Density functional computational thermochemistry: Isomerization of sulfineand its enthalpy of formation

Density functional (DFT), second-order perturbation theory (MP2), and coupl ed-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 iso desmic reaction CH2S + SO2 reversible arrow CH2SO + SO. Previous calculatio ns showed an inconsistency between the enthalpy of formation obtained using this methodology, Delta (f)H(298.15)degrees (1) = -52 +/- 10 kJ/mol, and t he value obtained employing the isomerization reaction CH2SO (1) reversible arrow HC(=O)SH (2) if Benson's estimate for the enthalpy of formation of i somer 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 u sing 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 format ion of 2, or oxygen, to assess the errors to be expected in the use of thes e 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 assess ment 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 iso merization reaction cannot be employed for the determination of the enthalp y of formation of sulfine. We ascribe this inadequacy to the errors introdu ced due to the change in the oxidation state of sulfur.