Further Studies on Temperature Dependences of Isotope Effects

Abstract

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, the phenomenon is not restricted to particular structural features in molecules or to isotopes of particular elements.