Hydroxyl radicals react with a-2-, a-3, and a-4-pyridyl l-oxide N-tert-butyl nitrones (POBN) with rate constants of 3.2 X lo9, 4.8 X lo9, and 3.5 X lo9 Ml, r espectively, via addition to two distinct sites. Addition to the pyridine ring yields short-lived radicals of the hydroxyazacyclohexadienyl type, while addition to the nitrone function in the side chain yields long-lived nitroxide radicals. The distribution of OH addition at the two molecular ...