Conformational Analysis of 1,4-Disilabutane and 1,5-Disilapentane by Combined Application of Gas-Phase Electron Diffraction and ab Initio Calculations and the Crystal Structure of 1,5-Disilapentane at Low Temperatures

Abstract

The gas-phase structures of the conformers of 1,4-disilabutane and 1,5-disilapentane have been analyzed from electron-diffraction data augmented by flexible restraints derived from ab initio calculations. This allowed the simultaneous refinement of 22 and 29 parameters for 1,4-disilabutane and 1,5-disilapentane, respectively. 1,4-Disilabutane has been found to be present in the vapour predominantly in the anti (A) form (76(2)% from the experiment, 83% predicted by theory). Consistency in the geometries is found between theoretical predictions and experimental findings, except for the torsion angle ∠(SiCCSi) of the gauche (G) conformer [exptl 78.5(21)°, theor 68.0°]. The AA conformer of 1,5-disilapentane was always found to be the lowest energy structure, while some doubt still remains about the ordering of the AG and G(+)G(−) conformers. The AA conformer is found to be the sole form present in the crystal [C2/c, a = 15.585(8), b = 4.704(3), c = 9.895(6) Å, β = 95.77(4)°, Z = 4]. Good agreement is found for geometrical parameters determined experimentally in the gas phase and solid state and calculated by ab initio methods. The following values represent the most important distances (r g/Å) and angles (∠g/deg) found for the gas phase and crystal structures. 1,4-Disilabutane GED (A/G, esd's correspond to 1σ): r(CSi) 1.882(1)/1.885(1), r(CC) 1.563(5)/1.563(5), r(SiH) both 1.499(3), ∠(CCSi) 110.7(2)/114.4(5), ∠(SiCCSi) 180.0/78.5(21). 1,5-Disilapentane GED [AA/G(+)G(−)]: r(CSi) 1.886(1)/1.888(1), r(CC) 1.537(2)/1.539(2), r(SiH) both 1.487(4), ∠(CCC)114.8(7)/118.8(7), ∠(CCSi) 114.1(4)/116.8(7), ∠(SiCCC) 180.0/60.9(10); X [%, AA/AG/G(+)G(−)] 28(4)/40(5)/26(6). 1,5-Disilapentane XRD: r(CSi) 1.868(2), r(CC) 1.527(2), ∠(CCC) 113.8(2), ∠(CCSi) 115.2(1), ∠(SiCCC) 180.0(1).

Intramolecular π–π Interactions in Flexibly Linked Partially Fluorinated Bisarenes in the Gas Phase

“…Ther elative energies (Table 1) reveal that the conformers with intramolecular p stacking are strongly preferred by about 10-18 kJ mol À1 for all three compounds,d espite Scheme 1. repulsive syn-pentane interactions [20] between the different phenyl groups.T he dihedral angles along the C-C-C/Si backbone differ from ideal local gauche conformations by about 108 8 [7: F(Ph f -Si-C-C) = 748 8, F(Si-C-C-Ph) = 548 8]. [13] Scheme 2.…”

Structures of Tetrasilylmethane Derivatives (XMe2Si)2C(SiMe3)2 (X = H, Cl, Br) in the Gas Phase, and their Dynamic Structures in Solution

“…The SARACEN [58][59][60] method was used for the refinement of experimental data, with the required restraints based upon comparison of calculations at the MP2, B3LYP, and M06-2X levels of theory, and with 6-31G(d) and aug-cc-pVDZ basis sets (using aug-cc-pVDZ-PP as a pseudopotential for Br in 4). Vibrational corrections were based upon data from SHRINK [51,52], calculated using force constants obtained from GAUSSIAN. As is common, because they are not particularly well defined from the GED data, restraints were placed upon the distance difference parameters, as well as parameters associated with hydrogen atoms.…”

Gas-phase structures of sterically crowded disilanes studied by electron diffraction and quantum chemical methods: 1,1,2,2-tetrakis(trimethylsilyl)disilane and 1,1,2,2-tetrakis(trimethylsilyl)dimethyldisilane

“…For each molecule parameters p 1 -p 4 are distances, while parameters p 5 -p 20 are angles. Different numbers of dihedral angles are required to describe 1 and 2, with p 21 -p 31 and p 21 -p 32 being used, respectively, to rotate the trimethylsilyl groups, the methyl groups, and one SiR(SiMe 3 ) 2 [R = H (1), CH 3 (2)] group with respect to the other.…”