Kinetic Study of the OH + Isoprene and OH + Ethylene Reactions between 2 and 6 Torr and over the Temperature Range 300−423 K

The rate constants for the OH + isoprene and OH + ethylene reactions have been measured in He with 10% O 2 ( P = 2−6 Torr) and over the temperature range 300−423 K using a discharge-flow system coupled with laser-induced fluorescence. The measured rate constants for the OH + ethylene reaction are in good agreement with previously reported values. The termolecular rate constant ( k 0 ) at the low-pressure limit for the OH + ethylene reaction was determined to be (2.62 ± 0.25) × 10 -29 cm 6 molecule -2 s -1 at 300 K where the uncertainty represents 2 standard errors. An Arrhenius expression of k 0 = (4.9 ± 0.2) × 10 -31 exp[(1210 ± 130)/ T ] cm 6 molecule -2 s -1 was obtained from a weighted linear least-squares fit of the k 0 data versus temperature. The observed negative activation energy (- E a / R ) is larger than the currently recommended value. Unlike the OH + ethylene reaction, the rate constant for the OH + isoprene reaction is independent of pressure between 2 and 6 Torr at 300 K. The measured rate constant of (1.10 ± 0.04) × 10 -10 cm 3 molecule -1 s -1 at 300 K and 2 Torr agrees well with those measured at higher pressures. However, the rate constant for the OH + isoprene reaction begins to show a pressure dependence at temperatures of 343 K and higher. At 343 K, the termolecular rate constant was observed to be (10.50 ± 1.57) × 10 -27 cm 6 molecule -2 s -1 . An Arrhenius expression of k 0 = (9.3 ± 5.4) × 10 -29 exp[(1560 ± 230)/ T ] cm 6 molecule -2 s -1 was obtained from a weighted linear least-squares fit of the k 0 data versus temperature. The negative activation energy for the OH + isoprene reaction is similar to that observed for the OH + ethylene reaction.