ChemInform Abstract: Ab initio MO Study of Diverse Si3H3+ Isomers.

Abstract

Structures and energies of many Si 3 H 3 + isomers were investigated theoretically at the MP2/6-31G ∗ level. The global minimum was the classical aromatic planar D 3h 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 2π aromaticity. Five planar Si 3 H 3 + 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 C 3v structure, derived from B 3 H 6 + , is 42.1 kcal/mol above the global minimum. However, for Ge, Sn, and Pb these A 3 H 3 + forms are more stable than the classical structures (5, J. Am. Chem. Soc. 1995, 117, 11361). In contrast to Si 3 H 3 + , C 3 H 3 + has only four isomers in the 189 kcal/mol range. The silicon analogues of the C 3 H 3 + acyclic structures, the prop-2-en-1-yl-3-ylidene cation and the 1-propynyl cation, are not favorable.