Dissociation of chloride from ortho-, meta- and para-chloromethylphenoxides: The enthalpy of formation of meta-quinomethane

The enthalpy of formation of m-quinomethane has been determined by energy-resolved collision-induced dissociation measurements involving chloro-substituted phenoxide ions. The chloride affinity of m-quinomethane is measured to be 202 ± 9 kJ/mol, whereas the chloride affinities of o- and p-quinomethane are both 57 ± 5 kJ/mol. The difference in the chloride affinities reflects the difference in the stabilities of the Kekule and non-Kekule structures. Combination of the dissociation energy of the meta-isomer with the gas-phase acidity of 3-(chloromethyl)phenol, measured by using the kinetic method to be 1432 ± 6 kJ/mol, results in an enthalpy of formation of 166 ± 13 kJ/mol for the m-quinomethane diradical. The measured value agrees with that predicted by bond additivity. The low bond dissociation energies for the o- and p-isomers, along with electronic structure calculations, suggest that the corresponding chlorine-substituted ions are best described as halide/diradical complexes.