A Simulation Study of the Fundamental Vibrational Shifts of HCl Diluted in Ar, Kr, and Xe: Anharmonic Corrections Effects

We have calculated the vibrational solvent shifts of the fundamental bands of HCl diluted in Ar, Kr, and Xe solutions at different thermodynamic conditions by means of the molecular dynamics technique and a model for the isotropic part of the interaction depending on the vibration. The theoretical vibrational shifts, which were compared with the available experimental data, have been determined by considering both, the usual linear Buckingham terms and the nonlinear anharmonic corrections, and the latter omitted in a previous work for the HCl in Ar and Kr. We have found that the Buckingham contributions dominate the solvent shifts of the fundamental bands of HCl in Ar, Kr, and Xe, although the anharmonic shifts’ present significant greater values than those obtained previously for N2 diluted in liquid Ar and pure liquid N2, both at normal conditions. We have analyzed the solvent shifts influence of the linear and quadratic (in the vibrational coordinate) oscillator-bath interaction terms and also the Dunham intramolecular potential effects on the anharmonic contributions. 1. Introduction The study of the fundamental and overtones infrared and Raman vibrational solvent shifts gives relevant information about the molecular interaction depending of the intramolecular vibration [1–7]. In an earlier work, according to Buckingham, it was deduced that the diatomic solvent shift associated to the vibrational transition of a cubic anharmonic oscillator presents a linear dependence with the vibrational quantum number [1]. This result, which is valid in principle for the fundamental band or lower overtones of several diatomics, would break down for the fundamental bands of some molecules and high overtones of much ones. Recently, Alessi et al. [6, 7] has deduced the non-linear ( ) correction (anharmonic correction) to the linear Buckingham solvent shift for a diatomic molecule perturbed by an atomic or molecular solvent. Alessi et al. [6, 7] analyzed the solvent shift of N2 diluted in liquid Ar and pure N2 liquid, both at normal conditions, founding that the non-linear solvent correction, also called anharmonic shift [6, 7], represents only the 1% of the fundamental shift of the isotropic Raman band. This result is due in part to the small anharmonicity parameter of N2 ( ) [6, 7]; however, because the HCl anharmonicity constant is clearly much larger than the N2 one, we expect a major contribution of non-linear solvent shift for the fundamental band of HCl diluted in Ar, Kr, and Xe. We have made a previous study [8] of the vibrational solvent shifts of HCl diluted