THERMAL RATE CONSTANTS, ENERGY-DEPENDENCE, AND ISOTOPE EFFECT FOR HALOGEN-HYDROGEN HALIDE REACTIONS

Thermal rate constants have been determined for the reaction Cl + DI and Br + HI in the temperature range 220-400°K. For Cl + DI, the effective reaction cross section reaches a maximum of 18.8 Å2 near 345°K. The isotope effect increases in favor of HI from 1.5 to 2.7 as the temperature decreases from 400 to 223°K. For Br + HI, the effective reaction cross section decreases slightly with increasing temperature. The cross section at enhanced collisional energy, with various rotational energies, is determined for the reaction Cl + HI (DI). It is found that a factor of 3.9 increase in translational velocity over room temperature actually decreases the cross section for Cl + HI and DI by factors of 9 and 6.6, respectively. At 11.4 kcal/mole relative translational energy, the cross section for the reaction Cl + HI increases by 1.2 as the rotational temperature increases from 223 to 295°K. The results are discussed in terms of a reaction model in which the attacking halogen atom is attracted to the halogen end of the hydrogen halide and then rotation of the hydrogen completes the reaction. A semiquantitative analysis shows that tunneling makes an important contribution to the isotope effect. © 1979 American Institute of Physics.