OH vibrational activation and decay dynamics of CH{sub 4}-OH entrance channel complexes

Infrared spectroscopy has been utilized to examine the structure and vibrational decay dynamics of CH{sub 4}-OH complexes that have been stabilized in the entrance channel to the CH{sub 4}+OH hydrogen abstraction reaction. Rotationally resolved infrared spectra of the CH{sub 4}-OH complexes have been obtained in the OH fundamental and overtone regions using an IR-UV (infrared-ultraviolet) double-resonance technique. Pure OH stretching bands have been identified at 3563.45(5) and 6961.98(4) cm-1 (origins), along with combination bands involving the simultaneous excitation of OH stretching and intermolecular bending motions. The infrared spectra exhibit extensive homogeneous broadening arising from the rapid decay of vibrationally activated CH{sub 4}-OH complexes due to vibrational relaxation and/or reaction. Lifetimes of 38(5) and 25(3) ps for CH{sub 4}-OH prepared with one and two quanta of OH excitation, respectively, have been extracted from the infrared spectra. The nascent distribution of the OH products from vibrational predissociation has been evaluated by ultraviolet probe laser-induced fluorescence measurements. The dominant inelastic decay channel involves the transfer of one quantum of OH stretch to the pentad of CH{sub 4} vibrational states with energies near 3000 cm-1. The experimental findings are compared with full collision studies of vibrationally excited OH with CH{sub 4}. In addition, ab initio electronic structure calculations have been carried out to elucidate the minimum energy configuration of the CH{sub 4}-OH complex. The calculations predict a C{sub 3v} geometry with the hydrogen of OH pointing toward one of four equivalent faces of the CH{sub 4} tetrahedron, consistent with the analysis of the experimental infrared spectra. (c) 2000 American Institute of Physics.