Raman Spectrometry A. C. Jones, Shell Development Co., Emeryville, Calif.
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PRESENT REVIEW covers the period from the end of the previous revieiT of Raman spectrometry (102) in this series, December 1961 to December 1963. As in the previous review, several hundred papers have not been cited in which Raman spectra have been simply reported, band assignments made, and/or complete vibrational analyses performed. Attention has been given in this review to those papers concerned with new aspects of Raman spectrometry and its applications. During the past two years, several reviews of Raman spectrometry have appeared. Green (89) and Mathieu (1%) have reported on recent progress in Raman spectromet,ry. The latter has given particular attent>ion to the theoretical calculation of the intensity of the Rainan effect'. Progress in the development of long-wavelength light' sources for exciting the Raman effect has been discussed by Stammreich (207) and by Delhaye (53) who has also reviewed progress in high speed, photoelectric recording of Raman spect'ra. Reviews of Russian work in vibrational spectroscopy (73), factors affecting vibrational bands (.219), the vibrational sl~ectra of molten salts (241) and of organic phosphorus compounds (172), and spectroscopic methods for the investigat,ion of internal rotation (166) have been published. Jones and Sheppard (104) reviewed the spectroscopic evidence supporting the occurrence of free rot'ation of some molecules in solution. h most welcome addition to the literature of Raman spectroscopy is the recently published book of 13randmueller and Moser (32). In addition to chapters on the theory of t,he Raman effect, molecular vibrations, the resonance Raman effect, and the calculation of Raman intensities by means of bond polarizabilities, the book contains extensive discussions of developments in instrumentation and experimental techniques up t,o about 1960. They have given particular attention to the measurement of intensity and depolarization ratios.. Another rrelcome addition is an English translation of Placzek's classic paper on t'he theory of Rayleigh and Raman scatt,ering (17 0 ) . Chaptprs on the subject of Raman spectroscopy have also appeared in recent books on physical methods (211 249). HE
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DEVELOPMENTS I N THEORY A N D PRACTICE OF RAMAN SPECTROMETRY
Theoretical Developments. During the past two years, papers on theoretical aspects of R a m a n spectrometry have been concerned principally with one of three subjects-Raman scattering from crystals, the intensity of R a m a n scattering from molecules, and t h e width of molecular Raman lines. Perhaps under the stimuli of the great interest in solid state physics in recent years and the advent of lasers as highly desirable sources of intense and well defined exciting radiation, considerable activity in theoretical studies of light scattering processes in crystalline media has developed. Raman (181) has discussed the spectroscopic properties of alkali halide crystals for which the first overtones of the lattice modes rather than the lattice modes themselves are allowed in the Raman effect. Such selection rules are based on the consideration of the symmetry properties of the crystal lattice. I3irman (20) has derived the infrared and Raman selection rules for single and multiple-phonon interactions-fundamental, overtone, and combination transitions-for crystals having the diamond and zincblende structures. With particular application to semiconducting crystals in mind, Loudon (133) has considered the firstorder Raman effect (Raman transitions for the fundamental frequencies of the lattice modes) occurring as the result of the mutual interactions of the exciting photons and of the lattice modes with the elections in the crystal. Ovander and Grechko (88, 161) have derived a theoretical treatment of the first-order Raman effect also applicable to polar and piezoelectric crystals. 130th theoretical developments show that for polar crystals, there is a contribution to Raman scattering from the long-range electrical forces in addition to the scattering calculated for homopolar crystals. This reviewer has not been able to determine whether the theories are equivalent, but Loudon points out that the two theories differ in some respects. One difference is dependence of the intensity of scattering upon the frequency of the exciting light vo when it approaches the frequency v e corresponding to an energy level in the crystal. Loudon's development gives the factor
1 v,-vo while Ovander's gives the factor 1 v c - v a 1-l for the dependence, With a somewhat different appioach, however, Ovander (169) did find a dein nonpolar pendence of l v e - v o crystals. For crystals containing paramagnetic ions, Leushin if 29) and Seiden (189) hare investigated theoretically the relationship between paramagnetic spin-lattice relaxation and the Raman effect. From the consideration of a crystal as a homogeneous medium with scattering in homogeneities being produced by the lattice vibrations, Strizhevskii (212) has derived a general expression for the tensor of Raman scattering in crystals which is an approximation for crystals analogous to the approximation of the polarizability theory for an isolated molecule. I3y a similar simple approach, Ovander (163) has derived expressions for the intensities and depolarization ratios as well as their dependence upon the angle of obs e n ation for the longitudinal and transverse vibrations of piezoelectric crystals with cubic axial symmetry. I n a more applied theoretical study, Kleinman and Spitzer (113) have used a valenceforce model and simple assumptions with regard to charge distribution and polarizability to calculate the spectral properties of a-quartz. They have calculated not only the vibrational frequencies but also the infrared and Raman intensities. For use in the calculation of the intensity of Raman lines by the classical bond polarizability approach, Sverdlov (213) has derived general formulas for the calculation of the tensor components of the polarizability derivatives with respect to the normal coordinates. Derived bond polarizabilities and their dependence upon bond lengths and bond angles are included in the formulation. Theoretical intensities in reasonable agreement with observed intensities have been calculated with this model of derived polarizabilities for cyclopentane and deuterated derivatives ( I D ) ,allene and its tetradeuterated derivative (SO), and toluene (118). Although Long et al. (132) have been able to attain a satisfactory calculation of inteniities with the simpler bond polarizability theory (no assumed dependence of bond polarizability upon bond angles and lengths) for CC14, CHCl,, and CDCl, when they used sum rules to
eliminate uncertaintics in t,he normal COordinate transformations. Taylor and Woodward (217) hEJve not, however, been successful in thc: application of the simple theory to the intensities of the totally symmetric frequencies for neopentane and its perdeuterated derivative. From theoreticay considerations, Kurowski and hlinc (116) have concluded that, contrary to common belief, in some cases thl: derived polarizability for a bond (‘an increase even though the bond has become more polar. Another effort in chssical theories ,of Raman intensities is ICrushinskii’s ( 1d$) detailed examination of the validity of the wavelength dependence of intensity obtained in Placzek’s classical theory of the Raman effect. Particular attention was given to the determination of the wavelength region in the neighborhood of an electronic absorption maximum wherein the theoretically determined wavelength dependence is not correct. Savin (188) has extended, with reasonable success, the application of the “metallic molecule” model to the correlation of Ramart band intensities and depolarization ratios for a number of aromatic compounds. I n this model the contributions of the n-electron systems are calculate’d on the basis of the energy levels for r-electrons in a conducting ring. F r 3m a theoretical treatment based on ii classical polarizability formulation f3r the intensity of Raman scattering, Kondilenko et al. (115) have shown that the intensity of a n overtone should increase more rapidly than that of the fundamental when the frequency of the exciting light approaches that of an absorption band. The experimental confirmation of their conclusions was quantitative for CCla and CHCla. The results for CS2 were not quantitatively in agreement with theory possibly because, as they pointed out, the effective absorption band is too close to t,he exciting frequencies for the theory to be valid. Theoretical consideraions of photon scattering processes (resonance, Rayleigh, and Raman scat’tering) at short wavelengths have been made by Heddler (95)and by Dalgarno and Williams (49). The calculated cross-section for Rayleigh scattering of light a t 1216 A. by hydrogen was found to be about 2 barns cm.2) by both groups and in agreement with the observed value (93). With respect to preijsure broadening of rotational and vibrEttiona1-rotational Raman lines for gases, Fiutak and van Kranendonk (7f)have examined in detail the theoretical application of t h e “impact theory” of pressure broadening for absorption and emiksion processes to Raman scattering. They found the expression for the shape of a nondegenerate Raman line to be the same expression as is obtained lor a n absorption
