Hydration of Phenylium: Oxonium or Carbenium Ion Formation?

The hydration of the phenylium ion (C 6 H 5 + ) is investigated to determine whether the most abundant product structure is best described as an oxonium (O‐protonated phenol) or carbenium (ring‐protonated phenol) ion. Using a combination of infrared multiple‐photon dissociation spectroscopy at the FELIX facility, collision‐induced dissociation experiments, and quantum chemical calculations, we have characterized the reaction products formed upon the addition of water to phenylium. The IR fingerprint of the hydrated ion reveals dominant spectral features consistent with carbenium isomers, particularly the ortho and par a forms, rather than the oxonium isomer. Reactivity measurements and branching ratios of the reaction between water and C 6 H 5 + produced by vacuum ultraviolet (VUV) dissociative photoionization of nitrobenzene at the SOLEIL synchrotron facility, as a function of internal energy show that protonated phenol forms efficiently in the gas phase. Computational modeling of the potential energy surface supports the preferential formation of carbenium structures and maps the key isomerization and dissociation pathways.