Synthesis, Structure and Conformational Properties of Fluoroformylchlorodifluoroacetyl Disulfide, FC(O)SSC(O)CF 2 Cl: Conformational Transferability in –C(O)SSC(O)– Compounds

Pure fluoroformylchlorodifluoroacetyl disulfide, FC(O)SSC(O)CF 2 Cl, has been prepared by the reaction of FC(O)SCl and CF 2 ClC(O)SH in quantitative yield. The conformational properties of the novel molecule have been studied by vibrational spectroscopy (IR – gas phase, Raman – liquid phase) and quantum chemical calculations (B3LYP and MP2 methods). The gaseous compound exhibits a conformational equilibrium at room temperature where the most stable form adopts a C 1 symmetry and a syn ‐periplanar ( sp ) orientation of both carbonyl groups with respect to the disulfide bond. A second form, observed in the IR spectrum of the vapor, corresponds to a conformer in which the carbonyl bond of the FC(O) moiety adopts an anti ‐periplanar ( ap ) position with respect to the S–S single bond, and gauche with respect to the ClC–C=O moiety in the chlorodifluoroacetyl group. The experimental free energy difference value Δ G 0 = G 0 ( ap – sp ) – G 0 ( sp – sp ) = 1.4(3) kcal/mol (IR) is reproduced well by the B3YLP/6‐311+G(3df) (1.1 kcal/mol) and the MP2/6‐31G* (1.8 kcal/mol) methods. In addition, the structure of a single‐crystal, grown in situ, was determined by X‐ray diffraction analysis at low temperature. The crystalline solid [monoclinic, P 2 1 /n , a = 5.579(3) Å, b = 16.615(7), c = 8.455(4) Å, β = 106.876(8)°] consists exclusively of molecules with the ( sp – sp ) conformation and the usual gauche orientation around the disulfide bond [φ(CS–SC) = 84.2°]. Conformational transferability is thus demonstrated once again for species that contain the –C(O)SSC(O)– group as part of a systematic program designed to analyze the behavior of this class of molecules. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)