Molecular structure and conformational preferences of 1-bromo-1- silacyclohexane, CH _2(CH _2CH _2) _2SiH-Br, as studies by gas-phase electron diffraction and quantum chemistry

摘要

The molecular structure of axial and equatorial conformer of the 1-bromo-1-silacyclohexane molecule, CH _2(CH _2CH _2) _2SiH-Br, as well as thermodynamic equilibrium between these species are investigated by means of gas-phase electron diffraction and quantum chemistry on the MP2(full)/SDB-AUG-cc-PVTZ level of theory. It is revealed that according to electron diffraction data, the compound exists in the gasphase as a mixture of conformers possessing the chair conformation of the six-membered ring and C _s symmetry and differing in the axial and equatorial position of the Si-Br bond (ax. = 80(5) mol %, eq. = 20(7) mol %) at 352 K, that corresponds to the value of A = (G _(ax) ° - G _(eq) °) = -0.82(32) kcal/mol. It is found that observed data agree well with theoretical ones. Using Natural Bond Orbital (NBO) analysis it is revealed that axial conformer of 1-bromo-1-silacyclohexane molecule is an example of the stabilization of the form that is unfavorable from the point of view of steric effects and effects of conjugations. It is concluded that stabilization is achieved due to electrostatic interactions.