Determination of the intermolecular potential energy surface for (HCl)2 from vibration–rotation–tunneling spectra.

An accurate and detailed semiempirical intermolecular potential energy surface for (HCl)2 has been determined by a direct nonlinear least-squares fit to 33 microwave, far-infrared and near-infrared spectroscopic quantities using the analytical potential model of Bunker et al. [J. Mol. Spectrosc. 146, 200 (1991)] and a rigorous four-dimensional dynamical method (described in the accompanying paper). The global minimum (De=-692 cm-1) is located near the hydrogen-bonded L-shaped geometry (R=3.746 Å, θ1=9°, θ2=89.8°, and [lowercase_phi_synonym]=180°). The marked influence of anisotropic repulsive forces is evidenced in the radial dependence of the donor–acceptor interchange tunneling pathway. The minimum energy pathway in this low barrier (48 cm-1) process involves a contraction of 0.1 Å in the center of mass distance (R) at the C2h symmetry barrier position. The new surface is much more accurate than either the ab initio formulation of Bunker et al. or a previous semiempirical surface [J. Chem. Phys. 78, 6841 (1983)]. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]