Part (ii) INFRARED AND RAMAN SPECTROSCOPY by P. J. Hendra (Department of Chemistry The University of Southampton) THE major experimental advance occurring this year has been the introduction of laser-Raman spectroscopy to Chemical laboratories. Approximately fifty papers have appeared wherein this new technique has been used and four commercial spectrometers are now available. This section is divided into an account of the developments in laser-Raman spectroscopy followed by a survey of the more conventional techniques in vibration and vibration-rotation spectroscopy. Laser Raman Spectroscopy.-As forecast in the last Annual Report, there has been very rapid development in the technique and application of Raman spectroscopy using laser sources. A review by Brandmuller ’ has ap-peared which covers developments until February 1967 whilst an account of Raman spectroscopy edited by Szymanski’ includes a chapter by Koningstein, devoted to the subject.Unfortunately this book seems to have suffered some delay in preparation since it contains few references more recent than 1965. During the period under review the number of laser-Raman spectrometers available to chemists has increased markedly and as a result a number of papers have appeared which demonstrate that many groups are currently applying this new technique to as wide a range of problems as possible. It is now well established that whereas the examination of all but colourless non-turbid liquids or crystals was experimentally difficult using conventional methods the laser source has completely transformed Raman spectroscopy.This transformation has been emphasised recently in a very interesting ~urvey.~ The first field where the laser source showed enormous advantages over the conventional Toronto Arc was in the study of deeply-coloured liquids and solids.’ During the year a number of papers have appeared containing routine spectra of almost opaque solid^,^ and even colorimetric reagent^.^ In fact the renaissance has been so profound that in normal circumstances better spectra can be obtained from many solids than from their solutions. The ease and routine nature with which chemists can use the commercial instruments (appreciable numbers of Perkin-Elmer LR1* Cary 81,T Huet,$ Coderg,§ and Spex(( J. Brandmuller Naturwiss. 1967,21 293.‘Raman Spectroscopy’ Ed. H. A. Szymanski Plenum Press New York 1967. ’ I. R. Beattie Chem. in Britain 1967 3 347. P. J. Hendra J. Chem. SOC. (A) 1967 1298. E. R. Lippincott T. E. Kenney T. R. Nanney and C. K. Weifenbach J. Chem. SOC. (4 1967,32. P. J. Hendra and Z. Jovic J . Chem. SOC. (A) 1967 1127. * Perkin-Elmer Corp. Beaconsfield Bucks. t Cary Instruments Monrovia California U.S.A. Fa. Huet Societe General d’optique Paris France. $ Fa. SociCtC de Conversion des Energies Clichy France. 11 Spex Industries Metuchen New Jersey U.S.A 190 P. J. Hendra instruments have now been delivered) and the various types of sample they are able to examine make Raman spectroscopy a potentially valuable analytical tool. The field which has benefited most from this renewed interest is co-ordination chemistry Here the problems in the measurement and use of vibrational spectra are twofold-firstly; the species are frequently deeply coloured and contain heavy atoms resulting in the need for far i.r.and laser-Raman techniques but having the intrinsic advantage that the Raman intensi-ties are high due to the high polarisability of heavy-atom systems. Secondly, the molecules frequently have a high degree of symmetry with the inevitable result that the observations of i.r. spectra alone give very little reliable and definitive information. This latter limitation is so severe in centrosymmetric square planar and octahedral systems that the Raman spectra if recordable should be an essential part of all studies of the vibrational characteristics of co-ordination compounds in future.A classic example of the limited value of i.r. spectroscopy alone is furnished by a recent study by Hartley and Ware6 of the metal cluster compounds containing the ion [Mo,C1,I4-. Whereas a previous i.r. study' had simply identified two absorptions and a recent study identified a band near 230 cm. ' with a stretching of the Mo-Mo bonds,' this more complete analysis was much more informative. The laser-Raman spectrum contained ten bands and crude force constants were calculable for the Mo-Mo bonds. A similar study of Ir4(C0)129 was equally valuable the Ir-Ir stretching force constant having a value close to 1-3 millidynes per A. Another field where Raman spectroscopy is making great advances is in the study of the vibrational spectra of polymers.Symmetric modes of highly stereoregular species are of course only apparent in the Raman effect yet the technique has contributed little information to date' due to experimental limitations. During the year a report has appeared of the vibrational Raman spectrum of isotactic polypropylene.' ' The spectrum is of superb quality. In addition a new effect has been noted12 in that dichroism in the intensity of the Raman bands of stretched polymers has been observed for polypropylene fibre and tape. Although the effect is related to that familiar in the i.r. the explanation is somewhat complex. It is easy to visualize that the dipole deriva-tive (dp/Lldq) is maximized in the same general direction as the vectors in a given vibrating system but it is by no means certain that (Ja/Jq) should obey this rule.A more recent study of the dichroism in the Raman spectrum of polyethylene fibres13 has shown that it is likely that (aa/aq) is parallel to the vectors in a given vibration and further that the assignment of the Raman ' D. Hartley and M. J. Ware Chem. Comm. 1967,912. ' R. J. H. Clark D. L. Kepert R. S. Nyholm and G. A. Rodley Syectrochim.