LABORATORY AND THEORETICAL-STUDY OF THE OXY RADICALS IN THE OH-INITIATED AND CL-INITIATED OXIDATION OF ETHENE

The products of the OH-initiated oxidation mechanism of ethene have be en studied as a function of temperature (between 250 and 325 K) in an environmental chamber, using Fourier transform infrared spectroscopy f or end product analysis. The oxidation proceeds via formation of a per oxy radical, HOCH2CH2O2. Reaction of this peroxy radical with NO is ex othernic and produces chemically activated HOCH2CH2O radicals, of whic h about 25% decompose to CH2OH and CH2O on a time scale that is rapid compared to collisions, independent of temperature. The remainder of t he 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 mo l(-1) for reaction 7, the barrier to decomposition of the HOCH2CH2O ra dical is found to be 10-11 kcal mol(-1). A study of the Cl-atom-initia ted oxidation of ethene was also carried out; the main product observe d under conditions relevant to the atmosphere was chloroacetaldehyde, ClCH2CHO. Theoretical studies of the thermal and ''prompt'' decomposit ion of the oxy radicals were based on a recent ab initio characterizat ion 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 t he fraction of activated radicals dissociating before collisional stab ilization was obtained by master equation analysis using RRKM theory. The barrier to HOCH2CH2O decomposition is inferred independently as be ing 10-11 kcal mol(-1), by matching both of the theoretical HOCH2CH2O decomposition rates at 298 K with the experimental results. The data a re discussed in terms of the atmospheric fate of ethene.