Conformations of silicon-containing rings Part 1. A conformational study on 1,3,5-trisilacyclohexane. Comparison of ab initio, semiempirical, and molecular mechanics calculations. Conformational energy surface of 1,3,5-trisilacyclohexane

The structure and relative energies for the basic conformations of 1,3,5-trisilacyclohexane (1) have been calculated by several methods and their performance compared. It is found that HF ab initio calculations using the basis set 6-31G*, the SV(P) basis set used by TURBOMOLE and the MM3 force field produce mutually fairly consistent results. MM2 performs not as well as MM3, but in many cases MM2 performs better than 3-21G. Three semiempirical methods (AM1, MNDO, and PM3) were tested. None of them was found to produce reliable results. It is found by ab initio (6-31G* and SV(P)) and MM3 calculations that the dihedral angles for the chair conformation are 52.7-53.1°, which makes (1) more flattened than cyclohexane, and thus (1) does not exhibit behaviour similar to cyclohexasilane, which is less flattened than cyclohexane. The ring flattening of (1) is mainly caused by the intrinsically large SiCSi bond angle (114.1-114.8°). The twist conformation of (1) is found by the same calculations to be 2.1-3.1 kcal mol^-1 higher in energy than the chair conformation, and the boat form is found to be 0.3-0.4 kcal mol^-1 higher than the twist form. These values are much closer to the values for cyclohexasilane than to those for cyclohexane. The conformational energy surface of (1) has been calculated by using MM3. The energy barrier from the chair to the twist conformation of (1) is found to be 5.5 kcal mol^-1.