Prototype Microsolvation of Aromatic Hydrocarbon Cations by Polar Ligands:  IR Spectra of Benzene⁺−LnClusters (L = H₂O, CH₃OH)

IR photodissociation spectra of clusters composed of the benzene cation (Bz + ) solvated by several water (H 2 O W) and methanol (CH 3 OH M) ligands are recorded in the O−H and C−H stretch ranges to investigate the microsolvation process of an aromatic hydrocarbon cation by polar ligands. The Bz + −L (L = W, M) spectra are consistent with H-bound dimer equilibrium structures, in which the ligand L approaches Bz + in the aromatic plane to form two H-bonds between the lone pairs of the oxygen atom of L and adjacent protons of Bz + . This charge−dipole structure corresponds to the global minimum of the dimer potential and is the only isomer identified in the IR spectrum. In contrast to the dimers, the spectra of larger Bz + −L n clusters ( n ≤ 4 for L = W, n ≤ 2 for L = M) provide clear evidence for the presence of two classes of isomers. Isomer class I corresponds to interior Bz + solvation and is characterized by cluster geometries in which n single ligands form separate H-bonds to the central Bz + cation. Isomer class II clusters show the onset of the formation of an H-bonded solvent network, because at least one ligand forms an H-bond to a second ligand. As the Bz + −L interaction is much stronger than the L−L interaction, class I isomers are probably more stable than class II isomers. Comparison between Bz + −L n and Bz−L n reveals the drastic influence of ionization of the aromatic solute on both the dimer properties and the microsolvation process.