Strain Energy Calculations of Caged Silanes

Abstract

Strain energies (SE) of typical caged silanes were evaluated at the B3LYP/6-31+G** and B3LYP/aug-cc-pVDZ levels, in combination with the isodesmic and homodesmotic reactions. The SE values of Si4H4 and Si8H8 are 512 kJ/mol and 336–338 kJ/mol, respectively, at the B3LYP/6-31+G** level, whereas the SE values of the caged silanes with five-number or larger rings are very small. In comparison, the SE value of Si8H8 is much smaller than 656–707 kJ/mol of cubane (C8H8), since the Si‒Si bond length (2.274 Å) of Si8H8 is much larger than the corresponding C‒C bond length of cubane. The large electron charge density at body-center of Si4H4 causes strong repulsion between the cage center and Si‒Si bond, which leads to the Si‒Si bond bending. The electron charge density at the face center of Si4H4 is more than two times of that of Si8H8, which also contributes significantly to the large SE difference between Si4H4 and Si8H8.