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 ar omatics, viz. benzene, phenol, pyrrole, imidazole, pyridine, indole, furane , and thiophene, to characterize the forces involved in such interactions a nd to gain further insight into the nature and diversity of cation-aromatic interactions. Such calculations may provide valuable information for under standing molecular recognition in biological systems and for force-field de velopment. B3LYP/6-31G** optimization on 35 initial structures resulted in 11 different finally optimized geometries, which could be divided into thre e types: NH4+-pi complexes, protonated heterocyclic-NH3 hydrogen bond compl exes, and heterocyclic-NH4+ hydrogen bond complexes. For NH4+-pi complexes, NH4+ always tilts toward the carbon-carbon bond rather than toward the het eroatom or the carbon-heteroatom bond. The calculated CHelpG charges sugges t that the charge distribution of a free heterocyclic may be used to predic t the geometry of its complex. Charge population and electrostatic interact ion estimations show that the NH4+-pi interaction has the largest nonelectr ostatic interaction fraction (similar to 47%) of the total binding energy, while the NH4+-aromatic hydrogen bond interaction has the largest electrost atic fraction (similar to 90%). A good correlation between binding energy a nd electrostatic interaction in the NH4+-pi complexes is found, which shows that nonelectrostatic interaction is important for cation-pi binding. The results calculated with basis sets from 6-31G to 6-311++G(2df, 2dp) show th at DeltaE(corr) and DeltaH(corr) do not require a basis-set superposition e rror (BSSE) correction, in view of experimental error, if a larger basis se t is used in the calculation. The calculated DeltaH(corr) values for the NH 4+-C6H6 complex with different basis sets suggest that the experimental Del taH may be overestimated.