CIO radical

The CIO radical in particular is implicated in environmentally sensitive reactions which lead to depletion of ozone and oxygen atoms in the... 

The great importance of the short-lived CIO radical has stimulated numerous investigations of its synthesis and molecular properties. Several routes are now available to this species (some of which have already been indicated above) ... 

The fundamental aspects of the problem are well established the measured concentrations of the CFCs indicate that they accumulate in the lower atmosphere and that they reach the stratosphere. As expected, chlorine atoms and CIO radicals are found in the stratosphere together with other species such as O, OH, HO2, NO, NO2, HCl, CIONO2, HOCl, etc. The observed concentrations are in reasonable agreement with the model predictions if the limitations of the models, as well as atmospheric variability, are taken into account. 

At more positive potentials, processes occur that depend on the composition of the electrolyte, such as the formation of H2S2Og and HS05 in sulphuric acid solutions, while the CIO radical is formed in perchloric acid solutions, decomposing to form C102 and 02. The formation of ozone has been observed at high current densities in solutions of rather concentrated acids. 

The CIO radical product of Equation (8.14) then reacts further, yielding the overall reaction ... 

CIO radicals during the first few milliseconds after the photoflash. A delay in the appearance of C102 is perhaps associated with a shift to the left in the equilibrium... 

Clearly, this discovery has an important bearing on the kinetics of the decompositions of chlorine oxides and of the halogen-sensitized decompositions of oxides, e.g. 03 and N20. Johnston et alf21 have recently shown that an additional, pressure-dependent reaction occurs between two CIO radicals, viz... 

Fe(NC>3)3 has the same reactivity as CAN, a one-electron oxidizing agent. The Fe(NC>3)3 will remove the electron highest in energy from the substrate. Such an electron would have to be one of the unshared electrons of the O atoms. After removal of an electron from 02, the C3-C5 bond can fragment to give a C5 radical, which can add to C9 and generate a new radical at CIO. The CIO radical then abstracts H from 1,4-cyclohexadiene. Sil is lost from 02 upon aqueous workup. 

Burkholder JB, Hammer PD, Howard CJ, Maki AG, Thompson G, Chackerian CJ (1987) Infrared measurements of the CIO radical. J Mol Spectrosc 124 139-161 Catlow CRA, Freeman CM, Islam MS, Jackson RA, Leslie M, Tomlinson SM (1988) Interatomic potentials for oxides. Phil Mag A 58 123-141... 

O( D) + Chlorojluoromethones. The reactions of O( D) with chloro-lluoromethanes have been studied by Donovan et al. (316), Gillespie and Donovan (396), and Fletcher and Husain (362). The reactions yield predominantly CIO radicals... 

The CIO radicals produced also react with NO to form N02, which eventually regenerates ozone by the sequence discussed before [(VII1-47) and (VI11-42)]... 

Molina, L.T., and M.J. Molina (1987) Production of CljOa from the self-reaction of the CIO radical, J. Phys. Chem. 91, 433-436... 

As discussed in the previous sections reaction of NO3 radicals with organic compounds results in the formation of HNO3 and organic nitrates. In the stratosphere CIO radicals react with N02 to give C10N02 which undergoes hydrolysis. 

G. Porter and F. J. Wright, Discussions Faraday Soc.y 14,23 (1953). The uncertainty in the frequency factor is due to the uncertainty of a factor of 3 in the absolute absorption coefficient of CIO radicals. Such a reaction being four-centered is unusual in having no activation energy, and the authors propose a precursor species, the dimer (C10)2. 

The resulting tertiary radical undergoes /3-splitting to form unsaturated Cio radicals that by hydrogen abstraction yield Ci0 cyclohexenes, in particular 1-butylcyclohexene and l-methyl-3-propylcyclohexene (and double bond isomers). 

Photolytic decomposition of the fluorocarbon molecule augments the natural concentration of the Cl and CIO radicals, thereby accelerating the odd oxygen (ozone plus atomic oxygen) conversion to O2. 

Atomic chlorine by reaction with hydrogen containing gases reforms HCl again whereas by reacting with oxone yields CIO radical which is further converted to chlorine nitrate, CIONO2, which is a stable edes. This compound can photodissociate to release chlorine atoms and can also react with water and hydroli2e... 

Three important recent results by Clyne and Coxon , who used a flow system and followed CIO spectroscopically, help to resolve the mechanistic difficulties in the CI2-O3 system. These results are (/) the reaction of Cl with O3 produces CIO in the ground vibrational level in amounts comparable to the initial Cl concentration, the rate coefficient being in excess of 4x 10 l.mole . sec (it) the reaction of CIO with O3 is immeasurably slow and (Hi) the products of the reaction of two CIO radicals are predominantly Cl and ClOO, at least at low pressures (a further discussion of this reaction is found in Section 2.2.4). 

The flash photolysis of CI2 in the presence of O2 was studied by Porter and Wright . Using the method of kinetic spectroscopy, they observed CIO radicals. Their observations were consistent with the mechanism, which was later corroborated by Burns and Norrish , viz. 

More recently, Clyne and Coxon have prepared CIO radicals from three different reactions in a flow system viz. 

A preliminary report and photograph of the spectrum attributed to this radical have been given elsewhere. It appears whenever chlorine is photolyzed in the presence of oxygen and has a half-life of a few milliseconds. The simple vibrational structure strongly suggests a diatomic molecule and, under the circumstances, the only possibility is the CIO radical whose occurrence in chemical reactions has frequently been postulated. Very little is known about the diatomic compounds of Group 6 of the periodic table with Group 7 and the interpretation of this spectrum is therefore of some interest. 

Furthermore, acetylene concentrations decayed faster than expected from removal by OH and Cl, indicating the presence of bromine atoms with mixing ratios of l-2pmolmol . The corresponding BrO mixing ratios in the range of several tens of pmol moP should cause loss of ozone by —90% per day by the BrO-BrO reaction cycle. Estimated concentrations of chlorine atoms were —1,000 times smaller than those of bromine (Jobson et al., 1994). There are some tentative indications of substantial concentrations of CIO radicals, similar to those of BrO, 21 12 pmol mol , at a detection limit of 20pmolmol per individual measurement. 

The CIO radical is the immediate culprit in the destruction of stratospheric ozone. Local increases in CIO concentration are directly correlated with decreases in O3 concentration. CIO catalyzes the destruction of ozone, probably by the mechanism... 

The concentration of Cl atoms is very small, so at any given time, the concentration of CIO will be very small. The probability of two CIO radicals finding each other to form ClOOCl is virtually zero. Even though this mechanism shows a catalytic cycle with Cl (starting the mechanism and being regenerated at the end), the middle step makes it highly unlikely. 

The presence of this hyperdense H-bond network also gives ice a particular chemical reactivity on its surface, which is characterized by the presence of numerous potential H-bond acceptors or donors that do not establish H-bonds. It thus has a central role in the seasonal ozone depletion that occurs mainly over the Antarctic, but also over the Arctic, where this type of PSCs that are made of ice Ih, called II PSC, play a fundamental catalytic role to transform inert chlorinated gas molecules into Cl or CIO radicals that attack and destroy O3. Much work still remains to be done to fully understand this catalytic process. Singular properties of ice are thus a direct consequence of the presence of such a dense H-bond network. This network is rigid. Its transformation in liquid water into an as dense but flexible H-bond network has dramatic consequences, giving liquid water numerous exceptional properties. This is the object of Ch. 9. 

---