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 S₄ symmetry conformation; the calculations indicate that the twist angle τ between the plane of the phenyl group and the plane defined by the Si−C bond and the S₄ axis is about 40°. 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 [Csákvári, É.; Shishkov, I. F.; Rozsondai, B.; Hargittai, I. J. Mol. 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, τ = 40 ± 2°, and the Si−Ph bond length, r_g = 1.881 ± 0.004 Å. The Si−Ph bond in tetraphenylsilane is marginally longer than the Si−Me bond in tetramethylsilane, r_g = 1.877 ± 0.004 Å 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 Å in the opposite sense, based on the covalent radii of C(sp³) and C(sp²). A delicate balance of subtle stereoelectronic effects, involving electron delocalization into the σ*(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 Å and 〈r_g(C−H)〉 = 1.102 ± 0.003 Å. The ipso ring angle of the phenyl groups is 117.5° from the DFT calculations, in close agreement with solid-state results.