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

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 C_3v 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.