, whatever the mechanism and whatever the final ordering, the orbitals are split and this is reflected in the PE spectrum, The theoretical background has been presented by Hoffmann in a very clear manner (130). He makes full use of the results of Heilbronner’s group, who have made the most thorough and elegant investigation of the phenomena. Their work has been summarized by Heilbronner (131). An earlier note (132) sums up the various theoretical consequences of different sorts of interactions and outlines the various compounds which will test them. Subsequent papers provide detailed experimental evidence and interpretations backed by suitable MO theory. The series is of particular value to the chemist interested in the wider applications of UV-PES because the molecules examined are relatively large organic molecules, albeit with a high degree of symmetry. They include 1,4-cyclohexadiene and bicyclo [2.2.2.Ioctadiene (1 $7) , 1,4-diaza-bicyclo [2.2.2.]octadiene (133, 134), l14,5,8-tetrahydronaphthalene (136), cis-cis-c~s-lJ4,7-cyclononatriene ( I % ) , bullvalene (137), barrelene (138), homofulvene (139), endo- and mo-cyclopropano-norbordierie (I@), various dicarbonyl compounds (141), and a number of compounds related to those listed. Splittings due to inter-
+
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
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action have been noted for 1,adimercapto ethane (90) and diphenylmethane and related systems, in which it is argued there is an interaction between the non-conjugated r systems of the phenyl rings (14.9). An apparent anomaly is found in the spectra of the alkyl bromides which apparently do not show a splitting of the bromine lone pair orbitals due to the interaction of one of them with the Walsh-type r orbital of the alkyl group. The magnitude splitting is that found for spin-orbit coupling, but one of the bands does show fine structure which can only arise from interaction. Brogli and Heilbronner have shown that the constancy is a balancing of the tendency to split by interaction by a tendency to reduce the spin-orbit splitting (143). Correlations. There have been attempts t o correlate well defined IP’s of a series of compounds with well known parameters. Thus the halogen lone pair IP for some simple series of halogen compounds has been correlated with electronegativity (144), and Kemp (23) has found that there are correlations with electronegativity and both the IP and the difference between the two upper T IP’s for 5-membered heterocycles. I n general, the spectra of series of compounds reflect changes in electronegativities (substituents of higher electronegativity giving higher IP’s and vice versa), and qualitative deductions employing this have been made in many of the works already cited, Similarly, for a series RX, where X has a readily identifiable lone pair orbital and associated IP, one might expect that there should be a correlation between IPX and some parameter which measures the electron releasing property of R. Linear correlations have been found between Taft U* values and IP for a number of alkyl compounds (3, 146). There is likely to be some error when the parameters obtained in solution studies, u * , are compared with those from the gas phase and this has been more fully dealt with by Cocksey et al. (146) who have tried to establish the relative importance of the various effects which lead to the u* values. Many workers have assumed that the shifts due to the various interactions and inductive, mesomeric, and electronegativity effects are additive. Some work has been undertaken to see whether this assumption has any quantitative support. Hashmall and Heilbronner (146) have found that the IP of the bromine lone pair in a series of alkyl bromides is shifted by -0.2 eV by a n a-methyl substituent and -0.09 eV by a p one. Meier and Simon have evolved a simple additivity rule for the approximation of IP’s of substituted aromatics and olefins (147). It is based on the analysis of an appreciable number of spectra and has statistically104R
set limits. Further correlations would seem to be possible. It is often assumed that the area under a peak in a W-PE spectrum is roughly proportional to the degeneracy of the orbital from which the electron is being ejected. It is known that the correlation is not absolutely true and sometimes is completely misleading. From the analytical viewpoint, it would be sufficient to determine experimentally whether proportionality factors can be established. Work has begun on the more difEcult theoretical trestment. Cox and Orchard have outlined an approach for dealing with open-shell molecules (148). Lohr and Robip have made a more extensive study of the photo-ionization crosssectional areas of n-electron systems (149). They have found that the cross-sectional area varies with photon energy, which is in agreement with the experimental results obtained with He 584 and 304 sources in the same instrument. They also conclude that the cross-sectional area depends to some extent on the symmetry of the molecular orbital from which the electron is being ionized; the one with least nodes has the highest ionization cross-sectional area.
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