Clusters of Ammonium Cation-Hydrogen Bond versus σ-Hole Bond

MP2/6-311++G(d,p) calculations were performed on the NH4+(HCN)(n) and NH4+(N-2)(n) clusters (n=1-8), and interactions within them were analyzed. It was found that for molecules of N-2 and HCN, the N centers play the role of the Lewis bases, whereas the ammonium cation acts as the Lewis acid, as it is characterized by sites of positive electrostatic potential, that is, H atoms and the sites located at the N atom in the extension of the HN bonds. Hence, the coordination number for the ammonium cation is eight, and two types of interactions of this cation with the Lewis base centers are possible: NHN hydrogen bonds and HNN interactions that are classified as sigma-hole bonds. Redistribution of the electronic charge resulting from complexation of the ammonium cation was analyzed. On the one hand, the interactions are similar, as they lead to electronic charge transfer from the Lewis base (HCN or N-2 in this study) to NH4+. On the other hand, the hydrogen bond results in the accumulation of electronic charge on the N atom of the NH4+ ion, whereas the sigma-hole bond results in the depletion of the electronic charge on this atom. Quantum theory of atoms in molecules and the natural bond orbital method were applied to deepen the understanding of the nature of the interactions analyzed. Density functional theory/natural energy decomposition analysis was used to analyze the interactions of the ammonium ion with various types of Lewis bases. Different correlations between the geometrical, energetic, and topological parameters were found and discussed.