Decay Reactions of Aryldiazenyl Radicals in Solution

The decay processes of aryldiazenyl radicals were studied by using a time-resolved ESR method. The first order rate constants of decomposition were determined at −117–−48 °C in cyclopropane solutions in the presence or absence of olefins. Aryldiazenyl radicals were relatively persistent and the k1 of phenyldiazenyl-d5 at −96 °C in cyclopropane was 34 s−1. The k1 of 20 aryldiazenyl radicals at −96 °C afforded a ρ value of +1.53 (r=0.91) in a Hammett plot. Activation energies ranged from 46.2 to 25.2 kJ mol−1 and the frequency factors, log A, from 13.2 to 8.2 s−1. The second-order decay rates were measured at the initial period of the decay process at lower temperatures, in a batch system (cyclopropane) and in a flow system (isohexane). The rate constant of phenyldiazenyl-d5 in cyclopropane was 9.2·10−5 M−1 s−1 at −93 °C and the Arrhenius parameters were Ea=23.5 kJ mol−1 and logA=12.77 M−1 s−1, respectively.