Crossed-beams and theoretical studies of hyperthermal reactions of O( ³P) with HCl and H₂O

The reactions of O(³P) with HCl and H₂O at hyperthermal collision energies (45 - 116 kcal mol^-1) have been investigated with crossed-molecular beams experiments and direct dynamics quasiclassical trajectory calculations. Both reactive systems may proceed by two analogous primary pathways, (1) H-atom abstraction to produce OH and either Cl or OH and (2) H-atom elimination to produce H and either ClO or HO₂. The H-atom elimination reactions are highly endoergic, and they have been observed experimentally for the first time. The H-atom abstraction reaction follows a stripping mechanism, in which the reagent O atom approaches the target molecule at large impact parameters and the OH product is scattered in the forward direction with respect to the initial direction of the reagent O atom. The H-atom elimination reaction requires low impact parameter collisions. The excitation function for ClO or HO₂ increases from threshold to a maximum around 115 kcal mol^-1 and then begins to decrease when the product can be formed with sufficient internal energy to undergo secondary dissociation. At collision energies slightly above threshold for H-atom elimination, the ClO or HO₂ product scatters primarily in the backward direction, but as the collision energy increases, the fraction of these products that scatter in the forward and sideways directions increases. The dependence of the angular distribution of ClO or HO₂ on collision energy is a result of the fact that only certain trajectories, where an H atom on the target molecule is oriented toward the incoming reagent O atom, can release enough energy in translation to lead to stable ClO or HO₂ at higher collision energies. These trajectories lead to forward and sideways scattering of ClO or HO₂. Moreover, these trajectories do not follow the minimum path and involve larger translational energy release. Therefore, they become dominant at higher collision energies because they lead to lower internal energies and more stable ClO or HO₂ products. The energy barrier for the O(³P) + H₂O → HO₂ + H reaction was determined experimentally to be 60 ± 2 kcal mol ^-1. Theory predicts that the H-atom abstraction and elimination reactions have comparable cross sections for hyperthermal O(³P) + HCl collisions but that H-atom elimination is only about 20 percent as probable as H-atom abstraction for hyperthermal O(³P) + H₂O collisions.