Density functional computational thermochemistry: solving the discrepancy between MO and DFT calculations on the enthalpy of formation of sulfine, CH 2 @S@O Oscar N. Ventura a, * , Martina Kieninger a , P.A. Denis a , Ra ul E. Cachau b a CCPG, DEQUIFIM, Facultad de Qu  ımica, Universidad de la Rep ublica, CC 1157, 11800 Montevideo, Uruguay b National Cancer Institute-SAIC, Advanced Biomedical Computing Center, P.O. Box B, Frederick, MD 21702-1201, USA Received 2 October 2001; in final form 24 January 2002 Abstract The enthalpy of formation of sulfine is computed at the density functional (DFT) level to solve the discrepancy between previously recommended theoretical values. In agreement with the most recent CBS-QB3 calculations, which predict a value of 30  6 kJ/mol, DFT calculations on isodesmic reactions predict a value of 38  10 kJ/mol. Previous estimations of 9  14 kJ/mol (at the MO level) and 52  10 kJ/mol (at the DFT level) are discussed and shown to be artifacts of the methods of calculation employed. Ó 2002 Published by Elsevier Science B.V. 1. Introduction Density functional theory (DFT) [1–4] was successfully employed in recent years for the study of thermochemical properties of species which bonding patterns are difficult to describe with traditional ab initio techniques. We have discussed several of these compounds in previous papers in this series [5–9]. For example, DFT was shown to be especially useful for the study of fluorine oxides, compounds for which DFT accuracy rivals or surpasses even that of CCSD(T). In our search for testing the limits of applicability of DFT for the computation of thermochemical properties, we became recently interested in sulfines. Many of these compounds, which parent molecule is methanethial, S-oxide (or sulfine) CH 2 @S@O, 1, have been prepared in the past [10]. The parent molecule was prepared in gas phase in 1976 [11] by flash vacuum pyrolysis of 1,3-dithiethane 1-oxide, and identified by its microwave [11] and photo- electron [12,13] spectra. Although no direct experimental data are available on D f H 0 298 ð1Þ, recent publications have recommended different values, on the basis of calculations done with different methodologies. Ruttink et al. [14] employed CAS-SDCI energy calculations on top of CASSCF/DZ(2df, 2d,p) + f(S) geometry optimizations to obtain the enthalpy of formation of 1 relatively to six experimentally well-known anchors: the isomer HCOSH, the protonated form 1H þ , and the decomposition products CH 2 þ SO; H 2 S þ CO, H 2 O þ CS and 2 April 2002 Chemical Physics Letters 355 (2002) 207–213 www.elsevier.com/locate/cplett * Corresponding author. Fax: +598-2-9248396/1906. E-mail addresses: oscar@bilbo.edu.uy, oscar@ccpg.fq.edu. uy (O.N. Ventura). 0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII:S0009-2614(02)00104-5