line except that the Raman scattering tensor replaces the dipole moment. The line shape is Lorentzian with its width defined b y a parameter called the optical cross section. For degenerate Raman lines, the theory shows that the broadening of polarized and depolarized lines is different. For example, when complete degeneracy occurs for a polarized line, pressure broadening does not occur. Further, when the components of a polarized line are not completely degenerate the limit of the broadening is the frequency separation of the components. This point is in agreement with the conclusion reached by Bazhulin (9) from an analysis of rotational and rotational-vibrational Raman spectra of gases. Mikhailov’s (146) observations for Raman lines of methane a t pressures from 14 to 250 atm. also illustrate the theoretical conclusions. He found that Y, (2914 cm.-l) was not broadened whereas p 2 (1535 cm.-1) and v 3 (3020 cm.-I) were broadened in a manner corresponding to optical cross sections comparable to kinetic cross sections. Also in their theoretical developments, Fiutak and van Kranendonk have obtained expressions for the effects of dipolar and quadrupolar interactions on the optical cross section. Van Kranendonk’s application of the theory to experimental data for HZ(25.5) showed that quadrupolar interactions accounted for the observed broadening. Both self-broadening and pressure broadening by a number of foreign gases have been investigated by Lazarev (127’). He concluded that the impact theory as applied to absorption and emission processes was consistent with experimental observation provided the optical cross section was larger than the kinetic cross section. His treatment did not work for the selfbroadening of hydrogen. A method for the calculation of the widths of the vibrational lines of gases has been developed and applied to benzene vapor ( f f 7 ) . Changes in the moments of inertia with vibration and Coriolis interaction are considered. Suitable experimental data for comparison with theory were not available. 1 theory of the width of vibrational Raman lines for liquids has been developed by Valiev. When vibrational relaxation and resonant exchange phenomena are considered alone (229), the calculated widths \vere narrower than the observed ones. With the inclusion of hindered Brownian rotation, however, observed widths were calculated (2.50-232). Satisfactory agreement with experimental observations \vas noted for the dependence of width upon the depolarization ratio and upon temperature. The theory also yielded information concerning the line shape and polarization of the Rayleigh scattered
light by spherical, symmetric-top, and asymmetric-top molecules (250). For liquids or solutions in which equilibria between molecular species can occur, Kreevoy and Mead (119) have suggested that the widths of Raman lines can be used to measure fast reaction rates and have developed a theoretical relationship between reaction rate and line width. They determined the rate for the association of trifluoracetate ion with protons and found values of the order of 10-l’ set.-' Stimulated Raman Effect. T h e most exciting discovery in many years of R a m a n spectroscopy has been t h e observation of the stimulated R a m a n effect by Woodbury and coworkers (65, 251). The phenomenon was first observed (65) for nitrobenzene used in the Kerr cell for a ruby laser operated in a pulsed reflector or giantpulse mode. Under such operation the duration of the laser pulse is of the order of 50 nanoseconds and peak power, a few megawatts. Raman shifts were observed for a number of other compounds (251) placed in a cell within bhe cavity of a ruby laser operated in the giant-pulse mode. The shifts were observed, however, for only one or two of the st,rongest Raman lines and sometimes multiples thereof for ring compounds-benzene, perdeuterohenzene, nitrobenzene, toluene, pyridine, l-bromonaphthalene, and cyclohexane. The unusual features of the Raman radiation are that the degree of beam collimation is essentially the same as for the ruby-laser beam, the intensity rapidly becomes a significant fraction (as high as 0.1 in some cases) of the intensity of ruby-laser radiation as the peak power is increased above a definite threshold level, and the widths of the Raman lines become very narrow a t high power levels. A\tstill higher power levels, i t has been found t,hat the intensity of the stimulated Raman line for nitrobenzene (a shift of 1345 cm.-l) in the Kerr cell was 25% of that of the ruby laser line ( 7 7 ) . I t has since been established that compounds not containing rings also exhibit the stimulated Raman effect. The phenomenon has been observed for Raman lines arising from the C-H &etching vibrations for acetone and 1,1,2,2-tet~rachloroethane ( 7 6 ) . These investigators have also noted tha,t a material exhibiting strong stimulated Raman acti1.ity supresses t,he stimulated Raman effect from a material of lesser activity when both are present in the laser cavity, probably because the more active material preferentially reduces the intensity of t,he ruby laser radiation by relatively more efficient’ conversion to Raman radiation. It has recently been demonstrated that the laeer cavity is not essential to the generation of stimulated Raman VOL. 3 6 , NO. 5 , APRIL 1964
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radiation. Both Eckhardt etal. (64) and Minck et al. (161) have observed stimulated Raman lines from samples excited outside the laser cavity by the focused light beams from giant-pulse lasers. Eckhardt and coworkers have shown that both lattice vibrations and internal vibrations of crystals can participate in the stimulated Raman effect. They observed both Stokes and anti-Stokes lines for the 1331 cm.-l lattice vibration of diamond, the 1085 cm.-l vibration of C03-* in calcite, and the 216 cm.-’ and 468 cm.-l vibrations of SS molecules in orthorhombic sulfur. The experimental observation of stimulated Raman scattering by lattice vibrations is satisfying since Loudon (134) has estimated from theoretical considerations that the threshold flux necessary for the stimulation lattice vibrations should be attainable with pulsed lasers. Minck et al. (161) concluded that the theory of nonlinear interactions of light and dielectric media derived by Armstrong et al. (3) can be extended to the stimulated Raman effect. As the result of four-photon interactions, a series of stimulated Raman lines should n u R ) where y o appear at frequencies ( u , and u R are the frequencies of the laser radiationand the Raman shift, respectively, and n assumes both positive and negative integer values representing anti-Stokes and Stokes Raman lines. For hydrogen they observed lines for n from - 2 to + 4 where v R is the frequency of the H-H stretching vibration. Although for deuterium their results indicated that both the stretching mode and a rotational mode participate, the observations have also been explained tentatively by four-photon interactions. Rivoire and Dupeyrat (184) have noted that, in 1934, Placzek (170) showed theoretically the existence of the stimulated Raman effect. The consideration of the molecular scattering of electromagnetic radiation in the formalism of Dirac’s, quantum mechanical treatment of the radiation field and the scattering molecule yields an expression for the transition probability of the Raman effect as the sum of two terms. One term, which applies to the ordinary or spontaneous Raman effect, is proportional to the intensity of the exciting radiation while the second term also involves the intensity of the Raman radiation and, therefore, represents the stimulated Raman effect. These terms show that the ratio of the stimulated and spontaneous Raman effects is the same as that for the anlaogous Einstein transition probabilities for emission and that the ratio varies inversely as the cube of the frequency of the Raman radiation. It would seem that the theoretical results of Placzek as well as the experimental results of Minck and coworkers for hydrogen serve to contradict Hellwarth’s (96) theoretical
