Molecular structure of 1,2-bis(trifluoromethyl)-1,1,2,2-tetramethyldisilane in the gas, liquid, and solid phases : Unusual conformational changes between phases

The molecular structure of 1,2-bis(trifluoromethyl)-1,1,2,2-tetramethyldisilane has been determined in three different phases (solid, liquid, and gas) using various spectroscopic and diffraction techniques. Both the solid-state and gas-phase investigations revealed only one conformer to be present in the sample analyzed, whereas the liquid phase revealed the presence of three conformers. The data have been reproduced using computational methods and a rationale is presented for the observation of three conformers in the liquid state

Conformational properties of 1-cyano-1-silacyclohexane, C5H10SiHCN: Gas electron diffraction, low-temperature NMR and quantum chemical calculations

The conformational preference of the cyano group of the 1-cyano-1-silacyclohexane was studied experimentally by means of gas electron diffraction (GED) and dynamic nuclear magnetic resonance (DNMR) as well as by quantum chemical (QC) calculations applying high-level coupled cluster methods as well as DFT methods. According to the GED experiment, the compound exists in the gas-phase as a mixture of two conformers possessing the chair conformation of the six-membered ring and Cs symmetry while differing in the axial or equatorial position of the substituent (axial = 84(12) mol %/equatorial = 16(12) mol %) at T = 279(3) K, corresponding to an A value (Gax – Geq) of −1.0(4) kcal mol−1. Gas-phase CCSD(T) calculations predict an A value of −0.72 kcal mol−1 at 279 K. In contrast, the low-temperature 13C NMR experiments resulted in an axial/equatorial ratio of 35/65 mol % at 120 K corresponding to an A value of 0.14 kcal mol−1. An average value for ΔG#e→a = 5.6 ± 0.1 kcal mol−1 was obtained for the temperature range 110–145 K. The dramatically different conformational behaviour in the gas-phase (GED) compared to the liquid phase (DNMR) suggests a strong solvation effect. According to natural bond orbital analysis the axial conformer of the title compound is an example of stabilization of a form, which is not favored by electrostatic effects and is favored predominantly by steric and conjugation effects.A.V.B. and Yu.F.S. are grateful to the Ministry of Education and Science of Russia (State Contracts N 14.B25.31.0013) for financial support. S.A.Sh. thanks the Russian Foundation for Basic Research (Grant 14-03-0023-a). I.A., S.O.W., and N.R.J. thank the Icelandic Centre for Research (RANNIS) for financial support, Grants No 080038021 and 100040022. R.B. acknowledges support from the Icelandic Research Fund, grant no. 141218051

Rannsóknir á stellingajafnvægi setinna silacyclohexan afleiða

The monosubstituted silacyclohexanes; C5H10SiHX with X = F, Cl, Br, I, SiH3, OMe, N(Me)2, tBu, CN and N3 (3-12) were synthesized with the main aim of investigating the molecular structure of their axial and equatorial chair conformers as well as the thermodynamic equilibrium between these species. These derivatives were intended for investigations by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR) and temperature dependent Raman spectroscopy. This work is an ongoing project and has as of yet not been completed for all the derivatives. When possible these experimental results in the gas and liquid phase were compared to the quantum chemical calculations available (MP2, DFT and composite methods) and to results for analogous cyclohexane derivatives. The A values for the monosubstituted silacyclohexanes were found to be substantially lower than for the corresponding cyclohexane analogues and in all cases except for one, they show a preference of the axial conformation. The following 1,1-disubstituted silacyclohexane rings, C5H10SiXY with X = Me and Y = F, X = Me and Y = CF3 as well as X = SiF3 and Y = F (13, 14 and 16) were also synthesized, so as to be investigated with respect to their conformational properties, using the same methods as described above. C5H10SiXY with X = Me and Y = D (15) was synthesized in order to be investigated by Raman spectroscopy to complete the investigation of C5H10SiHMe (1). The available results indicate that a simple additive model, which assumes that the conformational properties derived from the relevant monosubstituted rings are either complementary or competitive, cannot be applied and more sophisticated model is required. CF3Me2SiSiMe2CF3 (17) was synthesized and its seemingly unusual conformational properties were investigated by gas electron diffraction (GED), temperature dependent Raman spectroscopy and quantum chemical calculations (QC). The results indicate that there is only one conformer in the gaseous phase, while three rotamers were observed in the liquid phase. The novel compounds (C5H10SiH)2NMe (19) and (C5H10SiH)2NH (24) were also synthesized and the process of conformational analysis was initiated by performing preliminary QC calculations

Conformational Properties of 1‑Halogenated-1-Silacyclohexanes, C₅H₁₀SiHX (X = Cl, Br, I): Gas Electron Diffraction, Low-Temperature NMR, Temperature-Dependent Raman Spectroscopy, and Quantum-Chemical Calculations

The molecular structures of axial and equatorial conformers of <i>cyclo</i>-C₅H₁₀SiHX (X = Cl, Br, I) as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction, dynamic nuclear magnetic resonance, temperature-dependent Raman spectroscopy, and quantum-chemical calculations applying CCSD­(T), MP2, and DFT methods. According to the experimental and calculated results, all three compounds exist as a mixture of two chair conformers of the six-membered ring. The two chair forms of <i>C_s</i> symmetry differ in the axial or equatorial position of the X atom. In all cases, the axial conformer is preferred over the equatorial one. When the experimental uncertainties are taken into account, all of the experimental and theoretical results for the conformational energy (<i>E</i>_axial – <i>E</i>_equatorial) fit into a remarkably narrow range of −0.50 ± 0.15 kcal mol^–1. It was found by NBO analysis that the axial conformers are unfavorable in terms of steric energy and conjugation effects and that they are stabilized mainly by electrostatic interactions. The conformational energies for C₆H₁₁X and <i>cyclo</i>-C₅H₁₀SiHX (X = F, Cl, Br, I, At) were compared using CCSD­(T) calculations. In both series, fluorine is predicted to have a lower conformational preference (cyclohexane equatorial, silacyclohexane axial) than Cl, Br, and I. It is predicted that astatine would behave very similarly to Cl, Br, and I within each series