Molecular structure and conformation of trimethylsilylbenzene: a study by gas-phase electron diffraction and theoretical calculations.

The molecular structure and conformation of trimethylsilylbenzene have been investigated by gas-phase electron diffraction, molecular mechanics (MM3 force field), and ab initio MO calculations at the HF/6–31G* and MP2(f.c.)/6–31G* levels. The theoretical calculations show that the coplanar conformation of the molecule, with an Si-Me bond in the plane of the benzene ring, is a potential energy minimum. The perpendicular conformation, with an Si-Me bond in a plane orthogonal to the ring plane, is 0.2–0.5 kJ mol−1 higher in energy and corresponds to a rotational transition state. This low barrier makes the conformational space of the molecule almost evenly populated at the temperature of the electron diffraction experiment (305 K). A model approximating a freely rotating SiMe3 group is consistent with the experimental data. Important geometrical parameters from electron diffraction are 〈rg(C-C)〉 = 1.402 ± 0.003 Å, 〈rg(Si-C)〉 = 1.880 ± 0.004 Å, and <Cortho-Cipso-Cortho = 117.2 ± 0.2°. The corresponding re values from MP2 calculations are 1.400 Å, 1.887 Å, and 117.4°. The MO calculations also show that the Cipso-Cortho bonds are 0.011 Å longer than the other C-C bonds. The MM3 and MO calculations indicate that the lengths of the Si-Me and Si-Ph bonds differ by only a few thousandths of an ångström. This is less than what chemical expectation would suggest, but is in agreement with electron diffraction results from molecules containing either Si-Me or Si-Ph bonds.