Spectroscopy and dynamics of complexes of the hydroxyl radical with methane and nitrogen

The CH4 + OH → CH3 + H2O reaction coordinate has been probed by stabilizing complexes of CH4 and OH reactants in a shallow well in the entrance channel to the hydrogen abstraction reaction. These entrance channel complexes have been activated via the OH fundamental and overtone stretching transitions (νOH, 2ν OH) as well as the symmetric and asymmetric stretching modes of CH 4 (ν1, ν3) using infrared and/or stimulated Raman excitation. Vibrational excitation provides enough energy to surmount the relatively low barrier to reaction or, alternatively, to break the weak intermolecular bond, thereby initiating reactive and/or inelastic scattering processes between the CH4 and OH partners within the CH4-OH. The complexes were first identified via electronic spectroscopy in the OH A 2Σ+-X 2Π (1,0) and (0,0) regions. A cut off in laser-induced fluorescence above the OH A 2Σ+ (v ′ = 1) + CH4 asymptote as well as an onset of fluorescence above the OH A 2Σ+ (v ′ = 0) + CH4 asymptote permitted determination of the ground state binding energy. An IR-UV double resonance technique was utilized to obtain rotationally resolved infrared spectra of CH4 OH in the OH fundamental and overtone stretching regions as well as infrared spectra of the CH4-OD isotopomer in the OD overtone region. Pure OH stretching bands along with combination bands involving the simultaneous excitation of OH stretch and intermolecular bending motion were identified. The infrared spectra of CH4-OH in the pure OH stretching regions are consistent with a C3v geometry where the OH radical lies along the intermolecular axis and points toward CH4. This orientation is in accord with electronic structure calculations performed by Lendvay. The pure OD overtone spectrum of CH4-OD is analogous to that of CH4-OH. Homogeneous lifetime broadening of spectral features indicates rapid decay via vibrational predissociation and/or chemical reaction. The infrared spectrum of CH4-OD exhibits less extensive homogeneous lifetime broadening. Vibrational spectra of CH4-OH in the CH4 stretching regions indicate that the CH4 partner undergoes nearly free internal rotation within the complex. Stimulated Raman excitation of OH in the ν 1 symmetric stretching region reveals two blended Q-branch features attributed to different nuclear spin states of CH4. Infrared excitation in the ν3 asymmetric stretching region results in an enormously broad spectrum that extends 40 cm−1. CH4-OH undergoes a multitude of allowed transitions in this region associated with the rovibrational transitions of CH4. The ν1 and ν 3 spectra exhibit extensive homogeneous broadening due to rapid decay via vibrational predissociation and/or chemical reaction. A rotationally resolved infrared spectrum of OD-N2 has been observed in the OD overtone region. The pure OD overtone band of OD-N 2 as well as combination bands involving simultaneous OD stretch and intermolecular geared bending excitation were characterized. Direct time-resolved measurements yield a 150 ± 16 ns lifetime for OD-N2 upon OD overtone excitation. A decrease in lifetime was observed upon excitation of the lower energy geared bending state. The OD (v = 1) fragments from vibrational predissociation of OD-N2 are produced with high rotational excitation in accord with a vibrational to rotational and/or translational energy transfer mechanism