A DFT study on the deprotonation antioxidant mechanistic step of ortho-substituted phenolic cation radicals

The conformers of the 2-, 3- and 4-substituted phenolic cation radicals, 2-X-, 3-X- and 4-X-ArOH center dot+, and the respective phenoxyl radicals, ArO center dot, the intramolecular hydrogen bond strength (Delta H-intra) estimate along with the electronic effects of five electron withdrawing (EWG) and eight electron donating groups (EDG) on the gas-phase O-H proton dissociation enthalpies, (PDEs), of the short-lived, 2-X-ArOH center dot+, (involved in the single-electron transfer antioxidant mechanism), are studied at the DFT/B3LYP level of theory. EWG result to smaller PDEs, hence to stronger acidity; EDG to weaker acidity. The deprotonation antioxidant mechanistic step is not a rate-controlling step for 2-X-ArOH to scavenge free radicals. Approximate estimations of the Delta PDEs (hence acidities as well) can be derived from calculated structural and/or vibrational frequency values. Delta H(intra)s correlate reasonably with geometrical parameters for the closed-shell, neutral counterparts, in contrast with previous estimates. (C) 2005 Elsevier B.V. All rights reserved.