STRAIN ENERGIES OF SILICON RINGS AND CLUSTERS

Strain energies of silicon ring and cluster compounds can be calculate d as energy changes of homodesmotic reactions that convert cyclic stru ctures into acyclic molecules. The energy changes of these reactions c an be calculated by taking differences between ab initio energies of p roducts and reactants. Since homodesmotic reactions conserve bond type s and preserve atomic valence environments, one can anticipate cancell ation of much of basis set and electron correlation errors in individu al molecular energies when energy differences are taken. This study in volved nb initio geometry-optimized calculations at both the RHF and M P2 levels using the 6-31G* basis set. Calculated strain energies of t he cyclosilanes (SiH2)(n) can be compared with experimental estimates and with the well established strain energies of the cycloalkanes (CH2 )(pi). Strain energies of the polyhedral silanes (SiH)(2n) can be comp ared with those of the isostructural hydrocarbons. Except for tetrahed ral (SiH)(4) and (CH)(4), which have large and comparable strain energ ies, and cyclooctatetraene structures, which have negligible strain en ergies, the silicon clusters have uniformly smaller strain energies th an do the related hydrocarbons. These differences can be rationalized using the rule of additivity of individual ring strain energies, The r esonance energy of planar hexagonal (SiH)(6) is less that that for ben zene (CH)(6), but both of these quantities are modest stabilizing infl uences compared to the destabilizing strain energies associated with i someric structures, The relative energies of the sila analogs of the v alence isomers of benzene can be interpreted as resulting from differe nces in numbers of single and double bonds, the average energies assoc iated with these bonds, and resonance energies and strain energies. Th ese considerations allow an estimate of the energy of the Si double bo nd Si double bond: 101 kcal/mol.