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

Abstract

The geometric structure and conformational properties of thioanisole, C 6H 5SCH 3, 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–CH 3 bond ( φ(C–S) = 0°). The experimental potential function for internal rotation around the C(sp 2)–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 ( r a and ∠ h1 with 3 σ uncertainties) were derived: r(C–C) av = 1.395(3) Å, r(C(sp 2)–S) = 1.775(4) Å, r(S–CH 3) = 1.813(4) Å, ∠CSC = 104.5(5), ∠C2C1C6 = 119.3(3), ∠C1C2C3 = 120.2(4)°.