Molecular structure and conformational composition of decamethyl-n-tetrasilane, Si4Me10, by gas electron diffraction and density functional theory calculations

Density functional theory calculations on n-Si4Me10 at the B3PW91/6-311G* l evel indicate six minima on the potential-energy surface corresponding to t wo anti conformers with phi(SiSiSiSi) dihedral angles of +/- 160 degrees, t wo gauche conformers with phi = +/- 57 degrees, and two orthogonal conforme rs with phi = +/- 92 degrees. The relative energies of the three conformers were calculated to be 0.0 (anti), 2.9 (gauche) and 3.0 kJ mol(-1) (orthogo nal). Least-squares refinements on gas electron diffraction data recorded a t room temperature yielded the mole fractions chi(anti) = 0.51(6), chi(gauc he) = 0.32(8) and chi(ortho) = 0.17(14). The data thus confirm the presence of the anti and gauche conformers, while the presence of the ortho is prob able but not certain. The main bond distances and Valence angles of the mos t prevalent (anti) conformer are (calc./exp.): Si(1)-Si(2) = 237.1/235.0(6) ppm; Si(2)-Si(3) = 237.4/235.4(6) ppm; Si(1)-C = 189.5/189.3(2) ppm; Si(2) -C = 190.3/190.1(2) ppm; Si(1)Si(2)Si(3)= 112.5/112.4(5)degrees. (C) 2000 E lsevier Science S.A. All rights reserved.