Discussion of the Separation of Chemical and Relaxational Kinetics of Chemically Activated Intermediates in Master Equation Simulations

Chemical activation of intermediates, such as hydrogen abstraction products, is emerging as a basis for a fully new reaction type: hot beta-scission. While for thermally equilibrated intermediates chemical kinetics are typically orders of magnitude slower than relaxational kinetics, chemically activated intermediates raise the issue of inseparable chemical and relaxational kinetics. Here, this separation problem is discussed in the framework of master equation simulations, proposing three cases often encountered in chemistry: insignificant chemical activation, predominant chemical activation, and the transition between these two limits. These three cases are illustrated via three example systems: methoxy (CH3O), diazenyl (NNH), and methyl formate radicals (CH3OCO). For diazenyl, it is found that hot beta-scission fully replaces the sequence of hydrogen abstraction and beta-scission of thermally equilibrated diazenyl. Building on the example systems, a rule of thumb is proposed that can be used to intuitively judge the significance of hot beta-scission: if the reverse hydrogen abstraction barrier height is comparable to or larger than the beta-scission barrier height, hot beta-scission should be considered in more detail.