General mechanism of thermal dissociation and recombination reactions

General mechanism of thermal dissociation and recombination reactions 

The basic kinetic properties of unimolecular reactions can be understood in terms of simple reaction mechanisms that characterize the types of elementary processes involved. A dissociation reaction comprises three steps collisional activation to form excited molecules collisional deactivation (-1), 

The reverse reaction, recombination, comprises an association step that forms and redissociation (2) and collisional deactivation (-1) steps 

It can readily be seen why an energy-rich intermediate species must participate. In the dissociation direction, reaction can only occur after the molecule AB has accumulated, through collisions, enough energy to rupture the chemical bond holding A and B together in the recombination direction, the first species formed in an A-B encounter retains all of the energy of the newly formed bond until later events, i.e., subsequent collisions, cause it to be dissipated to the environment. 

The intermediate, energetically excited species AB does have different energy distributions among its internal quantum states in the dissociation and recombination directions. Nevertheless, it has been shown that (under steady-state conditions, which may always, as far as is known, be presumed to pertain for combustion) it is meaningful to treat the reaction scheme for dissociation and recombination reactions with the same formal rate coefficients fci, fc i, fc2 and fc 2 for the steps (1), (-1), (2), and (-2), respectively. 

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