IR spectroscopy of microsolvated aromatic cluster ions: Ionization-induced switch in aromatic molecule-solvent recognition

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 1RPD 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, pi-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. 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. 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. 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.