Effect of microhydration on the aromatic charge resonance interaction: the case of the pyrrole dimer cation

Charge resonance (CR) interactions between aromatic molecules are amongst the strongest intermolecular forces and responsible for many phenomena in chemistry and biology. Microhydration of an aromatic radical dimer cation allows investigation of the strong effects of stepwise solvation on the charge distribution and strength of the CR. We characterise herein the microhydration process of the pyrrole dimer cation (Py₂⁺), a prototypical aromatic homodimer with a strong CR. The NH and OH stretch vibrations (ν_NH/OH) of mass-selected bare and colder Ar-tagged hydrated clusters of Py₂⁺, Py₂⁺(H₂O)Ar (n ≤ 3, m ≤ 1), recorded by infrared photodissociation (IRPD) spectroscopy provide detailed insight into the preferred cluster growth and strengths of the various intermolecular interactions by comparison to dispersion-corrected density functional theory calculations. The analysis of systematic frequency shifts, structural parameters, binding energies, and charge distributions allows for a quantitative evaluation of the drastic effects of stepwise hydration on the strength and symmetry of the aromatic CR, the strengths of the various hydrogen bonds (H-bonds), and the competition between slightly noncooperative interior ion hydration and strongly cooperative formation of a H-bonded solvent network. The most stable Py₂⁺H₂O structure exhibits a strong NH⋯O ionic H-bond of H₂O to the antiparallel stacked Py₂⁺(a) core, thereby breaking the symmetry of the CR. Py₂⁺(H₂O)₂ prefers a highly symmetric C_2h structure with two equivalent NH⋯O H-bonds of Py₂⁺(a) and an optimised CR. Starting from n = 3, clusters with a parallel configuration, Py₂⁺(p), are more stable than those with Py₂⁺(a), further highlighting the strong impact of (micro-)solvation on the structural motif of the aromatic CR. The spectral and computational data demonstrate a linear correlation of ν_NH of the free Py unit with its partial charge, illustrating that IR spectroscopy is a powerful tool for probing the charge distribution in aromatic CR cluster cations. Comparison of Py₂⁺(H₂O) with neutral Py₂(H₂O) and Py⁺(H₂O) reveals the impact of the magnitude of positive charge and the number of acidic proton donors on the structure of the microhydration shell and strength of the various competing intermolecular bonds.