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

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.