A Theoretical Study of the Iminoxyl/Oxime Self-Exchange Reaction. A Five-Center, Cyclic Proton-Coupled Electron Transfer

In solution, the self-exchange reactions for oxygen-centered π-radicals, e.g., PhO• + PhOH ⇌ PhOH + PhO•, are known to occur with low activation enthalpies (E a ≈ 2 kcal/mol). For the PhO•/PhOH couple and, we conclude, for other O-centered π-radicals, exchange occurs by proton-coupled electron transfer (PCET) with the proton transferred between oxygen electron pairs while the electron migrates between oxygen orbitals orthogonal to the −O- - -H- - -O− transition state plane (Mayer et al. J. Am. Chem. Soc. 2002, 123, 11142). Iminoxyls, R2CNO•, are σ-radicals with substantial spin density on the nitrogen. The R2CNO•/R2CNOH self-exchange has a significant E a (Mendenhall et al. J. Am. Chem. Soc. 1973, 95, 627). For this exchange, DFT calculations have revealed a counterintuitive cisoid transition state in which the seven atoms, >CNO- - -H- - -ONC