Kinetics of the Reactions between Sulfide Radical Cation Complexes, [S∴S]+ and [S∴N]+, and Superoxide or Carbon Dioxide Radical Anions

The reaction of superoxide radical anions with sulfide radical cation complexes represents an important sulfoxide-forming process. Here, absolute rate constants for the reaction of sulfur−sulfur [>S∴SS∴NH2R]+) three-electron bonded sulfide radical cation complexes with superoxide and, for comparison, carbon dioxide radical anion have been measured by pulse radiolysis. For two different sulfur−sulfur bonded species, the intermolecular complex from dimethyl sulfide and the intramolecular complex from 1,5-dithia-3-hydroxycyclooctane, the rate constants for the reaction with superoxide are on the order of 1.6 × 1010 M-1 s-1 and with the carbon dioxide radical anion on the order of (6.5 ± 0.2) × 109 M-1 s-1. The fact that the stronger reducing carbon dioxide radical anion shows the lower rate constant can be rationalized by the higher internal reorganization energy of •CO2 - as compared to O2 •-. The rate constant for the reaction of superoxide with the sulfur−nitrogen bonded radical cation of Met-Gly, k = 5.3 × 109 M-1 s-1, is 3-fold lower as compared to that of the reaction with the sulfur−sulfur bonded radical cation complexes.