The relative rates of C-C and C-H β-scission reactions of i-butyl radicals were investigated with shock tube experiments at temperatures of (950 to 1250) K and pressures of (200 to 400) kPa. We produce i-butyl radicals from the decomposition of dilute mixtures of i- pentylbenzene and have observed the stable decomposition products propene and i-butene. These alkenes are characteristic of C-C and C-H bond scission, respectively. Propene was the main product, approximately 30 times more abundant than i-butene, indicating that C-C β- scission is the primary pathway. Uncertainty in our measured ratio is mainly due to a small amount of side chemistry, which we account for using a kinetics model based on JetSurF 2.01. Our data are well-described after adding chemistry specific to our system and adjusting some rate constants. With the kinetics model, we place an upper limit of 3.1 % on the branching fraction for C-H β-scission in the i-butyl radical. While this data agrees with previous high quality experimental results, many combustion kinetics models assume C-H branching values well above this upper limit, possibly leading to large systematic inaccuracies in model predictions. Some kinetics models additionally assume contributions from 1,2-H shift reactions which for i-butyl would produce the same products as C-H β-scission and our upper limit includes possible involvement of such reactions. Kinetics models should be updated to better reflect current experimental measurements.
Journal of Physical Chemistry A
kinetics, pyrolysis, combustion, modeling, shock tube