Structure and stability of radical cations from cyclic and open-chain dithia compounds in aqueous solutions

The formation of intra- and some intermolecular radical cation complexes has been observed during the oxidation of cyclic and open-chain organic dithia compounds (the two S atoms not adjacent) by hydroxyl radicals in aqueous solutions. These species are characterized by a new sulfur-sulfur bond established by interaction of the unpaired p electron from the oxidized sulfur atom with the free p-electron pair of a second sulfur atom. Two of these electrons form a sigma bond while the third is promoted to an antibonding sigma/sup */ level. At low solute concentration (typically around 10-4M) intramolecular complex formation predominates. The radical cation complexes show characteristic broad optical absorptions which are attributable to a sigma → sigma/sup */ transition. The position of the absorption maximum (reflecting the sigma/sigma/sup */ energy difference) and the extinction coefficients are related to the p-orbital overlap between the two interacting sulfur atoms. The extent of overlap depends on the internuclear distance, r(S therefore S), and particularly on the angular relationship between the interacting orbitals. Other structural parameters influencing lambda/sub max/ are the flexibility of the species formed and possible interactions between the sigma(S,S) bond with sigma(C,C) bonds. lambda/sub max/ is found to range from 400 to 650 nm, these particular values referring to the intramolecular radical cation complexes from1,5-dithiacyclooctane and 1,4-dithiacyclohexane, respectively. epsilon values are found up to 7 x 103 M-1 cm-1. The lifetimes of the radical cation complexes depend on the stability of the new sulfur-sulfur bond and may well extend into the millisecond range in aqueous solutions