Competition between π-π and NH···π Interactions in Pyrrole ⁺ -Benzene and Pyrrole ⁺ -Toluene Radical Cations Revealed by IR Spectroscopy

The charge resonance (CR) interaction is among the strongest intermolecular forces in aromatic dimer cations (∼100 kJ mol^-1). Its strength strongly depends on the ionization energy differences of the two interacting aromatic units (ΔIE). Therefore, it is the strongest in homodimers (A₂⁺) and forms π-stacked sandwich structures, like in the pyrrole dimer cation (Py₂⁺). In heterodimers (ΔIE ≠ 0), the CR is weakened, allowing other noncovalent forces, such as cation-π interactions, to compete in strength. Herein, we investigate the binding motifs of the pyrrole⁺-benzene (Py⁺Bz) and pyrrole⁺-toluene (Py⁺Tol) heterodimers, with ΔIE = 1.04 and 0.62 eV, respectively. The NH stretch vibrations (ν_NH) of mass-selected bare and colder Ar-tagged clusters of Py⁺Bz and Py⁺Tol, recorded by infrared photodissociation spectroscopy and analyzed using dispersion-corrected density functional theory calculations, provide detailed insight into the preferred binding motifs and their relative strengths. For both dimers, NH···π hydrogen bonding (H-bonding) dominates over the CR interaction, favoring T-shaped geometries over π-stacked structures because their large ΔIE values prevent the formation of a strong CR between the two π-systems. The systematic redshifts of ν_NH are correlated with the NH···π H-bond strength in Py⁺Bz and Py⁺Tol and thus enable quantitative evaluation of such cation-π interaction energies. A minor population of less stable π-stacked isomers is observed for both Py⁺Bz and Py⁺Tol. Local energy decomposition analysis reveals that the π-stacked structures are not stabilized by a strong CR but rather by weaker π-π stacking interactions governed by electrostatic, induction, and dispersion forces.