PHOTOINDUCED OXYGEN-TRANSFER FROM NO2 TO ETHYLENE IN THE VICINITY OF THE NO2 DISSOCIATION THRESHOLD - A LASER PHOTOCHEMICAL STUDY ON REACTANT PAIRS ISOLATED IN SOLID ARGON

The wavelength dependence of the photochemistry of C2H4.NO2 and C2D4.NO2 pairs, isolated in solid Ar, in the range 555-355 nm is reported. Continuous-wave and pulsed dye lasers were used to excite the reactants, and the products were monitored by FT infrared spectroscopy. Acetaldehyde, ethylene oxide, NO, and ethyl nitrite radical were the products observed at wavelengths longer than the 398-nm dissociation threshold of NO2. D isotope effects on the branching among these species derived from kinetic analysis of growth curves support our conclusion in previous work that the mechanism involves large-amplitude O transfer from NO2 to the C=C bond to yield a transient oxirane biradical. The latter acts as a common precursor of the observed products. A new species, ketene, appears at excitation energies just above the NO2 dissociation limit (385 and 355 nm). This product is assigned to a new path consisting of NO2 --> O(3P) + NO dissociation followed by O(3P) + ethylene addition. Comparison of product growth upon 385- and 420-nm excitation indicates that there is no abrupt change in total product yield, combined aldehyde and epoxide yield, or epoxide/aldehyde branching ratio as the photolysis photon energy is tuned through the 398-nm dissociation threshold. This suggests that large-amplitude O transfer continues to play a dominant role upon excitation of ethylene.NO2 collisional pairs even at wavelengths shorter than the NO2 --> O(3P) dissociation limit.