Introduction Isotope Effects in Unimolecular Reactions

As is implied by the name, a unimolecular reaction is one in which a single molecule of reactant decomposes or rearranges to give rise to product molecules. Ordinary thermal reactions can be modeled by a process which considers the reactant to be in thermal equilibrium with a transition state which then decomposes (rearranges) to give products. One can theoretically describe the process and its isotope effects using transition state theory. For unimolecular reactions, on the other hand, while there is still a transition state, it is not in thermal equilibrium with the reactant except for systems at high pressure. Consequently, a more elaborate theoretical framework is required to understand unimolecular reactions and their isotope effects. 

In the following section, the RRKM mechanism for gas phase unimolecular reactions will be introduced and the corresponding theoretical framework, including isotope effects, will be outlined. Subsequent sections will deal with some applications of this theoretical framework to systems which have been studied experimentally. 

The last parts of this chapter will deal with the so-called mass independent isotope effects which have been rationalized in recent years to be a consequence of unimolecular processes and which have become one of the most fascinating of present day isotope effect studies. 

Wolfsberg et at., Isotope Effects in the Chemical, Geological, and Bio Sciences, DOI 10.1007/978-90-481-2265-3 14, Springer Science+Business Media B.V. 2009 

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