Sulfur and phosphorus oxyacid radicals

We report a computational study of the little-studied neutral bisulfite, bisulfate, dihydro-phosphite and dihydro–phosphate radicals (HSO_x^•, H2PO_x^•, x = 3,4), calling special attention to their various tautomeric structures together with pK_a values estimated from the Gibbs free energies of their dissociations (at the G4 and CAM-B3LYP levels of density functional theory). The energetics of microhydration clusters with up to 4 water molecules for the S-based species and up to 8 waters for the P-based species were investigated. The number of microhydrating water molecules needed to induce spontaneous de-protonation is found to correlate the acid strength of each radical. According to computed Gibbs free reaction and activation energies, S- and P-centered radicals preferentially add to the double bond of propene (a lipid model), whereas the O-centered radical tautomers prefer H-abstraction. The likely downstream reactions of these radicals in biological media are discussed.