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treatment demonstrating that the stimulated Raman effect is a lasing phenomenon. Resonance Raman Effect. T h e resonance R a m a n effect refers t o the enhancement of the intensity of some Raman lines of a molecule as the wavelength of the exciting radiation approaches t h a t of a n electronic absorption band. T h e continued attention the effect has received is encouraging because study will lead t o a better understanding of the Raman effect and the interaction of light with matter. Shorygin (193) has reviewed his semiclassical theory of the resonance Raman effect and has discussed in considerable detail the factors affecting the derived polarizability as the exciting frequency approaches the frequency of the electronic absorption. With the inclusion of a damping factor, related to the width of the electronic level, the derived polarizability of the semiclassical theory has a finite value even very near resonance of absorption and exciting frequencies. The experimental observations of Shorygin and I v m o v a (194) of the Raman spectra of c6Ha-(CH=CH)5-C6H5 and C6H5-(CH=CH)~C& confirmed many features of the semiclassical theory of the resonance Raman effect. Their results showed that, when the life times of the vibrational sub-levels of the excited state were sufficiently long to produce vibrational structure in the electronic absorption band, the resonance increase in the intensities of Raman lines is more pronounced. Further, resonance intensity increases are much greater for overtone and combination Raman bands than for fundamentals. The pronounced dependence of the resonance effect upon specific vibrational sub-levels was also demonstrated. From a comparison of intensities of Raman lines excited with light of wavelengths between 4000 A. and 6000 A. from He and Hg sources, Tsenter and Bobovich (221) concluded that the fourth-power dependence upon the frequency of the Raman scattered light does not apply. I n the interpretation of their data, they considered one electronic transition with a factor for frequency dependence which has been shown to apply when the exciting frequency is relatively close to (certainly within 10,000 cm.-’) the frequency of the absorption band. The results leading to their conclusion, however, involved excitation frequencies 15,000 cm.-‘ to 25,000 cm.-’ from the electronic absorption band; therefore, their conclusion does not seem to be valid. On the basis that the relative intensity for the 1348 cm.-’ Raman line for the NO2 group of a qeries of aromatic nitrocompounds depend- only on the position of the elertronir absorption band with reqpect to the frequency of exciting light,
Chakraborty (42) found that his results for the frequency dependence of the intensity agreed more closely with Placzek’s theory (170) than with Shorygin’s (193). Clearly, the resonance Raman effect is not yet understood. In a somewhat different approach to the theory of the resonance Raman effect, Sushchinskii and Zubov (214) have developed a relationship for the dependence of Raman intensity upon the frequency of exciting radiation near an electronic absorption band in a manner paralleling the theoretical treatment of resonant fluorescence with damping taken into account. They found that the Raman intensity should be proportional to the usual term involving fourth-power of the frequency of the Raman radiation and the absorptivity at the frequency of the exciting radiation. Experimental results determined for Raman lines excited by Hg-arc lines from 5460 A. to 3100 A. were in qualitative agreement with their theory; however, no evidence was presented that would serve to prove their, theory superior to others. According to their treatment, depolarization ratios are independent of the frequency of the exciting light in agreement with the observed results for Raman lines of two polyenes. As discussed below, this observation also does not provide conclusive support for their theory. In the preceding review (102), there was an unresolved question concerning the explanation of the values of the depolarization ratios for corresponding Raman lines approaching 0.5 for a series of increasing conjugated compounds. One explanation was that it was due to an increasing resonance contribution from a single, highly polarized electronic transition since the absorption bands for the compounds approached the exciting frequency as conjugation was increased. Such increases in the depolarization ratio with increases in the degree of the resonance do exist and have been observed (156). The second explanation was that the increased conjugation increased the asymmetry of the derived polarizability ellipsoid, or, in other words, increased the degree of polarization of the electronic transitions contributing to the derived polarizability. Tsenter and Bobovich, who were initially responsible for the question, have shown the second explanation to be the correct one (222). They did this by determining that the depolarization ratios for each compound were independent of the frequency of the exciting light; therefore, the resonance effect could not be responsible for increasing depolarization ratios. h n interesting illustration of the value of the resonance Raman effect in the investigation of the nature of bonding in molecules is Zubov’s (157) work of the frequency dependence of the in-
tensity of Raman lines for C-H stretching modes of benzene, toluene, l-pentene, 1,5-hexadieneJ I ,3-pentadieneJ and isobutadiene. The intensities relative to the C-H lines for cyclohexane were determined for exciting light at wavelengths from 5640 A. to 3020 d. Zubov found that there was no enhancement of relative intensities at shorter wavelengths for benzene and toluene and only slight enhancement for the unconjugated olefins. For the conjugated olefins, on the other hand, striking resonance effects were o xerved indicating an unusual interaction between the Uelectrons of the C--I1 bond and the Telectrons of the double bonds in these compounds. Thompson and associates (69, 101) have investigated the effects of substituents upon the intensity of the Raman line for the C=O group of substituted methyl and ethyl benzoates and acetophenone and for the C r N group of benzonitrjle. After proper corrections for instruinental factors and medium effects were made, a correlation of corrected in Lensity was made with the Hammett inductive factors u H and the Taft resonance factors U R for the substituents and a factor (ve2 ~ , 2 ) 2 / ( ~ , 2 - v,*)4 for the resonance Raman effect. They found that the resonance Raman effect was the dominant factor affecting the Raman intensity. The same consideraticln should probably be taken for the observed intensity variations for the 1830 cm.-I line of methyl- and chloro-s ibstituted derivatives of styrenes (266) and for several Raman lines of mono- and disubstituted derivatives of benzene (255,239). Electronic Raman Effect. Since t h e discovery of t h e R a m a n effect in 1928, only on rare occasions has interest been shown in t h e observation of electronic transitions in t h e R a m a n effect. I n 1929 Raeetti (182) noted rotational Raman lines for NO representing the occurrence of a transition between the levels o l the doublet 217 ground state for thi3 molecule. The phenomenon was also discussed by Placzek (170). Subsequently, from theoretical considerations (41) i t was concluded that electronic transitions for atoms could be excited in the Raman effect only with photons of high energy; e.g., x-rays. The report (199) of electronic Raman lines for Sm(N03)3 was quickly shown to be a misinterpretation of the observed spectra (90, 198). I n view of this histor:y, it is surprising that papers have appeared independently and almost siniultaneously suggesting that the electronic Raman effect should be observable and useful for crystals (67) and reporting the observation of the electronic Raman effect for P r S 3 in crystalline PrC13 (97). Elliott and Loudon (67)have shown that transitions between electronic energy levels can be observed in the Raman effect
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and pointed out t h a t the Stark splittings by crystal fields for the ground states of rare earth ions having 4 j electrons would be of the order of a few hundred wavenumbers and, therefore, be readily observable. They further noted that the difference in selection rules would result in transitions not directly observable by absorption measurements. The expected intensities for such transitions in the Raman effect are comparable to Raman lines for crystal lattice vibrations. The observations of Hougen and Singh (97) for are in complete agreement with the predictions. Raman lines for the lattice vibrations were identified from a comparison of spectra of PC13 and the isomorphic L a c & crystals. The La+3 has no electronic transitions below 50,000 ern.-' At 77" K. a total of ten Raman lines were observed between 0 and 4000 em.-' and were assigned to transitions between known electronic levels. Yo electronic Raman lines were observed at room temperature. Intensity Measurement. T h e subject of t h e measurement of the intensities of R a m a n lines a n d their depolarization ratios was extensively reviewed two years ago (102). Among the more recent papers concerned with the measurement of Raman intensity is Golden and Crawford's (87) work on the measurement of absolute intensities in the gas phase. They have been concerned with instrumental corrections particularly applicable to measurements made with the Cary Model 81 instrument equipped with the multiplereflection cell for gases. Following essentially the approach of Yoshino and Bernstein (254), they accounted for partial polarization of the exciting light, polarizer imperfection, and convergence effects from measurements of the intensities of the v1 and u3 lines of methane which are complete polarized and depolarized, respectively. Polarization sensitivity of the spectrophotometer was determined independently. Spectral sensitivity of the instrument, including the variation in the reflectivity of the mirrors of the gas cell, was determined from otherwise corrected relative intensities of rotational lines of HP and Dz and vibrational lines of Oz, NzJ and other gases excited by both the 4047 A. and 4358 -1.mercury lines. Relative intensities were converted to an absolute scale with the calculated intensity for the rotational line of Hz for the transition, J = 1 + 3. The agreement between the absolute irrtensities of vibrational lines of HP and Dz determined by Golden and Crawford and by Yoshino and Bernstein with considerably different instrumznts gives one confidence that reliable measurements of absolute intensity can be made for gases.
