A step on the reaction path of some stepwise reactions in which the molecular entity C is already present in an encounter pair or encounter complex with A during the formation of B from A, e.g.
In this mechanism the chemical species C may but does not necessarily assist the formation of B from A, which may itself be a bimolecular reaction with some other reagent.
Pre-association is important when B is too short-lived to permit B and C to come together by diffusion. See also microscopic diffusion control, spectator mechanism.
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