Quantum dynamics study of kinetic isotope effects of OD with HBr and DBr.

We carried out a time-dependent, full dimensional, quantum dynamics wave-packet calculation to study the isotope effects for the OD + HBr and OD + DBr reactions. Reaction cross sections and rate constants for the OD + HBr (k(2)) and OD + DBr (k(4)) reactions are compared with the ones of OH + HBr (k(1)) and OH + DBr (k(3)). The comparisons of cross sections and rate constants show that OH/OD + HBr almost has the same reactivity, as does OH/OD + DBr. Nonetheless, the OH + HBr reaction still has the largest reactivity, then OD + HBr, OH + DBr, and OD + DBr has the smallest. Furthermore, the rate constants of OD + HBr/DBr reactions have a strong negative temperature dependence below 200 K and a nearly constant temperature dependence above 200 K, agreeing with the experimental results both qualitatively and quantitatively. Our calculated quantum primary kinetic isotope effects (k(1)/k(3) = 1.56 and k(2)/k(4) = 1.80) and secondary kinetic isotope effects (k(1)/k(2) = 1.11 and k(3)/k(4) = 1.29) are temperature independent, confirming the experimental measurements, and are in very good agreement with the experimental data.