THE MOLECULAR-STRUCTURE OF SILYL ACETYLENE IN A LIQUID-CRYSTAL SOLUTION DETERMINED BY ANALYSIS OF DIRECT COUPLINGS TO HYDROGEN NUCLEI, COMPARED WITH THE GAS-PHASE STRUCTURE FROM ELECTRON-DIFFRACTION AND MICROWAVE DATA

The molecular structure of silyl acetylene has been derived from direct dipolar couplings observed in the NMR spectra of solutions in nematic liquid crystal solvents, and compared with the gas phase structure determined by joint analysis of electron diffraction and microwave (rotation constant) data. In each analysis the appropriate corrections for harmonic vibrations consistent with the observed vibration frequencies have been made, so that each structure is formally on an rα or rα0 basis. The HSiC bond angle is significantly different in the liquid crystal solvent, 108.99(1)° compared with 109.22(7)° in the gas, which is consistent with a slight opening out of the SiH3 group, perhaps because of a weak donor-acceptor interaction between a -CN group of the solvent and silicon. Absolute values of bond lengths cannot be determined in the liquid crystal solvent, but making the assumption that the CC bond length is the same as in the gas phase (120.75 pm) we find that the Si-C bond is also the same length as in the gas phase, and the Si-H bond is only slightly lengthened: (rSi-C 182.46(26) pm; 182.68(7) pm in gas: rSi-H 148.76(10) pm; 148.03(13) pm in gas). The gas phase structure has rC-H 106.19(6) pm; in the liquid crystal the CH bond length appears much shorter, 103.14(14) pm, probably because of effects associated with the curvilinear motions of atoms in the bending vibrations. Strict linearity of the SiCCH group is assumed in all structures. © 1990.