Enhanced stability of ion-clathrate structures for magic number water clusters

The near threshold vacuum-UV photoionization of water clusters has been performed by using a resonance line emission of argon at 11.83 eV. The well-known intensity anomaly at the cluster ion (H₂O) ₂₁H⁺ is observed even in this threshold photoionization, for the first time, with very small excess energy. Structures for the water cluster ions (H₂O)₂₁H⁺ and (H ₂O)₂₈H⁺ which exhibit enhanced structural stability (magic number), are presented based on Monte Carlo simulations as well as on the analogy of our previous study on the stability of the (H ₂O)₂₀NH₄⁺ ion. The Monte Carlo calculations are carried out at the temperatures of 200, 150, 100, and 50 K for the ionized water clusters (H₂O)_n H⁺ around n=21 and also around n=28, which includes the ionic hydrogen-bond interactions between an H₃O⁺ ion and neutral H₂O molecules. The clusters (H₂O)₂₁H⁺ and (H ₂O)₂₈H⁺ have greater binding energies per molecule than their neighbors although the enhancement of the latter is somewhat temperature dependent. The calculations suggest that the stable structures for (H₂O)₂₁H⁺ and (H₂O) ₂₈H⁺ are represented by the ion-clathrate (ion-centered cage) configurations with either an H₂O⁺ or (H ₂O · H₃O)⁺ ion trapped inside the cage, respectively. The simulations for (H₂O)₂₀ · H ₃O⁺ also suggest that the cluster is especially stable due to the strong Coulombic interaction between the encaged H₃O ⁺ ion and the surrounding 20 water molecules which form a deformed pentagonal dodecahedral cage. The stability of the (H₂O) ₂₆(H₂O · H₃O)⁺ ion can also be explained consistently within the framework of the present ion-clathrate model.