Gas-phase reaction of NO3 radicals with isoprene: A kinetic and mechanistic study

The gas-phase reaction of NO3 radicals with isoprene was investigated under now conditions at 298 K in the pressure range 6.8 < P(mbar) < 100 using GC-MS/FID, direct MS, and long-path FT-IR spectroscopy as detection techniques. By means of a relative rate method, the rate constant for the primary attack of NO3 radicals toward isoprene was determined to be (6.86 +/- 2.60) x 10(-13) cm(3) molecule(-1) s(-1). The formation of the possible oxiranes, 2-methyl-2-vinyl-oxirane and 2-(1-methyl-vinyl)oxirane, was observed in dependence on total pressure. In the presence of O-2 in the carrier gas, the product distribution was found to be strongly dependent on the reaction pathways of formed peroxy radicals. If the peroxy radicals mainly reacted in a self-reaction, the formation of organic nitrates was detected and 4-nitroxy-3-methyl-but-2-enal was identified as a main product. On the other hand, when NO was added to the gas mixture and the peroxy radicals were converted via RO2 + NO --> RO + NO2, the formation of methyl vinyl ketone as the main product as well as 3-methylfuran and methacrolein was observed. From the ratio of the product yields if NO was added to the gas mixture it was concluded that the attack of NO3 radicals predominantly takes place in the 1-position. A reaction mechanism is proposed and the application of these results to the troposphere are discussed. (C) 1997 John Wiley & Sons, Inc.