Analytical three-body interaction potentials and hydrogen bond dynamics of hydrogen fluoride aggregates, (HF)(n), n >= 3

Abstract

Oligomeric aggregates of hydrogen fluoride are important prototype molecules for a detailed understanding of the structure, energetics, spectroscopy and dynamics of hydrogen bonding. The pairwise additive description of these oligomers is known to be inadequate. We have sampled the three-body potential for HF at 3000 (HF)(3) configurations selected by various classical and quantum sampling techniques, including dynamic sampling based on Voronoi step representation. The counterpoise-corrected Moller-Plesset second-order three-body energies using a double zeta Gaussian basis set with polarization functions (DZP + MP2) at these configurations are fitted by analytical 12-dimensional potentials. Cooperative effects are found to be sizeable and predominantly stabilizing in hydrogen fluoride ring aggregates. Test calculations with larger basis sets and for larger HF aggregates show that in combination with available high quality pair potentials, the analytical three-body terms give an excellent description of the (HF)(3) surface in the hydrogen bonding region and a good approximation for clusters up to at least the hexamer. Multidimensional vibrational quantum Monte Carlo calculations indicate that degenerate HF stretch excitation in (HF)(3) (3712 cm(-1)) is in close coincidence with (HF)(3) --> 3HF dissociation channels at low HF angular momentum, whereas degenerate DF stretch excitation in (DF)(3) (2725 cm(-1)) falls slightly below any (DF)(3) --> (DF)(2) + DF dissociation channels. The (HF)(3) potential surface, its stationary points, possible interconversion tunneling pathways, zero point energies, adiabatic channels, unusual isotope effects, fully centrifugal rotational states and the harmonic infrared spectrum are discussed in detail and compared to ab initio calculations and experiment. The applicability of the (1 + 2 + 3)-body approach for larger oligomers (3 < n < 8) is investigated with special emphasis on structure, energetics, infrared and microwave spectra, and predissociation. Neglect of four- and higher-body contributions and hydrogen exchange symmetry is found to affect some properties significantly, but the preference for simple ring structures remains pronounced. (C) 2001 Elsevier Science B.V. All rights reserved.