For liquid samples, on the 'other hand, it is apparent that the problems yet remain. The principal source of the difficulty is concerned with the effects of the refractive index of the sample. As pointed out in the previous review ( l o g ) , many workers have considered various ramifications of the refractive index effect. Recently, Michel and Gueibe (141) have considered two major aspects of the effect of refractive index, the optical effect and the internal field effect, as was done in the earlier work of Rea (185). The correction for optica1 effect, due purely to geometrical optical considerations involved in the irradiation of and the observation of a refracting medium, certainly must be applied when an external standard or intensity reference is used. T h e second effect, the internal field effect, is a real physical phenomenon which arises from the dependence of the effect,ive electric field of the exciting radiation upon the refractive index of the medium. Therefore, even when properly corrected for the optical effect, the observed intensity for a Raman line of a molecule will exhibit a dependence upon the refractive index of the medium in which the molecule is observed. Contrary to the proposal of Michel and Gueibe, it would seem desirable to eliminate the contribution of the internal field effect to Raman intensities in studies of molecular interactions and particularly in analytical applicat,ions. With respect to the opt'ical effect of the refractive index, an analysis of the influence of the geometry of irradiation upon the Raman intensity by Naberukhin (154) is very informat,ive. The polarization of the exciting light, 'the polarization sensitivity of the spectrophotometer, the direction of the exciting light, and the effect of refractive index on the direction of irradiation were taken into. account. He determined the general form of the correction factors for intensities based upon ratios of 1 3 ~ and ~ ) of ( 4 5 d 772) (452 where CY and y are the symmetric and the anisotropic parts, respectively, of the derived polarizability tensor. Intensities based on the second quantity are the "standard intensities" of Rea (183) and others. Saberukhin showed that both correction factors are dependent upon both the refractive index and the sample and the depolarization ratio of the Raman line contrary to Rea's assumption that thc influence of the depolarization ratio on the optical effect can be neglected. A s has been pointed out by several inves'tigators ( I O $ ) , the standard intensit,y scale becomes appropriate and convenient when exciting light polarized with its rlectric vector perpendicular to the (lire(-tion of observation. For this casc, Saberukhin found the correction factor to be appreciably simplified but still a func-
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tion of depolarization ratio. If, however, the spectrophotometer is not sensitivc to the degree or direction of polarizatiun of the Raman iadiation, the correction factor becomes very simple, and, for any given geometry, is a function only of the refractive index. Kow, TunnicJiff and Jones (223) have demonstrated t>hatthe polarization sensitivity of the spectrophotometer can be eliminat'ed instrumentally; therefore, it would seem that the advantages of these instriimental conditions to simplify the correction factor outweigh the loss of intensity of exciting light because of the polarizer. This conclusion supports the use of rat,ios of ( 4 5 ~ 2 7-yz) as t'he basis for standard intensities. Sabeukhin, on the other hand, concluded that, ( 4 h 2 1 3 ~ should ~) be the basis for standard intensities only because the correction factor in the general case is somewhat simpler, although still complicated, and somewhat less dependent upon depolarization ratios. Naherukhin's conclusion is justified if one were to insist upon using unpolarized exciting radiation. One of the major difficulties in the determination of standard intensity is the lack of agreement as to what corrections should be applied. For example, it will be fortuitous if the standard intensity data reported by Sidorov et al. (200) for the xylenes are correct. .\lthough relative values of "true" and observed depolarization rat,ios can be used to correct for the refractive index dependent upon convergence of citing light (f85),t'he observed depolarization ratios must, of course, he dc%ermined with the same instrument and excitation geomet'ry as the intensities are observed. For their correction factor, Sidorov and eoworkers used published values of depolarization ratios! Furthermore, their neglect of the opt'ical refractive index factor and t,he temperature factor is not justified. In many other recent studies of Raniari intensities, either no correction for refractive index or, a t best, a factor of n? for the effect upon the efficiency of observation has been made. I n most cases useful information has been obtained even though standard intensities have not been measured. Since tabulations of "standard intensities" (140, 153, 258) and depolariztition ratios (25, 256) for various of compounds are beginning to aiipear> however, universal agreement on t,he correct method for determining st aiidard int,ensit,ies is becoming highly t l w i rahle. 'I'wo invest'igations of the difficulties encountered in the measurement of Raman intensity from colored samples rvhicah absorb both the excit,ing and Raman d i a t i o n have recently been reported. This problem is of part,icular iinliortance in studies of t.he resonance
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Raman effect,. From a rather sophisticated treatment of the geometry for the exciting and Raman radiation, Behringer (15) obtained results quite similar to those from simpler models. Michel and Martin (142) have determined a coirection factor from a simple model and have successfully tested the correction with intensity observations for several binary mixtures containing small amounts of highly colored materials. The correction is accurate to about 5% over a range of up to 90% absorption of the Raman radiation. Another intensity measurement of interest in Raman spectral studies is the depolarization ratio. Koningstein and Hernstein (116) have proposed and tested a simple method for the ealculation of true depolarization ratios from observed values. Convergence of the exciting radiation and its dependence upon refractive index and t'he effective transmission of the polarizers have been considered. Their approach is dependent upon the assumption that the 459 em.-' Raman line of CCL is completely polarized and the 317 em.-' line is completely depolarized. Tests of the method indicated errors of about 4%. Solvent Effects. Information concerning the nature of molecular interaction is obtained from investigations of the solvent effects upon the features of many types of spectra. For Raman spectra, some effects such as shifts in the position and changes in the width of Raman bands can be determined in a straightforward manner and useful results can be obtained. The solvent effects on the intensities of Raman bands, however, are not readily determined. Part of the difficulty is due to the problem concerned with the measurement of intensities as discussed above. At the present time one cannot unambiguouslv assess the extent to which observed deviations of Raman intensities with variations in solvents and concentrations are due to instrumental factors, internal field effects, and molecular interactions. Thompson and coworkers (69, 101, 216) have determined intensities of t'he Raman lines for the C=O group of met,hvl benzoate, tmheC=N group of henzonitrile and acrylonitrile, and the C=C group of phenyl acetylene and propargyl chloride in a variety of solvents. They have carefully corrected their observed data for the known instrumental factors and have referred the int,ensities to the 459 em.-' line of CC1, in a medium of the same refractive index as that nf the system in which the Raman line for the molecule of interest was observed. With this intensity reference, one would expect that the internal field effect would be eliminated from the corrected intensity data. In all cases, however, a pronounced and consistent
dependence of the corrected intensities upon refractive index was found. Although it is possible that the dependence upon refractive index is an artifact of improper correction for instrumental fact,ors,it seems unlikely in view of their detailed consideration of the correction factors. The authors' conclusion that the observed dependence is real appears to be legitimate. Severtheless, even when all questions concerning the proper met'hods for correcting for the instrumental effects have been fully resolved, there will still remain the question of the int'ernal field effect. Namely, when one is interested in the interpretation of solvent effects on Raman intensity in t,erms of molecular interactions, how should the internal field effect be evaluated? Hernstein (17) proposed a scheme for the interpretation of the variation of Raman intensity with solvent in which the effective internal field strength has been caleulat,ed by the Onsager model as suggested by Pivovarov (169). He concluded that the effect of the internal field accounted for only a small part of the concent,ration dependent variations in the molar intensities of Raman lines for the components of the systems CC1,hexane and CSz-benzene. The intensity variations were, therefore, considered primarily due to molecular interactions which, he stated, could be explained by dispersion forces for the CC1,hexane system but not for the CSz-benzene system. Eernstein's approach requires further study, however, since Sokolovskaya (204) has shown that the internal field effects predicted from the Onsager model are far less than ~vouldbe necessary to account for observed relative intensities of Raman bands for CC12, CHCI,, and benzene in the vapor and liquid states. ;\nother approach to interpretation of t,he effects of the medium upon Raman intensities is a kinetic scheme suggested by Ryason (187). The scheme involves the assumptions of photon-molecule romplexes having lifetimes of t'he order of the time interval between intermolecular collisions (about 10-11 sec.) and of the intermolecular transfer of photons without depolarization. The scheme, therefore, appears physically unreal and can only be considered to be an empirical means for expressing observed solvent effects. I t can give no useful information, for example, about molecular interactions. The effect of solutes upon the Raman spectrum of water has enjoyed the attention of a number of groups of investigators in the past few years. The effects of dissolved alkali halides and temperature upon t,he intensities, posit,ions, and shapes of the Raman lines for the bending and stretching vibrations for HzO and D?O have been studied by S h u h and Hornig (19'7) and
