Intermolecular forces and bond length changes in high-pressure fluids.: Vibrational spectroscopic measurement and generalized perturbed hard fluid analysis

Pressure-dependent vibrational frequency shifts are used to measure intermolecular-interaction-induced forces and bond length changes in solution. The results are employed to quantitate changes in solute-solvent coupling as a function of solvent density using a generalized perturbed hard fluid (G-PHF) analysis of solvent mean-force-induced perturbations of molecular potential energy surfaces. Repulsive solvation forces are calculated using the cavity distribution function of a reference hard-sphere fluid. Attractive forces are treated using a generalized van der Waals mean-field approximation, which allows for both long-range and short-range solute-solvent cohesive interactions. The results indicate marked differences in the density dependence of the attractive solvation force along different types of bonds. Hydrogen stretch vibrations (i.e., C-H and O-H stretches) display a strongly nonlinear density dependence, which is indicative of a short-range attractive solute-solvent coupling mechanism and, thus, is consistent with enhanced C-H and O-H hydrogen-bond formation at high pressure (even in nonpolar solvents). Systems studied include iz-butanol, acetone, chloroform-d, ethane, and methyl iodide in the pure liquid state or dissolved in carbon disulfide, n-propyl bromide, n-propyl iodide, toluene, 2,3-dimethylbutane, methanol, tetrahydrofuran, or methylene chloride.