Theoretical Study of the Stability of Tautomers and Conformers of Isatin-3-Thiosemicarbazone (IBT)

Abstract

The isatin based thiosemicarbazones exhibit a number of biological applications. In this paper, we explore various rotamers and tautomers of the title molecule using density functional theory at the B3LYP/6-311++G** level. Investigation reveals that one tautomer dominates the gas phase, of which one of the rotamers constitutes 87% of the gas phase population. The Z configuration about the imine linkage is satisfactorily accounted for by the C=O…H hydrogen bonding that results in this planar structure. The orientation of the amine group is also explained by N-H…N hydrogen bonding. The weakening of the C=S bond by hyperconjugative interactions and extended conjugation involving the five membered ring are revealed by an NBO analysis. Comparison of computed geometrical parameters and vibrational frequencies with experimental ones shows good correlation.