Maximal Coordinator Number of Potassium, Rubidium, Caesium and Francium Ions in Gaseous Water

To understand the molecular and electronic structure of alkali metal ions, we carry out the MP2 calculation and demonstrate that the maximal coordinator numbers for the hydrated K⁺ and Rb⁺ are 8, while those for the hydrated Cs⁺ and Fr⁺ are 10. Furthermore, on the basis of the binding energy, the HOMO-LUMO gap and the electron affinity, the stability of the molecular and electronic structures of M⁺(H₂O)₈ (M = K, Rb, Cs, Fr) decreases with the increasing alkali metal atomic number and the stability of the molecular structures of M⁺(H₂O)_8-10 (M = Cs, Fr) decreases with the increasing cluster size.