Microsolvation of the pyrrole cation (Py+) with nonpolar and polar ligands: infrared spectra of Py+–Ln with L = Ar, N2, and H2O (n ≤ 3). Issue 5 (20th January 2017)

• Record Type: Journal Article
• Authors: Schütz, Markus; Matsumoto, Yoshiteru; Bouchet, Aude; Öztürk, Murat; Dopfer, Otto
• Is Part Of: Physical chemistry chemical physics. Volume 19:Issue 5(2017)
• Page Start: 3970
• Page End: 3986
• Publication Date: 2017-01-20
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/c6cp07251h
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : IR spectra and dispersion-corrected density functional calculations of pyrrole cluster ions with Ar, N2, and H2 O reveal the competition between H-bonding and π-stacking motifs of this prototypical heterocyclic aromatic cation in a hydrophobic and hydrophilic solvent. 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 2 A2 ground electronic state with nonpolar, quadrupolar, and dipolar ligands (L = Ar, N2, and H2 O) 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 (Δ ν 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⋯Ar) is competitive with π-stacking to the aromatic ring, and both Py + –Ar(H) and Py + –Ar(π) are observed. For L = N2 and H2 O, the linear NH⋯L 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 NH⋯L bonds. The latter are equivalent for Ar but nonequivalent for N2 . Py + –H2 O exhibits a strong and linear NH⋯O H-bond with charge-dipole configuration and C 2v symmetry. IRPD spectra of cold Py + –H2 O–L clusters with L = Ar and N2 reveal that Ar prefers π-stacking to the Py + ring, while N2 forms an OH⋯N2 H-bond to the H2 O ligand. The Δ ν NH frequency shifts in Py + –L n are correlated with the strength of the NH⋯L H-bond and the proton affinity (PA) of L, and a monotonic correlation between Δ ν 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⋯L H-bond, and the preferred ligand binding motif.