Kinetics for the Reactions of H₃O⁺(H₂O) with Isoprene (2-Methyl-1,3-butadiene) as a Function of Temperature (300-500...

We report kinetics studies of H₃O⁺(H₂O) with isoprene (2-methyl-1,3-butadiene, C₅H₈) as a function of temperature (300-500 K) measured using a flowing afterglow-selected ion flow tube. Results are supported by density functional (DFT) calculations at the B3LYP/def2-TZVP level. H₃O⁺ (n = 0) reacts with isoprene near the collision limit exclusively via proton transfer to form C₅H₉⁺. The first hydrate (n = 1) also reacts at the collision limit and only the proton transfer product is observed, although hydrated protonated isoprene may have been produced and dissociated thermally. Addition of a second water (n = 2) lowers the rate constant by about a factor of 10. The proton transfer of H₃O⁺(H₂O)₂ to isoprene is endothermic, but transfer of the water ligands lowers the thermicity and the likely process occurring is H₃O⁺(H₂O)₂ + C₅H₈ → C₅H₉⁺(H₂O)₂ + H₂O, followed by thermal dissociation of C₅H₉⁺(H₂O)₂. Statistical modeling indicates the amount of reactivity is consistent with the process being slightly endothermic, as is indicated by the DFT calculations. This reactivity was obscured in past experiments due to the presence of water in the reaction zone. The third hydrate is observed not to react and helps explain the past results for n = 2, as n = 2 and 3 were in equilibrium in that flow tube experiment. Very little dependence on temperature was found for the three species that did react. Finally, the C₅H₉⁺ proton transfer product further reacted with isoprene to produce mainly C₆H₉⁺ along with a small amount of clustering.