| Abstract
| - 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 suchinteractions and to gain further insight into the nature and diversity of cation−aromatic interactions. Suchcalculations may provide valuable information for understanding molecular recognition in biological systemsand for force-field development. B3LYP/6-31G** optimization on 35 initial structures resulted in 11 differentfinally optimized geometries, which could be divided into three types: NH4+−π complexes, protonatedheterocyclic−NH3 hydrogen bond complexes, and heterocyclic−NH4+ hydrogen bond complexes. For NH4+−πcomplexes, 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 heterocyclicmay be used to predict the geometry of its complex. Charge population and electrostatic interaction estimationsshow that the NH4+−π interaction has the largest nonelectrostatic interaction fraction (∼47%) of the totalbinding energy, while the NH4+−aromatic hydrogen bond interaction has the largest electrostatic fraction(∼90%). A good correlation between binding energy and electrostatic interaction in the NH4+−π complexesis found, which shows that nonelectrostatic interaction is important for cation−π binding. The results calculatedwith basis sets from 6-31G to 6-311++G(2df, 2dp) show that ΔEcorr and ΔHcorr do not require a basis-setsuperposition error (BSSE) correction, in view of experimental error, if a larger basis set is used in thecalculation. The calculated ΔHcorr values for the NH4+−C6H6 complex with different basis sets suggest thatthe experimental ΔH may be overestimated.
|
