Internal rotation and intramolecular hydrogen bonding in thiosalicylamide: gas phase electron diffraction study supported by quantum chemical calculations

Abstract

The molecular structure of thiosalicylamide (2-hydroxythiobenzamide) was investigated in the gas phase at 401 K by means of gas electron diffraction (GED) combined with quantum chemical (QC) calculations. Special attention was paid to the internal rotation of the thioamide group. Structural refinement was performed taking into account rovibrational corrections to the thermal-average internuclear distances calculated with harmonic and anharmonic (cubic) MP2/cc-pVTZ force constants in terms of static and dynamic models. It was shown that both models fitted the GED data equally well. The results of the GED refinement revealed that in the equilibrium structure, the thioamide group is twisted by about 30° with respect to the phenol ring plane. This is the result of an interatomic repulsion of hydrogen atom in the amide group from the closest hydrogen atom of the benzene ring, which overcomes the energy gain from the π−π conjugation of the thioamide group and the aromatic system of thiosalicylamide. Natural bond orbital (NBO) analysis and comparison of the thiosalicylamide molecular structure with those of related compounds revealed hydrogen-bonded fragment between the hydroxyl and thiocarbonyl groups. The structure of thiosalicylamide in the gas phase was found to be markedly different from that in the solid phase due to the effect of intermolecular hydrogen bonding in the crystal.