Structure and Conformation of Bis(methylthio)methane, (MeS)₂CH₂, Determined by Gas-Phase Electron Diffraction and ab Initio Methods

Abstract

The gas-phase structure and conformational properties of bis(methylthio)methane, (MeS)2CH2, have been determined by electron diffraction, augmented by results from ab initio molecular orbital calculations. The molecule was found to exist in the gas phase at ∼100 °C, predominantly in the G+G+ form (70(18)%), where the CH3 groups are on opposite sides of the SCS plane, with a smaller contribution from the GA form in the conformational mixture. The main conformer possesses C2 symmetry with a dihedral angle φ(CSCS) of 54(6)°. The second conformer has C1 symmetry with CSCS dihedral angles of 74° and 186° (ab initio values). Assuming entropy differences between the two conformers as obtained from MP2/6-311+G(d) calculations (ΔS = R ln 2 + 2.72 cal mol-1 K-1), this composition corresponds to an enthalpy difference of ΔH = 2.2(6) kcal mol-1. The corresponding ab initio/DFT enthalpy difference values at 373 K are 1.49 kcal mol-1 (HF), 2.38 kcal mol-1 (MP2), and 2.15 kcal mol-1 (B3LYP). The results for the main distances (rg) and angles (∠α) from the combined GED/ab initio (HF/6-311+G(d)) study of the G+G+ form of (MeS)2CH2 (with estimated 2σ uncertainties) are r(CCH3−S) = 1.805(2) Å, r(CCH2−S) = 1.806(2) Å, r(CCH3−H) = 1.108(5) Å, r(CCH2−H) = 1.098(5) Å, ∠(C−S−C) = 102.8(24)°, ∠(S−C−S) = 115.9(3)°, ∠(H−CCH2−H) = 107.5° (ab initio), and ∠(S−CCH3−H) = 108.9° (ab initio).