Design of NnH3n+1+ Series of Non-Metallic Superalkali Cations

Abstract

The species with lower ionization energy than alkali atoms are referred to as superalkalis. Typical superalkalis include a central electronegative core with excess metal ligands. We propose a new series of non-metallic NnH3n+1+ superalkali cations using MP2/6-311++G(d,p) level. These cations are designed by successive replacement of H-ligands of ammonium cation (NH4+) by ammonium (NH4) moieties. The resulting NnH3n+1+ cations, which can be expressed in the form of [NH4 (n-1)NH3]+ complexes, possess a number of unusual N-H N type of partially covalent H-bonds, with the interacting energy in the range 7.8-24.3 kcal/mol. These cations are stable against loss of a proton (NnH3n+H+) and loss of ammonia [(n-1)NH3+NH4+]. The vertical electron affinities (EAv) of NnH3n+1+ decreases monotonically from 4.39 eV from n = 1 to 2.39 eV for n = 5, which suggest their superalkali nature. This can be explained on the basis of electron localization on core (central) N-atom (Qc), as EAv correlates linearly with Qc. We have also demonstrated that this series may be continued to obtain new superalkali cations with even lower EAv, by exemplifying N9H28+ with the EAv of 1.84 eV. N9H28+ is stabilized by four partially covalent H-bonds (8.5 kcal/mol each) and four electrostatic H-bonds (0.4 kcal/mol each). This led to an exponential relation between EAv and n, which may provide an approximate EAv of for any value of n in NnH3n+1+ series.