((Fluoroformyl)imido)(trifluoromethyl)sulfur Fluoride, FC(O)NS(F)CF3: Unexpected Conformational Properties

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Abstract
The geometric structure and conformational properties of ((fluoroformyl)imido)(trifluoromethyl)sulfur fluoride, FC(O)NS(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 SN double bond (FC(O) group relative to sulfur substituents), and “syn” refers to the orientation of the CO bond relative to the SN bond. From the intensities of the CO bands in the IR (gas) spectrum, a composition of 86(8)%:14(8)% is derived. These ratios correspond to ΔG°(GED) = 0.79(36) and ΔG°(IR) = 1.09(35) kcal mol-1. The preference of a trans structure around the SN double bond is unexpected, since all imidosulfur compounds studied thus far possess a cis configuration. The conformational properties are reproduced qualitatively correctly by all theoretical calculations. The predicted energy differences ΔE(HF) = 2.41, ΔE(MP2) = 0.64, and ΔE(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)NS(F)CF3, with mixed substitution at sulfur and the FC(O) group bonded to nitrogen, prefers the trans structure.