Kinetics of the reactions between sulfide radical cation complexes, [S[3-electron bonded]S]{sup +} and [S[3-electron bonded]N]{sup +}, 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 and sulfur-nitrogen 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 {times} 10{sup 10}M{sup {minus}1} s{sup {minus}1} and with the carbon dioxide radical anion on the order of (6.5 {+-} 0.2) {times} 10{sup 9} M{sup {minus}1} s{sup {minus}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 {sup {sm{underscore}bullet}}CO{sub 2}{sup {minus}} as compared to O{sub 2}{sup {sm{underscore}bullet}{minus}}. The rate constant for the reaction of superoxide with the sulfur-nitrogen bonded radical cation of Met-Gly, k = 5.3 x 10{sup 9} M{sup {minus}1} s{sup {minus}1}, is 3-fold lower as compared to that of the reaction with the sulfur-sulfur bonded radical cation complexes.