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

The kinetics of the isoprene-OH reaction have been studied both experimenta lly and computationally. Experimental rate constants at pressures in the ra nge 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 dete rmined, which is consistent with previous results for the high-pressure lim iting rate constant. We present the first experimental observation of the f alloff 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 calcul ations to determine the relative branching among the four possible isoprene -OH adducts in the high-pressure limit. We find OW addition to the outer ca rbon positions dominates OH addition to the inner carbon positions. We have employed RRKM/master equation calculations to evaluate the pressure depend ence of the overall rate and the rates for the individual isomers in the pr essure range 0.25 - 1000 Torr. The excellent agreement between the calculat ed and experimental falloff behavior provides an independent test of the ab initio energetics and RRKM/ME treatment. The results shed light on the mec hanisms for oxidation of isoprene in the troposphere.