Thermal rate constants, energy dependence, and isotope effect for halogen–hydrogen halide reactions

Thermal rate constants have been determined for the reaction C1+DI and Br+HI in the temperature range 220–400 K. For C1+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 C1+HI (DI). It is found that a factor of 3.9 increase in translational velocity over room temperature actually decreases the cross section for C1+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 C1+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.