Intramolecular S•••O chalcogen bond in thioindirubin

Results of X-ray diffraction study and quantum-chemical calculations revealed that molecular conformation of thioindirubin molecule creates suitable conditions for formation of intramolecular C-H center dot center dot center dot O and S center dot center dot center dot O interactions. Analysis of molecular electrostatic potential (MEP) demonstrates existence of two areas of positive MEP (sigma-holes) in the outermost part of the sulfur atom on the continuation of the lines of the C-S bonds. One of these sigma-holes is oriented toward region of negative MEP around the oxygen atom of carbonyl group. Such situation corresponds to formation of sigma-hole or chalcogen bond. Existence of both types of bonding interactions is confirmed by topological analysis of electron density distribution using Atoms in Molecules (AIM) theory. Energies of the C-H center dot center dot center dot O hydrogen bond and the S center dot center dot center dot O sigma-hole bond derived from AIM and NBO theories are very close.