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Since carbon molecules with geodesic structures have been detected, the question for potential silicon-based analogues of these species has been raised. The existence of these hypothetical `silicon balls', however, has not been ascertained so far in spite of several theoretical as well as experimental attempts to do so. Several theoretical approaches have been put forward in the past to explain the experimentally observed features of Si N clusters and thus to account for the obvious dissimilarity between finite systems based on the isovalent elements silicon and carbon, but no comprehensive growth sequence model has been generally accepted for Si N so far. The present work is aimed at an exploration of closed-shell Si N systems, derived from highly symmetric cage-like geometries, in the size range N ≤26 by means of Hartree–Fock and post-Hartree–Fock procedures. Particular emphasis is placed on the question of the stabilities of these isomers as compared to the corresponding ground states, wherever available. Cohesive energy calculations suggest a tendency towards an increase of cluster stability with the number of constituents for cage-like Si N isomers up to N =10, while a trend towards destabilization with growing size is found for larger clusters.

The MP2(fc)/6-31G∗ structures, energies and frequencies of the trisilaallyl cation (1) and a number of its Si3H5+ isomers reveal considerable differences from the C3H5+ potential energy surface. Unlike the allyl cation, 1 is not a minimum. A “quasi-cyclic” form (7), with a much shorter Si(1)Si(3) separation and aromatic character, is the global Si3H5+ 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 mol−1 lower in energy than 7 at the CCSD(T)/6-311+G ∗ //MP2/6-316 ∗ level. Conjugation in the planar C2v 1 and hyperconjugation in the rotated 9 form are decreased relative to the carbon analogs. The remarkable stability of 5 and 7 can be attributed to their effective charge delocalization.