Higher oxides of chlorine: Absorption cross-sections of Cl2O6 and C12O4, the decomposition of Cl2O6, and the reactions of OClO with O and O3

T. J. Green, Meez Islam, C. Canosa-Mas, G. Marston, R. P. Wayne

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The absorption cross-sections of Cl2O6 and Cl2O4 have been obtained using a fast flow reactor with a diode array spectrometer (DAS) detection system. The absorption cross-sections at the wavelengths of maximum absorption (λ max) determined in this study are those of Cl2O6: (1.47±0.15) ×10-17 cm2UP>, at λmax =276 nm and T=298 K; and Cl2O4: (9.0±2.0) ×10-19 cm2 molecule-1, at λmax=234 nm and T=298 K. Errors quoted are two standard deviations together with estimates of the systematic error. The shapes of the absorption spectra were obtained over the wavelength range 200-450 nm for Cl2O6 and 200-350 nm for Cl2O4, and were normalized to the absolute cross-sections obtained at λ max for each oxide, and are presented at 1 nm intervals. These data are discussed in relation to previous measurements.The reaction of O with OClO has been investigated with the objective of observing transient spectroscopic absorptions. A transient absorption was seen, and the possibility is explored of identifying the species with the elusive sym-ClO3 or ClO4, both of which have been characterized in matrices, but not in the gas-phase. The photolysis of OClO was also re-examined, with emphasis being placed on the products of reaction. UV absorptions attributable to one of the isomers of the ClO dimer, chloryl chloride (ClClO2) were observed; some Cl2O4 was also found at long photolysis times, when much of the ClClO2 had itself been photolysed. We suggest that reports of Cl2O6 formation in previous studies could be a consequence of a mistaken identification. At low temperatures, the photolysis of OClO leads to the formation of Cl2O3 as a result of the addition of the ClO primary product to OClO.ClClO2 also appears to be one product of the reaction between O3 and OClO, especially when the reaction occurs under explosive conditions. We studied the kinetics of the non-explosive process using a stopped-flow technique, and suggest a value for the room-temperature rate coefficient of (4.6±0.9)×10-19 cm3 molecule-1 s-1 (limit quoted is 2 σ random errors).The photochemical and thermal decomposition of Cl2O6 is described in this paper. For photolysis at λ=254 nm, the removal of Cl2O6 is not accompanied by the build up of any other strong absorber. The implications of the results are either that the photolysis of Cl2O6 produces Cl2 directly, or that the initial photofragments are converted rapidly to Cl2. In the thermal decomposition of Cl2O6, Cl2O4 was shown to be a product of reaction, although not necessarily the major one. The kinetics of decomposition were investigated using the stopped-flow technique. Atelatively high [OClO] present in the system, the decay kinetics obeyed a first-order law, with a limiting first-order rate coefficient of 0.002 s-1.

Original languageEnglish
Pages (from-to)353-370
Number of pages18
JournalJournal of Photochemistry and Photobiology A: Chemistry
Issue number2-3
Publication statusPublished - 15 Mar 2004


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