UNCOMMON ELECTRONIC EFFECTS ON THE GAS-PHASE BRONSTED ACIDITY OF ISOMERIC HYDROXYPHENYLIUM IONS

The Bronsted-acid properties of gaseous hydroxyphenylium ions have been evaluated by kinetic FT-ICR experiments and compared with ab initio theoretical predictions. While ortho and para HOC6H4+ ions do not show any significant reactivity, the meta HOC6H4+ isomer exhibits a pronounced Bronsted-acid character by releasing efficiently to NH3 its HO proton instead of one of its more acidic ring protons adjacent to the formally vacant orbital. This peculiar behavior derives from extensive p-pi conjugative stabilization in meta -OC6H4+, absent in the ortho and para isomers, and to the operation of a substantial activation barrier (greater-than-or-equal-to 46 kJ mol-1) in the ring-proton transfer reactions from arylium ions.