A barrier-free atomic radical-molecule reaction: F + propene

The possible reaction mechanism of atomic radical F with propene is investigated theoretically by a detailed potential energy surface (PES) calculation at the UMP2/6-311++G-(d,p) and CCSD(T)/cc-pVTZ (single-point) levels using ab initio quantum chemistry methods and transition-state theory. Various possible reaction paths including addition-isomerization-elimination reactions and direct H-atom abstraction reactions are considered. Among them, the most feasible pathway should be the atomic radical F (F-2) attacking on the C=C double bond in propene (CH3CH=CH2) to form a weakly bound complex I1 with no barrier, followed by atomic radical F addition to the C=C double bond to form the low-lying intermediate isomer 3 barrierlessly. Starting from intermediate isomer 3, the most competitive reaction pathway is the dissociation of the C2-C3 single bond via transition state TS3-P5, leading to the product P5, CH3 + CHF=CH2. However, in the direct H-atom abstraction reactions, the atomic radical F picking up the b-allylic hydrogen of propene barrierlessly is the most feasible pathway from thermodynamic consideration. The other reaction pathways on the doublet PES are less competitive because of thermodynamical or kinetic factors. No addition-elimination mechanism exists on the potential energy surface. Because the intermediates and transition states involved in the major pathways are all lower than the reactants in energy, the title reaction is expected to be rapid. Furthermore, on the basis of the analysis of the kinetics of all channels through which the addition and abstraction reactions proceed, we expect that the competitive power of reaction channels may vary with experimental conditions for the title reaction. The present study may be helpful for probing the mechanisms of the title reaction and understanding the halogen chemistry.