Kinetic Investigations of the CH (X2Π) Radical Reaction with Cyclopentadiene.

The reaction of the ground state methylidyne radical (CH (X2Π)) with cyclopentadiene (C5H6) is studied in a quasi-static reaction cell at pressures ranging from 2.7 to 9.7 Torr and temperatures ranging from 298 to 450 K. The CH radical is generated in the reaction cell by pulsed-laser photolysis (PLP) of gaseous bromoform at 266 nm, and its concentration monitored using laser-induced fluorescence (LIF) with an excitation wavelength of 430.8 nm. The reaction rate coefficient is measured to be 2.70(±1.34) × 10-10 cm3 molecule-1 s-1 at room temperature and 5.3 Torr and found to be independent of pressure or temperature over the studied experimental ranges. DFT and CBS-QB3 methods are used to calculate the reaction potential energy surface (PES) and to provide insight into the entrance channel of the reaction. The combination of the experimentally determined rate constants and computed PES supports a fast, barrierless entrance channel that is characteristic of CH radical reactions and could potentially lead to the formation of benzene isomers.

Reactions of Atomic Carbon with Butene Isomers: Implications for Molecular Growth in Carbon-Rich Environments.

Product detection studies of C(3P) atom reactions with butene (C4H8) isomers (but-1-ene, cis-but-2-ene, trans-but-2-ene) are carried out in a flow tube reactor at 353 K and 4 Torr under multiple collision conditions. Ground state carbon atoms are generated by 248 nm laser photolysis of tetrabromomethane, CBr4, in a buffer of helium. Thermalized reaction products are detected using synchrotron tunable VUV photoionization and time-of-flight mass spectrometry. The temporal profiles of the detected ions are used to discriminate products from side or secondary reactions. For the C(3P) + trans-but-2-ene and C(3P) + cis-but-2-ene reactions, various isomers of C4H5 and C5H7 are identified as reaction products formed via CH3 and H elimination. Assuming equal ionization cross sections for all C4H5 and C5H7 isomers, C4H5:C5H7 branching ratios of 0.63:1 and 0.60:1 are derived for the C(3P) + trans-but-2-ene and the C(3P) + cis-but-2-ene reactions, respectively. For the C(3P) + but-1-ene reaction, two reaction channels are observed: the H-elimination channel, leading to the formation of the ethylpropargyl isomer, and the C3H3 + C2H5 channel. Assuming equal ionization cross sections for ethylpropargyl and C3H3 radicals, a branching ratio of 1:0.95 for the C3H3 + C2H5 and H + ethylpropargyl channels is derived. The experimental results are compared to previous H atom branching ratios and used to propose the most likely mechanisms for the reaction of ground state carbon atoms with butene isomers.

The reaction of anthracene with OH radicals: an experimental study of the kinetics between 58 and 470 K.

The first direct measurement of the reaction rate constant of a polycyclic aromatic hydrocarbon in the gas phase in the temperature range 58-470 K is reported. The reaction is OH+ anthracene and the experiment has been performed in a continuous flow Cinetique de Reaction en Ecoulement Supersonique Uniforme apparatus, which had to be modified for this purpose. Pulsed laser photolysis of H(2)O(2) has been used to generate OH radicals and laser-induced fluorescence to observe the kinetic decay of the radicals and hence determine the rate coefficients. The reaction is found to be fast, and the rate constant increases monotonically as the temperature is lowered. The rate coefficients match the expression k(cm(3) molecules(-1) s(-1))=1.12 x 10(-10)(T/300)(-0.46).

Electron attachment on HI and DI in a uniform supersonic flow: thermalization of the electrons.

In order to check the electron thermalization in the CRESU technique (Cinetique de Reaction en Ecoulement Supersonique Uniforme, e.g., "reaction kinetics in a uniform supersonic flow"), electron attachment on HI and DI has been studied in the 48-170 K range. Attachment to HI is exothermic and the reaction is expected to be fast and to proceed at a rate close to the capture limit. On the contrary, attachment to DI is slightly endothermic, and a strong positive temperature dependence of the measured rate coefficient is expected if the electrons are thermal. This dependence is not observed, and we conclude that the electrons are not in thermal equilibrium with the neutrals in the afterglow. A model, based on electron heating by superelastic collisions with the buffer gas, is proposed to explain this fact and implications for previously published results are discussed.