Kinetics of the OH-initiated oxidation of isoprene

Absolute rate coefficients have been determined for the reaction OH + C₅H₈ (1) and two of its isotopomeric variants. Reaction (1) was studied as a function of temperature and pressure in N₂, N₂/O₂ and He buffer gases. At room temperature, a rate coefficient, k₁, of (8.56±0.26)*10⁻¹¹ cm³ s⁻¹ independent of pressure and buffer gas identity was obtained. An Arrhenius fit to data over the temperature range 250 - 340 K gave a negative activation energy of -666±150 cal/mol. HO₂ production was inferred from observations of radical regeneration in the presence of NO. The absence of a kinetic isotope effect, together with the specificity of radical regeneration, indicates that reaction proceeds via an addition channel with no significant abstraction component. Our rate coefficient for reaction (1) is 15% lower than the value currently recommenced for atmospheric modeling.