One of the most important scientific discoveries of the twentieth century was the discovery of isotopes: variants of elements that differ only in the number of neutrons contained in their nuclei, and therefore have different masses. This seemingly subtle variation has profound effects that can be exploited in numerous ways. Some isotopes may be unstable and can fission, leading to radioactivity and nuclear reactions. Isotopic substitution in specific locations in molecules can also have profound effects. Absorption and emission spectra may be altered: the wavelengths of features may shift. Reaction rates and equilibrium concentrations may change. Some properties may change dramatically: the effects of changing the number of particles in a nucleus can cause population of some atomic or molecular energy states to appear or disappear altogether. Nonzero nuclear spin in some isotopes (and not in others) open molecules to study by new methods, such as Nuclear Magnetic Resonance (NMR). All these effects can be exploited in order to obtain a window into detailed reaction mechanisms, reaction rates, and to effect chemical separations otherwise impossible.
The recognition of the great utility of isotopic substitution in the last three-quarters of a century has also spurred much theoretical work seeking to understand the effects of isotopic substitution and to predict new effects that can be exploited for chemical studies.
Last Modified: June 28, 2012