Isotope effects, kinetic mass regarding

With thermal systems either in the gas phase or in solution, it is in ter -molecular isotope effects which are more commonly studied. Intramolecular isotope effects involve distinguishing and measuring two, or more, chemically identical but isotopically different products produced in the same reaction vessel from the same reactant. The situation is different in mass spectrometry. Intramolecular isotope effects are conveniently studied, because the chemically identical products are naturally separated according to their masses. Intermolecular isotope effects on ion abundances are also easily measured, but, as regards kinetics and mechanism of reaction, their value is limited. Whereas an intramolecular isotope effect (on ion abundances) reflects kinetic isotope effects, an intermolecular isotope effect (on ion abundances) reflects kinetic isotope effects, isotope effects on the internal energy distribution, P(E), and other factors as well and the effects cannot be easily separated (vide infra). 

The kinetic isotope effect (KIE) results in variation of the reaction rate constant as a function of isotopic variations in either of the reactants. A primary isotope effect occurs when the labeled bond is directly involved in the reaction coordinate and a secondary isotope effect occurs when the labeled bond is elsewhere in the ion or molecule. The effect is greater for larger changes in relative isotope mass. Often, a precise measure of the KIE can provide valuable information regarding the reaction mechanism. The predominant cause of the KIE is variation in the zero-point vibration energy (ZPE) as a function of isotope mass since changes to the ZPE may alter the reaction enthalpy of activation. Isotope substitution also effects changes in the translation, rotation and vibration partition functions, which in turn alters the reaction entropy of activation. 

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