Steps quadrimolecular

The molecularity of a step indicates how many reactant molecules participate in the step. For example, a step A— P is unimolecular, steps 2A— P and A + B — P are bimolecular. Trimolecular steps are rare, and quadrimolecular steps are unheard-of. 

A quadrimolecular step would require four molecules to collide with one another. Such an event is even much more improbable than a collision of three. Indeed, no quadrimolecular elementary step has ever been identified. 

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See also in sourсe #XX -- [ ]

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