Vibrational frequency shifts induced by molecular compression of pyridine in solution

Pressure-induced vibrational frequency shifts are calculated for a diatomic oscillator immersed in a benign solvent, employing a simplified version of the Schweizer–Chandler model for solute–solvent interaction. The repulsive contribution is determined from the pair distribution function for hard-sphere cavities. Interpolative evaluation of the pair distribution function is facilitated by noting that to an excellent approximation the pertinent expansion coefficients are merely linear functions of the reduced density. The treatment is applied to the quasidiatomic ring breathing vibrations of neat liquid pyridine, benzene, and toluene and to solutions of pyridine in several solvents including H2O, D2O, CH3OH, CHCl3, dimethylformamide, and toluene. The predicted pressure dependence of the ring breathing frequency is in the range ∂ν/∂P≈0.3–0.8 cm−1/kbar for all these systems. The corresponding compression of the mean ring radius is in the range 0.9 to 2.0×10−4 Å/kbar. Especially for the associated solvents, the dominant contribution (>90%) to ∂ν/∂P comes from the effective hard-sphere repulsion. Accurate values of the effective diameters thus can be evaluated from the observed pressure derivatives.