thus A0 = Z0, which varies as %/T, and hence the main temperature dependence of kf comes from the exponential term. For large molecules their orientation on collision becomes important and a correction to Ao is required. This is usually introduced as the steric factor or probability factor P, so that Ao = PZo, with P < 1. The steric factor requires a more complex treatment than that from simple collision theory and this is manifested within the transition-state model that involves a transition state or activated complex AB*.

7.7.3 Termolecular reactions

Recombination reactions involving a third body are termed termolecular reactions. The probability of a three-body collision at atmospheric conditions is very small (< 1%). The termolecular reaction involves two steps. First the two reacting bodies, A and B collide to form an activated body C*

If the activated body does not suffer a collision it will decompose to its constituent reactants. If it undergoes a deactivating collision with a catalytic body (usually N2 and O2 in the atmosphere) then it will become a stable body C

The overall reaction is the termolecular recombination reaction

Termolecular recombination reactions are thus also pressure dependent as the rate of deactivation of the activated body depends on the collision frequency of the three bodies. The general form of the rate constant (see JPL 2006) is kt([M],T)=(jj^jfd, (7.85)


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