The role of staple molecules in the formation of S···S contact in thioamides – experimental charge density and theoretical studies.

The reasons behind the formation of S‧‧‧S contacts in thioamides, the most important compounds with terminal sulfur atoms, were investigated by means of experimental charge density studies and theoretical calculations. As this interaction is to some extent similar to the much better-known halogen bond, geometrical analysis was performed using previously determined halogen bond formation criteria. To investigate the most representative thioamides, three compounds, namely, 6-amintothiouracil hydrate (ATU·H2O, 1), imidazolidinethione (IMT, 2) and thiazolidine thione (TT, 3), were selected. In all three structures, relatively short S···S contacts displaying different geometries were observed. Furthermore, different symmetry elements (mirror plane in ATU, inversion center in TT, and translation in IMT) determined the mutual orientation of the sulfur atoms in contact. The structural analysis and calculations proved that the isolated S∙∙∙S dimers are unstable and that they are stabilized by “staple” molecules, w...