Spectroscopic Characterization and Conformational Properties of S‐(Fluoroformyl)‐O‐(acetyl) Thioperoxide and S‐(Fluoroformyl)‐O‐(trifluoroacetyl) Thioperoxide

Abstract

AbstractThe conformational properties of FC(O)SOC(O)CH3 and FC(O)SOC(O)CF3 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 (C1 symmetry) is the most stable structure [73(5) % for FC(O)SOC(O)CH3 and 82(5) % for FC(O)SOC(O)CF3]. For FC(O)SOC(O)CH3 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°, C2 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)CH3 are ΔG° = G°(syn‐trans‐syn) – G°(syn‐gauche‐syn) = 0.83(17) kcal mol–1 and ΔG° = 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)CF3 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)CF3, ΔG° = 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)