Theoretical study on halide anion recognitions by pyrrole- and benzene-strapped calix[4]pyrroles

Abstract

Equilibrium geometries of two anion receptors, pyrrole- and benzene-strapped calix[4]pyrrole (PCP and BCP), and their complexes with halide anions (X−/receptor, receptor=PCP and BCP, X−=F−, Cl− and Br−) were optimized using density functional theory (DFT) at the B3LYP/6-311G(d,p) level of theory. Natural bond orbital (NBO) method, atoms in molecules (AIMs) theory, and energy decomposition analysis (EDA) have been used to characterize the orbital interaction between anion and receptors, to construct three-dimensional electron density (ED) maps of all complexes, and to investigate into the binding contribution to the anion recognitions. Two types of hydrogen bonds, N–H⋯X− and C–H⋯X−, were confirmed in complex structures, and the halide anions offer lone pair electrons to the binding σ* (N–H) or σ* (C–H) antibond orbital of receptors. These H-bonds in F−/receptor were obviously stronger than those in the other two (Cl−/receptor and Br−/receptor). The intermolecular interaction energies were predicted by using B3LYP/6-311G(d,p) methods with basis set superposition error (BSSE) correction. The order of the anion recognition selectivity was predicted as F−>Cl−>Br−, and PCP possessed a higher anion affinity than BCP under identical conditions. These calculation results were qualitatively in good agreement with the experimental results.