The nuclear field shift effect in chemical exchange reactions

Mass-independent isotope fractionations found in laboratory-scale chemical exchange experiments are reviewed. The classic theory of stable isotope fractionation in chemical exchange reactions has been established by Bigeleisen, Mayer, and Urey in 1947. In 1996, the conventional mass-dependent theory was expanded by Bigeleisen to include a mass-independent term named the nuclear field shift effect. The nuclear field shift is an isotope shift in orbital electrons, which results from the isotopic difference in nuclear size and shape. Since the revised theory was proposed, the mass-independent isotope fractionation of various elements, (e.g., Ti, Cr, Zn, Sr, Mo, Ru, Cd, Sn, Te, Ba, Nd, Sm, Gd, Yb, and U), found in chemical exchange systems has been successfully explained as the nuclear field shift effect. In this review article, from both theoretical and experimental viewpoints, origins of mass-independent isotope effects are discussed.