Structure and conformation of (trifluoromethyl)thiobenzene, C6H5SCF3: Gas electron diffraction and quantum chemical calculations

The geometric structure and conformation of (trifluoromethyl)thiobenzene, C6H5SCF3, has been determined by two independent gas electron diffraction (GED) studies at the Universities of Moscow and Ivanovo and by quantum chemical calculations. Both experimental studies result in perpendicular orientation of the SCF3 bond relative to the benzene plane. This result is confirmed by several quantum chemical calculations (HF/6-31G(d), B3LYP/cc-pVTZ, MP2/6-31G(d), MP2/6-311+G(d,p) and MP2/cc-pVTZ) which predict single-minimum potential functions for internal rotation around the C(sp2)S bond. A double-minimum potential function predicted by the B3LYP/6-31G(d) calculation is not compatible with the experiment. B3LYP and MP2 calculations with large basis sets (6-311+G(d,p) and cc-pVTZ) predict barriers to internal rotation around the C(sp2)S bond between 2.4 and 2.6 kcal/mol. The geometric parameters of both experimental studies agree with each other within the experimental uncertainties, except for the CF bond distance. The effect of fluorination of the methyl group in thioanisole, C6H5SCH3, i.e., conformational change from planar orientation of the SCH3 bond to perpendicular orientation of the SCF3 bond, is rationalized on the basis of a natural bond orbital (NBO) analysis. The predominant effect is loss of conjugation between the p-shaped sulfur lone pair and the π system of the ring upon fluorination.