The ratio (x1/x2)A/(x1/x2)B, where x is the abundance, expressed as the atom fraction of the isotope distinguished by the subscript numeral, when the two isotopes are equilibrated between two different chemical species A and B (or between specific sites A and B in the same or different chemical species). The term is most commonly met in connection with deuterium solvent isotope effects, when the fractionation factor expresses the ratio
for the exchangeable hydrogen atoms in the chemical species (or sites) concerned. The concept is also applicable to transition states. GOLD (1969).
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