Ab initio MO study of diverse Si₃H₃⁺ isomers

Abstract

Structures and energies of many Si₃H₃⁺ 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 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 C_3v 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.