Solvent-solute interactions and the Raman CH stretching spectrum of cyclohexane-d11: I. Solvent-dependence
We examine the gas-to-liquid frequency shifts and isotropic Raman linewidths for the axial and equatorial CH stretching vibrations of cyclohexane-d11 in 13 different solvents. The "perturbed hard-fluid model" of Ben-Amotz and Herschbach provides a consistent description of the observed shifts in terms of average repulsive and attractive solute-solvent interactions. The attractive part of the shift is dominated by dispersive interactions, with a small contribution from inductive forces. This analysis also reproduces the observed ratio of equatorial to axial attractive shift components if certain details of the cyclohexane-d11 geometry are accounted for when the repulsive part of the shift is calculated. The ratio of equatorial to axial peak linewidths is nearly the same for all solvents and is consistent with inhomogeneous broadening by attractive force fluctuations. A version of the Schweizer-Chandler theory of attractive force broadening, modified to incorporate interactions between the solute vibrational dipole and solvent dipoles, successfully describes the dependence of the observed linewidths on solvent dipole moment. However, the remaining portion of the linewidth does not correlate with solvent parameters characterizing dispersive force fluctuations in the manner predicted by the theory. © 1995 American Institute of Physics.
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