Isotope Effect and Heavy-Light-Heavy Reactivity Oscillation in the Cl + CHD₃/CHT₃ Reaction

Previous experiments and theories have shown the existence of heavy-light-heavy (HLH) reactivity oscillation in the Cl + CH₄ reaction and anticipated that similar oscillations should exist in many HLH reactions involving polyatomic reagents. However, the total reaction probabilities for the Cl + CHD₃ → HCl + CD₃ reaction exhibit only a step-like feature, and the total reaction probabilities for Cl + CHT₃ → HCl + CT₃ do not show any structure at all. Here, we report seven-dimensional state-to-state quantum dynamics studies for this reaction on the FI-NN PES, and we demonstrate that HLH reactivity oscillations also exist in these two reactions, manifesting as peaks in the reaction probabilities for low product rotational states. These oscillations, however, are obscured in the total reaction probability because of the higher excitation of j ≥ 2 product rotational states. Furthermore, the isotope replacement of nonreactive hydrogen with deuterium and tritium significantly enhances reactivity at collision energies above 0.112 eV, indicating an inverse secondary isotope effect on the probabilities, which is proved to be also caused by HLH mass combination. We also demonstrate that the highly rotational excitation of CHD₃ substantially enhances reactivity and the HLH oscillations, similar to HLH triatomic reactions. These observations are completely different from those in the H + CHD₃ reaction, which is also a late-barrier reaction. Therefore, the HLH mass combination is very important, which affects not only the reactivity oscillation but also the amplitude and product rotational state distribution and makes the initial rotation excitation play a pivotal role in the reaction.