Temperature Dependences of Kinetic Isotope Effects

Model‐reaction calculations are used to investigate the expected incidence of temperature‐dependence anomalies produced by the harmonic‐vibrational contributions to kinetic isotope effects. These “anomalies” are defined as deviations from a smooth‐monotonic log–log plot of isotopic rate‐constant ratio vs temperature. Such deviations include inflections, extrema, and crossovers (inversions in direction with respect to the infinite‐temperature, classical limit). The model systems studied represent a variety of mechanisms ranging from simple bond break to simultaneous rupture or formation of four bonds. The effects on the temperature‐dependence‐curve shapes of off‐diagonal force‐constant changes, variations in potential‐barrier curvature, and tunneling corrections are examined. The major emphasis is on carbon and hydrogen isotope effects although some nitrogen and oxygen effects are also considered. Out of the 476 isotopic reaction pairs investigated, approximately 25% were found to be anomalous. Several exa...

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