-Acta 1966,22,1967. * F. A. Cotton R. M. Wing and R. A. Zimmerman Inorg. Chem. 1967,6 11. C. 0. Quicksall and T. G. Spiro Chem. Cornm. 1967,839. l o J. R. Nielsen J . Polymer Sci. Part C Polymer Symposia 1963 19. l 1 R. F. Schaufele J . Opt. SOC. Amer. 1967,57 105. l2 P. J. Hendra and H. A. Willis Chem. and Znd. 1967,2146. l3 P . J. Hendra and H. A. Willis Chem.Comm. 1968,225 Part ( i i ) Infrared and Raman Spectroscopy 191 spectrum to fundamental modes is in error. It is suggested that the bands at 1133 and 1065 cm.- are assigned to B, and A respectively and not the reverse as previously widely thought. As the technique is applicable to thick fibres and sheets and requires no special sample preparation before examination it has great potential as an ancillary to i.r. dichroic measurements and has already been applied to polyethylene terephthalate and Nylon 66. During the later part of the year a report appeared14 describing preliminary experiments on adsorbed species. In many ways the results suggest that Raman spectroscopy may have intrinsic advantages over absorption measurements in this field since the background caused by the substrate material is remarkably unobtrusive.The work described was limited to physically adsorbed molecules but high sensitivity was demonstrated in that layers of bromine on silica gel down to thicknesses of the order of & monolayers gave spectra indicating the presence of bromine. Whereas in the i.r. the lowering of symmetry caused by the adsorption process makes many of the Raman bonds i.r. active the reverse does not seem to be true i.e. physically adsorbed centrosymmetric molecules show Raman spectra similar to those of the non-adsorbed species and mutual exclusion is retained. The demonstration of the effect in chemisorbed systems has appeared recently'' and is of interest since it is in this field that commercially-valuable heterogeneous catalytic problems may be accessible.There seems little reason to doubt that laser-Raman spectroscopy will be applied increasingly in this field since spectra of species adsorbed on silica carbon black and alumina have been recorded. In some cases electronic transitions can give rise to Raman emissions and some of these have been demonstrated and identified during the year by Koningstein.16 The effect is found to be of low intensity but the use of low temperatures and sophisticated optical and electronic design enables excellent spectra to be observed. Care is needed to distinguish the Raman effect from fluorescence but comparing spectra on the Stokes and anti-Stokes sides of the exciting line solves this problem. The trivalent cations of ytterbium europium, and neodymium in yttrium gallium garnet were studied in the first paper of a series.Although the helium-neon laser occupies the most important place in Raman spectroscopy examples are to be found of the use of other sources. Gee and Robinson17 have used an Argon-ion laser to obtain the Raman spectrum of crystalline benzene at 77" and 2 ' ~ . An output from the source of 250 milli-watts at 4880 8 enabled results to be recorded at a resolution of 0-65 cm.-' Brand-muller et have compared the performance of a quasi-continuous and continuous ruby lasers (at 6943 A) with the helium-neon gas laser but found them as yet inferior except for compounds such as p'-dimethylamino-p-nitro-l4 P. J. Hendra and E. J. Loader Nature 1967,216 789. l 5 P. J. Hendra and E. J. Loader Nature 1968,217,637.I' A. R. Gee and G. W. Robinson J . Chem. Phys. 1967,46,4847. J. A. Koningstein J . Chern. Phys. 1967,46 2811. J. Brandmuller K. Burchardi H. Hacker and H. W. Schrotter 2. angew. Phys. 1967,23 112. 192 P. J. Hendra azobenzene which are very deeply coloured. Perhaps the most elegant work in Raman spectroscopy using a laser source is that due to Porto and his co-workers on single crystals. A list of these will be found in ref. 1. A number of reports appeared at the 9th European Congress on Molecular Spectroscopy held in Madrid in September on work by other groups on single crystals. Their results are awaited with interest since the emphasis may then turn towards ranges of chemically interesting compounds. Tentative results on the Raman spectrum of a single crystal of a co-ordination compound have appeared.Ig The Raman spectra of gases are much more easily observed using laser sources than with gas-discharge lamps.It seems particularly advantageous to place the sample cell inside the resonant cavity of an argon-ion laser when vibration vibration-rotation and pure rotation lines can be observed using both photographic and recording spectrometers. The spectra of a number of gases were reported by Bernstein in Madrid (see above) but the most fascinating experiments described were those due to Barrett and Rigden.They examined gaseous CO in a cell within an argon-ion laser and observed the rotation lines. They then exposed the sample to a d.c. discharge and re-examined the spectrum whilst the discharge was in progress.They found prominent new bands in their Raman spectra neatly placed between the 'ordinary' rotational lines. The explanation of the effect was the subject of much discussion at the Conference. Structural Determinations using Infrared and Raman Spectroscopy.