Experimental and computational study of the OH-isoprene reaction: Isomeric branching and low-pressure behavior

The kinetics of the isoprene-OH reaction have been studied both experimentally and computationally. Experimental rate constants at pressures in the range 0.5-20 Torr have been determined at 295 K using pulsed photolysis/laser-induced fluorescence detection of the OH radical. A rate constant of (0.99 +/- 0.05) x 10(-10) molecules(-1) cm(3) s(-1) at 20 Torr in argon was determined, which is consistent with previous results for the high-pressure limiting rate constant. We present the first experimental observation of the falloff region for this reaction and have modeled the pressure dependence of the rates using the Tree formalism. Canonical variational transition state theory calculations were performed on the basis of recent ab initio calculations to determine the relative branching among the four possible isoprene-OH adducts in the high-pressure limit. We find OW addition to the outer carbon positions dominates OH addition to the inner carbon positions. We have employed RRKM/master equation calculations to evaluate the pressure dependence of the overall rate and the rates for the individual isomers in the pressure range 0.25 - 1000 Torr. The excellent agreement between the calculated and experimental falloff behavior provides an independent test of the ab initio energetics and RRKM/ME treatment. The results shed light on the mechanisms for oxidation of isoprene in the troposphere.