An analysis of electron diffraction data on bis(trimethylsilyl)acetylene taking into account nonlinear relations between Cartesian and internal vibrational coordinates

The electron diffraction data on bis(trimethylsilyl)acetylene were analyzed in terms of the one-dimensional dynamic model of free Si(CH3)(3) group rot ations about the Si-C=C-Si axis. The root-mean-square amplitudes and harmon ic shrinkage corrections were calculated taking into account nonlinear rela tions between Cartesian and internal vibrational coordinates at the level o f first-order perturbation theory (hl) and with the use of the traditional scheme (h0). The experimental r(alpha) distances were virtually independent of the approximation used to calculate vibrational effects. The r(h1) para meter values much better approximated the equilibrium geometry than the fam iliar r(alpha) = r(h0) parameters. The r(h1)-structure of bis(trimethylsily l)acetylene refined to Si-C(H-3) 1.877(4), Si-C=1.841(4), C=C 1.239(3), C-H (av.) 1.108(3) Angstrom, (H-3)C-Si-C=109.2(2)degrees and Si-C-H(av.) 111.3( 2)degrees. Electron diffraction data on silylacetylenes were systematized i n terms of r(g) parameters equal to r(h1) for bonded distances. (C) 2000 El sevier Science B.V. All rights reserved.