FTIR STUDIES ON THE PHOTOOXIDATION MECHANISMS OF CH3CL, CH3BR, CHBR3 AND CF3BR

The Photooxidations of CH3Cl, CH3Br, CF3Br, and CHBr3 in dry-air have been investigated at 298 K in a static reaction vessel by means of FTIR spectroscopy. Irradiation of the reaction mixtures has been performed by Hg-resonance lamps (184.9 nm and 253.7 nm lines) and broad band UV-fluorescence lamps (300-400 nm). The photooxidation mechanism of CH3Cl and CH3Br is governed by two different pathways: 1) photolytical release of halogen atoms and subsequent oxidation of the CH3 radical, 2) abstraction of an H-atom and oxidation of the respective CH2Cl and CH2Br-fragment. The second path leads to the formation of the formyl-halides HClCO and HBrCO which are transferred to CO and HCI or CO and HBr, respectively. Addition of trace amounts of NO to the gas mixture decreases the HClCO yield strongly. We suggest that NO reacts with the transient H2ClCO-radical to ClNO + H2CO. HBrCO, Br2CO and the final products CO, CO2, and HBr could be identified after Hg-resonance lamps induced photooxidation of CHBr3. The Cl2/fluorescence lamps induced photooxidation, however, yielded solely Br2CO, CO2, and HBr. In contrast to the labile species HCICO, HBrCO, and Br2CO, the photooxidation product of CF3Br, carbonyl fluoride (F2CO) is relatively inert under our reaction conditions.