| Abstract
| - The aminoxyl radical (>N−O•) BTNO (benzotriazole-N-oxyl) has been generated by the oxidationof 1-hydroxybenzotriazole (HBT; >N−OH) with a CeIV salt in MeCN. BTNO presents a broadabsorption band with λmax 474 nm and ε 1840 M-1 cm-1, and spontaneously decays with a first-order rate constant of 6.3 × 10-3 s-1 in MeCN at 25 °C. Characterization of BTNO radical by EPR,laser flash photolysis, and cyclic voltammetry is provided. The spontaneous decay of BTNO isstrongly accelerated in the presence of H-donor substrates such as alkylarenes, benzyl and allylalcohols, and alkanols, and rate constants of H-abstraction by BTNO from a number of substrateshave been spectroscopically investigated at 25 °C. The kinetic isotope effect confirms theH-abstraction step as rate-determining. Activation parameters have been measured in the 15−40°C range with selected substrates. A correlation between Ea and BDE(C−H) (C−H bond dissociationenergy) for a small series of H-donors has been obtained according to the Evans−Polanyi equation,giving α = 0.44. From this plot, the experimentally unavailable BDE(C−H) of benzyl alcohol canbe extrapolated, as ca. 79 kcal/mol. With respect to the H-abstraction step, peculiar differences inthe ΔS⧧ parameter emerge between an alkylarene, ArC(H)R2, and a benzyl alcohol, ArC(H)(OH)R.The data acquired on the H-abstraction reactivity of BTNO are compared with those recentlyreported for the aminoxyl radical PINO (phthalimide-N-oxyl), generated from N-hydroxyphthalimide(HPI). The higher reactivity of radical PINO is explained on the basis of the higher energy of theNO−H bond of HPI, as compared with that of HBT (88 vs ca. 85 kcal/mol, respectively), which isformed on H-abstraction from the RH substrate.
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