Weston (248). Both groups found that the effects produced by the solutes depended only upon the mion and that the magnitude of the chrtnges wese in the order I- > Hr- > C1- > F-. Changes produced by F - were small and in the opposite direction frori the changes produced by the other anigns. For the intensities of the strekhing bands of H20, on the other hand, Lauwers and van der Kelen (126) reported that, in addition to the effects, produced by the anions as noted abovc:, the cat,ions had some influence which was in the order Lf > N a + 3 K + . Liecause this order for cations is the same as the expected effect arising from refractive index of the solutions, the observed influence of the cations may be the rt:sult of improper corrections for the effects of refractive index upon observed Raman intensity. Another inconsistency in the observations of different groups was the effect of temperature on the intensities of Raman bands. Both Shultz and Hornig (197) and if'wton (648), who have corrected their intensit'y data for the instrumental factors and the refractive index, found that the effect of temperature increase lipon intensity is in the same direction as the effect of the electrolq tes. Walrafen (6,@), who made no corrections, found the effects to be in the opposite direction which is consistent with the changes in refractive indices. This particular point emphasizes the great i.nportance of the proper consideration of the effects of instrumental factors a.id the refractive index upon observed Filamari intensities because the relative changes produced by temperature and electrolytes are of great significance in the interpret'ation of the effect,s of electrolytes upon the structure of water. In many cases, however, i t is not necessary to make cnmplete and rigorour; corrections for all of the factors afffcting intensities in order to obtain useful information from Raman intensity data. This is illustrated by the work of Vollmar (240) on the investigation of the effects of cations upon the Raman band for the symmetric stretching :node of the nitrate ion in aqueous solutions. Pronounced changes in position and halfwidth were produced by all cations except NH4+. He, therl:fore, concluded that S H 4 + also did not cause a real perturbation of the intensity of the band and the obsermd variation of specific intensity for N114K03as a function of concentration w i ~ used s to obtain a correction factor as EL function of refractive index. Except for Ca(NO)? solutions, a reasonable correlation of changes of Raman intensity with shifts of the electronic abso1,ption bands of the nitrate ion suggested the existence of a resonance Ramrtn effect. The changes in band position and half-width were considered to reflect the tendencies
of t,he cations, respectively, to form ion-pairs with NOa- and to disrupt the structure of the water by cationic hydration. For some systems, the Raman intensity variations produced by molecular interactions are so large that they overshadow the effects of refractive index expeciallp when the change in the latter is small. Such is probably the case for the intensity variations of the C-0 stretching band of methanol in HrO and in acetone reported by Kecki (107). For solutions of methanol iu CCl, the change in refractive index with concentration is much larger; therefore, the observed intensity variations for this system may, in fact,, be due to intensity errors and internal field effects rather than molecular interaction. Thus, the observed results are not sufficient evidence to support the proposed unusual interaction between CCla and methanol. Similarly, the variety of spectral effects observed by Kecki and cow7orkers (208, 149, 150) for solutions of electrolytes in methanol and acetone are evidence for molecular interactions in a qualitative sense; but, interpretations based upon quantitative intensity measurements must be questioned. Other investigations of solvent effects have been reported for alcohols in polar and nonpolar solvents (dS7), for benzene systems ( 6 ) , for octyne-1 in solution in internal octynes ( I O ) , and for a variety of organic solutes in CCh, benzene, and methanol ( 5 , 234). I n spite of inadequate corrections of the inteneity measurements, evidence for molecular interact'ions has been obtained in many cases. Babich et al. (4) have invest,igated the effects of solvents upon the relative intensities of Raman lines for vibrational levels in Fermi resonance. Instruments and Techniques. T h e recently published book by Brandmueller and Moser (32) includes detailed descriptions of most of the significant, developments prior to 1960 in instrument'ation and techniques in Raman spectrometry. I n the past two years, considerable work on the development of light sources has appeared. Other areas of activity have been concerned with sampling problems particularly with respect to small samples and solids and with the excitat,ion of Raman spectra with a laser as the light source. The construction and operating characteristics of Hg-arc lamps having water-cooled electrodes and lamp bore have been reported by Tyulin and Tatevskii (227) and by Chisler (44). Wat,er-cooling of the lamps makes it possible to operat,e them a t higher power levels to attain a greater intensity of exciting light than without cooling. Tyulin and Tatevskii (228) described
t,he use of their lamp in a Raman spectrograph designed primarily for the observation of rotational Raman spect,ra of gases. Other Hg-arc lamps have been constructed by Bridoux and Delhaye (34) and Geppert and Scholz (78). There has been considerable interest. in recent years in the development of sources of Raman exciting light of wavelengths other than the 4358 A. line of mercury. Sources of light in the yellow and red spectral regions are of particular value for the Raman spectroscopy of colored and photosensitive compounds. Stammreich and coworkers (207-210) are among the leaders in this aspect of Raman spectroscopy. They have used sources containing helium or rubidium to generate exciting light in the red and near-infrared regions, respectively, to obtain Raman spectra of the halogens, interhalogen compounds, and FCr03- ion. h high intensity He lamp has also been developed by Kir'yanova et al. (112). Many other metallic elements besides Hg would be useful electrode materials for Raman light sources save for the fact that such lamps can be used only once because the glass electrodes are usually fractured when the molten metals solidify on cooling. Ablekov et al. ( 2 ) have solved this problem for zinc and cadmium by the use of amalgams. They used 20y0 Zn or Cd with mercury in lamps with Toronto-type elect,rodes to achieve high intensity sources for very narrow Zn .and Cd emission lines. Considerably greater flexibility in the choice of the metallic element used for the light source has been attained by means of electrode-less annular lamps energiied by high-frequency electrical power. Ham and Walsh (92) have obtained pot'assium and rubidium sources powered at microwave frequencies. They observed the Raman line for liquid bromine excited with both lamps. With the R b lamp, a resonance filter of R b vapor was used to reduce the intensity of the Rayleigh scattered exciting lines. One limitation of these lamps is the high cost of the powerful microwave power supplies necessary to produce high light intensities. Powell and coworkers (276, 177) have developed similar annular lamps powered a t radio-frequencies (about four megacycles). They have also . developed a relatively inexpensive R F power supply generating up to 3 kw. of power. They det'ermined t'hat' the RF-powered, electrode-less Hg source for 4358 A. exciting light was about 0.7 as efficient as a Hg-arc lamp with Toronto-type electrodes operated a t the same power level. Light snurces a t the following wide range of wavelengths have been attained with various elements: Hg, 2537, 4047, 4358, and 5461 .I.; R h , 4201, 4215, 7800, and 7947 A ; C s , 45553 VOL. 36, NO. 5 , APRIL 1964
