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

Abstract

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 ²A₂ ground electronic state with nonpolar, quadrupolar, and dipolar ligands (L = Ar, N₂, 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 (Δν_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 (NHAr) is competitive with π-stacking to the aromatic ring, and both Py⁺-Ar(H) and Py⁺-Ar(π) are observed. For L = N₂ and H₂O, the linear NHL H-bond is much more stable than any other binding site and the only observed binding motif. For the Py⁺-Ar₂ and Py⁺-(N₂)₂ trimers, the H/π isomer with one H-bonded and one π-bonded ligand strongly competes with a 2H isomer with two bifurcated nonlinear NHL bonds. The latter are equivalent for Ar but nonequivalent for N₂. Py⁺-H₂O exhibits a strong and linear NHO H-bond with charge-dipole configuration and C_2v symmetry. IRPD spectra of cold Py⁺-H₂O-L clusters with L = Ar and N₂ reveal that Ar prefers π-stacking to the Py⁺ ring, while N₂ forms an OHN₂ H-bond to the H₂O ligand. The Δν_NH frequency shifts in Py⁺-L_n are correlated with the strength of the NHL H-bond and the proton affinity (PA) of L, and a monotonic correlation between Δν_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 NHL H-bond, and the preferred ligand binding motif.