Halide anion recognition by calix[4]pyrrole: A quantum chemical study

The spherical recognition of halide anions by the calix[4]pyrrole molecule has been investigated with the aid of Hartree-Fock, hybrid DPT (B3LYP), and semi-empirical (PM3) quantum chemical methods. The free calix[4]pyrrole molecule adopts the 1,3-alternate conformation, as confirmed by an earlier X-ray crystal structure analysis, whereas a transition to a cone-like conformation occurs upon capture of a single halide anion. The calculated binding energies of the calix[4]pyrrole: halide anion complexes decrease monotonically along the group 17 anions, i.e. from fluoride to iodide, in agreement with the experimentally determined binding constants. Furthermore, a linear correlation of the calculated binding energy against both the standard reduction potential and the Pauling electronegativity has been found. Finally, a possible mechanism of halide anion capture by the calix[4]pyrrole receptor has been proposed on the basis of both theoretical calculations and crystallographic data.