A kinetic isotope effect (KIE) is a ratio of rate constants deviating from unity for a pair of reactions under identical conditions except for the isotopic composition of the reacting species, and is a consequence of the quantum-mechanical nature of nuclear motion. Historically, the theory of KIEs for reactions of gas-phase molecules was developed within the framework of transition state theory (TST) by the methods of statistical mechanics in terms of partition functions evaluated within the Born–Oppenheimer, harmonic-oscillator, rigid-rotor and ideal-gas approximations.^1,2  Neglecting tunnelling and transmission-factor contributions, the ratio of rate constants k/k′ (where the prime...