Rate Constants for the Gas-Phase Reactions of the Hydroxyl Radical with Isoprene, α- and β-Pinene, and Limonene as a Function of Temperature

Rate constants for the gas-phase reactions of the hydroxyl radical with the biogenic hydrocarbons isoprene, α- and β-pinene, and limonene were measured using the relative rate technique over the temperature range 295−364 K and at 760 Torr total pressure. OH was produced by the photolysis of H<INF>2</INF>O<INF>2</INF>, and helium was the diluent gas. The reactants were detected by on-line mass spectrometry, which resulted in high time resolution allowing for large amounts of data to be collected and used in the determination of the Arrhenius parameters. Many experiments were performed over the temperature range of interest leading to more accurate parameters than previous investigations, which have relied on rate constants measured at three or fewer temperatures. The following Arrhenius expressions have been determined for these reactions (in units of cm³ molecule^-1 s^-1):  isoprene, <EQUATION><EQNDATA>%@mt;sys@%2.56%@sx@%-0.31%@be@%+0.35%@sxx@%%@mx@%</EQNDATA> </EQUATION> × 10^-11 exp[(408 ± 42)/T]; α-pinene, <EQUATION><EQNDATA>%@mt;sys@%1.17%@sx@%-0.18%@be@%+0.21%@sxx@%%@mx@%</EQNDATA> </EQUATION> × 10^-11 exp[(436 ± 53)/T]; β-pinene, <EQUATION><EQNDATA>%@mt;sys@%1.47%@sx@%-0.21%@be@%+0.24%@sxx@%%@mx@%</EQNDATA> </EQUATION> × 10^-11 exp[(467 ± 50)/T]; limonene, <EQUATION><EQNDATA>%@mt;sys@%4.20%@sx@%-0.51%@be@%+0.59%@sxx@%%@mx@%</EQNDATA> </EQUATION> × 10^-11 exp[(401 ± 43)/T]. The Arrhenius parameters determined here for the reaction of OH with β-pinene are significantly different than the current recommendation, and the parameters given here for limonene are the first to be reported.