Raman spectra of aqueous lithium salt solutions (LiC1, LiBr, and LiC104) were measured from saturation concentrations to more dilute solutions at room temperature. In the saturated LiCl and LiBr solutions a polarized band at 380 and 335 cm-', respectively, was detected. This band was assigned to a symmetric vibrational mode from an inner sphere complex LiCVBr(OH2),+ where n is probably three. In more dilute aqueous LiCl and LiBr solutions the high frequency component disappeared, and a new polarized band at 255 cm-' (FWHH 60 cm-') was observed and assigned to the Li-0 symmetric stretching motion of Li-(OH2)4+. This is the first definitive report of the symmetric stretching motion of an univalent cation. Ab initio geometry optimizations and frequency calculations at the 3-21G and 6-31G* level were performed on the tetraaquolithium species and support our assignment.

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Raman frequency and intensity measurements have been performed on liquid water as H,O, D,O and HZ1'0 to study the effects of isotope substitution. Intensity data were collected digitally, normalized to account for the temperature and frequency factors and presented in isotropic and anisotropic forms. The frequency and intensity changes are consistent with the predictions of simple reduced mass calculations. In particular, there was no evidence to support the reported breakdown of the Born-Oppenheimer approximation as has been reported for H, "0. The isotope invariant sum rule was checked for H2I60, H,"O and D,O by relative intensity studies for the OH stretching region against an internal sulfate reference peak. Identical values for the isotope invariant sum were obtained for H,160 and H2"0 but the value for DzO was about 30% larger. The difference appears to have its origin in the more highly structured nature of D,O due to smaller anharmonic effects. Accurate frequency shifts among H, l6O, H,"O and D,O are also presented. Surprisingly, the greatest frequency shift which accompanied "0 substitution was in the low-frequency hydrogen-bonded region where the band at 192 cm-' for H,O shifted by 15 cm-I to 177 cm-' for H,"O. This result confirms previous observations and establishes the origin of this band as a hydrogen-bonded symmetric stretching mode which involves primarily oxygen displacement. Further support for this assignment comes from the observation that the band at 192 cm-' is slightly polarized. The effects of intermolecular coupling contribute to the band structure of the internal modes. Frequency differences in the OH stretching region of H,O and Hzl'O suggest that only about 50% of the anisotropic intensity is due to the Raman activity of the v3 antisymmetric stretching mode while the remainder is due to the symmetric stretching modes of intermolecularly coupled water molecules. A point-by-point comparison of the OH stretching region for the isotropic scattered intensity of H,O and H,"0 revealed that the complete region from 2800 to 3800 cm-I was shifted equally by 7 cm-', a fact that suggests that the peak maximum at about 3250 cm-a is just part of the v1 symmetric stretching mode and is not primarily due to 2v,. The effect of intermolecular coupling in the v, region of liquid water was confirmed by the difference in the frequency for the isotropic and anisotropic components for each of the isotopic forms of water. For H,"0 the peak maximum in the isotropic spectrum was at 1619 cm-' whereas the peak maximum in the anisotropic spectrum was at 1637 cm-with the result that in the measured I,, spectrum the peak maximum was at 1629 cm-'.

Recently it has become apparent that for quantitative studies the Z(w) Raman spectrum measured directly from the spectrometer must be corrected for temperature and frequency factors to give a reduced Raman spectrum, R(w), which is diectly proportional to the intrinsic molar scattering factor. Although generally applicable, the R(w) spectrum finds its greatest application in the low-frequency region of hydrogen-bonded and simple molecular liquids and solids. In this paper the general applicability of the R(w) format is discussed with respect to the low-frequency spectra observed for simple molecular and more complicated hydrogen-bonded liquids and solids. Examples are chosen from Sl(s), H,Oo,, SO,(,, and SO,(,, and CH,CN(,,. Further arguments are presented to stress the need to use temperature-and frequencycorrected band shapes for Raman component bands at non-zero frequency when attempting to deconvolute the Rayleigh band at zero frequency into component sub-bands.

The infrared and Raman spectra of anhydrous crystalline Li2C0 3 and Na 2 C0 3 were measured at 300 and at SocK. Bands observed in the vibrational spectra of Li2C03 were assigned according to the C 2h factor group symmetry. The doublet structure observed for each of the internal modes in the spectra of ::-Ja 2 C0 3 was interpreted in terms of an ordered arrangement of C03~ ions over two nonequivalent orientations within the primitive cell. Multiple internal reflection and polarized specular reflectance techniques were used to determine transverse optical (TO) and longitudinal optical (LO) mode frequencies in Li 2 C0 3 and Na2C0 3 • It was shown from these measurements that the infrared transmission spectra of these compounds exhibit band maxima which are admixtures of LO and TO modes. INTRODUCTIONcation) has the effect of doubling the number of vibrational bands. I4 . 15 Splitting from nonequivalent sites will Procedures for the determination of zero wave vector often be most apparent in the regions of non degenerate selection rules for optically active vibrational modes in modes that otherwise could only be split by correlation crystals are well established. I-II Although the vibra-field effects. tional frequencies of a molecular ion in the solid phaseRecently, Dubbledam and De Wolff 16 reported the remain similar to the hypothetical "free" ion fre-(average) crystal structure of anhydrous Na2C0 3 and quencies, two effects, each governed by the crystal suggested a modulated structure that could be exstructure, act to produce modification from the "free" plained by assuming an ordered arrangement of the ion spectrum: (a) symmetry of the site occupied by the C0 3 = ions over two nonequivalent orientations within molecular ion (static field effect) and (b) dynamic the primitive cell. The effects of this unusual structure coupling between identical vibrational modes of two or should be apparent in the vibrational spectrum of more molecular ions which are located on equivalent Na 2 C0 3 . Inasmuch as Li 2 C0 3 belongs to the same sites in the primitive unit cell (correlation field effect). crystal class I7 as Na2C03 but has only olle carbonate A complete factor group analysis is required to deter-orientation, it may serve to give a convenient normal mine all theoretically active normal modes. However, vibrational spectrum with which 1\a2C0 3 may be the vibrational spectrum can often be interpreted from compared. The Raman spectra of Li2C03 and Na2C0 3 simple site symmetry approximations because the have not previously been reported; however, Buijs and interactions that give rise to correlation field splittings Schutte lS and Tarte l9 have reported results from thin usually cause smaller perturbations than those due to film infrared transmission measurements on crystalline the static field. 7 -11 Li 2 C0 3 and ~a2C03. Buijs and Schutte 1S analyzed their Complications to the interpretation of the vibrational data in terms of static field effects, but no evidence was spectrum of a molecular ion can occur if the ion occupi...