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 [phi] expresses the ratio

[phi] = (xD/xH)solute/(xD/xH)solvent

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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