Laboratory and theoretical study of the oxy radicals in the OH- and Cl-initiated oxidation of ethene

The products of the OH-initiated oxidation mechanism of ethene have been studied as a function of temperature (between 250 and 325 K) in an environmental chamber, using Fourier transform infrared spectroscopy for end product analysis. The oxidation proceeds via formation of a peroxy radical, HOCH2CH2O2. Reaction of this peroxy radical with NO is exothernic and produces chemically activated HOCH2CH2O radicals, of which about 25% decompose to CH2OH and CH2O on a time scale that is rapid compared to collisions, independent of temperature. The remainder of the HOCH2CH2O radicals are thermalized and undergo competition between decomposition, HOCH2CH2O --> CH2OH + CH2O (6), and reaction with O-2, HOCH2CH2O + O-2 --> HOCH2-CHO + HO2 (7), The rate constant ratio, k(6)/k(7), for the thermalized radicals was found to be (2.0 +/- 0.2) x 10(25) exp[-(4200 +/- 600)/T] molecule cm(-3) over the temperature range 250-325 K. With the assumption of an activation energy of 1-2 kcal mol(-1) for reaction 7, the barrier to decomposition of the HOCH2CH2O radical is found to be 10-11 kcal mol(-1). A study of the Cl-atom-initiated oxidation of ethene was also carried out; the main product observed under conditions relevant to the atmosphere was chloroacetaldehyde, ClCH2CHO. Theoretical studies of the thermal and "prompt" decomposition of the oxy radicals were based on a recent ab initio characterization that highlighted the role of intramolecular H bonding in HOCH2CH2O. Thermal decomposition is described by transition state and the Tree theories. To quantify the prompt decomposition of chemically activated nascent oxy radicals, the energy partitioning in the initially formed radicals was described by separate statistical ensemble theory, and the fraction of activated radicals dissociating before collisional stabilization was obtained by master equation analysis using RRKM theory. The barrier to HOCH2CH2O decomposition is inferred independently as being 10-11 kcal mol(-1), by matching both of the theoretical HOCH2CH2O decomposition rates at 298 K with the experimental results. The data are discussed in terms of the atmospheric fate of ethene.