Energy dependence and isotope effect for the total reaction rate of Cl+HI and Cl+HBr

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Abstract
A laser initiated chemical reaction method has been used to determine the total reaction rates for Cl+HI and Cl+HBr and the dependence of the rates on collision energy and H–D isotopic substitution. The rate constants are k=1.64×10−10 cm3 molecule−1⋅sec−1 (σ=33.5 Å2) for Cl+HI and k=7.4×10−12 cm3 molecule−1⋅sec−1 (σ=1.44 Å2) for Cl+HBr. Measurements were all done at 295 °K in slowly flowing gases. Substituting H by D decreases the rate constant by a factor of 1.84 in the case of HI and by 1.5 in the case of HBr. The isotope effect may be a result of tunneling on corner cutting trajectories. The cross section decreases with increasing collision energy. This, together with the large reaction cross section, indicates the importance of an attractive potential in these systems.