Ionized phenol and its isomers in the gas phase
Résumé
Protonated 4-bromophenol and 4-bromoanisole produced by methane chemical ionization are found to easily be dehalogenated upon high (8 keV) or low (20-30 eV) energy collisional activation giving essentially phenol and anisole radical cations, respectively. Under similar conditions, protonated unsubstituted anisole is also readily demethylated generating the phenol ion but not cyclohexadienone ions. Other nonconventional isomers of ionized phenol are only detected by MS/MS/MS experiments performed on [M-CO].+ ions from salicylaldehyde. (U)B3LYP/6-311 ++G(d,p) and CASPT2/6-31G(d,p) calculations indicate the higher stability of the phenol radical cation with respect to the other six-membered-ring isomers. The least energy demanding fragmentation, namely, the decarbonylation, is shown to involve the intermediacy of six-membered ketones, open-chain ketenes, and five-membered cyclopentadiene isomeric ions. The rate determining step corresponds to the enol-keto interconversion with an energy barrier of about 276 kJ/mol relative to the phenol ion, which is markedly smaller than that required for hydrogen atom loss, deprotonation, or CO loss from an open-chain form. This suggests a crucial role played by the solvent in the readiness of the deprotonation of phenol ions in nonpolar media. The adiabatic ionization energy of phenol is evaluated as IEa(C6H5O) = 8.35 ± 0.2 eV (exptl: 8.49 eV), and the proton affinity of the phenoxy radical is evaluated as PA(C6H5O) = 863 ± 10 kJ/mol (exptl: 860 kJ/mol), PA(phenol) = 826 ± 10 kJ/mol (exptl: 818 kJ/mol), and PA(anisole) = 848 ± 10 kJ/mol.