Probing Ion–Receptor Interactions in Halide Complexes of Octamethyl Calix[4]Pyrrole

Abstract

Ion receptors are molecular hosts that bind ionic guests, often with great selectivity. The interplay of solvation and ion binding in anion host−guest complexes in solution governs the binding efficiency and selectivity of such ion receptors. To gain molecular-level insight into the intrinsic binding properties of octamethyl calix[4]pyrrole (omC4P) host molecules with halide guest ions, we performed cryogenic ion vibrational spectroscopy (CIVS) of omC4P in complexes with fluoride, chloride, and bromide ions. We interpret the spectra using density functional theory, describing the infrared spectra of these complexes with both harmonic and anharmonic second-order vibrational perturbation theory (VPT2) calculations. The NH stretching modes of the pyrrole moieties serve as sensitive probes of the ion binding properties, as their frequencies encode the ion−receptor interactions. While scaled harmonic spectra reproduce the experimental NH stretching modes of the chloride and bromide complexes in broad strokes, the high proton affinity of fluoride introduces strong anharmonic effects. As a result, the spectrum of F − •omC4P is not even qualitatively captured by harmonic calculations, but it is recovered very well by VPT2 calculations. In addition, the VPT2 calculations recover the intricate coupling of the NH stretching modes with overtones and combination bands of CH stretching and NH bending modes and with low-frequency vibrations of the omC4P macrocycle, which are apparent for all of the halide ion complexes investigated here. A comparison of the CIVS spectra with infrared spectra of solutions of the same ion−receptor complexes in d 3 -acetonitrile and d 6acetone shows how ion solvation changes the ion−receptor interactions for the different halide ions.

Infrared Spectrum of the Pyrene Anion in the CH Stretching Region

“…Because of the change in the transition moments used to calculate the spectra, we also reevaluated the intensities of transitions to any state involved in a Fermi resonance, replacing the deperturbed transition moments for the fundamental transitions with the corresponding harmonic values. A similar approach was taken in our recent study of host–guest complexes . To compare the harmonic and anharmonic calculated spectra with the experimental spectrum, we applied a scaling factor of 0.96088 to the harmonic frequencies.…”

Correction to “Probing Ion–Receptor Interactions in Halide Complexes of Octamethyl Calix[4]Pyrrole”

“…We wish to correct an error in the Supporting Information of the original paper concerning the description of the model for the behavior of NH stretching frequencies in the presence of dielectric media. There, the shift of the energy levels of an NH group in an electric field E⃗ was described using the treatment of a charged harmonic oscillator with second-order perturbation theory .…”