ENTROPIC AND ENTHALPIC EFFECTS OF 4-METHOXY SUBSTITUTION IN PHENOXYL RADICALS

Values of Delta H-3 = (-12.4 +/- 1.6) kJ mol(-1), Delta S-3 = (-18.5 +/- 5.6) J K-1 mol(-1) for reaction (3) (see text), corresponding to an O-H bond energy of 322.2 kJ mol(-1) in 1, and to a 14.5 J K-1 mol(-1) entropy loss for the CH3O-Ar (Ar = aromatic) libration in 2 relative to 1. are derived from the temperature dependence of the equilibrium constant K-3, determined by EPR spectrometry in benzene-toluene media, between 251 and 304 K. These results allow, for the first time, discrimination between enthalpic and entropic effects on the rates of (O)H-atom abstraction by peroxyl radicals from 4-methoxyphenols and related species.