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).