Ab initio studies of (H2O)20H+ and (H2O)21H+ prismic, fused cubic and dodecahedral clusters: can H3O+ ion remain in cage cavity?

The MP2/6-31G∗//HF/6-31G∗ level study on (H2O)20H+ clusters suggests that the edge-shared prismic structure is more stable than a fused cubic or a dodecahedral cluster by ∼7 and 22 kcal/mol, respectively. Only the dodecahedral cavity is large enough to enclose an H3O+ ion, and the hydronium ion originally placed in cavity either moves as a whole or loses its H+ ion to the surface during optimization and forms species like H5O2+, H7O3+ and H9O4+ ions on the surface. Based on HOH angular distortions and associated energy requirements we postulate that the edge-shared prismic or fused cubic clusters are not likely to form.