VIBRATIONAL FREQUENCY-SHIFTS IN NEHF AND ARHF COMPLEXES

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