Experimental charge density evidence for the existence of high polarizability of the electron density of the free electron pairs on the sulfur atom of the thioureido group, NH-C(S)-NH2, induced by N-H center dot center dot center dot S and C-H center dot center dot center dot S interactions

Abstract

The experimental charge density study of salicylaldehyde thiosemicarbazone (SalTSC) has been performed. The analysis of the crystal packing revealed that the sulfur atom is simultaneously engaged in six hydrogen-bonding interactions, two of the N-H center dot center dot center dot S and four of the C-H center dot center dot center dot S type. The strongest hydrogen bond, N2-H2n center dot center dot center dot S, leads to a centrosymmetric dimer. It has been established that the deformation density of the free electron pairs on the sulfur atom is inhomogeneously distributed within a torus. A relationship found between the deformation of the torus and the space directionality of the surrounding donor groups suggests that the sulfur atom of the thioureido moiety easily adjusts to the environment in order to increase the number of stabilizing contacts. A CSD study of the compounds containing a thioureido fragment, N-C(S)-N, confirmed the experimental results for SalTSC, i.e., about 60% of the 835 analyzed crystal structures form a dimer trough N-H center dot center dot center dot S interactions, whereas in about 50% of crystal structures, the S atom forms four of more D-H center dot center dot center dot S intermolecular interactions (D = C, N, or O).