THE MOLECULAR-STRUCTURES AND CONFORMATIONS OF BIS(DICHLOROSILYL)AMINEAND BIS(DICHLOROSILYL)METHYLAMINE IN THE GAS-PHASE - DETERMINATION BYELECTRON-DIFFRACTION AND BY AB-INITIO CALCULATIONS
H. Fleischer et al., THE MOLECULAR-STRUCTURES AND CONFORMATIONS OF BIS(DICHLOROSILYL)AMINEAND BIS(DICHLOROSILYL)METHYLAMINE IN THE GAS-PHASE - DETERMINATION BYELECTRON-DIFFRACTION AND BY AB-INITIO CALCULATIONS, Chemische Berichte, 128(8), 1995, pp. 807-815
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.