Ab initio MO study of diverse Si3H3+ isomers

Structures and energies of many Si3H3+ isomers were investigated theoretica lly at the MP2/6-31G* level. The global minimum was the classical aromatic planar D-3h structure (5). Isodesmic equations indicate the resonance stabi lization 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 pi aromaticity. Five planar Si3H3+ isomers display cy clic three-center-two-electron (3c-2e) delocalization, and eight minima hav e 3c-2e Si-H-Si bridged bonds. The planar tetracoordinated silicon and five -coordinated silicon also are represented. Eleven other minima were found w ithin a 46 kcal/mol range. An H-bridged C-3v structure, derived from B3H6+, is 42.1 kcal/mol above the global minimum. However, for Ge, Sn, and Pb the se A(3)H(3)(+) forms are more stable than the classical structures (5, J. A m. Chem. Sec. 1995, 117, 11361). In contrast to Si3H3+, C3H3+ has only four isomers in the 189 kcal/mol range. The silicon analogues of the C3H3+ acyc lic structures, the prop-2-en-l-yl-3-ylidene cation and the 1-propynyl cati on, are not favorable.