Solvent and pressure-induced perturbations of the vibrational potential surface of acetonitrile

Abstract

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, CH3CN (ν1, CH stretch; ν2, CN stretch; and ν4, 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.

Raman spectroscopy and theoretical modeling of HCl vibrational frequency shifts in high pressure argon

“…Previous studies have shown that the PHF model often provides excellent fits and predictions of frequency shifts in high pressure liquids up to the freezing density. 17,19,20 The rough agreement between the PHF curve in Fig. 3 and the highest density gas to liquid shift measured in cold liquid argon (ϳ100 K͒ by van der Veken, 10 confirms the expected upward curvature of the frequency shift at high density, as repulsive packing forces become increasingly significant.…”

“…17,[19][20][21][22] The resulting expression for the limiting density derivative of the frequency shift, ‫)⌬ץ/⌬ץ(‬ ϭ0 ͑which assumes pairwise additive solute-solvent interactions͒ is merely the Boltzmann average of the frequency shift, ␦(r), of a solvent atom located at the position r in the coordinate frame of the solute, 21…”

Solvent–solute interactions and the Raman CH stretching spectrum of cyclohexane-d11. II. Density dependence in supercritical carbon dioxide

“…7 ͒ To describe these different contributions to ⌬, Ben-Amotz and Herschbach have developed the ''perturbed hard-fluid model.'' 4,5 In the perturbed hard-fluid model, the repulsive contribution to the shift ⌬ R is evaluated using a hard-sphere approximation, with the solvent molecules represented by hard spheres of diameter S and the solute molecules by pseudodiatomics composed of two hard-sphere ''atoms'' with diameters 1 and 2 and an effective bond length r eff . The repulsive shift ⌬ R is given by 4,5 …”

“…[1][2][3][4][5][6] The average solvent force along a vibrational coordinate in the solute will shift the vibrational frequency and corresponding band center in the vibrational spectrum, whereas fluctuations in the solvent forces lead to vibrational energy relaxation and dephasing, which determine the linewidth of the band. Thus, by examining the solute vibrational spectrum in different solvents and different solvent conditions, e.g., temperature and density, one can attempt to determine the nature of the underlying forces and their dynamics.…”

Vibrational relaxation study in carbonyl containing molecule: role of hydrodynamic and dispersion forces

“…The coupling potential V can be expanded in a Taylor series 18,19 as a function of normal coordinates Q…”