Quantized Hydration Energies of Ions and Structure of Hydration Shell from the Experimental Gas-Phase Data

With previous data on alkali metal and halide ions included [Rais, J.; Okada, T. Anal. Sci. 2006, 22, 533], we analyzed rather broad data on ionic hydration from the point of view of gaseous cluster energetics. We have now added alkaline earth cations, Zn²⁺, H⁺, OH^-, Cu⁺, Ag⁺, Bi⁺, Pb⁺, and alkylammonium cations. The present analysis revealed the octa-coordinated nature of alkaline earth cations, which is not fully pronounced for Be²⁺ and Zn²⁺, existence of Eigen protonium complex, which is trigonally hydrated, and particular property of the first OH^-, H₂O cluster. Whereas these findings are generally in accordance with theoretical model calculation studies, we have foreseen in addition tetrahedral hydration for halide anions and Rb⁺ and Cs⁺, as well as for alkylammonium ions. The obtained picture of the quantized solvation of ions is mirrored in the ionization potentials of outer electrons of pertinent atoms. This is a second independent phenomenon, and together, they invoked a common pattern formation (“Aufbau”) obeying tetra- and octa-coordinated principles.