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