Vibrational frequency shifts in OC⋯HF and N2⋯HF

Abstract

The shift in the harmonic vibrational frequency of hydrogen fluoride on forming the weakly bound complexes OC⋯HF and N₂⋯HF has been obtained by ab initio quantum chemical techniques. We used two approaches: (i) through analytic second derivatives at the SCF and MP2 levels of theory, (ii) through a model based on perturbation theory and involving first and second derivatives of the 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 SCF and MP2 levels. The red shifts obtained for these complexes by these theoretical approaches are in agreement, but are larger than those observed spectroscopically. The computed red-shifts are reduced by the large-amplitude zero-point bending vibrations of HF in the complexes.