-As is always the case a large proportion of the effort in vibrational spectroscopy this year has been supplied to the solution of structural problems. The most thorough investigations are usually those involving an analysis of the complete spectrum followed by a structural diagnosis based on hypothetical models and the application of selection rules. A classical example of this approach was furnished by Clarke and Wood-ward2' who examined the cations of formula [(MeHg)],S+ and (MeHg),O+.The evidence (i.r. and Raman) strongly favours a pyramidal structure for the former but a planar (or nearly so) structure of the oxonium ion seems more probable. Another example of a structural determination typical of the work reported this year includes a far i.r. spectrum and assignment of the hyponitrite ion confirming its trans structure (C2,J.21 The method does have limitations however and on occasions results from other physical methods appear to disagree with the vibrational evidence. An example of this problem appeared during the year. Although trisilylamine is well established as a planar molecule,22 a paper appeared last year2 on the i.r. and Raman spectra of trisylyl phosphine suggest-l9 P. J. Hendra and E. R. Lippincott Nature 1966,212 1448. 2o J.H. R. Clarke and L. A. Woodward Spectrochim. Acta. 1968,23A 2077. 21 C. E. McGraw D. L. Bernitt and I. C. Hisatsune Spectrochirn. Acta 1967,23A 25. 22 K. Hedberg J . Amer. Chem. SOC. 1955,77,6491; E. A. V. Ebsworth J. R. Hall M. J. Mackillop, 23 G. Davidson E. A. V. Ebsworth G. M. Sheidrick and L. A. Woodward Spectrochirn. A k a , D. C. McKean N. Sheppard and L. A. Woodward Spectrochirn. Acta 1958,13 202. 1966 22 67 Part (ii) Infrared and Raman Spectroscopy 193 ing a similar structure and this has come under fire recently. A report appeared during the year on an electron diffraction study of (SiH,),P and (SiH,),As showing that the evidence clearly supported a pyrimidal C structure2, like trigermyl phosphine. However in a paper25 read before the American Chemical Society it was suggested that (SiH3)3P was planar.A further fascinat-ing twist to this story is that Goldfarb and Khare examined trisylamine in an Argon matrix and found that its i.r. spectrum could be interpreted in terms of a C3” structure but that methyldisilylamine is planar and dimethylsilylamine pyramidal.26 Clearly the energy difference between the two forms in these systems is small and crystal forces can distort the structure easily. In many cases it is not necessary to obtain the complete i.r. and Raman spectrum of a compound in order to discriminate between the likely structures. A very large number of examples of this type of work are to be found but there is room only for a few examples here. The selenium and tellurium halides have been repeatedly examined using i.r.and laser-Raman techniques during the year. Adams and suggest that the i.r. evidence is in favour of tellurium chloride and bromide having the TeCllCl- structure in the solid phase but that a molecular species is found in benzene solution. George et also examined the far i.r. spectra of the solids and came to a similar conclusion regarding SeCl, SeBr, TeCl, and TeBr,. The laser-Raman spectra29 of these species have been used as evidence for the existence of molecular species of C, symmetry in the solid phase. A comparison with far i.r. observations on the series SeBr, SeClBr, SeCl,Br, and SeCl seems to lend support to this conclusion. The reviewer who has a vested interest in the argument is not willing to adjudicate. During the year the i.r.spectrum of the P,I molecule was reported3’ and was used as evidence that the molecule did not exist in the ‘gauche’ configura-tion. A report3’ however appeared on the i.r. and Raman spectra of this com-pound the latter being obtained using helium discharge lamps giving an exciting line at 6678-15 A. The experiment was particularly interesting since the deeply-coloured compound was examined in solution and as a solid. The evidence strongly favours a trans PI2-PI structure of C, symmetry both in the solid state and in solution. Eleven spectral features were assigned to funda-mental modes. Attention was not confined to inorganic problems although much of the reports of the spectra of organic molecules was analytical and concerned with the presence of absence of functional groups.Two examples of gross structure determinations are as follows : 24 B. Beagley A. G. Robiette and G. M. Sheldrick Chem. Comm. 1967,601. ” A. H. Cowley and W. D. White Abtstracts 153rd Meeting Amer. Chem. SOC. 1967 L145. 26 T. D. Goldfarb and B. N. Rhare J . Chem. Phys. 1967,46 3379,3388. 27 D. M. Adams and P. J. Lock J . Chem. SOC. (A) 1967,145. 