MOLECULAR-STRUCTURE OF PHENYLSILANE - A STUDY BY GAS-PHASE ELECTRON-DIFFRACTION AND AB-INITIO MOLECULAR-ORBITAL CALCULATIONS

The molecular structure of phenylsilane has been determined accurately by gas-phase electron diffraction and ab initio MO calculations at th e MP2(f.c.)/6-31G level. The calculations indicate that the perpendic ular conformation of the molecule, with a SI-H bond in a plane orthogo nal to the plane of the benzene ring, is the potential energy minimum. The coplanar conformation, with a Si-H bond in the plane of the ring, corresponds to a rotational transition state. However, the difference in energy is very small, 0.13 kJ mol(-1), implying free rotation of t he substituent at the temperature of the electron diffraction experime nt (301 K). Important bond lengths from electron diffraction are: < r( g)(C-C) > = 1.403 +/- 0.003 Angstrom, r(g)(Si-C) = 1.870 +/- 0.004 Ang strom, and r(g)(Si-H) = 1.497 +/- 0.007 Angstrom. The calculations ind icate that the C-ipso-C-ortho bonds are 0.010 Angstrom longer than the other C-C bonds. The internal ring angle at the ipso position is 118. 1 +/- 0.2 degrees from electron diffraction and 118.0 degrees from cal culations. This confirms the more than 40-year old suggestion of a pos sible angular deformation of the ring in phenylsilane, in an early ele ctron diffraction study by F.A. Keidel, S.H. Bauer, J. Chem. Phys. 25 (1956) 1218. (C) 1998 Elsevier Science S.A. All rights reserved.