TRISILAALLYL CATION AND RELATED SI3H5- DEPARTURES FORM THE CORRESPONDING CARBON SPECIES( STRUCTURES )

The MP2(fc)/6-31G structures, energies and frequencies of the trisila allyl cation (1) and a number of its Si3H5+ isomers reveal considerabl e differences from the C3H5+ potential energy surface. Unlike the ally l cation, 1 is not a minimum. A ''quasicyclic'' form (7), with a much shorter Si(1)Si(3) separation and aromatic character, is the global Si 3H5+ minimum at MP2 and at B3LYP/6-311+G*. However, Raghavachari's (J . Chem. Phys., 95 (1991) 7373) double-bridged isomer (5) is 1.4 kcal m ol(-1) lower in energy than 7 at the CCSD(T)/6-311+G//MP2/6-31G* leve l. Conjugation in the planar C-2v 1 and hyperconjugation in the rotate d 9 form are decreased relative to the carbon analogs. The remarkable stability of 5 and 7 can be attributed to their effective charge deloc alization.