isotopic fractionation factor
The ratio \[\frac{(x_{1}/x_{2})_{\text{A}}}{(x_{1}/x_{2})_{\text{B}}}\] where \(x\) is the abundance, expressed as the atom fraction of the isotope distinguished by the subscript numeral, when the two @I03331@ 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 @D01648@ solvent @I03327@, when the fractionation factor \(\mathit{\Phi}\) expresses the ratio: \[\varPhi = \frac{(x_{\text{D}}/x_{\text{H}})_{\text{solute}}}{(x_{\text{D}}/x_{\text{H}})_{\text{solvent}}} \] for the exchangeable hydrogen atoms in the chemical species (or sites) concerned. The concept is also applicable to transition states.
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1115 [Terms] [Paper]