Marvin J. Stern

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.

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...

Further Studies on Temperature Dependences of Isotope Effects

It is shown that, although the temperature behavior calculated in the harmonic approximation for a specific isotope‐exchange reaction may be extremely sensitive to the force fields used in the computations, small force‐field changes do not affect significantly the over‐all prevalence of temperature‐dependence “anomalies.” In a series of calculations for a large number of 13C‐, 18O‐, and D‐isotope‐exchange equilibria involving organic and inorganic molecules, such “anomalies” are found in 50 of the 171 13C exchanges considered, in 84 of the 153 18O exchanges considered, and in 40 of the 120 D exchanges considered. When the present results are combined with previous results for 15N‐ and 18O‐exchange reactions involving simple inorganic molecules, “anomalies” appear in about 40% of the almost 700 reactions investigated. It is concluded that isotope‐effect temperature‐dependence “anomalies” are not due to unusual or fortuitous combinations of force constants, but represent a more general phenomenon. Further, ...