Rigid bender analysis of van der Waals complexes: The intermolecular bending potential of a hydrogen bond.

High resolution ir data on weakly bound OCOHF complexes formed in a slit supersonic expansion reveal a progression of extremely low frequency vibrational levels associated with the bending of the OCO–HF hydrogen bond. In a previous paper [J. Chem. Phys. 93, 7716 (1990)], we presented a spectroscopic analysis of the fundamental, combination and hot bands observed, corresponding to transitions between vlbend=00, 11, 20, 22, and 31, where vlbend denotes quanta of OCOHF skeletal bend excitation with l units of vibrational angular momentum. In this paper, we analyze the rotationally resolved data in terms of the rigid bender formalism of Hougen, Bunker and Johns to determine an explicit angular potential, V(θ), for the OCOHF complex in both the HF ground (vHF=0) and vibrationally excited (vHF=1) state.The OCOHF ground state (vHF=0) potential is dominated by quartic and sextic angular terms, and thus is surprisingly shallow with respect to the bending angle. This quasilinear vibrational behavior is characterized by wide amplitude bending wave functions with zero point motion extending from -38° to +38°. In contrast, the OCOHF excited state (vHF=1) exhibits a significantly bent equilibrium geometry with a hydrogen bond bend angle of 31°±5°, corresponding to a cylindrically symmetric, noncolinear minimum in the potential. This shift in equilibrium geometry upon vHF excitation is quantitatively responsible for promoting Δvbend=0,2,... combination band vibrational modes, in analogy with Franck–Condon progressions in a bent←linear electronic transition.The predissociation lifetimes for vHF=1 excited OCOHF vary systematically with vlbend, and can be analyzed in terms of a geometry dependent predissociation rate which increases with bending of the hydrogen bond angle. These empirical bending potentials are in qualitative agreement with, but quantitatively much shallower than predicted by previous electrostatic and ab... [ABSTRACT FROM AUTHOR]