THE MOLECULAR-STRUCTURES AND CONFORMATIONS OF BIS(DICHLOROSILYL)AMINEAND BIS(DICHLOROSILYL)METHYLAMINE IN THE GAS-PHASE - DETERMINATION BYELECTRON-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 i nvestigated by electron diffraction and ab initio calculations on a MP 2/6-31G level. Because the latter suggest the presence of a mixture o f two conformers for each molecule, attempts were made to fit the elec tron diffraction data in each case by two different models, one assumi ng the presence of only one conformation, and another allowing for two different conformers. The principal parameters (r(a)) from electron d iffraction 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 The ta(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 pos itions in which the Si-H bonds eclipse the N-H bond. Both of these lea d to nearly ideally staggered conformations of the two SiHCl2 groups w hen 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 sugges ted by ab initio calculation. For (HSiCl2)(2)NMe the experimental data could be suitably fitted by two conformers, in which the dichlorosily l groups are twisted by Theta(1) = 161.8(13)/Theta(2) = 63.1(23)degree s and Theta(1) = 156.1(9)/Theta(2) = 177.1(15)degrees from the positio ns in which the Si-H bonds eclipse the N-C bond. The first of these st ructures has an eclipsed Si(NSi)Cl conformation and an Si...Cl nonbond ed distance close to the sum of the appropriate van der Waals radii. M P2/6-31G calculations support the GED bond lengths and bond angles of both molecules, but indicate different conformations for the most sta ble equilibrium geometries: the computed twists are Theta(1) = 180/The ta(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 (HS iCl2)(2)NH.