Cryogenic Ion Vibrational Spectroscopy of Calix[4]pyrroles in Complexes With Anions

Calix[4]pyrroles (C4Ps) are macrocycles with concave binding sites that act as anion receptors. They can be synthesized with different cavities surrounding the binding site, resulting in a modification of the exposure of the bound ion to the chemical environment. We present cryogenic ion vibrational spectroscopy and density functional theory calculations on complexes of different C4Ps with halide ions. The NH…X^- distance (X = bound anion) and the vibrational mode couplings in the complex vary strongly with the proton affinity of the ion, as well as with the depth of the binding site. The spectra encode the interaction between the ion with the binding site and with the surrounding cavity or other chemical environment. They are well represented by vibrational perturbation theory.