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IR spectroscopy, mass spectrometry, and quantum chemical calculations are employed to characterize the intermolecular interaction of a variety of aromatic cations (A+) with several types of solvents. For this purpose, isolated ionic complexes of the type A+–L n , in which A+ is microsolvated by a controlled number (n) of ligands (L), are prepared in a supersonic plasma expansion, and their spectra are obtained by IR photodissociation (IRPD) spectroscopy in a tandem mass spectrometer. Two prototypes of aromatic ion–solvent recognition are considered: (i) microsolvation of acidic aromatic cations in a nonpolar hydrophobic solvent and (ii) microsolvation of bare aromatic hydrocarbon cations in a polar hydrophilic solvent. The analysis of the IRPD spectra of A+–L dimers provides detailed information about the intermolecular interaction between the aromatic ion and the neutral solvent, such as ion–ligand binding energies, the competition between different intermolecular binding motifs (H-bonds, π-bonds, charge–dipole bonds), and its dependence on chemical properties of both the A+ cation and the solvent type L. IRPD spectra of larger A+–L n clusters yield detailed insight into the cluster growth process, including the formation of structural isomers, the competition between ion–solvent and solvent–solvent interactions, and the degree of (non)cooperativity of the intermolecular interactions as a function of solvent type and degree of solvation. The systematic A+–L n cluster studies are shown to reveal valuable new information about fundamental chemical properties of the bare A+ cation, such as proton affinity, acidity, and reactivity. Because of the additional attraction arising from the excess charge, the interaction in the A+–L n cation clusters differs largely from that in the corresponding neutral A–L n clusters with respect to both the interaction strength and the most stable structure, implying in most cases an ionization-induced switch in the preferred aromatic molecule–solvent recognition motif. This process causes severe limitations for the spectroscopic characterization of ion–ligand complexes using popular photoionization techniques, due to the restrictions imposed by the Franck–Condon principle. The present study circumvents these limitations by employing an electron impact cluster ion source for A+–L n generation, which generates predominantly the most stable isomer of a given cluster ion independent of its geometry.

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Infrared photodissociation spectra of several phenol-L n cation clusters (Ph + -L n ; L ) He, Ne, Ar, N 2 , CH 4 ) are recorded in the vicinity of the O-H stretch vibration (ν 1 ) of bare Ph + . The Ph + -L n complexes are produced in an electron impact (EI) ion source, which generates predominantly the most stable isomer of each cluster ion. The spectra of all dimers (n ) 1) show strong ν 1 transitions (at 3537, 3534, 3464, 3365, 3365 cm -1 for L ) He, Ne, Ar, N 2 , CH 4 ), which are attributed to proton-bound structures based upon the complexationinduced redshifts, ∆ν 1 . A linear correlation between ∆ν 1 and the proton affinity of L is observed. In the case of Ph + -Ar, a weak transition at 3536 cm -1 is assigned to the ν 1 band of the less stable π-bound isomer. The analysis of photofragmentation branching ratios and systematic frequency shifts in the spectra of larger Ph + -L n clusters (n e 2 for CH 4 , n e 5 for Ar, n e 7 for N 2 ) provide information about the microsolvation process of Ph + in nonpolar environments. The ν 1 transitions of the most stable isomers display small incremental blueshifts with respect to the dimer transitions, suggesting that further solvation causes little destabilization of the intermolecular proton bond to the first ligand. In the case of the Ph + -(N 2 ) n complexes, the existence of two isomers is observed in the size range n ) 5-7. For several Ph + -L n clusters, the most stable cation structures produced in the EI source differ considerably from the geometries observed by resonant enhanced multiphoton ionization (REMPI) of the corresponding neutral precursors. The limitations of REMPI techniques (arising from the Franck-Condon principle) for the generation and spectroscopic characterization of cluster cations are discussed.

Structure of gas‐phase arenium ions: The long‐standing controversy about the geometry of protonated benzene is unambiguously solved by IR spectroscopy. The first high‐resolution gas‐phase spectrum of an arenium ion shows that the σ complex (not the π complex; see picture) is the most stable structure of C6H7+.