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

Abstract

The potential involvement of polysulfide radical anions Sn•− 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 Sn•− (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•− (C2h and C2v symmetry) and for S5•− (Cs 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 Sn•− (n = 6–8) are better described in terms of σ → σ* processes within a localized 2c-3e manifold.

Structure and morphology of red pigments based on sepiolite

“…•− ion radical could be identified by a strong absorption in the near IR region and proposed that the most stable cis-S 4 neutral molecule presented the best agreement with spectroscopic UV-vis and Raman data for red ultramarine, being the only candidate for red chromophores among the tetrasulfur species studied. The results of Chivers et al 26 using DFT and TD-DFT calculations confirm that both the cis and slightly less stable isomer trans of S 4…”

“…•− is poorly understood and continues to be conflicting. 26 Modelling tools using Hartree-Fock and Moller-Plesset theories 25 supported that the chain cis isomer of the neutral S 4 molecule is responsible for the red coloration in ultramarines. The absorption wavelengths of the sulfur chromophores species S 3 •− , S 4…”

“…Thus, many ionic species, both ionic and radical, can be formed from elemental sulfur. 16,24,26 The well-known chromophores S 2 ˙ − and S 3 ˙ − have formal valences of −1/2 and −1/3, respectively, while S 4 ˙ − has a valence of −1/4.…”

“…Thus, many ionic species, both ionic and radical, can be formed from elemental sulfur. 16,24,26 The well-known chromophores S 2…”

“…Hemolytic cleavage of the S–S bond can result in chain scission at several positions, giving rise to the various chromophore groups 3,24,26 as shown in eqn (3)–(6).[S 6 ] 2− → 2[S 3 ]˙ − [S 4 ] 2− → 2[S 2 ]˙ − …”