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 A ground electronic state with nonpolar, quadrupolar, and dipolar ligands (L = Ar, N, and HO) by infrared photodissociation (IRPD) spectroscopy of cold mass-selected Py-L (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) 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 = N and HO, the linear NHL H-bond is much more stable than any other binding site and the only observed binding motif. For the Py-Ar and Py-(N) 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 N. Py-HO exhibits a strong and linear NHO H-bond with charge-dipole configuration and C symmetry. IRPD spectra of cold Py-HO-L clusters with L = Ar and N reveal that Ar prefers π-stacking to the Py ring, while N forms an OHN H-bond to the HO ligand. The Δν frequency shifts in Py-L are correlated with the strength of the NHL H-bond and the proton affinity (PA) of L, and a monotonic correlation between Δν 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.

Aromatic Charge Resonance Interaction Probed by Infrared Spectroscopy

“…Specifically,t he N À Hb ond properties are r NH = 1.0061 and n NH = 3500 cm À1 .T here are two n NH modes,n amely the IRforbidden symmetric NÀHs tretch (a 1 )a nd the strongly IR active antisymmetric NÀHs tretch (b u ,2 88 km mol À1 ). [17] In line with the prediction, ionization of Py increases the acidity of the NÀHp roton and decreases n NH by 84 cm À1 .T he IRPD spectrum of Py 2 + exhibits as ingle transition at 3480 cm À1 assigned to n NH (b u )ofthe CR dimer calculated at 3500 cm À1 . This predicted trend is well reproduced by the experimental n NH frequencies of Py,P y + ,a nd Py 2 + included in Figure 4, when considering the widths of the experimental transitions in Figure 2( 5-30 cm À1 ,S upporting Information, Table S2).…”

“…All geometric data of Py 2 + are between those of Py and Py + , consistent with symmetric sharing of the excess positive charge (Supporting Information, Table S1). [17] Thelatter value is essentially unshifted from n NH of bare Py + because p-bonded ligands have essentially no impact on the NÀHb ond. They are split by less than 1cm À1 ,i ndicating that the coupling between both NÀHo scillators via the intermolecular CR bond is negligible.P lots of the computed N À Hb ond parameters versus the charge on the Py moiety show nearly linear dependencies between r NH , n NH ,a nd q (Figure 4).…”

“…[10d, 13a, 14] Unfortunately,most A 2 + dimers with strong IR chromophores and high sensitivity to charge (n OH/NH ), such as phenol or aniline dimer cations,prefer H-bonds (OÀH···O,N ÀH···N) to p-stacking and thus lack any CR. [17] This large frequency difference of Dn NH = À84 cm À1 observed for Dq =+eimplies that even small changes in the partial charge of Py result in significant frequencys hifts in n NH .H erein, an early linear correlation between n NH and q is demonstrated and this relationship is exploited to probe subtle effects of symmetry reduction, induced by 1) substitution of af unctional group (NH!NCH 3 )a nd 2) asymmetric solvation (by as ingle N 2 ligand), on the charge distribution. [16] Moreover,P yi sasmall heterocyclic arene and is therefore expected to exhibit strong CR stabilization.…”

“…[17] Briefly,Pyhas aplanar structure (C 2v )inthe neutral 1 A 1 and cationic 2 A 2 ground electronic states ( Figure 1, Supporting Information, Table S1). [17] Our calculations predict an antiparallel p-stacked sandwich structure with C 2h symmetry for Py 2 + in its 2 A u ground electronic state ( Figure 1, Supporting Information, Table S1). As aresult, the NÀHbond (r NH )elongates from 1.003 to 1.010 , and its stretch frequency is reduced from n NH = 3531 to 3459 cm À1 .These frequencies agree well with those measured by cavity ringdown spectroscopy of Py (3531 cm À1 ) [19] and those extracted from the IRPD spectrum of the p-bonded Py + -Ar dimer (3447 cm À1 ), shown in Figure 2( Supporting Information, Table S2).…”

“…Calculated structures of Py (C 2v , 1 A 1 ), Py + (C 2v , 2 A u ), Py 2 + -N 2 (C s , 2 A''), and (Py-NMPy) + (C s , 2 A''). IR spectrum of Py (cavity ringdown spectrum) [19] and IRPD spectra of Py + -Ar, [17] Py 2 + ,P y 2 + -N 2 ,a nd (Py-NMPy) + recorded in the NÀHstretch range (Supporting Information,T able S2) via the loss of the most weakly bound ligand (Ar,N 2 ,o rPy). Figure 2.…”