Simulation study of solvent shifts of vibrational overtone spectra

Abstract

Buckingham's theory of the solvent shift of vibrational spectral frequencies predicts that the shift of the v = 0 → n overtone transition is n times the shift of the fundamental v = 0 → 1. We test this prediction by molecular dynamics simulations using existing intermolecular potential models for liquid N2 and dilute N2 in liquid Ar, at standard state conditions. We extend Buckingham's theory by including additional intramolecular potential and perturbation terms which lead to solvent-induced anharmonicity, i.e. O(n 2) terms in the solvent shift. The simulations show that Buckingham's prediction is not accurate for N2 at standard liquid state conditions. We find that at these conditions there is a significant positive O(n 2) contribution to the solvent shifts and that for n ∼ 20 the shifts change sign from red to blue. Simulation results and indirect evidence from shock wave experiments with liquid N2 show that Buckingham's prediction is more accurate for high-pressure high-temperature conditions, where the shifts are blue and only slightly nonlinear in n.