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)

作者：Campanelli Anna Rita; Domenicano Aldo^*; Ramondo Fabio; Hargittai Istvan

来源：Structural Chemistry, 2011, 22(2): 361-369.

DOI：10.1007/s11224-010-9720-9

• Gas-phase electron diffraction
• MO calculations
• Phenylsilane
• Diphenylsilane
• Triphenylsilane
• Tetraphenylsilane
• Silicon-carbon bond
• Stereoelectronic effects
• pi -&gt
• sigma* Delocalization
• Negative hyperconjugation

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 tau = 39A degrees A A +/- A 3A degrees; 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 +/- A 0.004 , aOE (c) r (g)(C-C)> = 1.402 +/- A 0.003 , aOE (c) r (g)(C-H)> = 1.102 +/- A 0.003 , and a C-a-Si-H = 108.6A degrees A A +/- A 0.4A degrees. 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 pi -> sigma*(Si-C) delocalization. They also show that the mean lengths of the ring C-C bonds are about 0.003 larger than in unsubstituted benzene, due to a one hundredth angstrom lengthening of the C-ipso-C-ortho 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.

• 出版日期2011-4