Infrared Spectroscopy of the Entrance Channel Complex Formed Between the Hydroxyl Radical and Methane in Helium Nanodroplets.

ABSTRACT

The entrance channel complex in the exothermic OH + CH4 → H2O + CH3 reaction has been isolated in helium nanodroplets following the sequential pick-up of the hydroxyl radical and methane. The a-type OH stretching band was probed with infrared depletion spectroscopy, revealing a spectrum qualitatively similar to that previously reported in the gas phase, but with additional substructure that is due to the different internal rotation states of methane (jCH4 = 0, 1, or 2) in the complex. We fit the spectra by assuming the rotational constants of the complex are the same for all internal rotation states; however, subband origins are found to decrease with increasing jCH4. Measurements of deuterated complexes have also been made (OD-CH4, OH-CD4, and OD-CD4), the relative linewidths of which provide information about the flow of vibrational energy in the complexes; vibrational lifetime broadening is prominent for OH-CH4 and OD-CD4, for which the excited OX stretching state has a nearby CY4 stretching fundamental (X, Y = H or D).