Density functional theory (DFT) study on the interaction of ammonium (NH4+) and aromatic nitrogen heterocyclics

Abstract

A DFT calculation was performed at the B3LYP/6-31G* level on the complexes formed by NH₄⁺ and aromatic nitrogen heterocyclics, viz. pyrrole, imidazole, pyridine and indole, in order to investigate the mechanism and complexity of the interaction between the ammonium group and the aromatic heterocyclic in biomacromolecules. The optimized geometries suggested that there are two different types of complexes: one is a cation–π complex and the other is a hydrogen bond complex. A cation–π complex will be formed if the heteroatom has no localized lone-pair electrons. A hydrogen bond complex will be formed by proton transfer from NH₄⁺ to the heteroatom if the heteroatom has localized lone-pair electrons. In the case of the cation–π complex, the predicted geometries, atomic charges and thermodynamic parameters revealed that ammonium binds more strongly to heterocyclics than it binds to benzene. The calculated orbital coefficient and the optimized structures implied that NH₄⁺ interacts with the π electrons of the CC bond of heterocyclics to form a cation–π complex mainly through one hydrogen atom. Regarding the hydrogen bond complex, although the calculated binding strength is similar to that for the cation–π complex, the ΔH of the whole reaction process suggested that the formation of the hydrogen bond complex is favorable to the stability of the whole system. Calculated IR spectra showed that three groups of new bands appear when NH₄⁺ binds to heterocyclics. Normal mode analysis showed that these new bands are all related to the relative motion of the two parts in the formed complexes. All these results suggest that the NH₄⁺–heterocyclic system is a better model for studying the nature and complexity of the interaction between the ammonium group and the aromatic ring structure in biomolecules.