Direct dynamics studies for the reactions of CH₃CH₂Br with O (³P) and Cl (²P) atoms

A dual-level direct dynamics method is employed to perform the dynamics calculations for the multi-channel reactions CH₃CH₂Br + O(³P) → products (R1) and CH₃CH₂Br + Cl → products (R2). Four reaction channels, i.e., one α-hydrogen, two β-hydrogen, and one bromine-abstractions, are identified for each reaction. The geometries and frequencies of all the stationary points are optimized at the BH&H-LYP/6-311G(d, p) level. The complexes with energies less than those of the reactants or products are found at entrance or exit of each reaction channel, which indicate that the reactions may proceed via an indirect mechanism. The energy profiles are further refined at the G3//BH&H-LYP level. Then, the rate constants are calculated by canonical variational transition-state (CVT) theory incorporating the small-curvature tunneling method (SCT) correction in the temperature range of 220-2000 K. The theoretical rate constants are in good agreement with the experimental ones. Theoretical calculations show that the Br-abstraction channel should be negligible due to its much higher barrier height than the others. As to the three hydrogen-abstraction channels, α-hydrogen abstraction is the major pathway and the contribution of β-hydrogen abstraction become important with the temperature increasing.