TAUTOMERIC REARRANGEMENTS IN MONOCHALCOGENIDE AND DICHALCOGENIDE ANALOGS OF FORMIC-ACID, HC(X)YH (X, Y = O,S,SE,TE) - A THEORETICAL-STUDY

Theoretical calculations are reported at the Hartree-Fock (HF), MP2, a nd Becke3LYP (B3LYP) levels on a complete series of 16 chalcogenic der ivatives of formic acid HC(X)YH (X, Y = O, S, Se, Te) using all-electr on basis sets. The periodic variations observed on substituting the ch alcogens are discussed. The transition structures for the tautomeric r earrangement of these formic acid derivatives are also characterized. The variations in relative energies corrected for zero-point vibration s show that the barrier for tautomerism is reduced as the electronegat ivity of chalcogens is decreased. The trends of natural charges on ato ms of chalcogenides are described. At the correlated level of calculat ions both MP2 and B3LYP methods give comparable results. The solvent e ffects on tautomeric equilibrium are assessed by performing self-consi stent reaction field (SCRF) calculations at the HF level. A comparativ e study is provided for two solvation models: the electrostatic solvat ion model based on Onsager's reaction field theory and the self-consis tent isodensity polarized continuum model (SCI-PCM). The latter is sho wn to be a better model for solvation. The solvents with dielectric co nstants 2.0, 7.6, and 35.9 are shown to be less effective on the therm odynamic stabilities of these reactions. The dipole moments show signi ficant variations between solvents of lower dielectric medium, while t he variations are insignificant between solvents of higher dielectric media. A comparison of thermodynamic preferences for keto and enol for ms in monochalcogeno acetc acids with the solvent model SCI-PCM at the HF and B3LYP levels is also provided. Chemical shifts calculated usin g the GIAO method (at the B3LYP/6-311+G(2D,P)//B3LYP/6-31G(D) level) c orrelate well with the experimental results. However we conclude from these results that the thion form of CH3C(S)OH is less predominant.