Molecular structure and conformation of triphenylsilane from gas-phase electron diffraction and theoretical calculations, and structural variations in H4-n SiPh n molecules (n = 1-4)

The molecular structure of triphenylsilane has been investigated by gas-phase electron diffraction and theoretical calculations. The electron diffraction intensities from a previous study (Rozsondai B, Hargittai I, J Organomet Chem 334:269, 1987) have been reanalyzed using geometrical constraints and initial values of vibrational amplitudes from calculations. The free molecule has a chiral, propeller-like equilibrium conformation of C 3 symmetry, with a twist angle of the phenyl groups τ = 39° ± 3°; the two enantiomeric conformers easily interconvert via three possible pathways. The low-frequency vibrational modes indicate that the three phenyl groups undergo large-amplitude torsional and out-of-plane bending vibrations about their respective Si-C bonds. Least-squares refinement of a model accounting for the bending vibrations gives the following bond distances and angles with estimated total errors: r g(Si-C) = 1.874 ± 0.004 Å, rg(C-C)= 1.402 ± 0.003 Å, r g(C-H)= 1.102 ± 0.003 Å, and aC-Si-H = 108.6° ± 0.4°. Electron diffraction studies and MO calculations show that the lengths of the Si-C bonds in H4-n SiPh n molecules (n = 1-4) increase gradually with n, due to π → σ(Si-C) delocalization. They also show that the mean lengths of the ring C-C bonds are about 0.003 Å larger than in unsubstituted benzene, due to a one hundredth angstrom lengthening of the Cipso-Cortho bonds caused by silicon substitution. A small increase of r(Si-H) and decrease of the ipso angle with increasing number of phenyl groups is also revealed by the calculations. © 2010 Springer Science+Business Media, LLC.