ABINITIO CALCULATIONS ON SILICON ANALOGS OF THE ALLYL RADICAL

Calculations at the CI-SD/6-31G* level of theory have been performed in order to determine the effect on the A-H bond dissociation energy (BDE) in X-AH3 (A = C and A = Si) of replacing X = H3C- by X = H2C = CH- and H2Si = CH- and of replacing X = H3Si- by X = H2C = SiH- and H2Si = SiH-. The conjugative stabilization energies (CSEs) of the resulting allylic radicals have been obtained from the energies calculated to be required to twist a terminal AH2. group out of conjugation. Except for X = H2C = CH- and H2Si = CH- in X-CH2., these two definitions of allylic resonance energy give nearly identical values. The reason for the calculated difference between -DELTA-BDE and CSE in H2C = CH-CH2. and in H2Si = CH-CH2. is discussed. It is also found that, with the exception of H2Si = CH-SiH2., the calculated allylic resonance energies are generally those expected from the pi-BDEs in H2C = CH2, H2C = SiH2, and H2Si = SiH2. It is shown that pyramidalization at silicon lowers the resonance energy of H2Si = CH-SiH2. from that expected, based on the strength of the C-Si pi-bond in H2C = SiH2.