| Abstract | 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 (Ea ≈ 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 Ea (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<, lie in a plane (R = H, Me) or, for steric reasons, two planes twisted at 45.2° (R = Me₃C). The planar transition state has the two N-O dipoles close to each other and pointing in the same direction and an O- - -H- - -O angle of 165.4°. A transoid transition state for R = H lies 3.4 kcal/mol higher in energy than the cisoid despite a more favorable arrangement of the dipoles and a near linear O- - -H- - -O. It is concluded that iminoxyl/oxime self-exchange reactions occur by a five-center, cyclic PCET mechanism with the proton being transferred between electron pairs on the oxygens and the electron migrating between in-plane orbitals on the two nitrogens (RN-N = 2.65 Å). The calculated Ea values (8.8-9.9 kcal/mol) are in satisfactory agreement with the limited experimental data. |
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