Infrared spectroscopy has been utilized to examine the structure and vibrational decay dynamics of CH4–OH complexes that have been stabilized in the entrance channel to the CH4+OH hydrogen abstraction reaction. Rotationally resolved infrared spectra of the CH4–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 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, 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 CH4 vibrational states with energies near 3000 cm−1. The experimental findings are compared with full collision studies of vibrationally excited OH with CH4. In addition, ab initio electronic structure calculations have been carried out to elucidate the minimum energy configuration of the CH4–OH complex. The calculations predict a C3v geometry with the hydrogen of OH pointing toward one of four equivalent faces of the CH4 tetrahedron, consistent with the analysis of the experimental infrared spectra.

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Articles you may be interested inMode-selective O-H stretching relaxation in a hydrogen bond studied by ultrafast vibrational spectroscopy J. Chem. Phys. 125, 074504 (2006); 10.1063/1.2219111 Comparing the dynamical effects of symmetric and antisymmetric stretch excitation of methane in the Cl + CH 4 reaction J. Chem. Phys. 120, 5096 (2004); 10.1063/1.1647533 OH vibrational activation and decay dynamics of CH 4 -OH entrance channel complexes Intermolecular interaction in the CH 3 + -He ionic complex revealed by ab initio calculations and infrared photodissociation spectroscopy J. Chem. Phys. 110, 9527 (1999); 10.1063/1.478917 Structures and the vibrational relaxations of size-selected benzonitrile-( H 2 O ) n=1-3 and -( CH 3 OH ) n=1-3 clusters studied by fluorescence detected Raman and infrared spectroscopiesThe vibrational spectroscopy and decay dynamics of CH 4 -OH reactant complexes have been studied in the CH 4 symmetric and antisymmetric stretching regions ( 1 and 3 ). The vibrational spectra have been obtained using both infrared and stimulated Raman excitation with ultraviolet probe laser-induced fluorescence detection. Stimulated Raman excitation of CH 4 -OH in the symmetric stretching region reveals two blended Q branch features at 2912.5 and 2911.8 cm Ϫ1 . An extremely weak infrared spectrum is also seen in the CH 4 symmetric stretching region, which is induced by the presence of the nearby OH partner. Infrared excitation in the asymmetric stretching region results in an intense, yet enormously broad spectrum centered at 3020 cm Ϫ1 that extends over 40 cm Ϫ1 . The appearance of the spectra in the 1 and 3 regions has been explained in terms of a model in which the CH 4 unit undergoes internal rotation within the CH 4 -OH complex. The 1 features are attributed to transitions involving two different nuclear spin states of CH 4 . In the 3 region, the CH 4 -OH complex can undergo a multitude of allowed transitions, each associated with a rovibrational transition of free methane, which give rise to the enormous span of the spectrum. The vibrational spectra also exhibit extensive homogeneous broadening ͑у1 cm Ϫ1 ͒ arising from the rapid decay of vibrationally activated CH 4 -OH complexes due to vibrational predissociation and possibly reaction. The OH fragments are produced with minimal rotational excitation, indicating that the dominant inelastic decay channel involves near-resonant vibrational energy transfer within the CH 4 unit from the initially prepared CH stretch to an overtone bend (2 4 ) state.

OH(AfX) emission bands have been observed in the molecular beam jets produced by Space Shuttle engine exhaust using the GLO imager spectrograph located in the payload bay. Spectra were collected at a resolution of 4 Å for both daytime and nighttime solar illumination conditions, all at an altitude of ∼390 km. A spectral analysis is presented that identifies and quantifies four separate OH(A) excitation processes. These include (i) solar-induced fluorescence of the OH(X) in the exhaust flow, (ii) solar-induced photodissociation of H 2 O in the exhaust at the strong Lyman-R solar emission line (1216 Å), (iii) solar-induced photodissociation of H 2 O in the far UV, at shorter wavelengths than Lyman-R, and (iv) luminescent collisions between atmospheric species and exhaust constituents, most probably the reaction O + H 2 O f OH(A) + OH(X). Process (i) produces a very rotationally cold and spectrally narrow component due to the rapid cooling of the OH(X) in the supersonic expansion of the exhaust flow. Processes (ii) and (iii) produce extremely excited OH(A), not well characterized by thermal vibrational or rotational distributions. The O + H 2 O chemiluminescent reaction has a substantial activation energy, 4.79 eV, and is only slightly above threshold for the ram geometry, where the engine exhaust is directed into the atmospheric wind. Evidence for process (iv) is observed in the night ram but not the night perpendicular exhaust atmospheric interaction, consistent with the threshold energy. Through the use of a nonequilibrium spectral emission model for OH, the integrated intensity, spectral distribution, and OH(A) internal state characterization for each of the above processes was deduced. Additional confirmation of the analysis is provided through the use of a model simulation of the space experiment to predict the total integrated intensities for processes (i) and (ii), for which the underlying spectroscopy, absorption cross sections, and solar excitation intensities are well established. Analysis of process (iii) has established, for the first time, a value for the far-UV conversion efficiency of absorbed photons to OH(A) photons of 0.26, which is twice the established value for Lyman-R. Under the assumption that O + H 2 O collisions are the source of process (iv), the analysis has established a chemiluminescence cross section at ram conditions of 1.7 × 10 -2 Å 2 . Evidence of OH(A) emission bands from predissociated vibrational levels suggests that the total reaction cross section for process (iv) may be significantly higher. While this cross section assumes a single-step reaction of O with H 2 O, the possibility of a two-step process of O with other plume species has yet to be explored.

A rotationally resolved infrared spectrum of the pure OD overtone band of the CH 4 -OD reactant complex has been observed at 5165.68(1) cm -1 , shifted -9.03(1) cm -1 from the infrared overtone transition of OD. The spectrum exhibits homogeneous line broadening, which is estimated to have a zero power line width of 0.10(1) cm -1 , corresponding to a lifetime for CH 4 -OD (2ν OD ) of 53( 5) ps. The OD (V ) 1) product distribution from vibrational predissociation of the complex favors high rotor levels, indicating that the correlated CH 4 fragment is produced with one quantum of bending excitation. The CH 4 -OD results are compared with previous studies in this laboratory of the CH 4 -OH complex, and reveal significant changes in lifetime and vibrational predissociation mechanism upon deuteration.