Equilibria of simple thioenol/thiocarbonyl pairs. Comparison with the oxygen analogs and with the parent selenium and tellurium systems. A theoretical study

Abstract

Thiocarbonyl–thioenol and carbonyl–enol equilibria (CH₃C(X)R ⇌ H₂CC(XH)R, X = S, O) were calculated by high level ab initio and density functional quantum mechanical methods for R = H, Me, Et, i-Pr, i-Bu, t-Bu, SiH₃, CN, Ph, CH₂Y (Y = Ph, CN, CF₃, CHCH₂), cyclic (CH₂)_nCX, n = 4–6, (PhCH₂)₂CS, PhCH₂C(S)CHPh₂ and i-Bu₂CS. A detailed study when R = H, Me shows that B3LYP/6-31G(d,p) overestimates ΔH(thiocarbonyl–thioenol) by ca. 2 kcal mol⁻¹. Isodesmic equations are used to evaluate separately the effect of R on each of the four species. A good agreement exists between the theoretical and the available experimental values. The ΔH(thiocarbonyl–thioenol) values of ca. −5.5 to 8 kcal mol⁻¹ are much smaller than the ΔH(carbonyl–enol) values of ca. 5–17 kcal mol⁻¹. A correlation exists between the ΔH terms of the two series. For R = H, Me the ΔH(CH₃C(X)R − CH₂ C(XH)R) values at the B3LYP/6-31G(d,p) level decrease as X is changed to a heavier chalcogen; e.g., when R = Me, ΔH = 14.9 (X = O), 6.1 (X = S), 1.7 (X = Se), −0.2 (est., X = Te).