A @[email protected] in which \(\frac{k^{\text{l}}}{k^{\text{h}}}<1\), i.e. the heavier substrate reacts more rapidly than the lighter one, as opposed to the more usual 'normal' @[email protected], in which \(\frac{k^{\text{l}}}{k^{\text{h}}}>1\). The @[email protected] will normally be 'normal' when the frequency differences between the isotopic @[email protected] are smaller than in the reactants. Conversely, an inverse @[email protected] can be taken as evidence for an increase in the corresponding @[email protected] on passing from the reactant to the @[email protected]