SUMMARY

Isotope effects in spectroscopy, chemical equilibria, phase equilibria, physicochemical properties, reaction kinetics, and biology are reviewed. The theory of isotope effects is discussed in some detail.

1. INTRODUCTION

If naturally occurring water, containing deuterium in low concentration, evaporates then the concentration of deuterium in the liquid phase increases because H ₂O is more volatile than HDO. The viscosity of D ₂O is higher than that of H ₂O. The diffusion rate of ³⁶Ar is higher than that of the heavier argon isotope ⁴⁰Ar. The infrared spectrum of CH ₄ is different from that of CD ₄. The reaction of an organic compound becomes substantially slower if, in the rate-determining step of the reaction, a C-D bond is being broken instead of a C-H bond. Other examples abound.

These differences brought about by isotopic substitution in the physical and chemical properties of atoms and molecules are called isotope effects. The differences in the characteristic parameters of nuclear transformations (half-life, energy of radiation, cross section) are not considered as isotope effects. The molecules which differ in their isotopic composition are usually called isotopic molecules in the literature. However, according to the IUPAC terminology, a molecular entity that differs solely in isotopic composition (number of isotopic substitutions), e.g. CH ₄, CH ₃D, CH ₂D ₂, CD ₄ is called isotopologue. Isomers having the same number...