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

## TL;DR

This study uses IR spectroscopy to reveal how hydrogen bonding dominates over other interactions in pyrrole-benzene and pyrrole-toluene radical cations.

## Contribution

The paper quantitatively evaluates cation-π interaction energies using NH stretch redshifts in radical cation dimers.

## Key findings

- NH···π hydrogen bonding dominates over charge resonance in Py+Bz and Py+Tol dimers.
- T-shaped geometries are favored due to large ionization energy differences.
- π-stacked isomers are less stable and stabilized by weaker π-π interactions.

## Abstract

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
(νNH) 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 νNH 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.