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.