Ab Initio Study of the Effect of Heteroatoms and Bulky Substituents on the Strain Energies of Cyclosilanes

In an attempt to estimate the strain energy of three-, four-, and five-membered cyclosilanes substituted with a heteroatom or group, [R 2 Si] n X ( n = 2, 3, and 4; X = CH 2, NCH 3, O, SiH 2, PCH 3, and S; R = H and i -Pr), we have performed high-level ab initio molecular orbital calculations, some involving the ONIOM method. It is found that the hetero group X with a third-row element brings about the release of ring strain compared to the second-row analogues and that the effect of bulky substituents R on the strain energy works differently in the cyclosilanes with lone-pair electrons than in those without them.