Termination Rates by Time‐Resolved ESR. The N‐Hydropyridinyl Radical in Solution and the Influence of Non‐Uniform Radical Concentration on the Apparent Rate Constant

The rate constants (2 k ) for the second‐order self‐termination reaction of the N‐hydropyridinyl radical have been measured by time‐resolved ESR during intermittent UV photolysis of pyridine in various solvents containing propanol(‐2). The frequency factor and the activation energy are (1.8 ± 0.4) · 10 10 M −1 s −1 and (10.7 ± 0.4) kJ · mol −1 , respectively, in propanol(‐2)/acetone (1:1 by volume) as obtained from the Arrhenius plot of 2 A: in the temperature range between 293 and 230 K. The rate constants at 285 K are (7 ± 1) · 10 8 M −1 s −1 in H 2 O/propanol(‐2)/acetone (2:1:1 by volume), independent of the pH value (7.5 – 13.7), and (6 ± 1) · 10 8 M −1 s −1 in neat propanol(‐2), the latter value being essentially diffusion controlled. The rate constants measured in propanol(‐2)/acetone (1:1) containing various amounts of water correlate with Kosower's solvent polarity parameter. The self‐termination reaction ultimately leads to the destruction of the pyridine ring. Finally, a model is presented for the correction of the influence of non‐uniform radical concentration on apparent values of second‐order rate constants as determined by time‐resolved ESR during intermittent photolysis.