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