Organic molecules, photooxidations

The 02 ion appears to play an important role in a number of photooxidation reactions (see Section VI,C) for example, the photo-oxidation of alkenes over TiOz. However, it seems likely that OJ is not, in many cases, active in the oxidation step but further conversion occurs to give a mononuclear species, not detected directly, which then oxidizes the adsorbed hydrocarbons. Photo-oxidation of lattice oxygen in the M=0 systems (e.g., V2Os supported on PVG) gives rise to an excited charge transfer state such as V4 + -0 . This excited state can react as O- either by addition to a reactant molecule or by an abstraction reaction (see Section V of Ref. /). In the presence of oxygen, 03 is formed which then reacts further with organic molecules. 

Organic Systems. The photooxidation and reduction reactions for most organic compounds require two electron processes and are generally irreversible. However, several phenothiazine dyes, such as Thionine and Methylene Blue, function as reversible two electron redox systems. The reversible photobleaching of chlorophyll may also involve a one or two electron process although the exact mechanism is still in doubt. One electron redox processes for organic molecules are possible... 

For Fe2+ in octahedral coordination, we do not expect photooxidation of organic molecules via 02"- Fe2+ charge transfer. However, the possibility of photoreduction of organics via transitions from the Fe(3d) orbitals to the Fe(4s) orbitals or Fe(4s) conduction band should be considered. The lowest energy Fe(3d)- Fe(4s) transition is calculated to be 58600 cm"1. Although... 

In the precambrian (or on present day Mars), the absence, of an ozone layer allowed solar UV radiation with energies as high as 40,000 cm"1 (0.25 microns). Photoreduction transitions via the Fe(3d) to Fe(4s) transition may have been very significant. The photochemical oxidation of Fe2+, and the precipitation of FeOOH, may be the origin of the extensive precambrian banded iron formations (38-40). Moreover, the photooxidation of Fe2 may have reduced C02 to organic molecules (37.41) ... 

Gerischer, H. Heller, A. The role of oxygen in photooxidation of organic molecules on semiconductor particles, J. Phys. Chem 1991, 95, 5261. 

Only a few recent studies have dealt with the use of photoactive membranes prepared by immobilization of titania particles in polymeric membranes or deposition of porous titania coatings. " They were concerned with photooxidation applications like antifouhng or elimination of small organic molecules that cannot be stopped by conventional membrane treatments, but which are very harmful to the environment, like VOCs. 

The mixture of various catalytic multimers, like thioesters and phosphorylated organic molecules, presents the background for a protometabolism in the thioester world theory. Protometabolitic reaction pathways rapidly form networks. The latter would have included cyclic mass flows that are stabilized by interactions among metabolites. These pathways could have occurred in associations with pyrite, clays or iron dioxide flocks produced by UV photooxidation. 

Hanst, P. L., J. W. Spence, and O. Edney (1980). Carbon monoxide production in photooxidation of organic molecules in the air. Atmos. Environm. 14, 1077-1088. 

Triplet diradicals react with organic molecules in a very similar manner to RO radicals (Sect. 4.3.4) and are useful for photoinitiating free radical oxidations these photooxidations therefore closely resemble oxidations by other oxy radicals and will not be considered further. 

FIGURE 5.13 Organic radicals and products in the photooxidation of an organic molecule. 

Air atmospheric t,/2 2.4-24 h for C4H10 and higher paraffins for the reaction with hydroxyl radical, based on the EPA Reactivity Classification of Organics (Darnall et al. 1976) photooxidation reaction rate constant of 1.02 x 10-11 cm3 molecule-1 s-1 with OH radical with an estimated lifetime x = 14 h during summer daylight (Altshuller 1991). 

As a result of the above constraints, VOCs that for all practical purposes do not produce organic aerosol in the atmosphere include all alkanes with up to six carbon atoms (from methane to hexane isomers), all alkenes with up to six carbon atoms (from ethene to hexene isomers), and most other low-molecular-weight compounds. An important exception is isoprene, a five-carbon atom molecule with two double bonds (see Section 6.11) isoprene photooxidation produces SOA in laboratory chambers (Kroll et al. 2005, 2006) and its oxidation products have been detected in ambient aerosols (Claeys et al., 2004a Edney et al. 2005). In general, large VOCs containing one or more double bonds are expected to be good SOA precursors. A set of structure-SOA formation potential relationships have been proposed by Keywood et al. (2004) for cycloalkenes ... 

Some fraction of the RO. radical pool, usually 10% or less, produce alkyl nitrate, RONO., upon reaction with NO by 5.59b. This termination step removes both a radical and a molecule of NO,. About 8% of the RO. radicals formed from the OH-/i-butane reaction will lead to nitrate when reacting with NO. As noted in Section 5.8.1, this percentage increases with the size of the hydrocarbon molecule, reaching about 33% for n-octane. The organic components of RONO. can react with OH to continue the VOC photooxidation cycle, although the possibility of re-releasing NO, as a result of these reactions is uncertain at present. 

The phenanthrene acts as a sensitizer generating the singlet oxygen that in turn reacts with the ground-state molecule resulting in the oxidative transformations. This type of reaction has also been demonstrated in the photooxidation of anthracene associated with particulate organic matter. At this point, however, it is not clear why the reaction rates vary between sUica gel and alumina. 

---