Abstract
The conformational properties of FC(O)SOC(O)CH~3~ and FC(O)SOC(O)CF~3~ were studied by vibrational spectroscopy [IR (gas phase, Ar matrix), Raman (liquid phase)] and quantum chemical calculations (B3LYP and MP2 methods). For both compounds the syn‐gauche‐syn conformer with synperiplanar orientation of both carbonyl C=O bonds with respect to the S–O bond, and gauche orientation around the S–O bond (C~1~ symmetry) is the most stable structure [73(5) % for FC(O)SOC(O)CH~3~ and 82(5) % for FC(O)SOC(O)CF~3~]. For FC(O)SOC(O)CH~3~ two more conformers are observed in the Ar‐matrix spectra, 18(5) % syn‐trans‐syn with trans orientation around the S–O bond [φ(C–S–O–C) = 180°, C~2~ symmetry] and 9(5) % anti‐gauche‐syn with the FC(O) group oriented anti with respect to the S–O bond. The experimental free energy differences for these most stable forms of FC(O)SOC(O)CH~3~ are Δ__G__° = G°~(syn‐trans‐syn)~ – G°~(syn‐gauche‐syn)~ = 0.83(17) kcal mol^–1^ and Δ__G__° = G°~(anti‐gauche‐syn)~ – G°~(syn‐gauche‐syn)~ = 1.27(35) kcal mol^–1^, respectively. These values are well reproduced by the B3YLP/aug‐cc‐pVTZ method (0.76 and 1.23 kcal mol^–1^), but not by the MP2 approximation (2.72 and 1.28 kcal mol^–1^) For FC(O)SOC(O)CF~3~ only one more stable conformer, 18(5) % anti‐gauche‐syn, is observed. The experimental free energy difference between both rotamers of FC(O)SOC(O)CF~3~, Δ__G__° = G°~(anti‐gauche‐syn)~ – G°~(syn‐gauche‐syn)~ = 0.90(17) kcal mol^–1^, is reproduced very well by the B3YLP/6‐311++G** method (0.82 kcal mol^–1^) and reasonably well by B3LYP/aug‐cc‐pVTZ (1.17 kcal mol^–1^) and MP2/6‐31G* (1.23 kcal mol^–1^) calculations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)