Photodissociation spectroscopy of phenol dimer ion. Lack of resonance interaction between the aromatic rings

Electronic spectra of a homo-molecular but inequivalent dimer ion, (C6H5OH)2+, are measured by photodissociation spectroscopy. A broad band appeared in the 340–440 nm region is characteristic of the C ← X transition of a phenol ion chromophore, suggesting that the dimer ion consists of a phenol ion and a neutral phenol. No strong band is observed in the 450–1400 nm region, whereas other aromatic dimer ions usually show charge resonance bands in near-infrared region. Owing to the geometrical constraint of an OH · O hydrogen bond in (C6H5OH)2+, the two aromatic rings cannot be in a parallel configuration suitable for the resonance interaction.