Kinetic isotope effects in the reactions of bromine atoms with CH4 and CD4

The competitive reactions of Br atoms with CH₄ and CD₄ were studied over the temperature range of 562° to 637°K. Over this temperature interval, the kinetic isotope effect, k_H/k_D, varied from 3.05 to 2.47 for the reactions The rate constant ratio k_H/k_D, expressed in Arrhenius form, was found to equal (1.10 ± 0.05) exp (1030 ± 60/RT). A comparison is presented between the experimental result and the result obtained theoretically from absolute rate theory using the London-Eyring-Polanyi-Sato (LEPS) method of constructing the potential energy surface of the reaction. The agreement between theory and experiment is very poor, and this is believed to arise from the highly unsymmetrical nature of the potential energy surface involved in these reactions. A comparison is also presented between the k_H/k_D values obtained in the Br + CH₄–CD₄ experiments and the available data on the corresponding Cl + CH₄–CD₄ reactions.