Non-eyring temperature dependence of dynamic isotope effects

Our group has recently described a new form of kinetic isotope effect that arises from dynamic selectivity in the bifurcation of a reaction pathway on the slope of an energy surface. Since the selection between products does not occur at a potential energy saddle point, we proposed that the isotope effect is decided by dynamic factors that are not necessarily related to zero point energy effects on the surface. As an alternative explanation, it has recently been suggested that variational transition state theory can account for these isotope effects. The dynamic explanation and variational transition state theory explanation make distinct predictions as to the temperature dependence of these isotope effects. I describe here my studies of the temperature dependence of the intramolecular isotope effects for the reaction of singlet oxygen with gemtetramethylethylene- d6. The selectivity observed for this reaction across a broad temperature range is clearly a non-Eyring distribution and thus the mechanism cannot be adequately described solely using transition state theory or even some variation of it. After using these results to evaluate competing theories it was concluded that this mechanism could only be properly understood using a dynamic explanation.