SOLVENT AND PRESSURE-INDUCED PERTURBATIONS OF THE VIBRATIONAL POTENTIAL SURFACE OF ACETONITRILE

Raman-scattering studies at both ambient pressures and in a high-pressure diamond-anvil cell are used to measure gas-to-liquid vibrational frequency shifts of three normal modes of acetonitrile, CH3 CN (nu-1, CH stretch; nu-2, CN stretch; and nu-4, CC stretch) dissolved in various solvents (methylenechloride, chloroform, carbontetrachloride, toluene, pyridine, acetone, and methanol). The results are compared with calculated repulsive and attractive solvation force-induced perturbations of polyatomic vibrational potential surfaces. Repulsive solvation forces are modeled using recently developed analytical "hard-fluid" expressions for heteronuclear two-cavity distribution functions in hard-sphere fluids, while attractive forces are assumed to contribute a van der Waals(linearly density-dependent) mean field. Results for the CN and CC stretches of acetonitrile compare favorably with theoretical predictions, while the CH stretch appears to experience a nonlinearly density-dependent attractive frequency shift at high densities. Empirical attractive frequency-shift parameters, derived from gas-to-liquid shifts at 1 atm, agree reasonably well with those predicted using a simple dispersive and dipolar solvation force expression. Attractive solvation forces are found to correlate well with solvent polarizability (and solute bond polarizability derivatives). Dipolar solvation forces only appear to contribute significantly to the CN stretch.