Chlorine dioxide photolysis

We conclude our description of experiments involving the photon excitation by mentioning a single contribution by British researchers who combined GED with flash-photolysis [109]. These authors conducted diffraction studies on the decomposition of chlorine dioxide and biacetyl using electron pulses. 

The spectrum of chlorine dioxide (0C10) is characterized by a well developed progression of bands covering parts of the ultraviolet and visible regions (280 to 480 nm). Cross sections were measured by Wahner et al.(1987) and by Hubinger and Nee (4994). The lifetime of OCIO against photolysis is only a few seconds. 

The Radicals FOO and GlOO. It has recently been proposed that a radical containing one chlorine atom, previously thought to be the monoxide, CIO (4), is more likely to be the peroxide, ClOO (38). The radical is readily formed on photolysis of chlorine dioxide in y-irradiated KC104 at room temperature and also from rigid solutions of chlorine dioxide in sulfuric acid at 77°K. However, photolysis of C102 in a chlorate lattice does not result in the formation of this species. If the radical is indeed ClOO, this can be understood since chlorate is an extremely efficient oxygen atom acceptor, which would favor formation of CIO rather than ClOO. Also, many details of the ESR spectra are readily accommodated if ClOO is the correct formulation but very hard to understand if the species is CIO. 

Flash photolysis of chlorine dioxide, OOO, is also a good source of CIO radicals, " and Edgecombe, Norrish, and Thrush additionally showed that these radicals can be formed by the flash photolysis of CI2O. Several full kinetic studies of OO radical reactions using these methods have berai reported, and these early lesulte have been reviewed in detail." It has been shown that... 

As a gas, chlorine dioxide decomposes thermally and in the presence of ultraviolet light to produce the short-lived chlorine oxide radical, CIO. This same radical (a species with an unpaired electron) is also produced by the photolysis of chlorofluorocarbons such as CFCI3 and CF2CI2 and has been implicated in reactions leading to depletion of ozone in the earth s upper atmosphere (Section IX.F). 

Photolytic. Photodegrades under simulated atmospheric conditions to phosgene and nitrosyl chloride. Photolysis of nitrosyl chloride yields chlorine and nitrous oxide (Moilanen et al., 1978 Woodrow et ah, 1983). When aqueous solution of chloropicrin (10 M) is exposed to artificial UV light (X <300 nm), protons, carbon dioxide, hydrochloric and nitric acids are formed (Castro and Belser, 1981). 

Irradiation of gaseous formaldehyde containing an excess of nitrogen dioxide over chlorine yielded ozone, carbon monoxide, nitrogen pentoxide, nitryl chloride, nitric and hydrochloric acids. Peroxynitric acid was the major photolysis product when chlorine concentration exceeded the nitrogen dioxide concentration (Hanst and Gay, 1977). Formaldehyde also reacts with NO3 in the atmosphere at a rate of 3.2 x 10 cmVmolecule-sec (Atkinson and Lloyd, 1984). 

In this experiment, no carbon tetrachloride was detected but, on the basis of the mechanism proposed, carbon tetrachloride would have been an important reaction product, arising by the combination of chlorine atoms and trichloj-omethyl radicals. Further, the production of octachloropropane by the secondary photolysis of octachlorobutanone would involve the formation of decachlorobutane. In the presence of chlorine, only carbon tetrachloride was formed, whereas by interaction of the postulated CCI3CO- radical with chlorine, substantial amounts of trichloroacetyl chloride should have been observed. In the presence of oxygen, only carbon dioxide and phosgene were found and the yield of the latter was far too small to account for the loss of the radical products. 

Nitrogen dioxide is about 20 to 50% of the total nitrogen oxides NO, (NO, NOz, HN03, N2Os), while CIO represents about 10 to 15% of the total chlorine species CIO, (Cl, CIO, HCI) at 25 to 30 km. Hence, the rate of ozone removal by CIO, is about equal to that by NO, if the amounts of NO, are equal to those of CIO,. According to a calculation by Turco and Whitten (981), the reduction of ozone in the stratosphere in the year 2022 with a continuous use of chlorofluoromethanes at present levels would be 7%. Rowland and Molina (843) conclude that the ozone depletion level at present is about 1%, but it would increase up to 15 to 20% ifthechlorofluoromethane injection were to continue indefinitely at the present rates. Even if release of chlorofluorocarbons were stopped after a large reduction of ozone were found, it would take 100 or more years for full recovery, since diffusion of chlorofluorocarbons to the stratosphere from the troposphere is a slow process. The only loss mechanism of chlorofluorocarbons is the photolysis in the stratosphere, production of HCI, diffusion back to the troposphere, and rainout. 

Only a small percentage of the chlorine released by photolysis of CFCs is present in the active forms as Cl or CIO, however. Most of it is bound up in reservoir compounds such as hydrogen chloride and chlorine nitrate, formed respectively by hydrogen abstraction (equation 10) from methane and addition (equation 11) to nitrogen dioxide. Slow transport of these reservoir species across the tropopause, followed by dissolution in tropospheric water and subsequent rain-out, provide sink processes for stratospheric chlorine. 

A primary step, in which ethoxycarbonyl radicals are produced via the rupture of carbon-chlorine bonds, has been proposed by Pac and Tsutsumi for the continuous photolysis of ethyl chloroformate with a high pressure mercury arc. The pure compound decomposes to give ethyl chloride with smaller amounts of acetaldehyde, ethane, ethylene, hydrogen chloride, carbon monoxide and carbon dioxide. 

The structure —CHC1—CH2—CO—CH2 — was found by Kwei [99] in polyvinylchloride after photo-oxidation. Such j3 chloroketones decompose by the Norrish type I mechanism without loss of chlorine atoms. Hydrogen chloride is obtained only when polyvinylchloride is photo-oxidized above 30°C [98]. It seems that zipper dehydrochlorination plays little role in the reaction occurring on exposure to ultraviolet light at temperatures below 150°C in the presence of air [97], and that hydrogen chloride is mainly a product of thermal decomposition rather than photolysis [98], The following mechanism can be proposed which takes into account the experimental results namely, that chain scission and crosslinking occur simultaneously on irradiation at 253.7 nm [100] and that carbon dioxide is evolved, while an absorption band at 1775 cm-1 (ascribed to peracids) is detected in the infrared spectrum [98]. 

---