VIBRATIONAL FREQUENCY-SHIFTS IN OC...HF AND N2...HF

The shift in the harmonic vibrational frequency of hydrogen fluoride o n forming the weakly bound complexes OC...HF and N2...HF has been obta ined 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 involvin g first and second derivatives of the interaction energy with respect to displacements of the HF internuclear distance from equilibrium in t he free molecule. These derivatives were obtained from ab initio calcu lations at the SCF and MP2 levels. The red shifts obtained for these c omplexes by these theoretical approaches are in agreement, but are lar ger than those observed spectroscopically. The computed red-shifts are reduced by the large-amplitude zero-point bending vibrations of HF in the complexes.