28 J. W. George N. Katsaros and K. J. Wynne Znorg. Chem. 1967,6,903. 29 G. C. Hayward and P. J. Hendra J. Chem. SOC. (A) 1967,643. 30 R. L. Carrol and A. H. Cowley Chem. Comm. 1967 872. 31 S. G. Frankiss F. A. Miller H. Stammreich and T. Texeira Sans Spectrochirn. Acttr 1967,23A. 543 194 P. J. Hendra An investigation of the vibrational and n.m.r. spectra of the isocyanate dimer (CF,*SNCO) by Downs and Haas3 confirms that it has the cyclic uretidine 1 3 dione structure (I) with the CF groups occupying the ‘trans’ configuration whilst a laser-Raman study of hexanitro~obenzene~~ gave some evidence that it occurs as benzotrisfuroxan (11).N-0 The speed of application and convenience of the spectroscopic methods make them particularly valuable tools for structural studies. There have been many examples of the use of these attributes during the year. Thus during the year much interest has been shown in the nature of the chalcogen-chalcogen bond.34 Much of this has been of a theoretical nature and concerned with explaining the approximately 90” anhedral angle charac-teristic of the molecules having such bonds e.g. H,02 Se2(CF,)235 etc. An interesting report by P e d e r ~ e n ~ ~ appeared in the middle of the year in which the anhedral angle in hydrogen peroxide was deliberately altered.In fact i.r. spectra of solid sodium oxylate perhydrate (Na,C20 - H,O,) gave evidence for the hydrogen peroxide molecule having a planar trans structure. On the other hand the caesium salt appeared to contain skew molecules. Two elegant reports have appeared on compounds of xenon. In the first, Nelson and Pimentel,’ reacted the element with chlorine under discharge and condensed the product onto a cooled caesium iodide flat. A very complex absorption band was observed at 310 cm.-’ which they propose could be due to the asymmetric-stretching mode of linear XeCl molecules. As evidence they point out that (a) a 2 1 stoicheometric ratio of gases was used in the discharge (b) they do not observe any absorption attributable to a symmetric-stretching vibration and (c) the band has exactly the right contour for a linear molecule of this type when allowance is made for the proportions and masses of the seven isotopes of xenon and two of chlorine.The relative intensities and frequencies were shown to fit a linear model very closely. In the second investiga-tion Gasner and C l a a ~ e n ~ ~ examined the compound xenon hexafluoride. Although one might anticipate an octahedral structure the i.r. and laser-3 2 A. J. Downs and A. Haas Spectrochim. Acta 1967,23A 1023. 33 N. Bacon A. J. Boulton and A R Katritsky Tr~ins Frrraday SOC 1967,63 833 34 W. H. Fink and L. C. Allen J. Chem. Phys. 1967,46,2261.35 W. E. Palke and R. Pitzer J. Chem. Phys. 1967 46 3948. L. Pedersen and K. Morokuma, 36 B. F. Pedersen Acta. Chem. Scand. 1967,21,801. 37 L. Y. Nelson and G. C. Pimentel Inorg. Chem. 1967,7 1758. E. L. Gasner and H. H. Claasen Inorg. Chem. 1967,6 1937. J . Chem. Phys. 1967,46,3941 Part (ii) Infrared and Raman Spectroscopy 195 Raman spectra of the compound at 40" (solid) 54" and 92" (liquid) and 94" (vapour) suggest otherwise. Although the vapour densities of the system suggest monomeric character the relative intensities of some bands are definitely temperature sensitive. It is possible that the liquid resembles the crystal in containing tetramers. The line at 609 cm.-' in the vapour-phase Raman spectrum is weaker than that at 520 cm.-' and is broad but polarised.In addition the i.r. of the vapour has distinct absorptions at 613 and 520 cm.-' Clearly the structure cannot be centrosymmetric in the vapour phase or some unusual electronic effects must be interfering in this region of the spectrum. It is fascinating to note however that Bartell,' has predicted very recently the occurrence of a pseudo Jahn-Teller effect in XeF6 from theoretical considera-tions and he states that a distorted structure is the most probable. The structure of hydrogen-bonded systems has been a popular subject for study for a long time and three examples will be cited. Waldstein and Blatz4' have recorded Raman spectra of liquid acetic and formic acids whilst Jakobsen et d4' have reported the low-frequency absorption spectrum. The authors agree that acetic acid appears to exist as predominantly cyclic dimers whilst formic acid is polymeric.An investigation of the vibrational spectrum of acrylic acid and sodium a ~ r y l a t e ~ ~ shows that the acid has a 'pseudo' C, type of dimeric ring structure. The enthalpy charge on dimerization was estimated to be about 8; kcal. compared with about 7 for saturated acids. The difference in values is probably associated with the degree of conjugation in the un-saturated acids. The study of charge-transfer complexes using far i.r. techniques has been supplemented this year by the report of the laser-Raman spectra of these species.43* 44 The former paper and one due to Yagi et aL4' are concerned with the effect of a range of acceptor molecules on the vibrational spectrum of a halogen or interhalogen compound.In the familiar benzene-bromine complex Hassel and S t r ~ m m e ~ ~ have suggested from diffraction data that the bromine molecule lies on a centre of symmetry in the system. Person et u Z . ~ ~ have ex-amined the i.r. spectrum of the compound at -200" and have been able to explain their results in terms of a C structure. They explain that the crystal data obtained at -50" may be correct and a structure change occurs with temperature variation or that the crystal form was rhombohedra1 rather than monoclinic. Lively interest has been demonstrated this year in the spectra of the trihalide ions. In the salt NO'ClF the difluorochloride seems linear but in the rubidium jg L. S. Bartell J . Chem. Phys. 1967,46,4530.40 P. Waldstein and L. A. Blatz J . Phys. Chem. 1967,71 2271. 