Hydrogen atom abstraction kinetics from intramolecularly hydrogen bonded ubiquinol-0 and other (poly)methoxy phenols

The effect of methoxy substitution on the abstraction of the phenolic hydro gen atom involved in intramolecular hydrogen bonding by tert-butoxyl and cu myloxyl radicals has been investigated by laser flash photolysis. Also tran sition stale calculations for methoxyl radical and 2-methoxyphenol have bee n carried out by a density functional theory (DFT) method. Hydrogen atom ab straction is surprisingly easy from intramolecularly hydrogen bonded methox yphenols, in contrast to intermolecularly hydrogen bonded molecules. The ki netic solvent effect, investigated in six solvents with different hydrogen bond accepting properties, on the hydrogen atom abstraction reaction from o -methoxy phenols was shown to be smaller than for non-hydrogen bonded pheno ls, and is independent of further methoxy substitution. The high rate const ant for hydrogen atom abstraction from ubiqoinol-0 (2.8 x 10(9) M-1 s(-1) i n CCl4) and the small kinetic solvent effect make it a good antioxidant, ev en in a polar environment.