Tautomeric rearrangements in mono- and dichalcogenide analogs of formic acid, HC(X)YH (X, Y = O, S, Se, Te): a theoretical study

Abstract

Theoretical calculations are reported at the Hartree-Fock (HF), MP2, and Becke3LYP (B3LYP) levels on a complete series of 16 chalcogenic derivatives of formic acid HC(X)YH (X, Y = O, S, Se, Te) using all-electron basis sets. The periodic variations observed on substituting the chalcogens are discussed. The transition structures for the tautomeric rearrangement of these formic acid derivatives are also characterized. The variations in relative energies corrected for zero-point vibrations show that the barrier for tautomerism is reduced as the electronegativity of chalcogens is decreased. The trends of natural charges on atoms of chalcogenides are described. At the correlated level of calculations both MP2 and B3LYP methods give comparable results. The solvent effects on tautomeric equilibrium are assessed by performing self-consistent reaction field (SCRF) calculations at the HF level. A comparative study is provided for two solvation models: the electrostatic solvation model based on Onsager's reaction field theory and the self-consistent isodensity polarized continuum model (SCI-PCM). The latter is shown to be a better model for solvation. The solvents with dielectric constants 2.0, 7.6, and 35.9 are shown to be less effective on the thermodynamic stabilities of these reactions. The dipole moments show significant variations between solvents of lower dielectric medium, while the variations are insignificant between solvents of higher dielectric media. A comparison of thermodynamic preferences for keto and enol forms in monochalcogeno acetc acids with the solvent model SCI-PCM at the HF and B3LYP levels is also provided. Chemical shifts calculated using the GIAO method (at the B3LYP/6-311+G(2D,P)//B3LYP/6-31G(D) level) correlate well with the experimental results. However we conclude from these results that the thion form of CH₃C(S)OH is less predominant.