41 R. J. Jakobsen Y . Mikawa and J. W. Brasch Spectrochim. Acta 1967,23A 2199. 42 W. R. Feairheller jun. and J. E. Katon Spectrochim. Acta 1967,23A 2225. 43 P. Klaboe J . Amer. Chem. SOC. 1967,89 3667. 44 G. C. Hayward and P. J. Hendra Spectrichim. Acta 1967,23A 1937. Y . Yagi A. I. Popov and W. B. Person J . Phys. Chem. 1967,71,2439. 46 0. Hassel and K. 0. Stromme Acta. Chem. Scad. 1958,12,1146. 47 W. B. Person C. F. Cook and H. B. Friedrich J . Chem. Phys. 1967,46,2521. 4 196 P. J. Hendra and caesium salts the solid-phase spectra suggest otherwise.48 Two reports on some of the heavier anions have appeared one concerned with the i.r. and Raman (using discharge lamps)49 and the other the i.r.and la~er-Raman.~' Both seem to agree and again it is suggested that crystalline forces can distort an otherwise h e a r ion. The first paper4' is particularly interesting in that a new method of examining dark-coloured solids in the Raman effect using discharge lamps was utilised. It is unfortunate that this technique was not developed sooner from the point of view of the inorganic chemist. The study of the vibrational characteristics of aqueous solutions has always been a popular field amonst Raman spectroscopists. A number of papers have appeared this year of which an example is one due to Hester and Grossmans' where the laser-Raman spectra of In3 + solutions were examined. The tempera-ture-dependence was found to be most interesting since changes in the Raman spectrum at Av values below 500 cm.-' indicated the presence of highly-ordered clusters of water molecules around the cations.The ion In(H,O):+ can be clearly identified due to its production of an emission near Av = 400 cm.-l and this is found particularly in the room-temperature spectrum of the perchlorate. The very high intensity of the band is however used as evidence for ordered cluster formation around this octahedral complex cation. The intensity of this peak in In2(S04) increases rapidly as the temperature of the solution is lowered and is very high in the glass formed by cooling the solution to about -40". Goulden and Mannings2 have made a valuable contribution this year by pointing out clearly that i.r. spectra of aqueous solutions are readily obtainable.They show that 5 % w/v solutions in 50 pm path cells will give useful spectra of such ions as sulphate carbonate and the phosphates. Vibrational Spectra of Small Molecules.-As is always the case a large number of small molecules have been examined. In order to give some idea of the range of species and of the type of analysis typical of the work carried out and reported this year we include a table (Table 1) of about twenty examples of molecules multimers ions etc. Inevitably the largest proportion of molecules studied were inorganic and of particular interest to co-ordination chemists although those interested in organic compounds were not neglected. The thoroughness of study and analysis were varied. Some workers quote results measured on solid liquid, gaseous and even matrix-isolated species whilst others contented themselves with an examination of a single phase.The detailed analysis of lattice modes and interactions in the solid phase is on the increase. Vibrational Analysis and Force Constants.-It is quite impossible to review adequately the scope of systems whose vibrations have been analysed during 48 K. 0. Christe W. Sawodny and J. P. Guertin Inorg. Chein. !967,6 1159 49 A. G. Maki and R. Forneris Spectrochim Acta 1967,23A 867. 5 0 G. C. Hayward and P. J. Hendra Spectrochim Acta 1967,23A 2249. 5 1 R. E. Hester and W. E. L. Grossman Spectrochim Acta 1967 23A 1945. 5 2 J. D. S. Goulden and D. J. Manning Spectrochim. Acta 1967,23A 224 Part (ii) Infrared and Raman Spectroscopy 197 TABLE 1 Molecule Reference Comments C,H,Se 53 Planar selenophene molecule.Full assign-ment given. U.V. data included. Most results unique. 54 Divinyl ether. 1.r. and Raman study. C, rotational isomer most stable. Assignments proposed. c203 55 v q at 63 cm. - '-very flat potential well for this bending mode. Carbon suboxide. C,Hl,O 56 Di-tertiary butyl peroxide. Full assignment of all observed features. ClNF 57 Spectrum of gas trapped in Ar matrix. Full assignment given. NF,O 58 C, symmetry 6 fundamentals observed B = 0.19 cm.-' rNAF = 1.48A FZB-CN 59 1.r. spectrum 400&245 cm.- '. All bands assigned. N0,Cl and NO,F 60 Re-examination of spectra. New assign-ments. Force constants computed AH: (298") FNO = -25.8 kcal. mole-'. ClNO = +3.39 kcal. mole-'. F,P.S 61 C3, confusion in assignment resolved. vWF) 983 and vwc,) 768 cm.-'. 