The Origin of Primary Kinetic Isotope Effects

C-H Zero-point energy C-D Zero-point energy 

A Morse potential for a C-H bond showing that the activation energy for homolysis of a C-D bond is larger than for a C-H bond. 

For a bond breaking event, the stretching vibration of that bond is defined as the reaction coordinate. As was discussed in Chapter 2, the frequency of a stretching vibration is modeled by the classic equation for the stretching of a spring with a mass attached at both ends (Eq. 8.2). In Eq. 8.2 v is expressed in s , whereas in IR spectroscopy the frequency is expressed in cm , and is called v (where v= vie and c, the speed of light, is given in cm/s). 

Most isotope effects are attenuated from this value because reactions typically do not involve bonds that are completely broken in the transition state. An example of a reaction with a relatively large isotope effect is the hydroxylation reaction given in the Connections highlight on page 425. To understand any kinetic phenomenon, one always compares reactant with transition state. For isotope effects we compare the ZPEs of the various vibrations of the reactant and the activated complex. Usually the bond is only partially broken at the transition state, or another bond is starting to form at the transition state. Both of these will attenuate the isotope effect from that of total homolysis. To visualize this attenuation, we need to examine reaction coordinate diagrams and the associated vibrational modes. 

A potential energy well perpendicular to the reaction coordinate with the associated C-H and C-D vibrational states 

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