Microsolvation of the pyrrole cation (Py+) with nonpolar and polar ligands: infrared spectra of Py+-Ln with L = Ar, N2, and H2O (n ≤ 3)

The solvation of aromatic (bio-) molecular building blocks has a strong impact on the intermolecular interactions and function of supramolecular assemblies, proteins, and DNA. Herein we characterize the initial microsolvation process of the heterocyclic aromatic pyrrole cation (Py+) in its (2)A(2) ground electronic state with nonpolar, quadrupolar, and dipolar ligands (L = Ar, N-2, and H2O) by infrared photodissociation (IRPD) spectroscopy of cold mass-selected Py+ -L-n (n <= 3) clusters in a molecular beam and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level. Size-and isomer-specific shifts in the NH stretch frequency (Delta nu(NH)) unravel the competition between various ligand binding sites, the strength of the respective intermolecular bonds, and the cluster growth. In Py+ -Ar, linear H-bonding of Ar to the acidic NH group (NH center dot center dot center dot Ar) is competitive with pi-stacking to the aromatic ring, and both Py+ -Ar(H) and Py+ -Ar(pi) are observed. For L = N-2 and H2O, the linear NH center dot center dot center dot L H-bond is much more stable than any other binding site and the only observed binding motif. For the Py+ -Ar-2 and Py+ -(N-2)(2) trimers, the H/pi isomer with one H-bonded and one pi-bonded ligand strongly competes with a 2H isomer with two bifurcated nonlinear NH center dot center dot center dot L bonds. The latter are equivalent for Ar but nonequivalent for N-2. Py+-H2O exhibits a strong and linear NH center dot center dot center dot O H-bond with charge-dipole configuration and C-2v symmetry. IRPD spectra of cold Py+ -H2O-L clusters with L = Ar and N-2 reveal that Ar prefers pi-stacking to the Py+ ring, while N-2 forms an OH center dot center dot center dot N-2 H-bond to the H2O ligand. The Delta nu(NH) frequency shifts in Py+ -L-n are correlated with the strength of the NH center dot center dot center dot L H-bond and the proton affinity (PA) of L, and a monotonic correlation between Delta nu(NH) of the Py+ -L(H) dimers and PA is established. Comparison with neutral Py-L dimers reveals the strong impact of the positive charge on the acidity of the NH group, the strength of the NH center dot center dot center dot L H-bond, and the preferred ligand binding motif.