Probing Three-Body Intermolecular Forces: Near-Infrared Spectroscopy of Ar2HF and Ar2DF van der Waals Modes
J T. Farrell, David Nesbitt
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 ŅHF=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 ŅDF=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.
Journal of Chemical Physics
clusters, infrared spectroscopy, supersonic expansions, three-body forces, van der Waals
and Nesbitt, D.
Probing Three-Body Intermolecular Forces: Near-Infrared Spectroscopy of Ar<sub>2</sub>HF and Ar<sub>2</sub>DF van der Waals Modes, Journal of Chemical Physics
(Accessed October 4, 2023)