9694
J. Phys. Chem. 1996, 100, 9694-9697
Sulfine, CH2dSdO: Determination of Its Heat of Formation, Basicity, and Bond Strengths by Quantum Chemistry† Paul J. A. Ruttink Theoretical Chemistry Group, Department of Chemistry, Padualaan 14, State UniVersity of Utrecht, 3584 CH, The Netherlands
Peter C. Burgers* Hercules European Research Center BV, NijVerheidsweg 60, 3771 ME, BarneVeld, The Netherlands
James T. Francis and Johan K. Terlouw Department of Chemistry, McMaster UniVersity, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada ReceiVed: January 15, 1996; In Final Form: March 27, 1996X
Ab initio calculations executed at the CAS-SDCI/CASSCF/DZ(2df,2d,p)+f(S) level of theory have been employed to determine the heat of formation, ∆Hf, of the parent sulfine, CH2dSdO, 1, relative to six experimentally well-known anchors: ∆Hf(1) ) -3 ( 14 kJ/mol (0 K). The standard G2 and G2(MP2) methods yield lower values, -18 and -30 kJ/mol, respectively. It is inferred that the geometry optimization procedure in G2, which uses MP2(FULL)/6-31G*, does not accurately reproduce the experimental structure of 1. A larger basis set (MP2/6-311 + G(2df,2p) leads to the correct structure and to a ∆Hf(1) ) -2 kJ/mol (0 K). Our recommended value at 298 K, ∆Hf(1) ) -9 kJ/mol, represents a significant revision upwards of a previously estimated value (-50 ( 22 kJ/mol). The proton affinity of 1 (to generate CH2SOH+) is calculated to be 787 kJ/mol (298 K).
Introduction Sulfines (thiocarbonyl S-oxides) are four-centered heterocumulenes with general formula R1R2CdSdO.1 Many sulfines have been prepared and most of them appear to be stable species.1 However, it was not until 1976 that the parent molecule, CH2dSdO, 1, methanethial, S-oxide, or sulfine,2 was prepared in the gas phase by flash vacuum pyrolysis of 1,3dithiethane 1-oxide2 and identified by its microwave2 and photoelectron spectrum.3 This relatively short-lived (t1/2 ∼3060 min) intriguing molecule has a planar, but, as predicted by Walsh’s rules,1,4 bent geometry with a CSO angle of 114.7°2 and a dipole moment of 2.994 D. Sulfines and related compounds can occur naturally; for example, they are key compounds in allium chemistry, with Z-thiopropanal, S-oxide (ethylsulfine) being the lachrymatory factor of onions.5 Despite the above, virtually nothing is known about the thermochemical properties of sulfines, such as their heats of formation, ∆Hf (although an estimated value for ∆Hf for sulfine itself is available6), their bond dissociation energies, proton affinities (PA), or even their protonation sites. Recently, it was inferred from mass spectrometry based experiments7a that the most favorable protonation site of the parent sulfine, 1, most probably is the oxygen atom, not the carbon atom. It had been proposed previously8 that dimethyl sulfoxide, CH3S(dO)CH3•+, DMSO, radical cations dissociating with rate constants >106 s-1 (i.e., for fragmentations taking place at relatively large internal energies within the ion source of a mass spectrometer) cleave off CH3• to produce CH3SdO+, 3, carbon-protonated sulfine, as expected from a fast simple bond breaking reaction. It was later shown7 that, by contrast, DMSO ions dissociating with rate constants
