Aromatic Charge Resonance Interaction Probed by Infrared Spectroscopy

Charge resonance is a strong attractive intermolecular force in aromatic dimer radical ions. Despite its importance, this fundamental interaction has not been characterized at high resolution by spectroscopy of isolated dimers. We employ vibrational infrared spectroscopy of cold aromatic pyrrole dimer cations to precisely probe the charge distribution by measuring the frequency of the isolated N-H stretch mode (ν_NH ). We observe a linear correlation between ν_NH and the partial charge q on the pyrrole molecule in different environments. Subtle effects of symmetry reduction, such as substitution of functional groups (here pyrrole replaced by N-methylpyrrole) or asymmetric solvation (here by an inert N₂ ligand), shift the charge distribution toward the moiety with lower ionization energy. This general approach provides a precise experimental probe of the asymmetry of the charge distribution in such aromatic homo- and heterodimer cations.