Relative Rates of Reaction of Bromine Atoms with Isotopic Hydrogen Molecules

Abstract

Kinetic isotope effects in the reaction of bromine atoms with isotopic hydrogen molecules were measured in this investigation. Rate constant ratios were determined by isotopic analysis of H₂–HD, H₂–HT, or H₂—D₂ mixtures before and after reaction with bromine atoms. Over the temperature range of 438° to 623°K, the ratios of the second‐order rate constants were kH₂/Σk_HD = (1.14±0.01) exp(591±4/RT), kH₂/Σk_HT = (1.22±0.02) exp(658±16/RT), kH₂/kD₂ = (1.32±0.04) exp(1253±37/RT). A comparison of these results with the corresponding isotope effects obtained in the chlorine‐ and iodine‐atom experiments reveals surprisingly little difference in kinetic isotope effects. The theoretical calculation of the kinetic isotope effects was carried out using Bigeleisen's theory of isotope effects on reaction rates. The calculations were made using various methods of constructing the potential energy surface and it was found that none of the semiempirical methods used gave agreement to better than 40% to 80%.