Absolute and relative reference systems used to determine weight coefficients associated with AKIE_p for further calculation of isotopocule fractionation factors.

Molecule 1 illustrates the absolute reference system where each of the 6 atoms adopt absolute positions and based on which all 64 isotopocules of PCE are determined. This absolute reference system is described by WPCE. Molecule 2 illustrates the relative reference system based on which each reactive bond can be distinctly considered. Molecules 3 to 6 illustrate the result of transformation of the absolute system with a rotating reference system required to determine position-specific AKIEs related to the removed atom at absolute position X = C, E, F, and D, respectively. Clα1 will typically undergo a primary Cl isotopic effect while Clα2, Clβ1 and Clβ2 will undergo secondary Cl isotopic effects. Similarly, Cα will undergo a primary C isotopic effect and Cβ a secondary C isotopic effect. The green color indicates the absolute reference system; the red color indicates the relative reference system.