IR spectrum of the benzene–water cation: direct evidence for a hydrogen-bonded charge–dipole complex

The infrared spectrum of the benzene–water cation, C6H6+–H2O, was recorded in the O–H stretch region to obtain the first experimental information about its geometry and interaction strength. The spectrum is consistent with a charge–dipole structure in which the oxygen atom of H2O approaches the C6H6+ cation in the aromatic plane. The dissociation energy estimated from the spectral shifts is D0≈14±3 kcal/mol. The interaction in the C6H6+–H2O cation is rather different from the one in the neutral dimer, demonstrating the dramatic ionization-induced changes of the interaction between an aromatic hydrocarbon and a polar ligand.