Test of RRKM theory. Rates of decomposition in the series of chemically activated 2-n-alkyl radicals from C4 to C16

Abstract

The rates of decmposition of chemically activated 2‐n‐alkyl radicals in the series C 4 – C 16 were measured by an internal comparison method at 300°K in the pressure region centered at 0.01–0.1 torr. Activation was by H‐atom addition to the appropriate α‐olefin. The substrate species have a minimum energy of Ω 40 kcal mole −1 . The consequences of radical isomerization are discussed. A limitation of the present activation technique is illustrated. Theoretical Marcus‐Rice rate constants agree well with the experimental values. Earlier experimental evidence that all degrees of freedom of smaller radicals must be treated as active is now extended to the C16 member of the series, and the continued applicability of the conventional theoretical treatment is demonstrated for the very large species. Within the accuracy of the work, internal energy is shown to be effectively randomized in the time interval between stabilizing collisions, even for the largest member of the series. The observed rate constant decline from Ω 10 7 to 10 2 sec −1 , from smallest to largest member of the series, is mitigated somewhat by a concomitant increase in the average energy of the activated species due to intrinsic thermal energy. At a fixed energy value chosen, for example, as 47 kcal/mole−1, which is a representative value of the spectrum of energies of decomposing radicals over the whole series, it is estimated that the rate constants k 47 decrease from Ω 2× 10 8 to Ω 1 sec −1 in the series. Data for the decomposition of chemically activated 2, 3‐dimethylbutyl‐2 radicals are presented for a wide range of pressure. The deactivation efficiency of H2 relative to a strong collider is again found to be ∼ 0.2 , thus reconfirming the constancy of this value as independent of radical size for a homologous series of large radicals having near‐constant energetics.