How Does Ammonium Interact with Aromatic Groups? A Density Functional Theory (DFT/B3LYP) Investigation

DFT/B3LYP calculations were carried out on complexes formed by NH 4 + with aromatics, viz. benzene, phenol, pyrrole, imidazole, pyridine, indole, furane, and thiophene, to characterize the forces involved in such interactions and to gain further insight into the nature and diversity of cation−aromatic interactions. Such calculations may provide valuable information for understanding molecular recognition in biological systems and for force-field development. B3LYP/6-31G** optimization on 35 initial structures resulted in 11 different finally optimized geometries, which could be divided into three types: NH 4 + −π complexes, protonated heterocyclic−NH 3 hydrogen bond complexes, and heterocyclic−NH 4 + hydrogen bond complexes. For NH 4 + −π complexes, NH 4 + always tilts toward the carbon−carbon bond rather than toward the heteroatom or the carbon−heteroatom bond. The calculated CHelpG charges suggest that the charge distribution of a free heterocyclic may be used to predict the geometry of its complex. Charge population and electrostatic interaction estimations show that the NH 4 + −π interaction has the largest nonelectrostatic interaction fraction (∼47%) of the total binding energy, while the NH 4 + −aromatic hydrogen bond interaction has the largest electrostatic fraction (∼90%). A good correlation between binding energy and electrostatic interaction in the NH 4 + −π complexes is found, which shows that nonelectrostatic interaction is important for cation−π binding. The results calculated with basis sets from 6-31G to 6-311++G(2df, 2dp) show that Δ E corr and Δ H corr do not require a basis-set superposition error (BSSE) correction, in view of experimental error, if a larger basis set is used in the calculation. The calculated Δ H corr values for the NH 4 + −C 6 H 6 complex with different basis sets suggest that the experimental Δ H may be overestimated.