Structure and conformation of thioanisole: Gas electron diffraction and contradicting quantum chemical calculations

The geometric structure and conformational properties of thioanisole, C6H5SCH3, have been studied by gas-phase electron diffraction (GED) and quantum chemical calculations. The GED intensities are fitted very well with a single conformer with planar orientation of the S–CH3 bond (φ(C–S) = 0°). The experimental potential function for internal rotation around the C(sp2)–S bond derived with a dynamic model possesses a single minimum for planar conformation, but a second shallow minimum for perpendicular orientation cannot be excluded. Calculated potential functions depend strongly on the computational method. The MP2 approximation with small basis sets (6-31G(d)) predict a single minimum for perpendicular orientation, (φ(C–S) = 90°), with intermediate basis sets (6-311G(d,p)) an additional shallow minimum for planar orientation and with large basis sets (cc-pVTZ) the global minimum for planar orientation and an additional shallow minimum at φ(C–S) = 90°. The B3LYP/cc-pVTZ calculations result in a single minimum for planar orientation, in agreement with the GED experiment. The experimental and calculated barrier to internal rotation is about 1 kcal/mol. The following geometric parameters (ra and ∠h1 with 3σ uncertainties) were derived: r(C–C)av = 1.395(3) Å, r(C(sp2)–S) = 1.775(4) Å, r(S–CH3) = 1.813(4) Å, ∠CSC = 104.5(5), ∠C2C1C6 = 119.3(3), ∠C1C2C3 = 120.2(4)°.