Tore H. Johansen

1,1,1,3,3,3-Hexabromotrisilane: structure and conformation determined by gas-phase electron diffraction, ab initio molecular orbital and molecular mechanics calculations, and vibrational spectroscopy

Abstract The molecular structure of 1,1,1,3,3,3-hexabromotrisilane at 140°C has been studied using gas-phase electron diffraction. The two SiBr3 groups are both staggered relative to the central SiH2 group, but a twist of about 13° of the SiBr3 groups relative to the exactly staggered position reduces the symmetry to C2. From the vibrational spectra a distinction between C2 or C2v symmetry is possible, and point group C2v can be ruled out by arguments based on the selection rules. Bond lengths (rg) and valence angles ( o α ) are: r( Si  Si ) = 2.344(18) A , r( Si  Br ) = 2.205(4) A o SiSiSi = 112.9(19)° , o 〈 BrSiBr 〉 = 109.6(6)° (average BrSiBr angle), and o 〈 SiSiBr 〉 = 109.3(6)° (average SiSiBr angle). Error limits are given as 2σ where σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data. The infrared and Raman vibrational spectra have been assigned using normal coordinate calculations as well as ab initio calculations, and harmonic force constants are reported. In addition molecular mechanics and ab initio (RHF/SBK and RHF/STO-3G calculations were performed to assist the structural analysis and to compare with the diffraction results.

Vinyl dichlorosilane and vinyl dibromosilane (H2CCH–SiHX2, X=Cl, Br): conformational structure and vibrational properties determined by gas-phase electron diffraction, ab initio molecular orbital calculations, and variable-temperature Raman spectroscopy

Abstract The molecular structures, conformations, vibrational spectra, and torsional potentials of vinyl dichlorosilane (VDC) H2C CH–SiHCl2, and vinyl dibromosilane (VDB) H2C CH–SiHBr2, have been studied using gas-phase electron diffraction (GED) data at 23–25°C and variable-temperature Raman spectroscopy, together with ab initio molecular orbital calculations. The GED data were handled by a dynamic theoretical model using a cosine Fourier potential function in describing the torsional coordinate. According to the GED refinements, these molecules exist in the gas phase at room temperature as a mixture of two minimum energy conformers, syn (torsional angle φ(CCSiH)=0°) and gauche (torsional angle φ(CCSiH)≈120°). Relevant structural parameters for syn-VDC are as follows: Bond lengths (rg): r( Si–C )=1.847(5) A , r (Si–Cl) =2.042(2) A , r( C C )=1.357(7) A . Bond angles (∠α): ∠CSiCl=110.3(6)°, ∠CCSi=121.8° (calc.). Relevant structural parameters for syn-VDB are as follows: bond lengths (rg): r (Si–C) =1.827(9) A , r (Si–Br) =2.206(2) A , r (C C) =1.366(10) A . Bond angles (∠α): ∠CSiBr=110.1(8)°, ∠CCSi=121.7° (calc.). Uncertainties are given as 2σ (σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data). From the variable-temperature Raman investigation in the liquid phase, the energy differences are: VDC, Δ E° S−G =+0.11± 0.06 kcal mol −1 ; VDB, Δ E ° S−G =+0.23±0.07 kcal mol −1 . The Raman energies are average values obtained from two separate line doublets for each molecule, and they have been used in the GED least-squares refinements as valuable constraints.

Chloromethyldichloromethylsilane and chloromethyldimethylchlorosilane: structure, conformational composition and torsional potential determined by gas-phase electron diffraction and ab initio molecular orbital calculations

