Oxygenation of the phenylhalocarbenes. Are they spin-allowed or spin-forbidden reactions?

Oxygenation mechanisms of phenylhalocarbenes in vacuum are investigated through the use of density functional theory and CASSCF-PT2 approaches. Reactions with both substituted and unsubstituted phenylhalocarbenes are strongly exothermic. The reactions with nitrosubstituted halocarbenes are predicted to be spin allowed due to the practical degeneracy of the singlet and triplet carbene states. Conversely, for the unsubstituted compounds, the equilibrium between the triplet carbene and 3 O 2 requires a previous singlet–triplet state crossing. The large spin-orbit coupling at this crossing suggests a rather efficient tunneling rate. This would help to explain why the rate constant in solution in these cases is only one order of magnitude smaller than those involving a spin-allowed process. Moreover, the action of the solvent as a third body appears to play a fundamental role in prompting relatively smooth rates. Figure The oxygenation of phenylhalocarbenes is predicted to be spin-allowed when the phenylhalocarbene is substituted with electron-withdrawing groups at the para position of the aromatic ring and spin-forbidden for the unsubstituted compounds, in agreement with the experimental findings.