Shock tube study of the decomposition of cyclopentyl radicals

The kinetics of the thermal decomposition of cyclopentyl radicals has been studied in a single-pulse shock tube over a temperature range of 950–1116 K and pressures of 2.3–3.4 bar. Cyclopentyl radicals were generated by abstraction of H from cyclopentane by H atoms, which were produced by the thermal decomposition of hexamethylethane. Products were analyzed using dual-column gas chromatography under cryogenic conditions with simultaneous flame-ionization and mass-spectrometric detection. The main stable products resulting from the decomposition of cyclopentyl radicals are ethene and cyclopentene. These results are consistent with the interconversion of cyclopentyl and pent-4-en-1-yl radicals. The product branching ratio of ethene to cyclopentene is determined as [ethene]/[cyclopentene] = (8.2 ± 0.7)exp[−(1067 ± 88)/T), where the listed uncertainty (1σ) is from the statistical analysis. The overall accuracy in our measurement of the branching ratio is estimated at about 8% (1σ). In contrast to previous estimates, ring opening of cyclopentyl is found to be faster than ejection of a hydrogen atom. The former reaction is favored by about a factor of three. An RRKM/Master Equation treatment was adopted to assign the rate parameters to elementary reactions. Details of the model and its compatibility with the present and literature results are presented. Kinetic results for the pyrolysis of cyclopentyl and pent-4-en-1-yl radicals have been derived over relevant combustion conditions [0.1–1000 bar, 700–1900 K].