Conformations of silicon-containing rings.: II a conformational study on silacyclohexane.: Comparison of ab initio (HF, MP2), DFT, and molecular mechanics calculations.: Conformational energy surface of silacyclohexane

The structures and relative energies for the basic conformations of silacyclohexane 1 have been calculated using HE RI-MP2, RI-DFT and MM3 methods. All methods predict the chair form to be the dominant conformation and all of them predict structures which are in good agreement with experimental data. The conformational energy surface of 1 has been calculated using MM3. It is found that there are two symmetric lowest energy pathways for the chair-to-chair inversion. Each of them consists of two sofa-like transition states, two twist forms with C-1 symmetry (twist-C-1), two boat forms with Si in a gunnel position (C1 symmetry), and one twist form with C-2 symmetry (twist-C-2). All methods calculate the relative energy to increase in the older chair < twist-C-2 < twist-C-1 < boat. At the MP2 level of theory and using TZVP and TZVPP (Si atoms) basis sets the relative energies are calculated to be 3.76. 4.80, and 5.47 kcal mol(-1) for the twist-C-2, twist-C1, and boat conformations, respectively. The energy barrier from the chair to the twisted conformations of 1 is found to be 6.6 and 5.7 kcal mol(-1) from MM3 and RI-DFT calculations, respectively. The boat form with Si at the prow (C-s symmetry) does not correspond to a local minimum nor a saddle point on the MM3 energy surface, whereas a RI-DFT optimization under C-s symmetry constraint resulted in a local minimum. In both cases its energy is above that of the chair-to-twist-C-1 transition state, however, and it is clearly not a part of the chair-to-chair inversion.