ANOMALOUS TEMPERATURE DEPENDENCE OF KINETIC CARBON ISOTOPE EFFECTS AND PHENOMENON OF CROSSOVER

The effects of diagonal and off-diagonal stretching force constant variations on the temperature-dependent and temperature-independent factors in the intramolecular isotopic rate constant ratio for the decomposition of oxalic acid-13C have been explored. Crossover (change in sense with respect to unity) of the temperature-dependent factor is not observed when the reaction coordinate consists of a single internal coordinate displacement. When a zero frequency is achieved by employing one large off-diagonal force constant (two elements in the reaction coordinate), crossover is observed only if the basic diagonal force field is asymmetric at one or more isotopic bonds. If the reaction coordinate contains three or more elements (two or more non-zero off-diagonal force constants are employed), anomalies of temperature dependence can be generated with symmetric diagonal force fields. Two new techniques for study of the effects of systematic variation of the composition of the reaction coordinate are introduced and/or applied.