Phosphinoyl radicals: Structure and reactivity. A laser flash photolysis and time-resolved ESR investigation

The photochemistry of a series of bis(acyl)phosphine oxides and the rate constants of the reactions of their phosphorus radicals with n-butylacrylate, thiophenol, bromotrichloromethane, oxygen, and methyl viologen have been investigated by laser flash photolysis. The results were compared to a mono(acyl)phosphine oxide ((2,4,6-trimethylbenzoyl)diphenylphosphine oxide). The variation in reactivity of the different phosphorus radicals was correlated with the degree of radical localization and s-character on the phosphorus atom, as reflected by the P-31 hyperfine coupling constant, which was measured by time-resolved ESR. High P-31 hyperfine splitting is observed for high degree of spin localization in a sigma-orbital. It was shown that typical radical reactions (addition to monomers and oxygen, and atom abstractions) correlate well with the P-31 hyperfine coupling. An inverse correlation was observed for electron-transfer reactions (reduction of methyl viologen), were a higher p-character of a localized orbital enhances the reaction.