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4593, 8521, and 8934 A . ; Ti, 5350 A.; He, 5875 A . ; Cd, 6438 A . ; and K , 7664 and 7699 A. Many of the wavelengths correspond to resonance lines of the elements t.hereby offering the advantage of eliminating Rayleigh scatt'ered exciting light by the use of resonance filters. To absorb the Rayleigh scattered light from the 4358 A. H g line in a manner analogous to that of resonance filters, the filter cell must cont'ain excited mercury atoms in the vapor phase. Ostroy and McGinnis (160) have observed some absorption at 4358 -4. for mercury excited both by 2537 A. light and by electron bombardment. Papers concerned with a variety of other aspects of instrumentation for Raman spectroscopy have appeared. A high resolution grating spectrograph for the observation of Raman spectra excited at long wavelengths has been described by Stammreich and Forneris (208). I n the region from 6000 A. to 7000 A, a resolution of 0.3 crn.-l can be realized. Block and Mijlhoff (21) have considered the factors affecting the efficient collection of Raman radiation from cylindrical Raman t'ubes. A simple means for determining the spectral d i s t r i b h o n of the radiation of a tungsten lamp from its color temperature has been proposed by Preston (178). This method may be useful in the determination of the spectral sensitivity of photoelectric Raman spectrophotometers by means of a calibrated tungsten incandescent lamp. Two gioups of Russian investigators (137, 159) have shown that photomultiplier tubes of all types exhibit significant variations in the temperature dependence of response and spectral sensitivity. Schroetter et al. (191) and Migeon and Delhaye (143) have reported the design and performance of electronic and electromechanical integrators, respectively, for the measurement of the integrated intensity of Raman lines. S o t only are such devices time-saving but they are also said to give higher precision. Delhaye (64, 56) has described various instrument,al features for the oscillographic recording of Raman spectra at scanning speeds as high a5 1000 em.-' per second. hbramson et al. ( 2 ) have designed a photoelectric recording system with which the intensity of the Raman spectrum is measured with respect to the total intensity of the scattered radiation. The wavelength (or rvavenumber) axis of the recorded spectrum is determined from the channelled spectrum of a Fabry-Perot etalon observed simultaneously wit'h the same monochromator as is the Raman spectrum. The maximum errors are reported to be 0.5% for the intensity and 0.5 cm.-l for the spect,ral position. .I novel method for the measurement of depolarization ratios has been proposed by Fuhrer and Guenthard (76). With a rapidly ro302 R
ANALYTICAL CHEMISTRY
tating polarizer before the slit of the spectrophotometer, an ax. signal related to the depolarization ratio is obtained for Raman radiation excited by irradiation from a single direction. The authors have made a detailed analysis ?f the precision of such measurements. This technique, unfortunately, will not work with a cylindrically symmetric source of exciting light because the Raman radiation is always unpolarized (223). Bondarev (29) has proposed an approximate numerical method for the determination of the true contour of a Raman band from the observed contour which is a convolution of the true contour with the slit function. In his treatment the integral equation for the observed contour is approximated by a series of linear algebraic equations based on the assumption that the contours are symmetric. A more general treatment of this problem has been formulated by Khidir and Decius (111) in a manner in which it is not necessary t o assume that any of the contours are symmetric. They also obtained a series of linear equations from which the true band contour can be obtained by an iterative procedure. The accurate measurement of the positions of rotational Raman lines in the absence of suitable standard reference lines near the 4358 A. exciting line has been considered by Tyulin and Tatevskii (226) who have measured Raman shifts with a precision of a few hundredths of a wavenumber by a method involving the observed separation of corresponding Stokes and anti-Stokes lines. h general discussion of the problems associated with the observation of Raman spectra of samples other than coloiless liquids has been presented by Tobin (220). Experimental techniques for the observation of Raman spectra of small liquid and solid samples have been described by Schrader (190). He used a water-cooled, Hg-arc lamp designed so that the exciting radiation is collected from the end of the column of arc plasma and is focused on the small samples with a lens system. Hemispherical mirrors surrounding the sample concentrate exciting light on the sample. Useful Raman spectra can be obtained for 20 to 40 mg. of powdered solid pressed into a tablet in the shape of the slit image and for a5 little as 50 pl. of liquid in a capillary tube. An experimental technique for the observation of Raman spectra of qolid samples has also been reported by Behringer and Brandmueller (16). Powdered samples are compressed into tablets of a thickness dependent upon the opacity of the compressed material. With a two-tablet excitation unit, Raman spectra can be obtained from radiation either transmitted by the sample or reflected from the surface. With ap-
propriate adjustment of sample thickness, spectra of colored solids can also be obtained. Jones and Tunnicliff (224) have succeeded in designing and constructing a multi-pass Raman cell for liquids with which a fivefold increase in observed intensity can be realized with the Cary Model 81 Raman spectrophotometer. The design was achieved by means of ray tracing with a computer and illustrates the value of this approach as compared to trial-and-error experimental methods. Two cells for the observation of the Raman spectra of gases have been described by M a y and Stryland (138). With their lowtemperature cell, Raman spectra of gases refrigerated with liquid nitrogen and at pressures up to 100 atm. can be observed. The second cell, previously reported [see (102)], can be used for gases a t room temperature and a t pressures as high as 1000 atm. I n an investigation of the technique. for obtaining Raman spectra of solid materials with the Cary Model 81, Ferraro et al. (70) have studied the effects of sample preparation, particle size, sample thickness, and position upon the e 6 ciency of observation of Raman spectra. By techniques used for powdered samples, Karagounis and Issa (106) have obtained Raman spectra of molecules adsorbed on solids with high surface areas-porous glass, Aerosil, and powdered KBr. When about a monolayer of material was adsorbed, they found that Raman lines frequently appear a t frequencies for vibrational modes which are not normally Raman active because of symmetry considerations. They further noted that the fluorescence of adsorbed terphenyl, for example, was quenched and suggested that this technique would be useful for the observation of the Raman spectra of other fluorescing molecules. Similar observations of the Raman spectral features of adsorbed molecules have been reported by Pershina and Raskin (167). Both groups of workers found that the Raman spectra of molecules adsorbed at appreciably above a monolayer were essentially the same as for bulk samples. Since the invention of the laser, i t has been considered by many that the high degrees of monochromaticity, intensity, and collimation of the laser light beam make it a promising source of exciting light for Raman spectroscopy. One aspect of this subject is the stimulated Raman effect discussed earlier in this review. Because only one or two molecular vibrational modes appear to participate in the stimulated Raman effect foi a molecule, all of the Raman spectral information cannot be obtained from this phenomenon. I t is, therefore, encouraging that some efforts to excite the ordinary Raman effect with lasers have been successful. Porto
and Wood (175) observed spectra of CClr and benzene bu , concluded that, in its present state of development, the laser was not appreciably superior to other sources of long-wavelength exciting light. A much nore encouraging applicrttion of the lascr as a source for Raman spectioscopy was demonstrated by Danil’tseva, et al. (50). They obtained Raman spectra of powdered crystalline samples of derivatives of azobenzene which werct strongly colored yellow and red materials. It is apparent that, although idhelaser may not replace other bources for Raman excitation, it will perhzps be of great value for the excitation of Raman spectra for small samples of colored or fluorescing materials. Another interesting application for which a laser is a particularly suitable light source has been suggested by Komarov and From a theoretical Fisher (114). study of Rayleigh scattering, they concluded that the structure of liquids and solutions could be dwived from the angular dependence of the intensity and the shape of the Rayleigh line. The intense, collimate1 beam of highly monochromatic light from a laser is the appropriate source foi such measurements. Only one paper concerned with the techniques for quantitative analysis by Raman spectrometry has appeared in the last two years. Tunnicliff and Jones (125) have considered the many factors which can affect the observed Raman intensity and have devised an analytical scheme whereby the factors are eliminated or corrected by calibration. dlthough the scheme has been developed for use with the Cary Model 81 instrument, the coilsiderations are quite general and the techniques should be applicable to other instruments with little or no modification. Refractive index variations arc reduced by the use of a solvent. A strongly interacting solvent is prefer:tble to an inert one because molecular interactions of the sample will, then, tend to be swamped out by the irlteractions with the solvent. Effects of the latter interactions upon intensities are accounted for by calibration. Variations in geometry of excitation attd observation were provided for by the use of an internal standard, which may also be the solvent, with the exciting light polarized perpendicular to the direction of observation. Crimpensation for internal field effects on intensities because of small variations in the refractive index would be realized by the use of the internal standard. A base line technique for intensity measurements and the method of calibration served to eliminate many other f:tctors affecting observed intensities. The performance of the method was, demonstrated by the analysis of benzene-toluene
