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.