((Fluoroformyl)imido)(trifluoromethyl)sulfur fluoride, FC(O)N=S(F)CF3: Unexpected conformational properties

The geometric structure and conformational properties of ((fluoroformyl)imi do)(trifluoromethyl)sulfur fluoride, FC(O)N=S(F)CF3 are investigated by gas electron diffraction (GED) experiments, IR (gas) spectroscopy, and quantum chemical calculations (HF, MP2, and B3LYP with 6-31G* basis sets). The GED intensities are reproduced best with a mixture of 79(12)% trans-syn and 21 (12)% cis-syn conformers. "Trans/cis" describes the orientation around the S=N double bond (FC(O) group relative to sulfur substituents), and "syn" re fers to the orientation of the C=O bond relative to the S=N bond. From the intensities of the C=O bands in the IR (gas) spectrum, a composition of 86( 8)%:14(8)% is derived. These ratios correspond to DeltaG degrees>(*) over b ar * (GED) = 0.79(36) and DeltaG degrees>(*) over bar * (III) = 1.09(35) kc al mol(-1). The preference of a trans structure around the S=N double bond is unexpected, since all imidosulfur compounds studied thus far possess a c is configuration. The conformational properties are reproduced qualitativel y correctly by all theoretical calculations. The predicted energy differenc es DeltaE(HF) = 2.41, DeltaE(MP2) = 0.64, and DeltaE(B3LYP)= 0.28 kcal mol( -1) are larger or slightly smaller than the experimental values. Additional theoretical calculations (B3LYP) for several imidosulfur compounds reveal that only FC(O)N=S(F)CF3, with mixed substitution at sulfur and the FC(O) g roup bonded to nitrogen, prefers the trans structure.