Direct and Indirect Hydrogen Abstraction in Cl plus Alkene Reactions

Reactions between Cl atoms and propene can lead to HCl formation either by direct H abstraction or through a chloropropyl addition complex. Barring stabilizing collisions, the chloropropyl radical will either decompose to reactants or form HCl and allyl products. Using velocity-map imaging to measure the quantum state and velocity of the HCl products provides a view into the reaction dynamics, which show signs of both direct and indirect reaction mechanisms. Simulated trajectories of the reaction highlight the role of the direct H-abstraction pathways, and the resultant simulated scattering images show reasonable agreement with measurement. The simulations also show the importance of large excursions of the Cl atom far from equilibrium geometries within the chloropropyl complex, and these large-amplitude motions are the ultimate drivers toward HCl + allyl fragmentation. Gas-phase measurements of larger alkenes, 2-methylpropene and 2,3-dimethylbut-2-ene, show slightly different product distributions but still feature similar reaction dynamics. The current suite of experiments offers ready extensions to liquid-phase bimolecular reactions.