Photochemistry of tetrathionate anion in aqueous solutions: quantum-chemical mechanism of primary photoprocesses

Primary photoprocesses for tetrathionate anion (S 4 O 6 2− ) in aqueous solutions were studied theoretically using quantum chemistry methods. Optimal geometries of S 4 O 6 2− in water were calculated for the ground and singlet and triplet electronic excited states. The calculated energies of vertical electron transitions were compared with experimental absorption spectra and with intermediate absorption spectra obtained previously in the laser flash photolysis experiments. The formation of quasi-stable triplet biradical pairs (BPs), both symmetric 3 [S 2 O 3 •- …S 2 O 3 •- ] and asymmetric 3 [S 3 O 3 •- …SO 3 •- ] was predicted from the modeling of S 4 O 6 2− structure in the triplet state. The symmetric BP is formed by the splitting of S 4 O 6 2− into two strongly bound S 2 O 3 •- radicals anions, and the asymmetric BP is formed by the splitting of S 4 O 6 2− tetrathionate into weakly bound radicals anion S 3 O 3 •- and SO 3 •- . Possible pathways for the transformation of the BPs into free radical anions were assessed. The formation of free S 3 O 3 •- radical anion never reported in the literature was predicted. Calculations predict the formation of polythionates S 5 O 6 2− and S 6 O 6 2− as the most energetically favorable stable products.