Stereoelectronic effects in the Si-C bond: A study of the molecular structure and conformation of tetraphenylsilane by gas-phase electron diffraction and theoretical calculations

The molecular structure and conformation of tetraphenylsilane have been investigated by gas-phase electron diffraction and ab initio/DFT and molecular mechanics calculations. The structure of the free molecule is consistent with an Sq symmetry conformation; the calculations indicate that the twist angle tau between the plane of the phenyl group and the plane defined by the Si-C bond and the Sq axis is about 40 degrees. Analysis of the low-frequency modes indicates that the four phenyl groups undergo large-amplitude torsional and bending vibrations about the respective Si-C bonds. The electron diffraction intensities from a previous study [Csakvari, E.; Shishkov, I. F.; Rozsondai, B.; Hargittai, I. J. Mel. Struct. 1990, 239, 291] have been reanalyzed, using constraints from the calculations. A dynamical model accounting for the large-amplitude bending motion of the phenyl groups was used in the refinement: The new analysis yields accurate values for the twist angle of the phenyl group, tau = 40 +/- 2 degrees, and the Si-Ph bond length, r(g) = 1.881 +/- 0.004 Angstrom. The Si-Ph bond in tetraphenylsilane is marginally longer than the Si-Me bond in tetramethylsilane, r(g) = 1.877 +/- 0.004 Angstrom from the analysis of electron diffraction data taken with the same apparatus. This contrasts with chemical expectation, which would suggest a difference of 0.03 Angstrom in the opposite sense, based on the covalent radii of C(sp(3)) and C(sp(2)). A delicate balance of subtle stereoelectronic effects, involving electron delocalization into the sigma*-(Si-C) and 3d(Si) orbitals, appears to be responsible for the nearly equal length of the Si-C bonds in the two molecules. Other bond lengths from the present electron diffraction study are <r(g)(C-C)> = 1.401 +/- 0.003 Angstrom and <r(g)(C-H)> = 1.102 +/- 0.003 Angstrom. The ipso ring angle of the phenyl groups is 117.5 degrees from the DFT calculations, in close agreement with solid-state results.