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Isotope Effects and Reaction Mechanisms

Using NMR methodology developed in the Singleton group, we can measure kinetic isotope effects at natural abundance for each site in a molecule.  This has let us rapidly study reaction mechanisms throughout chemistry, resolving controversies, discovering new mechanisms, and uncovering new fine details of basic organic reactions. 

Kinetic isotope effects arise from changes in the zero-point energies of vibrational normal modes between starting materials and transition states.  By combining isotope effect measurements with theoretical calculations, we can get an experimentally-based picture of transition state geometries. Put another way, we can predict isotope effects extraordinarily well, but only if we have the correct mechanism and transition state geometry. Many of the current projects of group members employ isotope effects to study reaction mechanisms

 

• Kinetic isotope effects contain information about the vibrational normal modes in the same way as an IR spectrum does.

•  Conversely, isotope effects can be calculated from theoretical structures and frequencies.

•  The combination of the two provides information about the structure of the transition state.

 

Experimental Isotope Effects versus Predicted Isotope Effects
(transition state theory, harmonic approximation, 1-dimensional tunneling correction)

 

 

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