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 mechan ics (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 th e coplanar conformation of the molecule, with an Si-Me bond in the pla ne of the benzene ring, is a potential energy minimum. The perpendicul ar conformation, with an Si-Me bond in a plane orthogonal to the ring plane, is 0.2-0.5 kJmol(-1) higher in energy and corresponds to a rota tional transition state. This low barrier makes the conformational spa ce of the molecule almost evenly populated at the temperature of the e lectron diffraction experiment (305 K). A model approximating a freely rotating SiMe3 group is consistent with the experimental data. Import ant geometrical parameters from electron diffraction are [r(g)(C-C)] = 1.402 +/- 0.003 Angstrom, [r(g)(Si-C)] = 1.880 +/- 0.004 Angstrom, an d angle C-C-ortho-C-ispo-C-ortho = 117.2 +/- 0.2 degrees. The correspo nding r(e) values from MP2 calculations are 1.400 Angstrom, 1.887 Angs trom, and 117.4 degrees. The MO calculations also show that the C-ispo -C-ortho bonds are 0.011 Angstrom 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 angstrom. This is les s than what chemical expectation would suggest, but is in agreement wi th electron diffraction results from molecules containing either Si-Me or Si-Ph bonds. (C) 1997 Elsevier Science S.A.