Molecular geometry and ring deformation of trimethylsilylbenzenes: Gas-phase electron-diffraction study of 1,3-bis(trimethylsilyl)benzene and 1,3,5-tris(trimethylsilyl)benzene

Abstract

The molecular geometries of 1,3-C 6H 4(SiMe 3) 2 ( 2) and 1,3,5,-C 6H 3(SiMe 3) 3 ( 3) have been determined by gas-phase electron diffraction. The mean CC bond lengths are slightly larger than those in benzene, and the endocyclic ipso bond angles C6C1C2 are considerably less than 120°, in accord with the electron-releasing inductive effect of the SiMe 3 substituent and the predictions of the VSEPR model. The SiC (methyl) bonds are longer than the SiC(aryl) bonds by about 0.02 Å (see below). Only limited information concerning conformational preference could be obtained from the electron-diffraction data but this did not hinder the determination of the other structural characteristics. The following main bond length ( r g) and angle parameters (with estimated total errors) have been obtained at nozzle temperatures of 86 and 105°C, respectively. For 2: (CC) mean 1.405±0.003, (SiC) mean 1.87 9± 0.004, Δ(SiC)  SiC(methyl)  SiC(aryl) = 0.018 ± 0.008, SiC(methyl) 1.884 ± 0.004, SiC(aryl) 1.866 ± 0.007 Å C6C1C2 116.2±0.6°, C4C5C6 119.5° (assumed), CSiC 109.5° (assumed); for 3: CC 1.410±0.003, (SiC) mean 1.881 ± 0.004, Δ(SiC) 0.016±0.006, SiC(methyl) 1.885±0.004, SiC(aryl) 1.869±0.006 Å, C6C1C2 117.0±0.6°, CSiC 109.5° (assumed). On the basis of available structural information on three trimethylsilyl-substituted benzene derivatives, and assuming additivity, the angular ring deformation impact of the trimethylsilyl substituent has been estimated. The predicted endocyclic angles for trimethylsilylbenzene are (starting from the ipso angle) 116.8, 122.2, 119.9, and 119.0°.