The ratio
,
where
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.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1115
