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.

OH stretching vibrations of 2-naphthol-(H2O)n (n=0–3 and 5) hydrogen-bonded clusters in the S0 state have been observed by infrared-ultraviolet (IR-UV) double-resonance spectroscopy. In bare 2-naphthol, cis- and trans-isomers were identified by the comparison of the observed OH frequencies with those obtained by ab initio calculations with the HF/6-31G basis set. The OH stretching vibrations (νOH) of hydrogen-bonded 2-naphthol-(H2O)n show characteristic shifts depending on the cluster size. They are classified into hydrogen-bonded νOH, and νOH free from the hydrogen bond. The cluster structures were also examined by comparing the observed IR spectra with simulated ones. It was found that the clusters with n=2 and 3 form ring structures, while the cluster with n=5 exhibits an ice (I) structure.

We applied autoionization-detected infrared (ADIR) spectroscopy in order to observe OH stretching vibrations of jet-cooled 1-and 2-naphthol cations. In this technique, high Rydberg states, the vibrational levels of which are essentially the same as those of the corresponding bare molecular ion, were prepared by two-color doubleresonance excitation. Vibrational transitions in the ion core of the high Rydberg states were measured by detecting the vibrational autoionization signal. For rotational and structural isomers of naphthol, similar lowfrequency shifts of the OH frequencies upon ionization were found. The OH frequency shifts of naphthols were much smaller than that found for phenol, although both molecules have quite similar OH frequencies in their neutral ground state. This remarkable difference in frequency shifts was qualitatively explained in terms of the charge delocalization in the aromatic ring. In addition, the OH stretching vibrations of the 1-and 2-naphthol-Ar cluster cations were observed by infrared photodissociation spectroscopy. It was found that perturbations from the Ar atom to the hydroxyl group of naphthol are negligible in both the neutral and cationic ground states.

The IR spectra for various sizes of pyrrole clusters were measured in the NH stretching vibration region by infrared cavity ringdown spectroscopy. The hydrogen-bonded structures and normal modes of the pyrrole clusters were analyzed by a density functional theory calculation of the B3LYP/6-311+G(d,p) level. Two types of pulsed nozzles, a slit and a large pinhole, were used to generate different cluster size distributions in a supersonic jet. A rotational contour analysis of the NH stretching vibration for the monomer revealed that the slit nozzle provides a warmer jet condition than the pinhole one. The IR spectra, measured under the warmer condition, showed the intense bands at 3444, 3392, and 3382 cm(-1), which were assigned to hydrogen-bonded NH stretching vibrations due to the dimer, the trimer, and the tetramer, respectively. On the other hand, the IR spectra measured under a lower temperature condition by a pinhole nozzle showed a broad absorption feature in addition to sharp bands. This broad absorption was reproduced by the sum of two Gaussians peaks at 3400 and 3372 cm(-1) with widths of 30 and 50 cm(-1) (FWHM), respectively. Compared with the spectra of the condensed phase, two bands at 3400 and 3372 cm(-1) were assigned to hydrogen-bonded NH stretching vibrations of larger clusters having liquid-like and solid-like structures, respectively.

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