A computational and experimental re-examination of the reaction of the benzyloxyl radical with DMSO

Abstract

A detailed density-functional theory study on the reaction of the benzyloxyl radical (BnO) with dimethyl sulfoxide (DMSO) was performed. The calculations reveal that BnO transfers a hydrogen atom to DMSO via a proton-coupled electron transfer reaction to form benzaldehyde, in agreement with the mechanism proposed in the literature for the reaction of ethoxyl radical with substituted pyridines. The radical formed from this reaction, DMSO-H, is predicted to be unstable owing to population of the SO antibonding π-orbital and decompose without barrier to form dimethyl sulfide and hydroxyl radical. The calculated exergonicity of the reaction is 16.6kcal/mol. Our previous laser-flash photolysis (LFP) studies reported a kinetic deuterium isotope effect (KDIE), kH(DMSO)/kD(DMSO-d6), of 0.91 for this reaction, which is consistent with the foregoing mechanism, and with the value calculated in this work of 0.77. New LFP data reported herein for the reaction of BnO-d2 with DMSO give an experimental KDIE of 3.06, compared with a calculated value of 2.8. Taken collectively, our computational and experimental re-examination of the reaction between BnO and DMSO leads to the conclusion that BnO transfers a hydrogen atom to, rather than abstracts a hydrogen atom from, DMSO.