Design of the NnH3n+1+ series of “non-metallic” superalkali cations

Abstract

Species with a 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” N_nH_3n+1⁺ superalkali cations based on the results of the MP2/6-311++G(d,p) computations. These cations are designed by successive replacement of H-ligands of ammonium cations (NH₄⁺) by ammonium (NH₄) moieties. The resulting N_nH_3n+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 interaction energy in the range of 7.8–24.3 kcal mol⁻¹. These cations are stable against loss of a proton (N_nH_3n + H⁺) and loss of ammonia [(n − 1)NH₃ + NH₄⁺]. The vertical electron attachment energies (EA_vs) of N_nH_3n+1⁺ decrease monotonically from 4.39 eV for n = 1 to 2.39 eV for n = 5, which suggests their superalkali nature. This can be explained on the basis of electron localization on the core (central) N-atom (Q_c), as the EA_v correlates linearly with Q_c. We have also demonstrated that this series may be continued to obtain new superalkali cations with even lower EA_v, by exemplifying N₉H₂₈⁺ with an EA_v of 1.84 eV. N₉H₂₈⁺ is stabilized by four partially covalent H-bonds (8.5 kcal mol⁻¹ each) and four electrostatic H-bonds (0.4 kcal mol⁻¹ each). This led to an exponential relation between EA_v and n, which may provide an approximate EA_v for any value of n in the N_nH_3n+1⁺ series.