OxH2x+1+ Clusters: A New Series of Non-Metallic Superalkali Cations by Trapping H3O+ into Water

The term superalkali refers to the clusters with lower ionization energy than alkali atoms. Typical superalkali cations include a central electronegative core with excess metal ligands, OLi3+, for instance, which mimic the properties of alkali metal ions. We report a new series of non-metallic superalkali cations, OxH2x+1+ (x = 1-5) using ab initio MP2/6-311++G(d,p) level. These cations are designed by successive replacement of H-ligands of hydronium cation (OH3+) by ammonium (OH3) moieties followed by their geometry optimization. The resulting OxH2x+1 + clusters, which can be expressed in the form of OH3 + (x-1)H2O complexes, possess a number of electrostatic as well as partially covalent H-bonds, with the interacting energy in the range 5.2-29.3 kcal/mol as revealed by quantum theory of atoms in molecules analyses. These cations are found to be stable against deprotonation as well as dehydration pathways, and their stability increases with the increase in x. Interestingly, the vertical electron affinities (EAv) of OxH2x+1 + clusters decreases rapidly from 5.16 eV for x = 1 to 2.67 eV for x = 5, which suggest their superalkali nature. It is also possible to continue this series of non-metallic superalkali cations for x > 5 with even lower EAv, down to an approximated limit of 1.85 eV, which is obtained for OH3 + trapped into water cavity implicitly using polarizable continuum model. The findings of this study will not only provide new insights into structure and interactions of OxH2x+1 + clusters but also reveal their novel properties, which can be exploited their interesting applications.