Electronic features and hydrogen bonding capacity of the sulfur acceptor in thioureido-based compounds. Experimental charge density study of 4-methyl-3-thiosemicarbazide

Sulfur atom is generally considered as a weak hydrogen bonding acceptor which is mostly explained by its low electronegativity. This is a background, and motivation, for this study that aims to provide additional insight into electronic features of the sulfur acceptor. The analysis of sulfur's electronic features uses high-resolution, low-temperature X-ray diffraction datasets on 4-methyl-3-thiosemicarbazide (MeTSC) and salicylaldehyde thiosemicarbazone (SalTSC). The total electron density distribution was modeled using Hansen-Coppens multipole model. The fact that MeTSC crystallizes with two crystallographically independent (but chemically equivalent) molecules, engaging in partly different D-H center dot center dot center dot S interactions, has, quite conveniently, enabled the examination of fine effects of the D-H center dot center dot center dot S interactions on the lone pairs electron density of the two S acceptors. The deformation density distribution, topological analysis of the total experimental charge density rho(r) and its Laplacian (del(2)rho(r)), and, finally, electrostatic potential all point to the great flexibility of the S acceptor and its ability to adjust to the spatial distribution of the interacting donor groups. Toroidal accumulation of the S lone pairs electron density, strong value of the negative electrostatic potential even at a distance of 3.1 angstrom from the S nucleus, and interpenetration of the van der Waals spheres for the donor and S acceptor atoms are all an indication of sulfur's capacity to simultaneously engage in a greater number of interactions than conventional acceptors such as O and N.