Generation and decay of aryl sulfinyl and sulfenyl radicals: A transient absorption and computational study

Absorption spectra and extinction coefficients of phenylsulfinyl and phenylsulfenyl (thiyl) radicals are determined by nanosecond laser photolysis in various solvents. Direct observation and characterization of arylsulfinyl radicals from the photolysis of several aromatic sulfoxides provides the strongest evidence to date for a-cleavage as the predominant primary photochemical process for these compounds. The absorption spectrum of phenylsulfinyl, with lambda(max) = 300 and 450 nm and epsilon = 1.1 x 10(4) and 1.3 x 10(3) M-1 cm(-1), is practically independent of solvent. Quantum yields of free sulfinyl radicals range from 0.09 to 0.18 in various solvents. Recombination rate constants very near diffusion control indicate that there is a large spin-orbital coupling in the radical pair. Rate constants for the reactions of arylsulfinyl radicals with stable nitroxide radicals are among the fastest known, but reactivity with O-2 is very modest. Computations indicate that the singly occupied molecular orbital is a pi* orbital largely localized on the sulfur and oxygen atoms.