Special Topic 5.2 Isotope effects and tunnelling

Isotopes play a multifaceted role in mechanistic chemistry. Isotopic labelling at specific sites of a reagent is often used to support or exclude a hypothetical reaction mechanism based on the location of the isotopic label in the products. Kinetic isotope effects, that is, changes in rate constants that are induced by isotopic substitution, may be seen in time-resolved experiments or may express themselves as changes in product quantum yields. They hold important information regarding the nature of the rate-determining step that limits the overall rate in a sequence of reactions. 

Deuterium isotope effects are studied most often, because deuterium is relatively cheap, the synthesis of deuterium-labelled compounds is frequently straightforward and the effects are large and thus easy to measure with sufficient accuracy. Kinetic isotope effects can be attributed largely to the difference in the zero-point energies of the C—H and C—D stretching vibrations in the reactant, E0 = hvl2. The vibrational 

A case in point is the photoenolization reaction of o-alkylphenyl ketones (Section 6.3.6) and related compounds. Detailed studies of the temperature dependence of both the primary photoreaction386 3X8 and of the back transfer reaction in the ground state389 have provided convincing evidence for tunnelling contributions in these reactions. 

The wealth of information on reactive intermediates is covered in textbooks of physical organic chemistry and the specialized books quoted above. Many reactions involving such intermediates will be discussed in Chapter 6. 

Time-resolved IR studies have further probed the equilibrium between the spin states of 

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