mixtures in a solvent of acetonitrile. With a single set of calibration data obtained only for the individual components, successful analyses were obtained with two considerably different cells-the standard 5-ml. cell and the multipass cell (S2.4) described above. The fivefold gain in intensity realized with the multipass cell compensates for the loss in intensity because of the fivefold dilution of the sample. With both cells, however, errors of only 1% were found. Furthermore, the closure to about 1% demonstrated the reliability of the method. As pointed out, the method is not suitable for colored samples without corrections for the effects of color on observed intensity, and i t cannot cope with excessively fluorescent samples. OF RAMAN SPECTROMETRY Raman spectrometry can serve the chemist in the solution of problems involving the determination of the composition of mixtures and the structure of molecules and mixtures. It can also give information on the nature of molecular interactions and of chemical bonding. An attempt has been made in this review to cite papers illustrating such applications. A great number of papers concerned primarily with the assignment of bands and the vibrational analysis of infrared and Raman spectra have not been cited even though they represent a valuable application of Raman spectrometry. Such papers are beyond the scope of this review and would be appropriately considered in a review of vibrational spectroscopy. Organic Compounds. R a m a n and infrared spectral studies as functions of pressure and temperature for gases and as a function of temperature for liquids and solids can give information concerning the conformation of molecules. Such investigations have been reported for butadiene (10, 135) and othei dienes (203). I t has been noted that the depolarization ratio for the C=C band is much lower for cis-isomers than for trans-isomers of dienes (27). Relative intensities of Raman lines for biphenyl have been used to determine the angle between the rings (26). The temperature dependence of the intensities of Raman lines for some halogenated butanes (139) has shown the existence of three rotational isomers for each compound. Estimates of the relative amounts of the isomers were obtained from the relative intenqities. Only two isomers have been reported for 1,4-dichlorobutane (51). h’oack and Jones (156) have determined that the positions and relative intensities of the infrared and Raman bands for the C=C and C=O groups of a,P-unsaturated ketones can be used to identify the cisand trans-conformations. They have examined the influence of alkyl subAPPLICATIONS
stituents in various positions upon the stability of the two isomers. The conformation of cis- and trans-isomers of 1cyano-2-methycyclohexanol and 1cyano-2-chlorocyclohexanol have been determined for their Raman spectra (8). The rotational isomerism of a variety of vinyl ethers have been investigated and the influence of substituents upon the Raman spectra examined (74, 144, IYi, 1’74, 195). Similar attention has been given to vinyl sulfides (173, 196). Nukada (157) has concluded that only one rotational isomer exists for dimethoxymethane, ethoxymethoxymethane, and diethoxymethane but two isomers exist (158) for the corresponding 1,l-dialkoxyethanes. The choice of one of several possible structures .for a molecule can of ten be made from the consideration of the number of observed infrared and Raman bands and the polarization of the latter. I n this manner, I3egun and Fletcher (12) have concluded that the planes of the C 0 2 groups are perpendiculat rather than coplaner for the oxalate ion in aqueous solutions. Similarly, Lhe structure of the trifluoroacetate ion is such that the axis of the CF3 group does not lie in the plane of the COZgroup (186). Among other examples are that the skeletal structure of E(OCH3)d is planar but B[?j’(CH&]r is not (11), that the AIHl group of A1H3.2[N(CH3)2] is planar (72, 94), that solid tetrameric tert-butyllithium has a structure in which the Li atoms and the a-carbon atoms are a t the vertices of concentric inteipenetrating tetrahedia (247), and that the tricyanomethanide ion, C(CN),-, is planar (131, 147). In an investigation of the vibrational spectra of numerous substituted pyridines and the corresponding pyridinium compounds, Spinner (205, 206) has found little effect produced by the formation of the cations except for 2- and 4-aminopyridine. The formation of amidinium structures for the cations of these two molecules has been postulated to explain the spectral changes. Vibrational spectra of cyclic trimers and tetrameis of ditnethyl and diphenylphosphinoborines have shown that the P-B ring bonds are single bonds rather than double bonds (4.9). From the intensity variations for the Raman spectra of derivatives of N-benzoylaniline, Theiy et al. (218) concluded that substitution in either ring tends to reduce conjugation between the rings. Jost and Harrand (105) observed that the effect of conjugation upon Raman intensity is much less for two C=O bonds than it is for a C=O and a C=C bond. A correlation between the Hamett Ufactor and the poqition of the Raman band of the C S S group in p-substituted phenyl isothiocyanates has been noted (168). Briner et al. (35-37) have found that the characteri4c spectral VOL. 36, NO. 5, APRIL 1964
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feature for the ozonides of a number of olefins is an intense infrared band near 1160 cni.-l but no corresponding Raman line at the same frequency. From a study of the Raman spect'ra of allylalkyl silanes, germanes, and stannanes, a number of variations of spectial features have been observed to follow the order-C, Si, Ge, Sn (66, 128). Spectral correlations have been reported for the number and positions of substituents in substituted furans (202). Raman spectral studies of mixtures of pyridine with a variety of compounds have led to the conclusions that pyridine forms hydrogen-bonds with alcohols and with CHC13 but arts as the electron-donor in forming a chargetransfer complex with SO2 (31). The behavior of the N-H stretching frequencies of primary amines dissolved in pyridine (280) and of pure liquid amines (250) has been explained on the basis t,hat only one S-H of the amino group is hydrogen-bonded. The Raman spect,ra of a number of derivatives of nitroarninobutadiene have been found to give evidence for an intramolecular, six-membered hydrogen-bonded ring involving the amino and nitro groups (24). The comparison of the Raman spectra of (CH3)3SnOHand (CH3)&C1 has shown that, the hydroxide dissociates slightly in solution (122). Evidence for R3Sn+ ions has also been reportfed for ICH3)3SnOH, [(CHI)&OH]r.(CH3)3Sn13r,and [(CH3)&n1 3 0 I h in the solid state ( H I ) and for (CeH5)3SnF (120). .lpparent ionization cons t a n h for thioglycolic acid and cysteine have been determined in concentrated solutions from the variation in the intensity of the Raman band for t,he S H group as a function of pH of the solut,ions (68). From the vibrational spectra of the 13F3 compIexes with dimethyl and diet'hyl ethers and tetraisohydrofuran for both 13IO and t,opes, isotopic partit'ion functions have been determined (13, 1 4 ) . From these functions w r e derived t'he corresponding equilibrium constants for isotopic exchange. The rwrilting values were in good agreement, with known experirnmtal values. Inorganic Compounds. Just as for orgmic moleculcs. the comparison of both the infrared and Raman sprctra in t h r light of sclrction rules and symmctry consider:it,ions can somctimw l., Short, E. L., Waters, D. N., J . Iizorg. IVucl. Chem. 25, 975 (1963). (153) %\loser,H., Weber, U., Proc. Intern. Meeting Mol. Spectry., 4th, Bologna, 1969 3, 1116 (,Pub. 1962). (154) Naberukhln, Yu. I., Opt. i Spektroskopiya 13, 498 (1962). 55) Nixon, J., Plane, R . A., J . Am. Chem. SOC.84, 4445 ( 1 962). 56) Koack, K., Joneci, R. N., Can. J . Chem. 39, 2201, 2225 (1961). 57) Nukada, K., Bull. Chem. SOC. J a p a n 34, 1615, 1624 (1961). 58) Ihid., 35, 3 (1962). 59) Osherovich, A. L., Glukhovski, B. M., Shpakov, P:. S., Pribory i Tekhn. Eksperim. 7, 149 (1962). (160) Ostroy, E. E., McGinnis, E. A., Proc. Penn. Acad. Sci. 35, 172 (1961). (161) Ovander, L. N.,Fiz. Tverd. Tela 3, 2394 (1961); 4, 1466 (1962); 5, 21 (1963). (162) Ihid., 4, 1471 (1962). (163) Ovander, L. N., Opt. i Spektroskopiya 12, 718 (1962:l. (164) Paet,zold, R., Ainoulong, H., Z . Anorg. Allgem. Chem. 317, 288 (1962). (165) Paetzold, R., Rcsensch, E., Ibid., 315, 64 (1962). (166) Pentin, Yu. A., Tatevskii, T’. M., I z v . Akad. X a u k SSiJR, Ser. Fiz. 26, 1241 (1962). (167) Pershina, E. V., Raskin, Sh. Sh., Dokl. A k a d . IVauk ,SSSR 150, 1022 (1963). (168) Pisarcik, M., Sh. Prac Chem. Fak SVST 1962, 49, 53. (169) Pivovarov, T’. M , Opt. i Speklroskopiya 9, 139 (1960). (170) Plsczek, G., “Haiidbuch I ologie,” E. Mark, ed., p. 209, Akadeniische Ysrlagsgesellschaft; Leipzig, 1934 [Trancilation, U.S. A t . Energy Comm. UCRL-256(L), (1962)l. (171) Popov, E. M., Andreev, N. S., Kagan, G . I., Opt. i flpektroskopiya 12, 37 (1962). (172) Popov, E. M., Kabachnik, M. I., Mayants, L. S.,Usp. K h i m . 30, 846 (1961). (173) Popov, E. M., Kagan, G. I., Opt. i Spektroskopiya 11, i30 (1961). (174) Ibid., 12, 194 (1962). (175) Porto, S. P. S., Wood, D. L., A p p l . Optics, Suppl. 1, 139 (1962); J . Opt. SOC.A m . 52, 251 (196:l). (176) Powell, F. X., Fletcher, O., LipDinrott, E. R., Reo. i 3 C i . Instr. 34, 36 (1963). (177) Powell, F. X., Lippincott, E . R., Steele, D., Spectrochim. Acta 17, 880 ( 1 9 6-1,~ . \ - -
(178) Preston, J., Brit. J . A p p l . Phys. 14, 43 (1963). (179) Prokof’eva, 3.I., Sverdlov, L. M., Opt. i Spektroskopiya 13, 324 (1962). (180) Puranik, P. S., Rarniah, K. T’., Proc. Indian Acad. Sci. 54A, 146 (1961). (181) Rarnan, C. V., Ihid., p. 253. (182) Rasetti, F., Phyqg. Rev. 34, 548 ( 1929). (183) Kea, D. G., J . Opt. Soc. Am. 49, 90 (1959). (184) Rivoire, G., Dupeyrat, R., Compt. Rend. 256, 1947 (1963).
