A pre-association mechanism in which one of the molecular entities,
C, is already present in an encounter pair with
A during formation of
B from
A, but does not assist the formation of
B, e.g.
S05810.png
The formation of
B from
A may itself be a bimolecular reaction with some other reagent. Since
C does not assist the formation of
A, it is described as being present as a spectator, and hence such a mechanism is sometimes referred to as a spectator mechanism.
See also: microscopic diffusion control
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1165 (https://doi.org/10.1351/pac199466051077)