F. Grandinetti et M. Speranza, UNCOMMON ELECTRONIC EFFECTS ON THE GAS-PHASE BRONSTED ACIDITY OF ISOMERIC HYDROXYPHENYLIUM IONS, Chemical physics letters, 229(6), 1994, pp. 581-586
The Bronsted-acid properties of gaseous hydroxyphenylium ions have bee
n 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 pronou
nced Bronsted-acid character by releasing efficiently to NH3 its HO pr
oton instead of one of its more acidic ring protons adjacent to the fo
rmally 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 barrie
r (greater-than-or-equal-to 46 kJ mol-1) in the ring-proton transfer r
eactions from arylium ions.