Positive ions of SiH4: Jahn-Teller-like distortions for {{rm SiH}_{4}^{+}} and eventually molecular collapse on further ionisation.

We have recently studied by quantum-chemical calculations some fifteen tetrahedral and octahedral molecules. These seemingly disparate numerical tabulations were, it was demonstrated, pulled together by comparison with model semiclassical scaling laws for (i) nuclear–nuclear repulsion energy at the equilibrium geometry and (ii) total energies likewise. Here, we again appeal to such model scaling predictions, but now for positive ions of SiH₄. We then report Hartree-Fock equilibrium geometries and MP2 corrections. For ${{rm SiH}_{4}^{+}}$ , we assumed the symmetry to be C_2v , by analogy with ${{rm CH}_{4}^{+}}$ for which experimental confirmation of this symmetry is available. Larger distortions, but still for C_2v symmetry, are found from our quantum-chemical studies in the case of ${{rm SiH}_{4}^{2+}}$ . But for ${{rm SiH}_{4}^{3+}}$ there is a marked tendency to return to a configuration quite close to tetrahedral symmetry. But non-convergence is found for ${{rm SiH}_{4}^{4+}}$ . Finally relations to the admittedly simplistic semiclassical geometry scaling predictions of Lawes and March are conjectured.