title: Microsolvation of the pyrrole cation (Py+) with nonpolar and polar ligands: infrared spectra of Py+-Lnwith 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 its2A2ground electronic state with nonpolar, quadrupolar, and dipolar ligands (L = Ar, N2, and H2O) by infrared photodissociation (IRPD) spectroscopy of cold mass-selected Py+-Ln(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 (ΔνNH) 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 = N2and H2O, the linear NHL H-bond is much more stable than any other binding site and the only observed binding motif. For the Py+-Ar2and Py+-(N2)2trimers, 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 C2vsymmetry. IRPD spectra of cold Py+-H2O-L clusters with L = Ar and N2reveal that Ar prefers π-stacking to the Py+ring, while N2forms an OHN2H-bond to the H2O ligand. The ΔνNHfrequency shifts in Py+-Lnare correlated with the strength of the NHL H-bond and the proton affinity (PA) of L, and a monotonic correlation between ΔνNHof 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.
authors.affiliation: Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany. dopfer@physik.tu-berlin.de.
authors.forename: Markus
authors.identifier: http://orcid.org/0000-0002-3773-6009
authors.initials: M
authors.lastname: Schütz
authors.affiliation: Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga, Shizuoka, 422-8529, Japan.
authors.forename: Yoshiteru
authors.identifier: http://orcid.org/0000-0001-5296-3077
authors.initials: Y
authors.lastname: Matsumoto
authors.affiliation: Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany. dopfer@physik.tu-berlin.de.
authors.forename: Aude
authors.identifier: http://orcid.org/0000-0002-4765-8876
authors.initials: A
authors.lastname: Bouchet
authors.affiliation: Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany. dopfer@physik.tu-berlin.de.
authors.forename: Murat
authors.identifier: http://orcid.org/0000-0002-4040-3379
authors.initials: M
authors.lastname: Öztürk
authors.affiliation: Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany. dopfer@physik.tu-berlin.de.
authors.forename: Otto
authors.identifier: http://orcid.org/0000-0002-9834-4404
authors.initials: O
authors.lastname: Dopfer