The weakly exothermic rearrangement of methoxy radical (CH3O⋅) to the hydroxymethyl radical (CH2OH⋅)

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Abstract
Although the CH3O⋅ and CH2OH⋅ radicals have long been considered critical intermediates in combustion and atmospheric processes, only very recently has the potential significance of the isomerization CH3O⋅→CH2OH⋅ been appreciated. This isomerization and related aspects of the CH3O⋅/CH2OH⋅ potential surface have been studied here using nonempirical molecular electronic structure theory with moderately large basis sets and with incorporation of electron correlation. The vibrational frequencies of CH3O⋅, CH2OH⋅ and seven other stationary points on the potential energy hypersurface have been predicted, both to compare with results from spectroscopy and to provide estimates of zero‐point vibrational corrections. In general, there is reasonable agreement with those vibrational frequencies of CH3O⋅ and CH2OH⋅ which are known from experiment. Our ab initio calculations predict that CH3O⋅ lies 5.0 kcal mol−1 higher in energy than CH2OH⋅ with a barrier to rearrangement to CH2OH⋅ of 36.0 kcal mol−1. Rearrangement of CH3O⋅ to CH2OH⋅ via a dissociation–recombination mechanism is energetically more costly (by 6.1 kcal mol−1). The Jahn–Teller distortion of CH3O⋅ from point group C3v is described in some detail. Barriers to inversion and rotation in CH2OH⋅ are predicted and compared with the results of ESR experiments. Finally, the dissociation of CH3O⋅ and CH2OH⋅ to yield formaldehyde plus H⋅ are each predicted to involve modest reverse activation energies.