Vibrational frequency shifts in NeHF and ArHF complexes

Abstract

The hydrogen fluoride vibrational frequency shift in the NeHF and ArHF complexes in their linear equilibrium configurations has been calculated using a theoretical model derived from quantum mechanical perturbation theory and involving first and second derivatives of the inert gas interaction energy with respect to displacements of the HF internuclear distance from equilibrium in the free molecule. These derivatives were obtained from ab initio calculations at the self-consistent field (SCF) and Møller-Plesset (MP)2 levels of theory. The calculated shifts are compared with harmonic frequency shifts computed from ab initio analytic second derivatives at the SCF and MP2 levels of theory. The frequency shifts for these complexes obtained by these two theoretical approaches are in reasonable agreement for ArHF but differ for NeHF whose shift is nearly zero. The basis set dependence of the shift was investigated at the SCF level for NeHF; large basis sets are required to reproduce the correct sign of the shift. The MP2 predictions for linear ArHF overestimate the experimental red shift; this discrepancy is attributed to the variation of the shift with vibrational bend angle.