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
β-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 (CH3Ȯ), diazenyl (ṄNH),
and methyl formate radicals (CH3OĊO). For diazenyl,
it is found that hot β-scission fully replaces the sequence
of hydrogen abstraction and β-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 β-scission:
if the reverse hydrogen abstraction barrier height is comparable to
or larger than the β-scission barrier height, hot β-scission
should be considered in more detail.