A theory of vibrational frequency shifts revisited: application to the linear FArH⋯N2, FArH⋯P2, FArH⋯CO and FArH⋯OC complexes

Abstract

The shift in the harmonic vibrational frequency of the H–Ar and Ar–F vibrations of HArF on forming the stable weakly bound linear complexes FArH⋯N2, FArH⋯P2, FArH⋯CO and FArH⋯OC was determined by ab initio computations at the MP2/6-311++G(2d,2p) level of theory. These frequency shifts were found to be in good agreement with predictions from a model based on perturbation theory and involving first and second derivatives of the interaction energy with respect to displacement of the H–Ar or Ar–F bond length from equilibrium in the isolated HArF molecule. In all four complexes red shifts of the Ar–F stretch are obtained, while blue shifts of the Ar–H stretch are obtained, except for FArH⋯P2 for which a large red shift is computed. These vibrational characteristics are rationalized by considering the balance between the interaction energy derivatives obtained from the perturbative model. The bond length changes on complexation are also well predicted by this simple model.