Computational characterization of sulfur-cxygen-bonded sulfuranyl radicals derived from alkyl- and (carboxyalkyl)thiopropionic acids: evidence for sigma-type radicals.

Abstract

Sulfide radical cations are known to stabilize via sulfur-oxygen bond formation with carboxylic acid functions. However, structural information on the resulting sulfuranyl radicals has so far only been provided by ESR spectroscopy for more stable, aromatic-substituted species, while the rather short-lived aliphatic sulfuranyl radicals have largely been characterized by time-resolved UV spectroscopy only. Therefore, we have obtained theoretical structural information on S-O-bonded sulfuranyl radicals from three model compounds, tert-butyl 2-(methylthio)peroxybenzoate, 3-methylthiopropionic acid, and 3,3'-thiodipropionic acid, using density functional theory, semiempirical, and molecular mechanics methods. All S-O-bonded species exist predominantly as three-electron-bonded sigma-radicals with an estimated heterolytic bond dissociation energy of the S therefore O bond on the order of 23-27 kcal mol(-1). Characteristic optical absorption bands, vibrational frequencies, and hyperfine coupling tensors are evaluated to facilitate the identification of such radicals by time-resolved UV, IR, and ESR spectroscopy.