Molecular structure and conformation of p-bis(trimethylsilyl)benzene: a study by gas-phase electron diffraction and theoretical calculations.

The molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two Si—Me bonds in the plane of the benzene ring, to be energy minima. The perpendicular conformations, with two Si—Me bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconversion barrier, 0.3-0.5 kJ mol-1. The calculated lengths of the Si—Me and Si—Ph bonds differ by only a few thousandths of an angstrom, in agreement with electron diffraction results from molecules containing either Si—Me or Si—Ph bonds. The geometrical distortion of the benzene ring in p-bis(trimethylsilyl)-benzene may be described by superimposing independent distortions from each of the two SiMe3 groups. The electron diffraction intensities from a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mol. Struct. 1982, 95, 187) have been reanalyzed, imposing constraints from the theoretical calculations, and using a model based on a 1: 1 mixture of the two coplanar conformers. The effective torsion angles of the SiMe3 groups may indicate nearly free rotation. Important geometrical parameters from the present electron diffraction analysis are <rg(C—C)> = 1.407 ± 0.003 Å, <rg(Si—C)> = 1.881 ± 0.004 Å, <rg(C—H)> = 1.108 ± 0.003 Å, and <Cortho—Cipso—Cortho = 117.2 ± 0.3°. While the mean bond lengths are virtually the same from the previous and present analyses, the new ipso angle is in better agreement with the MO calculations [HF, 116.9°; MP2(f.c.), 117.1°; B3LYP, 116.9°].