Low-Pressure Study of the Reaction of Cl Atoms with Isoprene

Abstract

The kinetics of the reaction of chlorine atoms with isoprene (1) has been studied using the discharge-flow mass-spectrometric method: Cl + C5H8 (+M) → C5H8Cl (+M) (1a); Cl + C5H8 → C5H7 + HCl (1b). As a result of direct and relative measurements, the overall rate constant k1 = (6.7 ± 2.0) × 10-11 exp{(485 ± 85)/T} cm3 molecule-1 s-1 was obtained at a pressure of 1 Torr of helium over the temperature range 233−320 K. k1 was found to be pressure-independent (within 10%) at T = 298 K in the pressure range 0.25−3 Torr. Both HCl and C5H8Cl adduct were detected as products of reaction 1. The measurements of the HCl yield as a function of temperature led to the branching fraction for the H-atom abstraction channel (1b): k1b/k1 = (1.22 ± 0.4) exp{−(595 ± 90)/T} (P = 1 Torr, T = 233−320 K); k1b/k1 = 0.169 ± 0.022 at 298 K. The results are compared with those reported in a recent study4 carried out at 298 K. In addition, the rate coefficient has been measured for the reaction Cl + Br2 → BrCl + Br (2), which was used as the reference in the relative study of reaction 1. Combining the present results obtained in the range 233−320 K with those available from a previous study in the range 298−401 K,10 the following Arrhenius expression is recommended: k2 = (2.3 ± 0.4) × 10-10 exp{(135 ± 60)/T} cm3 molecule-1 s-1.