Summary. The rate constants of the unimolecular decomposition of the pivaloyl radical (AD) and of the bimolecular self terminations of pivaloyl (k,) and t-butyl radicals (k,) in liquid mcthylcyclopentane arc determined by ESR.-spectroscopy :The viscosity dependence of K, is analysed with respect to diffusion control of tlie reaction.Comparison of k D values of different acyl radicals reveals a strong dependence of the activation energies on radical structure. . The studies were limited to small temperature ranges and few activation parameters were reported.In this paper, we extend our previous studies of photochemically produced radicals and their reaction rates [lo-121 to a determination of the absolute rate constants of self termination of the pivaloyl (R1= (CH,),CCO) and t-butyl (R2-(CH,),C.) radicals and of decomposition of pivaloyl (R1 --f R2 + CO) in a wide temperature range. The radicals are produced by photocleavage of di-t-butylketone in methylcyclopentane solutions. Steady state and time resolved ESR.-spectroscopy is applied.The particular chemical system was chosen for several reasons: Firstly, the radical reactions are rather simple and well known at room temperature [13] [14]. Secondly, the high quantum yield of cleavage leads to strong ESR.-signals of R1and R2 [lo] [ll]. Thirdly, though the self termination of t-butyl (R2) has been reported for several systems at or near room temperature 1-31 [5] 1-12] [15] the data show a considerable scatter and no activation parameters have been given. These would be highly instructive with respect to the possibIe diffusion control of this reaction in solution and for a comparison with tlie recent gas phase values [16]. Finally, the rate constant for decomposition of pivaloyl, a reaction reported for liquids [17], is of interest because a comparison with the known constants of other l)

Es wird die Kinetik des Zerfalls des Pivaloyl‐Radikals und des Kettenabbruchs durch Dimerisierung für beide Titelradikale untersucht.