Consider, for example, the dihydrogen molecules H2, HD, and HT, in which D represents deuterium 2H1 and T is tritium 3H1 (H is of course 1H1). The minimum vibrational energy each molecule can possess is highest for H and smallest for T. But the energy required to cleave the bond between the two atoms depends mainly on electronic factors that are not affected by isotopic mass, so as the diagram shows, the energy that must be added in order to break the pair of atoms is smallest for H–H and greatest for H–T.
The kinetic isotope effect forms the basis for an experimental way of determining if the breaking of a particular bond is the rate-determining step in a mechanism. By examining the effect of a deuterated solvent such as HDO, D2O or C2H5OD, one can establish the role of proton transfer involving the solvent in a mechanism.
In the case of dihydrogen, the dissociation energy is so much greater than these zero-point energies that the differences are of little practical importance. But for deuterium-substituted C–H or O–H bonds which are much weaker, the effect is much greater. A reaction in which one of these bonds breaks to form the transition state will be significantly slowed down if the H is replaced by D.
Because the magnitude of this effect, known as the kinetic isotope effect, depends on the relative masses of the isotopes, it is most apparent when H and D isotopes are compared.
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1797 | The first recorded use of clay minerals as heterogeneous catalysts by Bondt et. al. who studied the dehydration of alcohols. |
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