Hydrogen Bonding in Host-Guest Complexes – Cryogenic Ion Vibrational Spectroscopy of Calix[4]pyrroles in Complexes With Anions*

Octamethyl-calix[4]pyrrole (OMC4P) is a prototypical anion receptor for halide ions, particularly fluoride and chloride. We present cryogenic ion vibrational spectroscopy and density functional theory calculations on complexes of OMC4P with halides, nitrate, and formate. For the halide complexes, the NH…X^- distance (X = F, Cl, Br) and the vibrational mode couplings in the complex vary strongly with the proton affinity of the ion. As a result, harmonic calculations are unable to recover the infrared spectrum for X = F, while the spectra for the complexes involving the heavier halides are represented reasonably well by this approximation. Anharmonic VPT2 calculations allow a near-quantitative description of all halide-OMC4P complexes. The OMC4P complex with nitrate shows a single binding motif of the ion with one NO group captured by the binding site. In contrast, formate can bind in two distinguishable motifs, and we observe evidence for both.