Kinetics of the Self Reaction of Cyclopentadienyl
Radicals
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Abstract
The
kinetics of the self-reaction of cyclopentadienyl radicals
(c-C<sub>5</sub>H<sub>5</sub>) was studied by laser photolysis/photoionization
mass spectroscopy. Overall rate constants were obtained in direct
real-time experiments in the temperature region 304–600 K and
at bath gas densities of (3.00–12.0) × 10<sup>16</sup> molecules cm<sup>–3</sup>. The room-temperature value of
the rate constant, (3.98 ± 0.41) × 10<sup>–10</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>, is significantly higher than the rate constants for most hydrocarbon
radical–radical reactions and coincides with the estimated
collision rate. The observed overall c-C<sub>5</sub>H<sub>5</sub> +
c-C<sub>5</sub>H<sub>5</sub> rate constant demonstrates an unprecedented
strong negative temperature dependence: <i>k</i><sub>1</sub> = 2.9 × 10<sup>–12</sup> exp(+1489 K/T) cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>, with estimated
uncertainty increasing with temperature, from 13% at 304 to 32% at
600 K. Formation of C<sub>10</sub>H<sub>10</sub> as the primary product
of cyclopentadienyl self-reaction was observed. In additional experiments
performed at the temperature of 800 K, formation of C<sub>10</sub>H<sub>10</sub>, C<sub>10</sub>H<sub>9</sub>, and C<sub>10</sub>H<sub>8</sub> was observed. Final product analysis by gas chromatography/mass
spectrometry detected two isomers of C<sub>10</sub>H<sub>8</sub> at
800 K: naphthalene (major) and azulene (minor)