Temperature Dependences of Isotope Effects

It is shown that the harmonic-vibrational contributions to an isotope effect can by themselves lead to several types of temperature-dependence “anomalies,” including inflections, maxima, minima, and “cross-overs,” in a plot of the logarithm of an isotopic equilibrium-constant or rate-constant ratio vs temperature (or vs logarithm of temperature). In a series of calculations for a large number of 15N and 18O isotope-exchange equilibria among fourteen simple oxynitrogen compounds, such “anomalies” are found in 15 of the 91 15N exchanges considered and in 97 of the 153 18O exchanges considered. The significance of the prevalence of temperature-dependence “anomalies” to equilibrium and kinetic isotope-effect studies is discussed.