Conformational preference of the silyl group. Ab initio and molecular mechanics studies

Abstract

The energy difference between the equatorial and axial conformations (the A-value) for the methyl and silyl substituents in monocyclohexane has been estimated by ab initio molecular orbital and molecular mechanics methods. MP4(SDTQ)/6-31G ∗//MP2/6-31G ∗ results predict the A-values to be 1.90 kcal mol −1 for the methyl group, and 1.69 kcal mol −1 for the silyl group, respectively. After including zero-point vibrational energies, thermal vibrational energies, and entropy corrections, the A-values become 2.14 kcal mol −1 for the methyl group and 1.90 kcal mol −1 for the silyl group, respectively. These values are about 0.5 kcal mol −1 higher than those observed by NMR experiments. However, MM3 estimates the A-values of the methyl and silyl groups as 1.78 and 1.16 kcal mol −1, respectively. The slightly smaller A-value of the silyl group with respect to the methyl group appears to be ascribed to the reduced steric interactions due to the longer CSi bond compared to the CC bond. A perfect correlation between the A-values and the gauche/anti energy differences may indicate that the MM3 calculated A-values entirely depend on the selected values of the torsional parameters.