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

被引:0
作者
Anna Rita Campanelli
Aldo Domenicano
Fabio Ramondo
István Hargittai
机构
[1] University of Rome “La Sapienza”,Department of Chemistry
[2] University of L’Aquila,Department of Chemistry, Chemical Engineering and Materials
[3] Budapest University of Technology and Economics,Materials Structure and Modeling Research Group of the Hungarian Academy of Sciences and Department of Inorganic and Analytical Chemistry
来源
Structural Chemistry | 2011年 / 22卷
关键词
Gas-phase electron diffraction; MO calculations; Phenylsilane; Diphenylsilane; Triphenylsilane; Tetraphenylsilane; Silicon–carbon bond; Stereoelectronic effects; π → σ* Delocalization; Negative hyperconjugation;
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摘要
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 C3 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: rg(Si–C) = 1.874 ± 0.004 Å, 〈rg(C–C)〉 = 1.402 ± 0.003 Å, 〈rg(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−nSiPhn 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.
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页码:361 / 369
页数:8
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