Chlorodifluorothioacetic Acid, CF2ClC(O)SH: Synthesis, Characterization, X-ray Structure and Conformational Properties

Chlorodifluoroacetic anhydride, [CF2ClC(O)]2O, and chlorodifluoroacetyl chloride, CF2ClC(O)Cl, react with hydrogen sulfide to yield chlorodifluorothioacetic acid, CF2ClC(O)SH. This novel thiocarboxylic acid has been obtained in pure form and characterized by NMR, IR and Raman spectroscopy. By extrapolation of the vapour pressure curve, log p = 6.102–2068/T (p [bar], T [K]), the boiling point was determined to be 66(1) °C. The melting point of the pure substance is –83(2) °C. The conformational properties of the gas-phase molecule have been studied by vibrational spectroscopy [IR (vapour), Raman (liquid)] and quantum chemical calculations [B3LYP and MP2 methods with 6-311++G(d,p) and 6-311++G(3df,2p) basis sets]. The most stable form has C1 symmetry with a synperiplanar (syn) orientation of the C=O double bond with respect to the S–H single bond whereas in the chlorodifluoroacetyl group the gauche orientation of the ClC–C=O moiety is preferred (gauche-syn form). The analysis of the IR spectrum of gaseous CF2ClC(O)SH suggests the presence of two other conformations in equilibrium at room temperature. These forms have been fully characterized by high-level quantum chemical calculations. The structure of a single crystal, grown by a miniature zone-melting procedure using focused infrared laser radiation, was determined by X-ray diffraction analysis at low temperature. The crystalline solid [monoclinic, space group C2/c, a = 17.6165(14), b = 5.8848(5), c = 11.0877(9) A, β = 113.5330(11)°] consists exclusively of molecules adopting the gauche-syn conformation. These molecules associate, forming pairs of cyclic dimers in the crystal through S–H···O=C hydrogen bonds.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)