title: Conformations of silicon-containing rings. 5. Conformational properties of 1-methyl-1-silacyclohexane: gas electron diffraction, low-temperature NMR, and quantum chemical calculations.
abstract: The molecular structure of 1-methyl-1-silacyclohexane 3 has been determined by gas electron diffraction (GED). The conformational preference of the methyl group was studied experimentally in the gas phase (GED) and in solution (low-temperature (13)C NMR) and by quantum chemical calculations (HF, MP2, and B3LYP with 6-31G basis sets and mPW1PW91/6-311G(2df,p)). Both experimental methods result in a preference of the equatorial position of the methyl group, 68(7)% in the gas phase at 298 K and 74(1)% in solution at 110 K. The calculations predict 68-73% equatorial conformer at room temperature. From coalescence temperatures, Gibbs free energies of activation for ring inversion DeltaG++ (eq --> ax) = 5.81(18) and DeltaG++ (ax --> eq) = 5.56(18) kcal mol(-1) were derived. The calculated values for DeltaG++ (eq --> ax) are 5.92 (B3LYP) and 5.84 kcal mol(-1) (mPW1PW91).
authors.affiliation: Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland. ingvara@raunvis.hi.is
authors.forename: Ingvar
authors.initials: I
authors.lastname: Arnason
authors.forename: Agust
authors.initials: A
authors.lastname: Kvaran
authors.forename: Sigridur
authors.initials: S
authors.lastname: Jonsdottir
authors.forename: Palmar I
authors.initials: PI
authors.lastname: Gudnason
authors.forename: Heinz
authors.initials: H
authors.lastname: Oberhammer