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 ha ve been investigated by gas-phase electron diffraction, ab initio MO calcul ations 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 a nti-coplanar conformations, with two Si-Me bonds in the plane of the benzen e ring, to be energy minima. The perpendicular conformations, with two Si-M e bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconver sion barrier, 0.3-0.5 kJ mol(-1). The calculated lengths of the Si-Me and S i-Ph bonds differ by only a few thousandths of an angstrom, in agreement wi th electron diffraction results from molecules containing either Si-Me or S i-Ph bonds. The geometrical distortion of the benzene ring in p-bis (trimet hylsilyl) -benzene may be described by superimposing independent distortion s from each of the two SiMe3 groups. The electron diffraction intensities f rom a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mel. Struc t. 1982, 95, 187) have been reanalyzed, imposing constraints from the theor etical 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 i ndicate nearly free rotation. Important geometrical parameters from the pre sent electron diffraction analysis are [r(g)(C-C)] = 1.407 +/- 0.003 Angstr om, [r(g)(Si-C)] = 1.881 +/- 0.004 Angstrom, [r(g)(C-H)] = 1.108 +/- 0.003 Angstrom, and LC,angle cortho -Cipso,, -Cortho = 117.2 +/- 0.3 degrees. Whi le the mean bond lengths are virtually the same from the previous and prese nt analyses, the new ipso angle is in better agreement with the MO calculat ions [HF, 6.9 degrees; MP2(f.c.), 117.1 degrees; B3LYP, 116.9 degrees].