Gas-phase photochemical reactions

Organic aerosols formed by gas-phase photochemical reactions of hydrocarbons, ozone, and nitrogen oxides have been identified recently in both urban and rural atmospheres. Aliphatic organic nitrates, such dicarboxylic acids as adipic and glutaric acids, carboxylic acids derived from aromatic hydrocarbons (benzoic and phenylacetic acids) and from terpenes emitted by vegetation, such as pinonic acid from a pinene, have been identified. The most important contribution in this held has been that of Schuetzle et al., who used computer-controlled... 

Gaseous hydrogen peroxide is a key component and product of the earth s lower atmospheric photochemical reactions, in both clean and polluted atmospheres. Atmospheric hydrogen peroxide is believed to be generated exclusively by gas-phase photochemical reactions (lARC, 1985). Low concentrations of hydrogen peroxide have been measured in the gas-phase and in cloud water in the United States (United States National Library of Medicine, 1998). It has been found in rain and surface water, in human and plant tissues, in foods and beverages and in bacteria (lARC, 1985). 

Chlorination of Alkanes. Free-radical chlorination is the most commonly used method for the chlorination of a saturated hydrocarbon.31 106-108 111 112 Both thermal and photochemical processes may be carried out in the liquid or vapor phase. The liquid-phase photochemical procedure is preferred for polychlorination gas-phase photochemical reactions can yield either mono- or polychlorinated product. 

Generally speaking, atoms in doublet and triplet states are reactive and tend to form chemical bonds either with themselves or with other atoms or molecules. Hence the photochemist is concerned in practice with gas phase photochemical reactions of mercury, cadmium, zinc, and the noble gases whose atoms exist normally in singlet states. 

Kasting (2001) argues in support of the view of Farquhar et al (2000) (but see also Ohmoto et al, 2001) that sulfur isotope fractionation changed around 2.3 Ga. This opinion is based on the claim, from comparison of sulfur isotopes, that so-called mass independent fractionation occurred as a result of gas-phase photochemical reactions, particularly photolysis of SO2. Such fractionation would be much more likely to occur in a I0W-O2 atmosphere in which sulfur was present in a variety of oxidation states. Thus, the claim that fractionation changed around 2.3 Ga ago can be seen as supporting the notion that there was a substantial rise in O2 around this time. This, however, raises the question if cyanobacterial oxygen production had been sufficient to create the mbisco fingerprint in carbonates as early as 2.7-3.0Ga ago, why did the rise of free O2 only occur 400-700 Myr later ... 

SOA compounds formed by gas-phase photochemical reactions of hydrocarbons, ozone, and nitrogen oxides have been identified in both urban and rural atmospheres. Most of these species are di- or polyfunctionally substituted compounds. These compounds... 

Photolyses of several prototypical sulfoxides and related compounds have been carried out in recent years. Generally, 193 or 248 nm excitation has been used. Because of this, the lack of a solvent bath to absorb excess vibrational energy efficiently, and the generally unimolecular nature of gas phase photochemical reactions, the investigations have centered on the types of photodissociations that are observed and whether one or two carbon-sulfur cleavages occur in the primary photochemical event. 

Most gas-phase photochemical reactions occur in the stratosphere, and nonphotochemical reactions generally occur in the troposphere. 

Approximately 100,000 tons of benzyl chloride is produced annually using this gas-phase photochemical reaction. 

Research Opportunities. The presence of a long-lived fluorescing state following either 532 nm or 1064 nm excitation of PuF6(g) provides a valuable opportunity to study the extent to which electronic energy in a 5f electron state is available in photochemical and energy transfer reactions. Such gas phase bimolecular reactions would occur in a weak interaction limit governed by van der Waals forces. Seen from the perspective of potential photochemical separations in fluoride volatility... 

Air t1/2 = 6 h with a steady-state concn of tropospheric ozone of 2 x 10-9 M in clean air (Butkovic et al. 1983) t/2 = 2.01-20.1 h, based on photooxidation half-life in air (Howard et al. 1991) calculated atmospheric lifetime of 11 h based on gas-phase OH reactions (Brubaker Hites 1998). Surface water computed near-surface of a water body, tl/2 = 8.4 h for direct photochemical transformation at latitude 40°N, midday, midsummer with tl/2 = 59 d (no sediment-water partitioning), t,/2 = 69 d (with sediment-water partitioning) on direct photolysis in a 5-m deep inland water body (Zepp Schlotzhauer 1979) t,/2 = 0.44 s in presence of 10 M ozone at pH 7 (Butkovic et al. 1983) calculated t,/2 = 59 d under sunlight for summer at 40°N latitude (Mill Mabey 1985) t,/2 = 3-25 h, based on aqueous photolysis half-life (Howard et al. 1991) ... 

If released to the atmosphere, o-limonene is expected to rapidly undergo gas-phase oxidation reactions with photochemically produced hydroxyl radicals, ozone and, at night, with nitrate radicals. Limonene can react with ozone, forming submicron particulates that could impact asthmatics and those with other respiratory ailments. 

Other gas-phase photochemical production methods which have been used for the synthesis of PAN follow a general reaction scheme in which halogens remove the aldehydic proton from the corresponding aldehyde (X = C1, Br). Reaction 19.12 and Reaction 19.13 are the key processes leading to the formation of PAN in the troposphere when X is replaced by hydroxyl radical. 

The global phenomenon is extremely difficult to model, as it involves both photochemical reactions, gas-phase thermal reactions, heterogeneous reactions (especially with ice particles), phenomena of convective and diffusive transport inside... 

Eyring s TST has provided the basic conceptual framework for the interpretation of the rates of nearly all chemical reactions on a bulk scale. He quickly applied his new theory to homogeneous gas-phase thermochemical reactions, photochemical reactions, heterogeneous catalysis, and reactions in solution [19]. He even considered such topics as viscosity and diffusion [19]. 

Multi-phase photochemical reactions take place across phase boundaries. In most cases, substrate molecules are either dissolved in different liquid phases or one reactant is in the gas phase and interacts with the liquid phase. For this reaction category, phase transfer kinetics, in addition to the actual photochemical reaction, have to be taken into account, assuming microreactors to be ideal reaction engineering tools due to their low mass transfer limitations. 

The first is a pyrolytic approach in which the heat dehvered by the laser breaks chemical bonds in vapor-phase reactants above the surface, allowing deposition of the reaction products only in the small heated area. The second is a direct photolytic breakup of a vapor-phase reactant. This approach requires a laser with proper wavelength to initiate the photochemical reaction. Often ultraviolet excimer lasers have been used. One example is the breakup of trimethyl aluminum [75-24-1] gas using an ultraviolet laser to produce free aluminum [7429-90-5], which deposits on the surface. Again, the deposition is only on the localized area which the beam strikes. 

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

Photochemical Reactions. The photochemistry of chlorine dioxide is complex and has been extensively studied (29—32). In the gas phase, the primary photochemical reaction is the homolytic fission of the chlorine—oxygen bond to form CIO and O. These products then generate secondary products such as chlorine peroxide, ClOO, chlorine, CI2, oxygen, O2, chlorine trioxide [17496-59-2] CI2O2, chlorine hexoxide [12442-63-6] and... 

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