THE MOLECULAR-STRUCTURES AND CONFORMATIONS OF BIS(DICHLOROSILYL)AMINE AND BIS(DICHLOROSILYL)METHYLAMINE IN THE GAS-PHASE - DETERMINATION BY ELECTRON-DIFFRACTION AND BY AB-INITIO CALCULATIONS

The gas-phase molecular structures of bis(dichlorosilyl)amine, (HSiCl2)(2)NH, and bis(dichlorosilyl)methylamine, (SiHCl2)(2)NMe, have been investigated by electron diffraction and ab initio calculations on a MP2/6-31G* level. Because the latter suggest the presence of a mixture of two conformers for each molecule, attempts were made to fit the electron diffraction data in each case by two different models, one assuming the presence of only one conformation, and another allowing for two different conformers. The principal parameters (r(a)) from electron diffraction for (HSiCl2)(2)NH are (the appropriate ab initio calculated values are given in brackets []): r(Si-N) 171.9(2) [172.7], r(Si-Cl) 204.1(1) [204.5] pm; Si-N-Si 126.5(8) [127.4], N-Si-Cl 107.3(5) [110.8] and Cl-Si-Cl 106.5(3) [107.5]degrees. For (SiHCl2)2NMe: r(Si-N) 171.1(3) [171.5], r(Si-Cl) 204.0(9) [205.2], r(N-C) 149.7(10) [148.3] pm; Si-N-Si 124.9(13) [123.4], N-Si-Cl 109.7(5) [110.7] and Cl-Si-Cl 107.1(3) [107.2]degrees. For (HSiCl2)(2)NH three different conformers were found to fit the experimental data with the single-conformation model. In two of the conformers, the dichlorosilyl groups are twisted by Theta(1) = 146.0(10)/Theta 2 = 155.2(20)degrees and Theta(1) = 101.1(49)/Theta(2) = 196.8(12)degrees respectively counterclockwise from the positions in which the Si-H bonds eclipse the N-H bond. Both of these lead to nearly ideally staggered conformations of the two SiHCl2 groups when viewed along the Si...Si axis. In contrast, the third conformation is nearly eclipsed, with torsion angles Theta(1) = 131.4(15)/Theta(2) = 80.4(26)degrees. Planarity of the Si2NH group was assumed as suggested by ab initio calculation. For (HSiCl2)(2)NMe the experimental data could be suitably fitted by two conformers, in which the dichlorosilyl groups are twisted by Theta(1) = 161.8(13)/Theta(2) = 63.1(23)degrees and Theta(1) = 156.1(9)/Theta(2) = 177.1(15)degrees from the positions in which the Si-H bonds eclipse the N-C bond. The first of these structures has an eclipsed Si(NSi)Cl conformation and an Si...Cl nonbonded distance close to the sum of the appropriate van der Waals radii. MP2/6-31G* calculations support the GED bond lengths and bond angles of both molecules, but indicate different conformations for the most stable equilibrium geometries: the computed twists are Theta(1) = 180/Theta(2) = 180 degrees and Theta(1) = 177.0/Theta(2) = 183.0 degrees for (HSiCl2)(2)NH and (HSiCl2)(2)NMe, respectively. Rotation about the Si-N bonds is a very facile process, cf. the computed [MP2/6-31G*//MP2/6-31G* + ZPE(SCF/6-31G*)] rotational barrier of 2.9 kJ . mol(-1) for (HSiCl2)(2)NH.