The intermolecular large amplitude motions of the HF...ClF complex were studied theoretically. The ab initio intermolecular potential energy surface, calculated at the MP2/DZ+(2d1f/2p1d)//HF/ DZP level of theory including BSSE correction, was used in the solution of the dynamical equations. The frequency for the van der Waals stretching and the frequency and the intensity of the HF...Cl bending were calculated. The frequency results were compared with reported theoretical and the experimental harmonic values. The harmonic stretching frequency calculated is 97.60 cm-1, which is in excellent agreement with the experimental harmonic value of 100 ± 2 cm-1. The frequency value calculated including anharmonicity is 89.38 cm-1. The calculated fundamental frequency of the HF...Cl bending vibration is 13.12 cm-1, while harmonic ab initio calculations predicted a value of 72 cm-1. This significant difference is due to tunneling motion through the HF...Cl angle, which has a barrier to internal rotation of 151 cm-1. The intensities of these transitions at two temperatures have also been calculated. At 10 K, only the fundamental transition is significant, whereas the hot band transition of 81.30 cm-1 between the third and fourth levels dominates the spectrum at 300 K.
tunneling; van der Waals complexes; spectroscopy; ab initio calculations
