Introduction

The isotope effect is a very familiar phenomenon in chemical reactions. The isotope composition of molecules affects their transformation rates in the course of chemical reactions as well as the equilibrium ratio of molecules. Both kinetic and equilibrium isotope effects can originate from the difference between isotopic masses. Isotope substitution changes molecular vibration frequencies, the energy of the molecular ground state (zero point vibration energy), the molecular momentum of inertia, and the effective mass for movement along a reaction coordinate. Atomic masses play a significant role in quantum tunnelling reactions. These isotope mass effects are well-known and are widely used in chemistry for separating isotopes, enriching compounds with definite isotopes, investigating mechanisms of chemical reactions, elucidating structures of transition states and other details of molecular dynamics in the elementary steps of chemical transformations. There are many monographs and reviews discussing comprehensively isotope mass effects (see, e.g., [1–3]).