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

The photochemistry of sodium thiosulfate (S₂O₃^2-) in aqueous solutions is rather complicated. Several sulfur-containing radical anions are formed upon photoexcitation. Any of them are rather common (SO₃•^-, SO₂^•-, and SO₅^•-); others are rare (S₂O₃^•-, ^•S₄O₆^3-, and S^•-) or never documented (S₂O₅^•-). In order to support the identification of intermediate radical anions, quantum-chemical (QM─quantum mechanical) calculations of the geometric and electronic structures of S₂O₃^•-, S₂O₅^•-, and ^•S₄O₆^3- 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 S₂O₃^•-, S₂O₅^•-, and ^•S₄O₆^3- was achieved. It was shown that S₂O₅^•- and ^•S₄O₆^3- can exist in two isomeric forms with different spectral properties. These isomers are S₂O₃O₂^•-; SO₃SO₂^•- for the case of S₂O₅^•- and (S₂O₃)₂^•3-; (S₃O₃^2-...SO₃^•-) for the case of ^•S₄O₆^3-.