(185) Robinson, E. A., Can. J . Chem. 40, 1725 (1962). (186) Robinson, R. E., Taylor, R. C., Spectrochim. Acta 18, 1093 (1962). (187) Ryason, P. R., J . Mol. Spectry. 8. 164 11962). (188) Savin, F., Opt. i Spektroskopiya 15,20 (1963). (189) Seiden, J., Compt. Rend. 255, 1721 (1962). (190) Schrader, B., Z. Anal. Chem. 197, 295 (1963). (191) Schroetter, H. W., Weber, U., Brandrnueller, J., Moser, H., Proc. Intern. Meeting Mol. Spectry., 4th, Bologna, 1969 3, 1320 (Pub. 1962). (192) Schug, K., Katain, L. I., J . Phys. Chem. 66, 907 (1962). (193) Shorygin, P. P., Pure A p p l . Chem. 4, 87 (1962). (194) Shorygin, P. P., Ivanova, T. M., Dokl. Akod. N a u k S S S R 150. 533 (1963); Opt. i Spektroskopiya 15, 176 (1963). (195) Shorygin, P. P., Shkurina, T. S . , Shostakovskii, M. F., Gracheva, E. P., Izv. Akad. N a u k SSSR, Old. K h i m . N a u k 1961. 1011. 96) Shorygin, P. P., Shostakovskii, M. F., Prilezhaeva, E. N . , Shkurina, T. N., Stolyarova, L. G., Genich, A. P., Ibid., p. 1571. .97) Shultz, J. W., Hornig, D. F., J . Phys. Chem. 65, 2131 (1961). 98) Sibaiya, L., Phys. Rev. 60, 471 118411 j - _ _ -
199) S:baiya, L., Venkatararniah, H. S., Ibid., 56, 381 (1939). 200) Sidorov, N . K., Stal’rnakhova, I. S., Bratanova, L. I., Opt. i Spektroskopiya 13. 783 (1962). (201) Simon, A., Pisrhtechan, A,, J. Prakt. Chem. 14, 196 (1961). (202) Sobolev, E. V., Aleksanvan, V. T.. Karakhanov, R. A., Bel’ikii, I. F.; Ovodova, \.. A., Z h . Strukt. K h i m . 4, 358 (1963). (203) Sobolev, E. V., Aleksanyan, 5’. T., Naryshkina, T. I., Ibid., p. 354. (204) Sokolovskaya, A. I., Opt. i Spektroskopiya 11, 478 (1961). (205) Spinner, E., J . Chem. SOC. 1962, 3119; 1963, 3860, 3870. (206) Spinner, E., White, J. C. B., Ibid., 1962,3115. (207) Starnmreich, H., Pure A p p l . Chem. 4, 97 (1962). (208) Starnmreirh, H., Forneris, R., Spectrochim. Acta 17. 775 (1961). (209) Starnmreich; H., Forneris, R., Yara, T., Ihid., p. 1173. (210) Stamrnreich, H., Sala, O., Bassi, D., Ibid., 19, 593 (1963). (211) Stoicheff, B. P., “Methods of Experimental Physics,” D. Williams, ed., T’ol. 3, p. 111, Academic Press, New York, 1962. (212) Strizhevskii, 5’. L., Fiz. Tverd. Tela 3,2929(1961); 5, 1511(1963). (213) Sverdlov, L. M., Opt. z Spektroskopiya 11, 774 (1961); 14, 731 (1963); 15. 133 (1963). (214) Sushchinskii, M. M., Zubov, T’. A,, Ibid., 13, 766 (1962). (215) Tanaka, M., Balasubrarnanyam, K., Bockris, J. O’M., Electrochim. Acta 8, 621 (1963). (216) Tare, S. A., Thompson, H. W., pectrochim. Acta 18, 1095 (1962).
(217) Taylor, R. A,, Woodward, L. A., Proc. Roy. SOC.(London) 2644, 558 (1961). (218) Thery, B., Harrand, M., Grarnmaticakis, P., J . Phys. ( P a r i s ) 24, 297 (1963). (219) Thompson, H. W., Spectry. Rept. Conf. Organ. Hydrocarbon Res. Group Inst. Petrol., London 1962, 233. (220) Tobin, M . C., Develop. A p p l . Spectry. 1, 205 (1962). (221) Tsenter, M. Ya., Bobovich, Ya. S., Dokl. Akad. N a u k SSSR 146, 333
(1962). (222) Tsenter, M. Ya., Bobovich, Ya. S., Opt. i Spektroskopiya 12, 54 (1962). (223) Tunnicliff, D. D., Jones, A. C., J . Opt. SOC.Am. 51, 1430 (1961). (224) Tunnicliff, D. D., Jones, A. C., Spectrochim. Acta 18, 569 (1962). (225) Ihid., p. 579. (226) Tyulin, V. I., Tatevskii, V. M., Opt. i Spektroskopiya 14, 436 (1963). (227) Ibid.. rn .i82 (228j Ibid., 15, 38 (1963). (229) Valiev, K. A., Ibid., 11, 465 (1961). (230) Ibid., 13, 505 (1962). (231) Valiev, K. A., Zhur. Eksperim. i Teor. Fiz. 40, 1832 (1961). (232) 1-aliev, K. A., Eskin, L. D.. Oot. i Spektroskopiya 12, 758 (1962) ’ ‘ (233) Van Kranendonk, J., Can. J . Phys. 41. 433 (1963). (2341 Venkateswarlu, K., Jhagatkhezan, S., Opt. i Spektroskopiya 13, 775 (1962). (235) Ibid., p. 778. (236) Venkateswarlu, K., Mariam, S., Acta Phys. Austriaca 15, 367 (1962). (237) Ibid.. 16. 125 (19631. (238) Venkateswarlu, K.; Mariam, S., Z . Physik 168, 195 (1962). (239) Venkateswarlu, K., Radhakrishnan, M., Spectrochim. Acta 18, 1433 (1962). (240) J’ollmar, P. M., J . Chem. Phys. 39, 2236 (1963). (241) Wait, S. C., Janz, G. J., Quart. Revs. 17, 225 (1963). (242) Walrafen, G. E., J . Chem. Phys. 36, 90 (1962). (243) Ibid., p. 1035. (244) Ibid.. 37. 1468 (1962). i245\ Ibid.: 39: 1479- i1963j. (246) Waliafen, G. E., Irish, D. E., Young, T. F., Ibid., 37, 662 (1962). (247) Weiner, M., Voael, G., West. R.. Inorg. Chem. 1, 654 (1962). (248) Weston, R. E., Jr., Spectrochim. Acta 18, 1257 (1962). (249) Wilson, M. K., “Determination of Organic Structures by Physical Methods,” F. C. Nachod, W. D. Phillips, eds., \-oI. 2, p. 181, Academic Press, New York, 1961. (250) Wolff, H., Staschewski, D., 2. Elektrochem. 66, 140 (1962). (251) Woodbury, E. J., Ng, W. K., Proc. I R E 50, 2357 (1962). (252) Woodward, L. A., Taylor, M. J., J . Chem. SOC.1962, 407. (253) Woodward, L. A., Ware, M. J., Spectrochim. Acta 19, 775 (1963). (254) Yoshino, T., Bernstein, H. J., J . Mol. Spectry. 2 , 213 (1958). (255) Yu, P. S.,Nikitin, 1’.N., 1-ol’kenshtein, M. V., Zh. Fiz. K h i m . 36, 681 (19621. (256) Zubov, 1.. A,, Opt. i Spektroskopiya 13, 861 (1962). (257) Ibid., 14, 578 (1963). 7
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