Ab initio MO study of diverse Si<SUB>3</SUB>H<SUB>3</SUB><SUP>+</SUP> isomers

Structures and energies of many Si3H3+ isomers were investigated theoretically at the MP2/6-31G&#8727; level. The global minimum was the classical aromatic planar D3h structure (5). Isodesmic equations indicate the resonance stabilization energy to be half that of the analogous cyclopropenyl cation. The next lowest energy minimum, with a divalent silicon and a bridging hydrogen, also exhibits the 2&#960; aromaticity. Five planar Si3H3+ isomers display cyclic three-center-two-electron (3c-2e) delocalization, and eight minima have 3c-2e Si-H-Si bridged bonds. The planar tetracoordinated silicon and five-coordinated silicon also are represented. Eleven other minima were found within a 46 kcal/mol range. An H-bridged C3v structure, derived from B3H6+, is 42.1 kcal/mol above the global minimum. However, for Ge, Sn, and Pb these A3H3+ forms are more stable than the classical structures (5, J. Am. Chem. Soc. 1995, 117, 11361). In contrast to Si3H3+, C3H3+ has only four isomers in the 189 kcal/mol range. The silicon analogues of the C3H3+ acyclic structures, the prop-2-en-1-yl-3-ylidene cation and the 1-propynyl cation, are not favorable