R. M. Hexter

Intermolecular Coupling of Vibrations in Molecular Crystals: A Vibrational Exciton Approach

A vibrational exciton theory is developed which parallels in many ways the electronic exciton theory originally put forward by Davydov and amplified by Craig and Hobbins and by Fox and Schnepp. The kernel of the theory is the adoption, following Davydov again, of a transition‐dipole—transition‐dipole interaction as the potential which perturbs the isolated molecule energies and which thereby couples the motions of pairs of molecules in a crystal. As a result of the quantitative application of this theory, molecular dipole derivatives of several of the parallel modes of methyl chloride are obtained from the correlation field splittings of the corresponding fundamentals in the spectrum of solid methyl chloride. An isotope effect upon correlation field splittings is reported and is accounted for in terms of the same theory. A general method of testing the theory in terms of this isotope effect is suggested. The effect of intermolecular transition dipole coupling on intensities is derived and compared with th...

Infrared Spectroscopic Evidence for the Rotation of the Ammonia Molecule in Solid Argon and Nitrogen

High‐resolution infrared spectra (of the symmetric bending mode) of ammonia suspended at high dilution in solid argon and nitrogen have been obtained. More than a half dozen extremely sharp bands have been observed in the 950–1040 cm—1 region (in the spectrum of ammonia) in these materials under conditions where there should be little if any absorption due to polymeric species. The frequency separations of the bands, their temperature dependence, and their extreme sharpness appear to be compatible with a model in which the ammonia molecule executes quantized rotation in these solids.

Solid‐State Vibrational Spectra of the Methyl and Methyl‐d3 Halides

The vibrational fundamentals of solid CH3Cl, CH3Br, and CH3I and of their fully deuterated counterparts have been examined at 77°K under high resolution. CH3Cl35 has been synthesized and its ν3 spectrum compared to that of ordinary CH3Cl, permitting the assignment of the CH3Cl35 and CH3Cl37 contributions in the ν3 multiplet structure. The relative intensities of absorption in 1:1 CH3X:CD3X solid solutions have been measured to test the relationship between splitting, frequency, and absorption intensity predicted by the dipolar correlation model. The test proved to be insensitive. A study of the ν3 absorption of each component in CH3Cl:CD3Cl solid solutions throughout the concentration range has shown that multiplet splitting persists at concentrations as low as 1%.

Infrared Spectrum of Single Crystals of LiOH, LiOD, and LiOH·LiOD

High‐resolution infrared absorption spectra of single crystals of LiOD and of the solid solution LiOH·LiOD are reported. These are compared with each other and with the spectrum of monocrystalline LiOH. Comparisons with the spectra of Mg(OH)2 and Ca(OH)2 are also made. The differences in selection rules for the two types of hydroxides are discussed together with differences in spectra to be expected upon changing the mass of the cation and upon deuterium substitution. The discussion is in terms of the current theory of the spectra of such crystals. It is concluded that the complex spectrum of these minerals in the high‐frequency region (2000–5000 cm—1) is entirely due to OH— (or OD—) ion motion. The motion is quite localized; the motions of the several OH— ions of the crystal are poorly coupled. Difficulties with previous assignments due to unusual temperature dependencies of intensities are also discussed. The restricted rotational character of the localized motion of the OH— ions, previously proposed, m...

Infrared Studies of Crystal Benzene. I. The Resolution and Assignment of v20, and the Relative Magnitudes of Crystal Fields in Benzene

Under the higher resolution available with double pass prism and small grating infrared spectrometers, it has finally been possible to observe splitting of the degenerate fundamentals of benzene in the solid phase. Three components were found for v20 at 1032.6, 1034.7, and 1039.4 cm—1, as predicted by Zwerdling and Halford. These are assigned to the absorptions along the b, c, and a axes, respectively, by a mixed crystal study of benzene‐d6 in benzene combined with the polarized work of Zwerdling and Halford. The crystal splitting of v20 was also observed in the benzene‐d6 crystal, showing components at 806.5, 808.5, and 812.2 cm—1 assigned to absorptions along the b, c, and a axes, respectively. The magnitudes of the static and correlation fields are thus found to be similar, contrary to the ideas advanced by Zwerdling and Halford, and long accepted intuitively. The effect of temperature on the appearance of the spectrum was studied. An explanation for the changes observed is advanced on the basis of the...