Conformational analysis—XV : Force field calculations and NMR determination of conformational equilibria of organosilicon compounds

Abstract

Force field calculations of the conformational energies of fifteen silanes are described. The calculated structures of silaethane, 2-silapropane and 2-methyl-2-silapropane are in acceptable agreement with available microwave data. The calculated torsional barriers of silaethane 2-silapropane; 2-methyl- 2-silapropane and 1-silapropane are consistent with reported barriers. In 2-silabutane and related compounds, the gauche conformation is more stable than the anti conformation as a result of attractive van der Waals energy terms. The strain energies of the two eclipsed conformations of 2-silabutane are identical and substantially lower than the strain energies of the two eclipsed conformations of butane which are of unequal energy. The shape of the torsional curve for 2-silabutane differs dramatically from that of butane. 1-Silabutane is stable in the anti conformation and the gauche-anti energy difference is similar to butane. The two eclipsed conformations of 1-silabutane stand in the same order as those for butane but are of higher energy. A comparison of the torsional curve for 1-silabutane with butane illustrates the steeper barriers for the former compound. Conformational equilibrium constants for 2-silabutane, 1-silabutane, and several compounds containing the 2-silabutane structure are obtained by NMR analysis of vicinal coupling constants and are in agreement with the calculated force field values. The conformational preferences of SiH 3, SiH 2, SiH(CH 3) 2, and Si(CH 3) 3 on cyclohexane are calculated. Unique features of silacyclohexane and the conformational preferences of hydrogen, methyl, and t-butyl on this ring are discussed.