Termination rates by time‐resolved ESR. The N‐hydro‐4‐acetylpyridinyl radical in solution

The second‐order rate constants for the self‐termination reaction of the N‐hydro‐4‐acetylpyridinyl radical in propanol(‐2) and propanol(‐2) acetone have been measured by time‐resolved ESR during intermittent photolysis. The radical was generated from 4‐acetylpyridine via different photochemical reactions. The rate constants were measured in the temperature range ‐10 to 26°C. The frequency factor and the activation energy from the Arrhenius plot are (1.1 ± 0.4) · 1010M−1· s−1and 16.6 ± 0.6 kJ · mol−1, respectively. The Arrhenius parameters were independent of the solvent composition and the method of radical generation. The termination reaction is attributed to the formation of the respective pinacol by the combination of two radicals. The reaction rate is not diffusion‐controlled. Possible mechanisms are proposed and discussed.