Reaction of deuterium atoms with methyl bromide studied by gas phase e.p.r. spectroscopy

Abstract

Gas-phase electron paramagnetic resonance spectroscopy has been used to study the reaction of deuterium atoms with methyl bromide in a discharge flow system in the 1-10 Torr pressure range at 297 K and between 297 and 480 K at 3.1 ± 0.1 Torr. The initial steps in the reaction are D + CH₃Br = CH₃+ DBr (7), D + DBr = D₂+ Br (10), with k₇= 5.41 ± 0.18 × 10¹³ exp(–17.9 ± 1.1 kJ mol ^–1/RT) cm³ mol^–1 s^–1, the total rate of disappearance of deuterium atoms being several times greater than k₇ and inversely dependent on total pressure. This was due to a chain reaction in which the rate-determining step is diffusion of bromine atoms to the wall. Methyl radicals are removed primarily at the surface in this system and the reactions D + CH₃= CH₃D^*(4), CH₃D^*= CH₂D + H (5), are relatively minor processes. From the kinetics of hydrogen atom production a value of k₄= 7 × 10¹² exp(–9.1 ± 1.5 kJ mol^–1/RT)cm³ mol^–1 s^–1 is obtained.