isotopic fractionation factor

https://doi.org/10.1351/goldbook.I03343
The ratio \[\frac{\left(\frac{x_{1}}{x_{2}}\right)_{\text{A}}}{\left(\frac{x_{1}}{x_{2}}\right)_{\text{B}}}\] 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: \[\mathit{\Phi }=\frac{\left(\frac{x_{\text{D}}}{x_{\text{H}}}\right)_{\text{solute}}}{\left(\frac{x_{\text{D}}}{x_{\text{H}}}\right)_{\text{solvent}}}\] 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 [Terms] [Paper]