Probing Three-Body Intermolecular Forces: Near-Infrared Spectroscopy of Ar2HF and Ar2DF van der Waals Modes

Published

December 1, 1996

Author(s)

J T. Farrell, David Nesbitt

Abstract

Four intermolecular cibrational states of the weakly bound complexes Ar₂HF and Ar₂DF have been studied via high resolution infrared spectroscopy. The vibrations are accessed as combination bands built on the v=1 HF or DF intramolecular stretch. These van der Waals vibrational states correlate adiabatically with j=1 motion of a hindered HF/DF rotor, corresponding to liberal motion either in, or out of, the molecular plane. The vibrational origins of the Ar₂HF in-plane and out-of-plane bends are 4008.9669(24) and 4035.17483(86) cm^-1, respectively, which are 62.375 and 88.584 cm^-1 above the origin of the intermolecular ground state in the Ņ_HF=1 manifold. For Ar₂DF, the in-plane and out-of-plane origins are 2939.83569(4) and 2967.10025(5) cm^-1, respectively, which correspond to intermolecular bending frequencies in the Ņ_DF=1 manifold of 44.851 and 72.116 cm^-1. Two-dimentional angular calculations are presented which solve fro the hindered rotor HF/DF eigenfunctions and eigenvalves on a pairwise additive potential constructed using a ridgid Ar₂ framework; the predicted bending frequencies reproduce the correst energy ordering of the vibrational frequencies by Ernesti and Hutson [Phs. Rev. A, 51 239, (1995)] on the full pairwise additive surface are found to be as much as 11% higher than the experimental values, and therefore indicate the presence of three-body repulsive contributions to the true angular potential. Inclusion of conventional three-body dispertion and induction terms into the pure pairwise additive potential shifts the predicted frequencies in the correct direction, but only account for roughly 1/3 of the observed discrepencies. The majority of the vibrational shifts can be ascribed to three-body exchange effects, i.e., the strongly anisotropic interaction of the HF/DF dipole with an exchange quadrupole formed by Ar-Ar. Inclusion of all three nonadditive terms (dispersion, induction, and exchange) inproves the agreement with experiment by more than an order of magnitude. The empirically determined nonadditive terms in the Ar₂HF/DF intermolecular potential are in reasonable agreement with ab initio predictions.

Citation

Journal of Chemical Physics

Volume

105

Issue

No. 21

Pub Type

Journals

Keywords

clusters, infrared spectroscopy, supersonic expansions, three-body forces, van der Waals

Citation

Farrell, J. and Nesbitt, D. (1996), Probing Three-Body Intermolecular Forces: Near-Infrared Spectroscopy of Ar₂HF and Ar₂DF van der Waals Modes, Journal of Chemical Physics (Accessed July 18, 2024)