Aqueous phase reactions photochemistry

It is possible that colloidal photochemistry will provide a new approach to prebiotic syntheses. The work described previously on redox reactions at colloidal ZnS semiconductor particles has been carried on successfully by S. T. Martin and co-workers, who studied reduction of CO2 to formate under UV irradiation in the aqueous phase. ZnS acts as a photocatalyst in the presence of a sulphur hole scavenger oxidation of formate to CO2 occurs in the absence of a hole scavenger. The quantum efficiency for the formate synthesis is 10% at pH 6.3 acetate and propionate were also formed. The authors assume that the primeval ocean contained semiconducting particles, at the surface of which photochemical syntheses could take place (Zhang et al 2007). 

Nitrate and nitrite photochemistry might also play a role in atmospheric hydrometeors. Nitrite photolysis has been shown to account for the majority of hydroxyl photoformation in irradiated fog water from a polluted site [ 14]. In addition, the generation of mutagenic and carcinogenic compounds from amino acids and amines dissolved in fog water [147] is a process that can be linked with nitrite photochemistry [20,141]. Furthermore, the formation of atmospheric nitrophenols partially takes place in aqueous solution. Reactions in the aqueous phase can account for about 30% of the atmospheric sources of mononitrophenols and for the vast majority of the dinitrophenol ones [ 148], and irradiation of nitrate and nitrite can possibly play a role in the process (see Sect. 3.2). Mono- and dinitrophenols are toxic compounds, and their occurrence in rainwater is thought to be a contributory factor in forest decline [149-151]. 

Allmand and Franklin studied the photochemistry of HCI-O2 mixtures, and their results corroborated those of earlier workers, particularly those of Richardson . For concentrated aqueous solutions, reaction occurred with radiation between 2540 and 3650 A, the products being CI2 and presumably hypo-chlorous acid. At 2600 A the quantum efficiency was about 0.2, but it fell off at longer wavelengths. For gaseous mixtures no reaction took place unless the mixtures were saturated with water vapor presumably the reaction occurred in a water film and not in the gas phase. 

Clouds are multiphase chemical systems coupling the chemistry in the gas phase to that in the aqueous phase. Small clouds do not greatly ahenuate the intensity of solar radiation and the gas-phase photochemistry continues, but reaction rates change because of the redistribution of gases in the presence of liquid water. 

Iron complex photolysis is mie of the processes that produce reduced iron (Fe(n)) in a highly oxidizing enviromnent like the atmospheric aqueous phase. There are numerous other processes such as reactions with HO species or Cu(I)/ Cu(n) which can reduce or oxidize iron in the troposphere. These reactions can take place simultaneously and cause iron to undergo a so-caUed redox-cycling [167]. Because of the large number of complex interactions in the atmospheric chemistry of the transition metal iron, it is useful to utilize models to assess the impact of the complex iron photochemistry. 

Figure 14 shows simulated concentration time profiles for the Fe(III) ligands pyruvate and oxalate, which have mostly lower concentrations during the daytime, when the photochemistry as described here is active. Thus, it has to be emphasized that Fe(III) complex photolysis reactions can be a major sink for the carboxylate species besides radical reactions, and it is crucial not to neglect these reactions when the fate of carboxylic acids in the atmospheric aqueous phase is considered. 

In the context of this work we found it interesting to make the transition from ion spectroscopy in the gas phase to ion photochemistry in liquid solution. While the basic processes are still the same the different environment drastically modifies the results known from gas phase studies. The mobility of ions and electrons in solution is greatly different, the energy of the spectroscopie states is changed, and reactive interaction with the solvent or other added species becomes possible. A very first step in this direction of preparative ionic photochemistry was reported before from our laboratory treating the benzene molecule as an example. More results have now been obtained relating to the mechanism of ion fonnation and its yield, the nature of the intermediates, and the conditions favouring product yield. As a first case the conversion of benzene to phenol and biphenyl in aqueous solution was studied followed by the study of reactions of benzene derivatives in water and other solvents /3/. 

---