Theoretical Investigation for 1H NMR chemical shift of the N-H…S proton due to intramolecular hydrogen bonding formation in di thiocarboxamides using Post Hartree-Fock and density functional theory

Abstract

A three hypothetical derivatives are derived from β-diketones were studied in this work. The molecules were substituted with different arbitrary selected substituents of wide range of Hammett constants that span the negative through the positive sigma para values range. To determine the tautomeric form of the molecules the non-substituted molecule was freely geometry optimized in the gas phase at the MP2/6-311++G(d,p) and the ωB97XD /6-311++G(d,p) levels of theory. The substituted molecules were then subjected to geometry optimization at and the ωB97XD /6-311++G(d,p) level of theory. The optimized structures used to calculate the 1H NMR chemical shifts at the MPW1PW91/6-311+(2d,p) level of theory.2

Our calculations show that the enolic form enaminothione is the most stable tautomer than others derivatives of these molecules. The cis isomer of the di enaminothione is most stable than the trans isomer. A fair correlation between the energy of the intra molecular hydrogen bond (R²= 0.756) expressed by the N…S distance and the ¹H chemical shift of the N-H…S proton.