Reaction branching and extreme kinetic isotope effects

Reaction branching and extreme kinetic isotope effects 

We have just mentioned that extreme KIEs are often accompanied by anomalous temperature effects, but these effects can be also due to reaction branching , which can fully mimic the effects caused by tunneling. 

Appearance of reaction branching, accompanied by extreme KIEs, often serves as evidence of the existence of short-lived intermediates. Thus, efforts were made to procure evidence for the existence of carbanions in proton 

Early literature explains extremely high isotope effects as being due solely to tunneling. Today, however, it is known that unusually high primary KIEs, as well as ratios of Arrhenius pre-exponential factors much less than 1, may be due to reaction branching if the rate of the reaction studied increases with temperature much more than the rate of the parallel reaction. Let us consider the composite reaction  

Physical Chemistry, 4th ed., Prentice-Hall, Englewood Cliffs, NJ 1972, p. 332. 

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Thihhlin, A. and Ahlherg, P. (1989). Reaction branching and extreme kinetic isotope effects in the study of reaction mechanisms. Chem. Soc. Rev. 18, 209 -224... 

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