Density functional evaluation and a feasibility study of intramolecular thione-thiol tautomerization
Performance of various density functionals for use in geometry and energetics calculation of thiocarbonyl systems is evaluated. The three biologically significant systems such as 2-methyl-3-thiosemicarbazide, 1-acetyl-2-thiohydantoin and bornane-2-thione are selected for the geometry analysis. Energy of tautomerization of thioacetone and enthalpy of formation of 2-thiohydantoin are considered for analyzing the performance in energetics. Minnesota functionals such as M05-2X, M06-2X and range separated hybrid functional wB97 give best results in both geometry and energetics calculations. Studies on basis set effect showed reliable performance by 6-31 + G* and 6-31++G** over higher basis sets. Feasibility of tautomerization in the aforementioned thiones has been investigated using the best performing density functionals. The proposed thione–thiol tautomerization step during PPC-DC catalyzed decarboxylation reaction has also been investigated and the computed high energy barrier shows less feasibility of the reaction.