On the role of bimolecular reactions in chemical activation systems

The kinetics of chemically activated intermediates in complex-forming bimolecular reactions can often be described in terms of the branching ratio between their unimolecular decomposition channels and the collisional stabilization under steady-state conditions (D/S). This requires that the stabilized part of the population does not contribute to the decomposition rate, which is true only for reaction times shorter than the thermal lifetime of the intermediate. As these intermediates, however, are often radicals or other highly reactive species, they can undergo consecutive bimolecular reactions. Here two limiting cases are conceivable: (i) If the bimolecular steps are slow, mainly stabilized intermediates react, and their population is prevented from being built up, i.e. their thermal decomposition is suppressed. In this case the steady-state description in terms of D/S remains adequate also for reaction times longer than the thermal lifetime of the intermediate. (ii) If the bimolecular reactions are extremely fast, even highly excited intermediates react and are removed from the population. Consequently, the observed relative yield of the unimolecular channels, Phi, drops below the steady-state value Phi(ss) = [1 + (D/S)(-1)](-1). The branching between decomposition and stabilization of a chemically activated intermediate, therefore, can depend on the rate of its bimolecular consecutive reactions, a fact which is often overlooked in the kinetic analysis of such systems. In the present work we investigate the dependence of Phi on the rate of these bimolecular steps in a general way by means of a master equation. A simple method is presented to estimate the parameter range, where the steady-state approach in terms of D/S remains adequate if bimolecular reactions of the intermediate occur. The problem is exemplarily treated for the reaction between chemically activated but-2-yl radicals and hydrogen atoms.