OH vibrational activation and decay dynamics of CH4-OH entrance channel complexes

Infrared spectroscopy has been utilized to examine the structure and vibrat ional decay dynamics of CH4-OH complexes that have been stabilized in the e ntrance channel to the CH4+OH hydrogen abstraction reaction. Rotationally r esolved infrared spectra of the CH4-OH complexes have been obtained in the OH fundamental and overtone regions using an IR-UV (infrared-ultraviolet) d ouble-resonance technique. Pure OH stretching bands have been identified at 3563.45(5) and 6961.98(4) cm(-1) (origins), along with combination bands i nvolving the simultaneous excitation of OH stretching and intermolecular be nding motions. The infrared spectra exhibit extensive homogeneous broadenin g arising from the rapid decay of vibrationally activated CH4-OH complexes due to vibrational relaxation and/or reaction. Lifetimes of 38(5) and 25(3) ps for CH4-OH prepared with one and two quanta of OH excitation, respectiv ely, have been extracted from the infrared spectra. The nascent distributio n of the OH products from vibrational predissociation has been evaluated by ultraviolet probe laser-induced fluorescence measurements. The dominant in elastic decay channel involves the transfer of one quantum of OH stretch to the pentad of CH4 vibrational states with energies near 3000 cm(-1). The e xperimental findings are compared with full collision studies of vibrationa lly excited OH with CH4. In addition, ab initio electronic structure calcul ations have been carried out to elucidate the minimum energy configuration of the CH4-OH complex. The calculations predict a C-3v geometry with the hy drogen of OH pointing toward one of four equivalent faces of the CH4 tetrah edron, consistent with the analysis of the experimental infrared spectra. ( C) 2000 American Institute of Physics. [S0021-9606(00)00315-9].