Conformational properties of six-membered heterocycles: accurate relative energy differences with DFT, the importance of dispersion interactions and silicon substitution effects.

Density functional methods were evaluated in their ability to predict relative conformational energies of a test set of monosubstituted cyclohexanes and six-membered heterocycles. It is shown that while popular density functionals like B3LYP are unreliable for predicting accurate conformational energies for the axial/equatorial equilibrium of monosubstituted cyclohexanes, 1-silacyclohexanes and tetrahydropyrans, density functionals that take into account dispersion interactions like M06-2X and B2PLYP-D result in energy differences close to CCSD(T)/CBS results. Using the M06-2X density functional, we have then investigated the conformational properties of a large number of monosubstituted silacyclohexanes, with the number of silicon atoms ranging from 1 to 6. Our calculations suggest remarkably different conformational properties when compared to cyclohexane. The carbon/silicon exchange in a cyclohexane ring often has systematic, yet counterintuitive effects on the conformational properties. Dispersion interactions are shown to be especially important for accurate relative energy calculations of polysilacyclohexanes.