Abstract The molecular structure and conformation of chloromethyldichloromethylsilane (TCS), ClH2CSiCl2CH3, and chloromethyldimethylchlorosilane (DCS), ClH2CSiCl(CH3)2, have been studied using gas-phase electron diffraction at 24°C and ab initio molecular orbital calculations. These molecules exist in the gas phase as a mixture of two conformers, anti (A, torsion angle ∅(ClCSiX) = 180°, X = CH3 (TCS) or Cl (DCS)) and gauche (G, torsion angle (∅(ClCSiX) close to 60°). Some relevant structural parameters for TCS (gauche) are: bond lengths (rg), r( SiC(Cl) ) = 1.877(5) A , r( SiCH 3 ) = 1.854(5) A , r( CCl ) = 1.787(7) A , r( 〈 SiCl 〉 ) = 2.046(3) A (average value); bond angles (∠α), ∠CSiC = 113.6(23)°, ∠SiCCl = 110.4(6)°, ∠ 〈 CSiCl 〉 = 108.8(5)°, ∠ClSiCl = 107.9(12)°; torsion angle, ∅(G) = 51(5)°. For TCS the experimental gas-phase composition (%) was (anti/gauche) 16 84 with error limits ± 15%. With ΔS° A-G = −R ln 2 + ΔS∗ , where ΔS∗ is the entropy difference from vibrational and rotational partition functions calculated from ab initio, an estimated conformational energy difference ΔE°A-G = 0.6(± 0.7) kcal mol−1 was obtained for TCS. Some relevant structural parameters for DCS (anti) are: bond lengths (rg), r( SiC(Cl) ) = 1.888(3) A , r( SiCl ) = 2.078(5) A , r( CCl ) = 1.793(14) A ; bond angles (∠α), ∠ CSiC = 111.5°(assumed), ∠SiCCl = 110.7(10)°, ∠ClSiC(Cl) = 104.0°(assumed), ∠SiCH = 112.3(15)°; torsion angles, ∅(A) = 161(3)°, ∅(G) = 70(8)°. For DCS the anti conformer showed an apparent deviation from the exactly staggered value of 180°, but this is a result of large-amplitude torsional motion about the SiC bond. For DCS the experimental composition was (anti/gauche) 59 41 with error limits ± 16%. Using ΔS° A-G = −R ln 2 + ΔS∗ an estimated conformational energy difference ΔE°A-G = −0.6(± 0.4) kcal mol−1 was obtained from this composition. Error limits are given as 2σ (σ includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data). Full geometry optimizations were performed for all conformers of both molecules employing ab initio molecular orbital HF/6–31G(d) level of theory, while conformational energy calculations were performed at the MP2(fc)/6–311 + G(d,p)//HF/6–31G(d) level, with scaled zero-point vibrational energy corrections from frequency calculations at the HF/6–31G(d) level. The theoretical energies and geometries are compared with the experimental results.

Symmetrically substituted silanes: (XH2C)2SiH2, (XH2C)2SiX2, (X2HC)2SiH2 and (X2HC)2SiX2 with X F, Cl or Br. Conformational energies, structures and torsional force constants obtained by molecular-mechanics calculations

Abstract A series of twelve symmetrically halogen-substituted silanes (XH 2 CSiY 2 CH 2 X and X 2 HCSiY 2 CHX 2 , with Y  H or X, and X  F, Cl or Br) have been investigated using molecular-mechanics calculations, with bonding parameters and interaction potentials derived from earlier gas-phase studies on halogenated alkanes. For the molecules FH 2 CSiH 2 CH 2 F and FH 2 CSiF 2 CH 2 F the low-energy conformers are GG and GG″, while for the molecules F 2 HCSiH 2 CHF 2 and F 2 HCSiF 2 CHF 2 only AA is a low-energy form. The low-energy form of the molecules (X = Cl or Br) XH 2 CSiH 2 CH 2 X and XH 2 CSiX 2 CH 2 X is GG. For Cl 2 HCSiH 2 CHCl 2 low-energy forms are AG and GG, while for Cl 2 HCSiCl 2 CHCl 2 low-energy forms are AA, AG and GG. Only GG is a low-energy conformer in Br 2 HCSiBr 2 CHBr 2 . The conformers AA, AG, GG and GG″ have staggered terminal groups relative to the central group. However, in Br 2 HCSiH 2 CHBr 2 one of the low-energy forms found is AS, possessing one eclipsed terminal group, while the other low-energy form is GG.