Nuclear coupling of 33S and the nature of free radicals in irradiated crystals of cystine dihydrochloride: Part II, charged radicals.

The (33)S hyperfine structure has been observed in negatively charged cystine molecule-radicals RCH(2)(SS)(-)CH(2)R. The coupling, found to be axially symmetric about the SS bond, proved the orbital of the unpaired electron to be an antibonding SS sigma() orbital with the spin density shared equally between 3p(sigma()) orbitals of the two sulfurs. The (33)S hyperfine structure could not be detected for a second radical produced by irradiation of the cystine hydrochloride crystal at 77 degrees K, but interpretation of the proton hyperfine structure and g tensor observed by others led to the conclusion that this radical is the positively charged cystine molecule RCH(2)S(+)SCH(2)R with the spin density concentrated in a 3p orbital of one S and perpendicular to the SS bond.