Torsional vibrational modes of (HF)3: IR–IR double resonance spectroscopy and electrical interaction theory

Abstract

A concerted theoretical and experimental effort has been carried out to characterize and assign in-plane and out-of-plane torsional vibrational modes of hydrogen fluoride trimer. These vibrations are large amplitude motions which sample sizable regions of the intermolecular potential energy surface. The cyclic (HF)3 cluster was modeled as a three-dimensional system of fully coupled hindered rotors. Vibrational energy levels, wave functions, and oscillator strengths moments were calculated for each of the two vibrational systems. Potential energy surfaces for the interaction of the coupled rotors were calculated via a multipole moment and multipole polarizability electrostatic analysis. Complete vibrational manifolds for the two systems are presented. An important feature is that two vibrational bands, an in-plane overtone E′(vA′=0,vE′=2) and an out-of-plane combination E′(vA″=1,vE″=1), were calculated to exist within the frequency range of the CO2 laser. Using a two-laser, double-resonance technique, two rovibrational transitions were observed, corresponding to these predicted bands. Theoretical determinations of oscillator strengths and the effects of deuterium isotopic substitution on the vibrational energy levels were experimentally verified to establish the assignment of the observed transitions to their specific vibrational modes. The theoretical analysis was continued to examine the anharmonicity, amplitude of vibrational motion, and oscillator strengths of the vibrational modes of the cluster in detail. The experimentally determined linewidths of the observed transitions were compared to the previously reported linewidth of the (HF)3 predissociative H–F stretching mode.