Infrared spectroscopy of the hydroxyl radical-carbon monoxide reactant complex and the isoelectronic hydroxy radical-nitrogen complex as a probe of the hydroxy radical + carbon monoxide goes to hydrocarboxyl radical reaction pathway

Infrared spectroscopy has been used to characterize a hydrogen-bonded complex between the OH and CO reactants that lies directly along the OH + CO ↔ HOCO → H + CO2 reaction coordinate. The spectroscopic experiments have accessed the pure OH overtone stretch as well as combination bands involving intermolecular bend and spin-orbit excitation of the linear OH-CO reactant complex. The geometry, OH spectral shift, binding energy, and vibrational frequencies derived from these measurements are in good accord with complementary ab initio calculations inspired by the experimental work. The experimentally observed geared bend and H-atom bend vibrations are of special interest because they probe portions of the reaction path leading to trans-HOCO formation. Infrared excitation of the OH-CO complex also supplies sufficient energy to surmount the low barrier to reaction or break the weak intermolecular bond, thereby initiating reactive or inelastic scattering dynamics. The rotational distribution of the inelastically scattered OH (v = 1) fragments is bimodal, revealing that vibrational predissociation proceeds by vibration-to-vibration and vibration-to-rotation and/or translation energy transfer mechanisms. The relative intensities of the OH overtone and combination bands are shown to differ in infrared action and fluorescence depletion measurements, providing preliminary evidence that the OH-CO reactant complex decays by both inelastic and reactive decay processes upon geared bend or H-atom bend excitation. Analogous spectroscopic and dynamical studies of the isoelectronic OH-N 2 complex in the OH overtone region provide a valuable point of comparison, since this nonreactive system decays exclusively by vibrational predissociation through a vibration-to-vibration mechanism. Infrared spectroscopic studies of the OH-N2 complex are used as the basis for a model of the coupling between the electronic angular momentum of the OH radical and the vibrational angular momentum of the intermolecular bending modes, which is directly applicable to the OH + CO system