Odd oxygen species

The ratio is clearly pressure dependent in the lower stratosphere [02] and [M] are fairly large and /3 is small (due to absorption above the required wavelengths), so the dominant odd-oxygen species is ozone. At higher altitudes both [02] and [M] fall and the photolysis rate increases so that O is the dominant species in the atmosphere. The net flux of radiation in the band 240-290 nm is nearly zero at the surface of the Earth, which is then shielded from this radiation. 

The formation of odd oxygen species under pressurized conditions is possible even in the wavelength region that is somewhat longer than the dissociation threshold at 242.4 nm this is important as the initial reaction in the laser-induced oxidation of hydrocarbons in the hydrocarbon/02/supercritical C02 mixtures at 248 nm [5],... 

Figure 5.2. Stratospheric chemical cycle affecting odd oxygen species in the stratosphere. The numbers in boxes represent concentrations (cm-3) calculated at 25 km altitude while the numbers associated with the arrows account for the reaction fluxes (cm-3s-1) between different compounds (24 hour global average conditions). Note that the figure extends beyond the simple pure oxygen chemistry case and that NO2 is identified as an odd-oxygen reservoir. (See following sections for more details from Zellner, 1999).

This process dominates the odd oxygen destruction near the tropopause because it is the most effective catalytic cycle involving only ozone as the reactive odd oxygen species. Most of the other HOx, NOx, and C10x cycles (see below) also require reaction with atomic oxygen, which is present only in very small amounts at low altitudes. For example, the following cycles are important in the middle and upper stratosphere, and in the mesosphere... 

Here the processes (2) and (3) rapidly interconvert the odd-oxygen species 0 and O3 which are formed by process (1) and destroyed by reaction (4). This latter process only removes about one-quarter of the stratospheric ozone (odd oxygen) and the remainder is largely accounted for by catalytic cycles of the type... 

In order to calculate the steady-state concentration of ozone in the stratosphere, we need to balance the rate of production of odd oxygen with its rate of destruction. Chapman originally thought that the destruction was due to the reaction O + 03 —> 2O2, but we now know that this pathway is a minor sink compared to the catalytic destruction of 03 by the trace species OH, NO, and Cl. The former two of these are natural constituents of the atmosphere, formed primarily in the photodissociation of water or nitric oxide, respectively. The Cl atoms are produced as the result of manmade chlorofluorocarbons, which are photodissociated by sunlight in the stratosphere to produce free chlorine atoms. It was Rowland and Molina who proposed in 1974 that the reactions Cl + 03 —> CIO + O2 followed by CIO + O —> Cl + O2 could act to reduce the concentration of stratospheric ozone.10 The net result of ah of these catalytic reactions is 2O3 — 3O2. 

The authors, on the basis of their experimental results, pointed out that olefin radical cations-allylically methylated and/or stabilized by heteroatoms or n conjugation [136] do not react with an active oxygen species, suggesting, in spite of their low oxidation potentials, that several factors may contribute to determine this behavior. Thus, one of the important factors is the presence of an independent 7t-system in which odd-electron density is not delocalized, whereas, a second and a third factor can be the insufficient steric hindrance to block the attack of oxygen... 

This channel reduces significantly the calculated ClO/HCl ratio in the upper stratosphere and, hence, the sensitivity of ozone to chlorine. There are also other slow processes that yield hydrogen chloride. Like HNO3, HC1 provides a relatively inert reservoir which sequesters a photochemically active species and hence reduces the rate of its catalytic reaction with odd oxygen. 

The catalytic destruction of odd oxygen by chlorine species must also be considered, particularly in the upper stratosphere where this cycle is quite effective (Stolarski and Cicerone, 1974 Molina and Rowland, 1974a) ... 

In most regions of the atmosphere, the interconversion between O and O3 is so rapid that the two species are often considered together as the chemical family, Ox = O + O3, which is denoted odd oxygen. ... 

NO is one of the simplest odd-electron species in which the presence of an unpaired electron reduces the bond order to 2.5 (whereas it is around 3 in NO ). Due to these chemical properties, NO is reluctant to dimerize, has paramagnetic properties, and reacts with atoms and free radicals. In the gaseous phase NO reacts with oxygen to form nitrogen dioxide (NO2), while in the aqueous phase NO reacts with oxygen to form nitrite. In vivo, nitrite reacts with oxyhemoglobin, resulting in the formation of its stable metabo-... 

FIGURE 4.10 Simplified schematic of the CIO, ozone depletion cycle. Cycling between Cl and CIO continually converts odd oxygen to even oxygen. The reservoir species CIONO., HCl. and HOCI sequester activ e chlorine and diminish the effectiveness of the CIO, cycle. 

The lifetime of ozone in aqueous systems depends principally on the pH. Ozone decomposition is catalyzed by HO" in a bimolecular process whose rate is dependent on both [O3] and [HO"]. However, the kinetics are complicated by the existence of a radical chain process involving HO and other intermediate odd-electron species. Forni et al. (1982) indicated that the initiation step for the decomposition reaction appeared to be an oxygen atom transfer to form molecular oxygen and the anion of H2O2 ... 

Correspondingly, quenchers of benzylic radical also suppress the alkylation. However, the situation is more complicated here. Thus, while benzylic radicals are efficiently trapped by oxygen (see below), their reaction with other known radical traps (e.g. electron-poor alkenes) does not succeed either with benzyl or with allyl radicals, while this is perfectly feasible with simple alkyl radicals. Thus, even if delocalized radicals would be expected to diffuse out of cage more efficiently, and thus become more available for trapping by species different from the acceptor anion radical, this factor is more than overcome by adverse factors such as the lower intrinsic rate of addition to alkenes by these stabilized species and the stabilization offered by the interaction between the two ic-delocalized odd electron species, the radical and the anion radical, which are initially formed face-to-face and tend to react that way. 

The symmetric form of chlorine dioxide (OCIO) is different from the unstable species ClOO in Reaction (7.42b). However, the presence of OCIO provides an important indication of the amount of CIO and therefore the destruction of odd oxygen in the ozone hole. 

That is, the two diatomic molecules collide in space and rearrange to form two new molecules, OH. Notice that this is not the hydroxide ion It is a combination of one oxygen atom and one hydrogen atom, and as an uncharged diatomic molecule it has an odd number of electrons. Such odd-electron species are rare in main-group compounds. Typically, odd-electron molecules are reactive and short-lived they are called free radicals, or more simply, radicals. 

These experiments having eliminated participation of both the biradical and the hypothetical peracid analog there remains a large area where there are persistent indications of involvement of odd-electron species in oxidation processes. Single electron transfers occur in many of the same situations in which free radical initiation and photosensitization occur. There have been cases of this kind where superoxide radical ion, O2-, has been observed, and some of its interactions with singlet oxygen have been studied. 

Free radicals are species that contain unpaired electrons The octet rule notwithstand mg not all compounds have all of their electrons paired Oxygen (O2) is the most famil lar example of a compound with unpaired electrons it has two of them Compounds that have an odd number of electrons such as nitrogen dioxide (NO2) must have at least one unpaired electron... 

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