  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.
