Topological and spectroscopic study of three-electron bonded compounds as models of radical cations of methionine-containing dipeptides

Small models of radical cations of methionine-containing dipeptides, which are stabilized by formation of two-centre three-electron (2c–3e) S∴X bonds (X = S, N and O), were investigated at the BH&HLYP/6-31G(d) level and by means of topological tools. The SX distance is not so important for stability but the relative orientation of both fragments is. The AIM and ELF topological analyses shows that the nature of the S∴X bond varies with X, from purely 2c–3e in S∴S+ entities to electrostatic in S∴O+ ones. The σSX → σ SX ∗ wavelengths, obtained at the TD-BH&HLYP/cc-pVTZ level, strongly depend on X and on conformation.