Sulfur-Containing Radical Anions Formed by Photolysis of Thiosulfate: Quantum-Chemical Analysis.

The photochemistry of sodium thiosulfate (S 2 O 3 2- ) in aqueous solutions is rather complicated. Several sulfur-containing radical anions are formed upon photoexcitation. Any of them are rather common (SO 3 • - , SO 2 •- , and SO 5 •- ); others are rare (S 2 O 3 •- , • S 4 O 6 3- , and S •- ) or never documented (S 2 O 5 •- ). In order to support the identification of intermediate radical anions, quantum-chemical (QM─quantum mechanical) calculations of the geometric and electronic structures of S 2 O 3 •- , S 2 O 5 •- , and • S 4 O 6 3- were performed. Two different approaches, time-dependent density functional theory and complete active space self-consistent field, were applied to identify the method optimal for the reproduction of the experimental electronic absorption spectra. Several of the most commonly used functionals were considered. The best agreement with the experimentally observed spectra of reference compounds (common sulfur-containing anions and radical anions) was achieved for the WB97X-D3 functional. Using this approach, satisfactory agreement between experimental and calculated spectra of S 2 O 3 •- , S 2 O 5 •- , and • S 4 O 6 3- was achieved. It was shown that S 2 O 5 •- and • S 4 O 6 3- can exist in two isomeric forms with different spectral properties. These isomers are S 2 O 3 O 2 •- ; SO 3 SO 2 •- for the case of S 2 O 5 •- and (S 2 O 3 ) 2 •3- ; (S 3 O 3 2- ...SO 3 •- ) for the case of • S 4 O 6 3- .