Réactions des radicaux chlorofluorométhyl péroxyles avec no : étude cinétique dans le domaine de température compris entre 230 et 430 K

Les constantes de vitesse absolues des réactions d'une série de radicaux chlorofluoromethyl peroxyles CX3O2 avec NO ont été déterminées pour des températures comprises entre 230 et 430 K, par photolyse laser pulsée et spectrométrie de masse résolue dans le temps. Les réactions sont rapides, environ deux fois plus rapides que la réaction équivalente de CH3O2 et présentent un coefficient négatif de température. Les expressions des constantes de vitesse sont les suivantes, les unités étant exprimées en cm3 molecule-1.s-1 : [math] Aucun effet de pression significatif n'a pu être détecté entre 1 et 10 Torr. Des mesures de rendements de NO3 ont montré que CX3O et NO3 sont les produits principaux de la réaction dans le domaine de pression étudié

Photolyse de l’ammoniac à 206,2 nm en présence d’isobutane réaction des radicaux NH

La réaction photochimique de l’ammoniac en présence d'isobutane est étudiée en phase gazeuse par excitation à 206,2 mm. Un schéma réactionnel est déduit de l’évolution des rendements quantiques des produits, en fonction de divers paramètres. Les résultats montrent que la réaction du radical NH2 avec l’hydrocarbure joue un rôle important dans la séquence des réactions radicalaires. Il est également montré que la réactivité relative des hydrogènes primaire et tertiaire de l’isobutane est très différente avec H et NH2

Gas-Phase Reactivity of the HCO Radical with Unsaturated Hydrocarbons: An Experimental and Theoretical Study

cited By 14International audienceThe kinetics and mechanism of the reactions of HCO radicals with a series of unsaturated hydrocarbons were studied experimentally by the flash photolysis-laser resonance absorption technique and theoretically, in the particular case of ethylene, by ab initio SCF MO calculations with double f basis sets. Experiments were carried out in the temperature range 350–510 K by photolyzing acetaldehyde for the generation of HCO. The Arrhenius expressions obtained are the following, in cm3molecule-1s-1 units: ethylene, (1.5 ± 0.6) X 10-13 exp[-(2750 ± 75 K/T)]; propene, (1.7 ± 0.9) X 10-13 exp[-(2700 ± 100 K/T)]; isobutene, (5.45 ± 2.9) X 10-13, exp[-(3125 ± 130 K/T)]; 1-butene, (3.8 ± 2.7) X 10-13 exp[-(3000 ± 180 K/T)]; 2-butene, (3.3 ±2.8) X 10-13 exp[-(3000 ± 220 K/T]; and 1,3-butadiene, (5.8 ± 1.3) X 10-13 exp[-(2050 ± 50 K/T)]. The uncertainties quoted are equal to 2σ. The reactivity of the HCO radical with respect to that of unsaturated hydrocarbons is compared to that of other radicals. The end-product analysis in the photolysis of the acetaldehyde-olefin (or butadiene) system has shown that the reaction essentially proceeds via an addition mechanism, rather than a hydrogen atom transfer from HCO to the double bond, which is also energetically favorable. This is the first characterized reaction of HCO which does not involve such a hydrogen transfer from the radical. These results are supported by quantum mechanical calculations of the potential energy surfaces involved in the two possible reaction channels. These calculations show in particular that the energy barrier is much higher for the hydrogen atom transfer channel than for the addition process. In addition, calculated activation energies and preexponential factors are in good agreement with experimental results. © 1986, American Chemical Society. All rights reserved

UV absorption spectra of methyl-substituted hydroxy-cyclohexadienyl radicals in the gas phase

UV absorption spectra of five methyl-substituted hydroxy-cyclohexadienyl radicals, formed by the addition of the hydroxyl radical (OH) to toluene (methyl benzene), o-, m- and p-xylene (1,2-, 1,3- and 1,4-dimethyl benzene, respectively) and mesitylene (1,3,5-trimethylbenzene), have been determined at 298 K, 1 atm pressure (N-2 + O-2), and the corresponding absolute absorption cross-sections measured, using laser flash photolysis and time-resolved UV absorption detection. As observed for other cyclohexadienyl-type radicals, a strong absorption band is present in the 260-340 nm spectral region, with maximum cross-sections in the range (0.9-2.2) x 10(-17) cm(2) molecule(-1). The shape of the band varies significantly from one radical to the next for the series of aromatic precursors investigated. The nature and yields of hydroxylated ring-retaining oxidation products, identified in previous studies of the OH-initiated oxidation of aromatic hydrocarbons, and the results of theoretical density functional theory (DFT) calculations indicate that one or more possible isomers of the various OH-adducts may contribute to the observed spectra. Isomers where the OH-group is ortho- (or both ortho- and ipso-) to a substituent methyl-group are likely to be the most abundant but other isomers may also be formed to a significant extent. Nonetheless, the present study provides absorption spectra of the adduct radicals formed from the gas phase addition of OH to the aromatic hydrocarbons considered, near room temperature and I atm pressure. (c) 2005 Elsevier B.V. All rights reserved