62 Bands in solid HF dimer trimer and chain polymers identified. SeF and WF6 63 Vibrational analysis and Coriolis constants applied to force field. s6 64 D3d chair ring. Raman data incomplete. Full assignment give 198 P. J. Hendra Table 1-continued C2D,Se 65 Complete assignment. Some modes identi-fied by using co-ordination compounds. 66 Discussion on 'interaction constants' to be used in Urey-Bradley Force Field calculations. BaCl - 67 Raman of BaC12/KC1 melt systems. Full assignment of 3 bands. Fourth funda-mental found by quenching melt and examining solid in the i.r. Structure Dlh Ga2CI 68 Approximate force constants calculated after assignment of i.r. and Raman bands.C4HC17 69 Chlorocyclobutane and deuteriated ana-logues. Study of ring puckering (v = 165 cm.- '). Potential neergy function for this mode discussed. sc1; SeCl,+ TeCl; 70 Raman spectra recorded of AsFi-salts. Force constants computed. D2S 71 v1 and v2 analysed. Rotational constants calculated. 72 Re-investigation of i.r. spectrum. Fermi resonance between 2v2 and v1 postulated. 53 A. Trombetti and C. Zauli J . Chem. Soc. (A) 1967,1106. 5 4 A. D. H. Clague and A. Danti J . Mol. Spectroscopy 1967,22,371. 5 5 R. L. Redington Spectrochim. Acta 1967,23A 1863. 5 6 D. C. McKean J. L. Duncan and R. K. Hay Spectrochim Acta 1967,23A 605. 5 7 J. J. Comeford J. Chem. Phys. 1966,45,3463. 't3 E. C. Curtis D. Philipovich and V. H. Moberley J . Chem. Phys.1967,46,2904. 5 9 N. B. Colthup Spectrochim. Acta 1967,23A 2167. 6 o D. L. Bernitt R. H. Miller and I. C. Hisatsune Spectrochirn. Acta 1967,23A 237. 6 1 R. G. Cavell Spectrochim. Acta 1967,23A 249. '* J. S. Kittelberger and D. F. Hornig; ;I. Chem. Phys. 1967 46 3099. 6' S. Abramowitz and I. W. Levin lnorg. Chem. 1%7,6 538. 64 L. A. Nimon V. D. Neff R. K. Cantley and R. 0. Buttlar J . Mol. Spectroscopy 1967,22 105. 6 5 J. R. Allkins and P. J. Hendra Spectrochim. Acta 1967,23A 1671. 66 K. Shimizu and H. Shingu Spectrochim. Acta 1966 22 1999. 67 J. R. Chadwick P. J. Cranmer and H. C. Marsh J . Znorg. Nuclear Chem. 1967,29 1532. '* I. R. Beattie T. Gilson and P. Cocking J . Chem. Soc. (A) 1967 702. J. L. Hollenberg J. Chem. Phys. 1967,47 3271 Part (ii) Infrared and Raman Spectroscopy 199 the year.One field in which significant progress has been made is in the use of the spectra of crystals to support assignments. Thus Sbrana et al.73 were able to show a number of errors in the previous assignments of the spectra of pyrimidine by examining single crystals of the protonated and tetra-deuteriated molecules in polarised i.r. radiation. Similar approaches to the spectra of anthraq~inone,~~ 1,2,5-0xadiazole,~ and some pho~ophonitrilics~~ have enabled the assignments to be corrected and put on a firmer experimental footing. Yamada and S u z ~ k i ~ ~ went a step further and examined a bulk crystal in the i.r. using attenuated total reflectance. Although the spectra of oriented polymers have already been recorded by this method this is the first report of the effect using a single crystal.The previous assignments were by no means unanimous and a new set of proposals are given. Other papers which must be mentioned include ones on the mechanical calculation of force constants by Becker and Mattes,78 Bruton and Wood-ward,79 and a note by Llewellyn Jones" on the transferability of force constants. It has become customary amongst many co-ordination chemists interested in carbonyl compounds to relate force constant and 'bond strength'. Jones points out that this is most hazardous unless the types of system compared are very similar e.g. the K- values in M(CO),L cannot be compared meaningfully with those in M(C0)6. Further one cannot use 'approximate' force constants interactions must be allowed for since these are highly signifi-cant.In another paper Jones discusses the connection between force constants and bond order in the dichalcogenides of carbon.8' In a similar vein Wing and Crockerg2 point out the value of i.r. intensities for structural diagnosis and plea for their use rather than absorption frequencies. One cannot help asking if force constants are nm-transferable and cannot be related to bond strengths of what value are they? If these restrictions apply vigorously then frequencies calculated from 'apparently reasonable' force constants must be suspect in value especially since it remains virtually impossible to forecast interaction constants in most cases. Vibration Rotation Spectra.-The study of rotation vibration is a perennial feature in the i.r.field. An example where detailed structural information was obtained is an account of the spectrum of germyl isothiocyanateg3 and its tri-69 J. R. Durig and A. C. Morrissey J . Chem. Phys. 1967,46 4854. ' O W. Sawodny and K. Dehnicke Z . Anorg. Chem. 1967 349,169. 71 R. E. Miller and D. F. Eggers jun. J . Chem. Phys. 1966,45 3028. 72 J. W. Nebgen F. I. Metz and W. B. Rose J . Mol. Spectroscopy 1966 21 99. 7 3 G. Sbrana G. Adembri and S. Califano Spectrochim. Acta 1966 22 1831. 74 C. Pecile and B. Lunelli J . Chem. Phys. 1967,46,2109. 7 5 G. Sbrana M. Ginannerchi and M. P. Mazocchi Spectrochim. Acta 1967,23A 1757. " U. Stahlberg and E. Steger Spectrochim. Acta 1967,23A 627. 77 H. Yamada and K. Suzuki Spectrochim. Acta 1967,23A 1735. " H. J. Becker and R.Mattes Spectrochim. Acta 1967,23A 2449. 