Clusters of Ammonium Cation—Hydrogen Bond versus σ‐Hole Bond

Abstract

MP2/6-311++G(d,p) calculations were performed on the NH4 (+) ⋅⋅⋅(HCN)n and NH4 (+) ⋅⋅⋅(N2 )n clusters (n=1-8), and interactions within them were analyzed. It was found that for molecules of N2 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 HN 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: NH⋅⋅⋅N hydrogen bonds and HN⋅⋅⋅N interactions that are classified as σ-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 N2 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 σ-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.

Experimental Charge Density Evidence for Pnicogen Bonding in a Crystal of Ammonium Chloride

“…In all cases, the ammonium cation displays a net charge that is significantly reduced from its formal value of + 1, with the experimental electron density giving the best match to the calculated limit value of approximately + 0.87 e in its clusters. [33] Even when aware of a known fallacy of atomic charges, especially in relation to s-hole interactions, [10] one can still view the modest charge transfer from the chloride anion to the ammonium cation in NH 4 Cl ( % 0.2-0.4 e) as an indicator of rather weak hydrogen bonds between them, [47] which may be quantified from the topological analysis of an electron density distribution by using the above-mentioned Espinosa's correlation. [30,31] Topological analysis of the total of five electron densities (Table 2) within the AIM theory [29] gives much more consistent results than was observed for the net atomic properties (charges and volumes).…”

“…Black points designate the extremes of electrostatic potential at H and N atoms, which are 0.286 and 0.263 a.u., respectively. [33] (Reprinted with permission from Ref. [33].)…”

“…[25,33] The choice of ammonium chloride, a classic example of an order-disorder phase transition in a crystal [34] and the first ferroelastoelectric confirmed by experiment, [35] as a model compound is due to its very simple unit-cell composition with the NH 4 + cation involved in hydrogen and pnicogen bonds only.…”

“…[33] (Reprinted with permission from Ref. [33].) cules (AIM) theory, [29] this approach provides a full set of bonding interactions in a crystal, all unambiguously revealed by a bond critical points (bcps; 3, À1) search and characterized by the values of electron density, its laplacian, etc.…”

Properties of cationic pnicogen-bonded complexes F4-nHnP+:N-base with H–P···N linear andn= 1–4

“…Similarly, only a relatively few studies of pnicogen-bonded anionic and cationic complexes have been reported in the literature [23][24][25][26][27][28]. We recently reported two systematic studies of cationic P···N pnicogen-bonded complexes.…”