Feshbach resonances in the F + CHD → HF + CD reaction.

The signature of dynamics resonances was observed in the benchmark polyatomic F + CH/CHD reactions more than a decade ago; however, the dynamical origin of the resonances is still not clear due to the lack of reliable quantum dynamics studies on accurate potential energy surfaces. Here, we report a six-dimensional state-to-state quantum dynamics study on the F + CHD → HF + CD reaction on a highly accurate potential energy surface. Pronounced oscillatory structures are observed in the total and product rovibrational-state-resolved reaction probabilities. Detailed analysis reveals that these oscillating features originate from the Feshbach resonance states trapped in the peculiar well on the HF(' = 3)-CD vibrationally adiabatic potential caused by HF chemical bond softening. Most of the resonance structures on the reaction probabilities are washed out in the well converged integral cross sections (ICS), leaving only one distinct peak at low collision energy. The calculated HF vibrational state-resolved ICS for CD( = 0) agrees quantitatively with the experimental results, especially the branching ratio, but the theoretical CD umbrella vibration state distribution is found to be much hotter than the experiment.