79 M. J. Bruton and L. A. Woodward Spectrochim. Acta 1967,23A 175. 'O L. H. Jones Znorg. Chern. 1967,6 1269. " L. H. Jones Znorg. Chem. 1967,6,429. e.' G. Davidson L. A. Woodward K. M. Mackay and P. Robinson Spectrochim. Acta 1967, R. M. Wing and D. C. Crocker Inorg. Chem. 1967,6,289. 23A 2383 200 P. J. Hendra deuteriated derivative. The i.r. and Raman spectra were recorded and assigned and the Ge-H modes near 2130,870 and 645 cm.-' were examined in detail in the gas phase. The results were interpreted as indicating an angle of ca. 156" between the Ge-N and N - C bonds. In small systems it is possible to record rotational constants. An example of this type of work appearing this year was the molecule 15N180 where values re = 1.507 A o = 1818.94 cm.-', and D = 4.7 x lo- cm.-' were c a l c ~ l a t e d .~ ~ Another example which poses interesting questions is an examination of the spectrum of b ~ t a d i e n e . ~ ~ The analysis of the rotational fine structure lead to the proposition of a carbon-carbon length of 1-464 f 0.003 A and a C = C angle of 123.2 f 0.2". The C-C single-bond length is significantly shorter than the value obtained from electron-diffraction data. Occasionally seemingly anomalous behaviour is noted in these investiga-tions. One sample from this year's work is contained in a paper by Durig and Wertzg6 on the i.r. spectra of HNCS and DNCS. It was observed that large gas-solid shifts occurred on freezing these gases and also that the vibration-rotation results from the N-H stretching band gave rotational constants of 43.45 & 0-5 cm.-' for the ground state but 36-67 & 0.5 for the first excited vibrational state.This fall in value seems very large. An explanation of an apparent anomaly was also offered during the year by Brown and Sheppard and by Sanb~rn.~' They explain the shape of the C=C and C=N stretching bands in gas-phase i.r. spectra of acetylenes and nitriles by invoking the occurrence of a multiple Q-branch structure associated with 'hot bands'. The reason may well be due to interaction of the C-C=X deformation and C=X stretching modes. A similar explanation for v2 in C F C S H was given by Sanborn. Another interesting paper appearing recently concerned the rotation of monomeric hydrogen fluoride in rare-gas lattices.88 There is a low barrier to rotation as demonstrated by the contour of the vibration-rotation band in the i.r.An interesting observation is that the centre of gravity does not co-incide with the centre of a lattice site i.e. there is a low potential maximum at this position. The effect seems to increase from argon to xenon. Also during the year a paper appeared on the pressure induced rotational quadropole spectra of hydrogen ~hloride.~' Intensities of the AJ = 2 lines were observed for HCl and HCl/SF mixtures and HBr and HBr/SF,. The molecular quadropoles were calculated as 5-8 x e.s.u. cm.2 for HCl and HBr respectively. It is claimed that these are unique results. C \ and 5-5 x 84 J. L. Griggs jun. N. K. Narahari Rao L. H.Jones and R. M. Potter J . Mol. Spectroscopy, 8 5 A. R. H. Cole G. M. Mohay and G. A. Oxborne Spectrochim. Acta,,1967,23A 909. 86 J. R. Durig and D. W. Wertz J . Chem. Phys. 1967,46,3069. 87 J. K. Brown and N. Sheppard Spectrochim. Acta 1967,23A 129; R. H. Sanborn ibid. p. 1999. 88 M. T. Bowers G. I. Kerley and W. H. Flygare J . Chem. Phys. 1966,45,3399. 8 9 S . Weiss and R. H. Cole J . Chem. Phys. 1967,46 644. 1967 22 383 Part ( i i ) Infrared and Raman Spectroscopy 20 1 Infrared Intensities.-Perhaps the major development during the year has been the expansion of interest in the intensities of i.r. bands in solids. A typical example of the type of work confined to simple molecular systems is that due to Ross and S~hnepp,~' who examined nitrogen and carbon dioxide as solids, and developed a theory to explain the intensities of the bands.In addition steady progress is apparent in the scope of investigations of gas-phase absorp-tion intensities. The advantage in examining gases is that intermolecular forces do not perturb the molecular motions. One of the major problems however in this field is the necessity of using high pressures to broaden the rotation-vibration lines. This broadening is needed because the slit-width of even the best commercial i.r. spectrometers is quite large and certainly greater than the width of a vibration-rotation band at low pressures. Pressure broaden-ing is of course caused by intermolecular collisions and therefore the use of this technique tends to destroy the advantage of measuring spectra in the gas phase.An example of the pressure-broadening technique was furnished by Morcillo et aL9' who examined CH2F2 CH,Cl, and CF,Cl,. They conclude, from their studies based on a new theory to explain absorption intensities, that (ap/d4) is not directed along the bonds but is strongly deflected towards the most polarisable atom in a molecule. A report appeared during the year in which intensities of the v3 bonds in methane nitrous oxide and acetylene were measured usiag a method which does not require pressure broadening. Hunter92 used a constant-volume gas cell containing the gas and recorded the pressure rise when filtered i.r. rotation was allowed to enter the cell and be absorbed by the gas. The pressure rise was minute but by using a moving diaphragm as half of a condenser he was able to measure the rise accurately.The amount of energy absorbed was measured by using electrical heating until the same pressure rise as before was produced. No corrections for overtones or combinations were included and the band pass of the filters was 3-5 p. The intensity data are probably accurate to within 7 % and are compared with previously published work. Hunter's table is reproduced in Table 2 and is self explanatory. TABLE 2 Intensity data in literature cm.