Structures and Spectroscopic Properties of Polysulfide Radical Anions: A Theoretical Perspective

The potential involvement of polysulfide radical anions S^•- is a recurring theme in discussions of the basic and applied chemistry of elemental sulfur. However, while the spectroscopic features for n = 2 and 3 are well-established, information on the structures and optical characteristics of the larger congeners (n = 4-8) is sparse. To aid identification of these ephemeral species we have performed PCM-corrected DFT calculations to establish the preferred geometries for S^•- (n = 4-8) in the polar media in which they are typically generated. TD-DFT calculations were then used to determine the number, nature and energies of the electronic excitations possible for these species. Numerical reliability of the approach was tested by comparison of the predicted and experimental excitation energies found for S₂^•- and S₃^•-. The low-energy (near-IR) transitions found for the two acyclic isomers of S₄^•- (C_2h and C_2v symmetry) and for S₅^•- (C_s symmetry) can be understood by extension of the simple HMO π-only chain model that serves for S₂^•- and S₃^•-. By contrast, the excitations predicted for the quasi-cyclic structures S^•- (n = 6-8) are better described in terms of σ → σ* processes within a localized 2c-3e manifold.