Substituent effects on the tautomerism of monochalcogenocarboxylic acids XC(=O)YH (X = H, F, NH2, OH, CN, and CH3; Y = S, Se, and Te): A theoretical study

Substituent effects on the conformational and energetic properties of keto-enol tautomerism of monochalcogenocarboxylic acids XC(=O)YH (X = H, F, NH2, OH, CN, and CH3; Y = S, Se, and Te) were investigated systematically by theoretical calculations. The relative stabilities of the tautomers, associated barrier heights and transition state geometries were reported. Results from different levels of calculations show that the keto forms are thermodynamically more stable than the enol forms by over 3 kcal/mol. Generally, compared with the H-substituted systems, the relative energies between the keto and enol isomers and activation energies of the keto-enol isomerizations are not affected much by the substituents NH2, CN, and CH3, which could form conjugations with the C=O group of the keto forms or the C=Y moieties of the transition states and enol forms. However, both the relative energies and activation energies would be increased by the strong electron-withdrawing substituents OH and F. The factors that influence the kinetic and thermodynamic properties of the tautomerizations were analyzed. Analysis of the transition state geometries show all these 1,3-H shift transition states are more similar to the products than the reactants, and the F- and OH-substituted ones are later than others. (C) 2008 Elsevier B.V. All rights reserved.