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

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.