Strain Energies of Silicon Rings and Clusters

Abstract

Strain energies of silicon ring and cluster compounds can be calculated as energy changes of homodesmotic reactions that convert cyclic structures into acyclic molecules. The energy changes of these reactions can be calculated by taking differences between ab initio energies of products and reactants. Since homodesmotic reactions conserve bond types and preserve atomic valence environments, one can anticipate cancellation of much of basis set and electron correlation errors in individual molecular energies when energy differences are taken. This study involved ab initio geometry-optimized calculations at both the RHF and MP2 levels using the 6-31G** basis set. Calculated strain energies of the cyclosilanes (SiH2)n can be compared with experimental estimates and with the well established strain energies of the cycloalkanes (CH2)n. Strain energies of the polyhedral silanes (SiH)2n can be compared with those of the isostructural hydrocarbons. Except for tetrahedral (SiH)4 and (CH)4, which have large and comparable strain energies, and cyclooctatetraene structures, which have negligible strain energies, the silicon clusters have uniformly smaller strain energies than do the related hydrocarbons. These differences can be rationalized using the rule of additivity of individual ring strain energies. The resonance energy of planar hexagonal (SiH)6 is less that that for benzene (CH)6, but both of these quantities are modest stabilizing influences compared to the destabilizing strain energies associated with isomeric structures. The relative energies of the sila analogs of the valence isomers of benzene can be interpreted as resulting from differences in numbers of single and double bonds, the average energies associated with these bonds, and resonance energies and strain energies. These considerations allow an estimate of the energy of the SiSi double bond: 101 kcal/mol.