Measurable consequences of a dip in the activation barrier for an adiabatic chemical reaction

Abstract

A dip in the activation barrier is shown to give rise to a pseudo-bound activated complex, whose existence has an important effect on the chemical reaction cross-section and rate constant. Quantum-mechanical theory is applied to the model reaction A + BC → AB + C with the atoms constrained to be colinear but with the system free to rotate. At energies in the region of the dip, the reaction cross-section shows a well resolved vibrational-rotational resonance structure, based on the states of the activated complex. The most prominent lines may contribute 10-2 å 2 to the reaction cross-section. The rate constant, k, follows an Arrhenius temperature dependence at both high and low temperature limits. The activation energy coincides in the first case with the top of the activation barrier and in the second case with the zero point level of the complex. This behaviour is qualitatively similar to that predicted for simple tunnelling through a single humped potential barrier. Measurement of k over a temperature range extending to both high and low temperature limits for two isotopic substitutents would allow a quantitative test by which the resonance tunnelling mechanism discussed in this paper could be recognized.