-' atm.-' New results v,-CH 324,326,358 3 70 v3-N,0 1867,1650,1617,997,1850 1760 v~-C~H 590,280,325 420 Using i.r. dispersion and at 2 9 4 " ~ pressure-broadened absorp-tion techniques. 0. Schnepp J . Chem. Phys. 1967,46,3983,3991. . 91 J. Morcillo J. J.Zamorano and J. M. V. Heredia Spectrochim. Acta 1966 22 1969. 92 T. F. Hunter J . Chem. SOC. (A) 1967,374 202 P. J. Hendra Vibrational Spectra of Polymers.-Apart from the reports of the laser-Raman spectra of polymers a considerable number of papers on the i.r. spectra have appeared. A revision in the assignment of the spectral features to the vibrations of polyethylene has been discussed by Snyder93 confirming that the study of polymers and especially the more complex ones is still in its infancy. This statement particularly applies to the identity of the lower-frequency bands. At the International Conference on Molecular Spectroscopy held in September in Madrid these bands in solid polymers were discussed in detail. A paper by Tasumi and Krimmg4 has appeared in which the bands in solid polyethylene are identified dispersion curves produced and solid phase chain-chain interactions discussed.Zerbi et have examined molten polypropylene and solid polyvinylidene fl~oride.’~ The former is considerably different from the solid but still contains helical moieties (normal mode calculations support 5 repeat units per helical section) whilst the fluoride seems to occur in two forms, one planar zig-zag and the other less symmetrical but including a glide-plane. Another typical example of structural determination in polymeric systems was one due to Matsura and Miyazawa” who showed that in polyethylene-glycol (liquid) a random coil structure is likely. Probably the low melting point is connected with this coiling. Yet another field where spectroscopic examination is of value is in the decomposition of polymers and an example has appeared recently.Luongog8 irradiated ?oly-(3-methylbut-2-ene) with electrons of 1 MeV. It is suggested that the decay occurs as follows and since an increase in vinylidene saturation I 6-C-C + 2Me.+H’- CH, H CH,H I II I and a small increase in trans unsaturation is observed coupled with a yield of hydrogen and propene the first mechanism is the most likely. Infrared Spectroscopy of Adsorbed Species-Since numerous papers have appeared in the field and no new developments seem to be apparent it is only possible to select typical examples. In one such a classic system carbon monoxide on palladium was e~amined.~’ Two bands were observed at 4.8 p M groups.This latter 7 - O \ due to M-CEO groups and at 5.2 p from M 93 R. G. Snyder J . Mol. Spectroscopy 1967,23,224. 94 M. Tasurni and S. Krirnrn J . Chem. Phys. 1967,46,755. 95 G. Zerbi M. Gussoni and F. Ciampelli Spectrochim. Acta 1967 23A 301. 96 G. Fortili and G. Zerbi Spectrochim. Acta 1967,23A 285. 97 M. Matsura and T. Miyazawa Spectrochim. Acta 1967,23A 2433. 98 J. P. Luongo J . Polymer. Sci. Part B Polymer letters 1967 5,281. 99 J. K. A. Clarke G. Farren and H. E. Rubalcava J . Phys. Chem. 1967,71,2376 Part (ii) Infrared and Raman Spectroscopy 203 band seems to remain after continued pumping whilst the former is varied in intensity with the coverage of the palladium on its support (in this case silica). The authors suggest that this latter observation may be caused by the crystallite size of the metal on its substrate.In low concentrations of palladium the particles are small and it is suggested this leads to a spacing of the metal atoms unfavourable to the formation of bridges. A further quite typical example of the work published this year is again on the re-examination of a classic system -NH on porous glass."' Examination of ammonia over dehydroxylated fluoridated and deuteriated porous glasses are included but the i.r. spectra are confined to the 3400 cm.- region. Evidence B is put forward for the formation of 'B-NH, \ -SiNH2 \N-H and / B / / physically adsorbed ammonia groups on the*surface. Finally a paper on physically adsorbed species and surface reactions ; Young and Sheppard''' have shown amongst other things that acetaldehyde adsorbed on silica heated to 120" gives a spectrum convincingly identified as that due to crotonaldehyde. Two excellent books have appeared on this subject during the year one by Hare102 and the other by Little.lo3 loo M. J. D. Low N. Ramasubramanian and V. V. Subba Rao J . Phys. Chem. 1967,71 1726. l o 2 M. L. Hare 'Infrared Spectroscopy in Surface Chemistry' Marcel Dekker New York 1967. lo' L. H. Little 'Infrared Spectra of Adsorbed Species' Academic Press New York 1966. R. P. Young and N. Sheppard J . Catalysis 1967,7 223; Trans. Faraday SOC. 1967,63 2291