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

DFT/B3LYP calculations were carried out on complexes formed by NH4+ 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: NH4+−π complexes, protonated heterocyclic−NH3 hydrogen bond complexes, and heterocyclic−NH4+ hydrogen bond complexes. For NH4+−π complexes, NH4+ 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 populati...