His work on the magnitude of kinetic isotope effects is still the basis of understanding in the field.

This kinetic isotope effect can be used to study reaction mechanisms by analyzing how the differently-massed atom is involved in the process.

For example, they can lead to a very direct interpretation of the isotope effect.

In such a case, the change is termed a primary isotope effect.

Thus, the magnitude of the kinetic isotope effect can be used to elucidate the reaction mechanism.

Then, analysis of the experimental data for the reaction allowed them to choose which pathway was most likely based on the observed isotope effect.

As a result, very large kinetic isotope effects are observed that can not be accounted for by differences in zero point energies.

Thiemens says the new isotope effect provides a simpler explanation.

So while one could calculate (I suppose) the isotope effect on atomic radius, it would be very small.

This was indicated by the isotope effect observed in superconductors.