Conformational preferences of fluorocyclohexane and 1-fluoro-1-silacyclohexane molecules: ab initio study and NBO analysis

The molecular structure of the axial and equatorial conformers of fluorocyclohexane and 1-fluoro-1-silacyclohexane, H₂C(CH₂CH₂)₂XH–F (X = C or Si), as well as the thermodynamic equilibrium between these species were investigated by means of quantum chemical calculations up to MP2/aug-cc-pVTZ level of theory. According to MP2 data, these compounds consist of a mixture of conformers with chair conformation and C _s symmetry differing in the axial and equatorial position of the C–F/Si–F bonds (axial = 42/56 mol%, equatorial = 58/44 mol%) at T = 298 K. This corresponds to a free energy difference of A = (G _ax ? G _eq) = 0.19/?0.13 kcal mol^?1 for X = C/Si. NBO analysis revealed that the axial conformer of 1-fluoro-1-silacyclohexane is an example of stabilization of the form that is unfavorable in terms of conjugation effects; stabilization is achieved mainly due to steric interactions. The equatorial conformer of fluorocyclohexane is an example of stabilization of the form that is unfavorable in terms of electrostatic interactions; stabilization is achieved due to steric and conjugation effects.