GAS-PHASE MOLECULAR-STRUCTURES OF BIS(CHLOROMETHYLSILYL)AMINE AND BIS(CHLOROMETHYLSILYL)METHYLAMINE BY ELECTRON-DIFFRACTION AND AB-INITIO CALCULATIONS - EXPERIMENTAL SUPPORT FOR N(N)-SIGMA-ASTERISK(SI-CL) HYPERCONJUGATION
H. Fleischer et al., GAS-PHASE MOLECULAR-STRUCTURES OF BIS(CHLOROMETHYLSILYL)AMINE AND BIS(CHLOROMETHYLSILYL)METHYLAMINE BY ELECTRON-DIFFRACTION AND AB-INITIO CALCULATIONS - EXPERIMENTAL SUPPORT FOR N(N)-SIGMA-ASTERISK(SI-CL) HYPERCONJUGATION, Journal of the Chemical Society. Dalton transactions, (4), 1998, pp. 593-600
The gas-phase molecular structures of NH(SiHMeCl)(2) 1 and NMe(SiHMeCl
)(2) 2 have been investigated by electron diffraction (ED) and by ab i
nitio geometry optimisations at the MP2/6-31G level. Ab initio calcul
ations suggest six different conformers to be present for both compoun
ds. They also predict that all conformers exhibit planar or nearly pla
nar configurations at the N atom and that all the Si-CI bonds are more
or less orthogonal to the SiNSi plane, a feature that is supported by
the ED refinement. Natural bond orbital analysis and an SCF deletion
calculation showed an energy contribution of the n(N)-sigma(Si-Cl) hy
perconjugative interaction of more than 80 kJ mol(-1), thus rationalis
ing conformations that optimise an overlap of the two appropriate orbi
tals. The principal parameters (r(a)) from electron diffraction for NH
(SiHMeCl)(2) (with estimated standard deviations given in parentheses
and ab initio-calculated values for the lowest-energy conformer given
in square brackets) are: r(Si-N) 171.2(2) [173.7], r(Si-Cl) 206.2(1) [
207.7], r(Si-C) 184.5(3) [186.4] pm; Si-N-Si 128.8(12) [131.3], N-Si-C
l 112.9(4) [111.3], N-Si-C 115.7(9) [112.0] and CI-Si-C 102.6(6) [107.
3]degrees. For NMe(SiHMeCl),: r(Si-N) 171.5(1) [173.7], r(Si-CI) 207.1
(1) [208.1], r(Si-C) 185.2(2) [186.8], r(N-C) 147.4(6) [148.1] pm; Si-
N-Si 124.9(6) [123.7], N-Si-Cl 108.3(5)[110.4], N-Si-C 115.1(8) [112.4
] and Cl-Si-C 107.3(7) [106.7]degrees.