Nature of Isomeric-Transition-Induced Bromine-82 Reactions with CH4 and CD4

Abstract

The role of excess kinetic energy and thermal processes in the 82Br reaction in gaseous CH4 and CD4 activated by 82mBr isomeric transition (IT) was determined using rare-gas and bromine additives. Possible isotope effects were investigated between (n, γ-induced 80Br and 80mBr and between CH4 and CD4 target molecules. The “freeze–thaw” technique and (n, γ) “in-reactor” technique were employed in determining the 82Br organic yields. The effects of He, Kr, Xe, and Br2 additives on the reaction of CH4 and CD4 with 82Br activated by the (IT) process are presented. The data appear to extrapolate, at zero mole fraction CH4, to 3.7% ± 0.5% for the He, Kr, and Xe additives and to 0% for Br2. This would indicate that 7.4% minus 3.7% is formed via excess kinetic-energy processes and that 3.7% of the organic 82Br is formed via thermal (kinetic-energy-independent) processes, probably involving Br+ ions in their first two excited states, P13 and P03. The data suggest the absence of a target molecule isotope effect between CH4 and CD4 in the 82Br(IT)-activated process, but does suggest a target molecule isotope effect for (n, γ-activated bromine reactions. The data for bromine reactions involving excess kinetic energy were analyzed using the kinetic theory of hot reactions. The reactivity integral values I were found to be very low. The kinetic theory showed good internal consistency for (IT)-activated bromine reactions. Experimental data showed the absence of any isotope effect between (n, γ)-induced 80mBr and 80Br reaction in CH4. The use of HBr as a source molecule was reinvestigated in the CD4 system and found to give organic yields higher than those of Br2 systems at low mole fractions. This difference diminished at higher HBr concentrations.