Density functional computational thermochemistry: Isomerization of sulfine and its enthalpy of formation

Density functional (DFT), second-order perturbation theory (MP2), and coupled-cluster [(CCSD(T)] calculations using Pople's basis sets up to 6-311++G(3df,2pd) and Dunning's correlation consistent basis sets have been employed to determine the enthalpy of formation of sulfine, CH2SO, 1, using the isodesmic reaction CH2S + SO2 reversible arrow CH2SO + SO. Previous calculations showed an inconsistency between the enthalpy of formation obtained using this methodology, Delta (f)H(298.15)degrees (1) = -52 +/- 10 kJ/mol, and the value obtained employing the isomerization reaction CH2SO (1) reversible arrow HC(=O)SH (2) if Benson's estimate for the enthalpy of formation of isomer 2 (thioformic acid) was employed. This result was particularly vexing, since the computed enthalpy of formation of I was reasonably in agreement with Benson's own estimate. In this paper we extended our previous study using the reactions HC(=O)-XH + RH reversible arrow H2CO + R-XH with R = H, Me, Et, Pr, and i-Pr. X was either sulfur, to obtain the enthalpy of formation of 2, or oxygen, to assess the errors to be expected in the use of these reactions for the evaluation of Delta H-f degrees. The result, Delta (f)H(298.15)degrees (2) = -121 +/- 8 kJ/mol, arrived at after a critical assessment of B3LYP, MP2, and CCDSD(T) results, is in complete agreement with the value of - 126 +/- 4 kJ/mol estimated by Benson. This implies that the isomerization reaction cannot be employed for the determination of the enthalpy of formation of sulfine. We ascribe this inadequacy to the errors introduced due to the change in the oxidation state of sulfur.