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), 2-imidazolidinethione (IMT, 2), and 2-thiazolidinethione (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, which are any molecules present in the crystal structure that interact with both molecules forming the S···S contact. Several types of staple molecules were identified, differing in the area of interaction with the S···S dimer molecule. The analysis of the data in the Cambridge Structural Database showed that the staple structures can be found in 77% of all structures with short S···S contacts (shorter than 3.4 Å) and in more than half of the structures with the contacts within the van der Waals radius limit. The calculations show that the smaller the distance between sulfur atoms in the S···S dimer, the greater the amount of energy needed for dimer stabilization. Consequently, the presence of a staple is essential.