Rapid photolysis reactions

As noted above, there have been many reports of rapid time-resolved spectroscopy achieved using a step-scan interferometer, most of which involve the investigation of rapid photolysis reactions. We will give two examples of this type of measurement, one of a relatively ligand-exchange reaction and the second of a more complex biochemical system. 

Experimentally, while the determination of absorption cross sections is fairly straightforward, measuring primary quantum yields is not, due to interference from rapid secondary reactions. As a result, in cases where quantum yield data are not available, calculations of maximum rates of photolysis are often carried out in which it is assumed that (A) = 1.0. It should be emphasized in such cases that this represents only a maximum rate constant for photolysis the true rate constant may be much smaller, even zero, if photophysical fates of the excited molecule such as fluorescence or quenching predominate. 

Alkyl nitrites (RONO) absorb light strongly in the actinic region, dissociating to form RO + NO. Because of this rapid photolysis, other reactions such as that with OH cannot compete, and alkyl nitrites have not been generally observed in the troposphere at significant concentrations. 

Because of the rapid photolysis of NO, during the day (see Chapter 4.G), competing reactions of NO, are important primarily at night. Some of the most important dark reactions are discussed in the following sections. For detailed treatments of nitrate radical chemistry, the reader should consult the extensive reviews by Wayne et al. (1991) and Atkinson (1991). 

One of the most important reactions of NO, is that with N02 to form N205, discussed earlier. This is the only known source of N205 in the atmosphere, and because NO, is only present at significant concentrations at night due to its rapid photolysis, the formation of N2Os is restricted to the dark. 

As discussed in Section D, BrONOz undergoes even more rapid heterogeneous reactions than C10N02, forming HOBr, and the photolysis of both BrONOz and HOBr is relatively fast. Thus bromine spends more time in its catalytically active form, making it very effective in ozone destruction. 

The hydrolysis reaction (46) of N2Os under many conditions in the atmosphere becomes limited by the rate of N2Os formation, which only occurs at a significant rate at night (because of the rapid photolysis of the N03 precursor during the day). Hence under these conditions, reactions (39), (41), and (42) followed by photolysis of the chlorine-containing products become primarily responsible for the removal of gas-phase NOy and increase in CIO (Keim et al., 1996). 

The radical reactivity of Craq002+ has been especially well documented. This complex is ideal for mechanistic studies, not only because of the convenient combination of lifetime and reactivity, but also because it is nearly inert to visible-light photolysis. Photochemical generation of reactive partners in the presence of Craq002+ is thus possible, and rapid radical reactions can be observed and studied directly, as shown on the examples of acylperoxyl radicals, NO, and NO2. 

In the photolysis of ozone, only emission from 02(1A9) can be detected,70 and the absence of 02(1S,+) is now understood in terms of the considerable reactivity of 02(123+) with ozone61 (see Sect. V-B-l). has been detected in the vacuum UV photolysis products of 0277 although absolute measurements of the excitation efficiency have not, so far, proved practicable. The limits of sensitivity of the photomultiplier detector to 1.27 [x emission suggest that reaction (19) could proceed perhaps 20 times more rapidly than reaction (20) work is at present in progress in an attempt to measure k12 directly. 

Lipscomb, Norrish, and Thrush [195] also observed Oj when C102 was flash photolyzed. CIO., possesses a number of strong absorption bands between 300 and 400 nm, which can yield only 0(3P) atoms, and, on flashing, was completely destroyed within a few fistc by photolysis and the rapid [209] reaction... 

However, since the photolysis reaction is performed with the compound dissolved in C6F6, it has proved impossible to determine whether this is an inter- or intra-molecular reaction. Exchange of the fluoride anion for chloride occurs rapidly in chloroform in the presence of ArCl [77,78]. These reactions are summarised in Eqn. (14). 

Amorphous Ti02/MV systems behave quite differently upon photo-excitation. MV2+ is not adsorbed on amorphous Ti02 and so instantaneous formation of MV+ is not observed. However, in the presence of electron donors such as polyvinyl alcohol (PVA), or halide ions, a slow growth of MV+ formation is observed over a period of several us (Fig. 10). In the case of PVA, a permanent reduction of MV2+ is observed as reported by GrStzel et al. (16), but in the case of Cl no permanent reduction is observed. Pulse photolysis studies show large yields of initially formed MV+, but rapid back reaction of MV+ and Cl2 yield the starting materials. In 0.1 M HClO the reduction of MV2+ is not efficient, as electron transfer from ClO - to the Ti02 hole is not efficient. 

A few years later the author noticed a report by Kraeutler that stated that methylcobalamine is stable under photolysis in aqueous solution in the absence of radical scavengers, but that the presence of CO in moderate concentration (0.03 M) leads to acetylcobalamine in high yield.11 The explanation (Scheme 7) was again unusual in view of the known propensity of methyl and acetyl radicals for rapid coupling reactions. 

The use of short light pulses to study rapid chemical reactions in solution dates from the work of Norrish and Porter [15]. In a flash photolysis experiment a light... 

When released to the atmosphere, RDX is degraded by reacting with photochemically generated hydroxyl radicals (Atkinson 1987 HSDB 1994). The half-life for this reaction in the vapor phase was estimated to be 1.5 hours (Atkinson 1987 HSDB 1994). No data were located on photolysis of RDX in the atmosphere. However, it is expected that photolysis of RDX is an important fate process in the atmosphere since RDX absorbs ultraviolet wavelengths between 240 and 350 nm (Army 1986e) and it undergoes rapid photolysis in water (Army 1980a). 

Of the various troposphereic intermediates we have discussed, NO3 isuniqueinthatits reactions are more important at night than during daytime. Its daytime concentration is extremely low due to rapid photolysis by visible light... 

PROBABLE FATE photolysis dissolved portion should undergo rapid photolysis to quinones, when released to air, may undergo direct photolysis, although adsorption can slow this process, direct photolysis is important near surface of waters half-life for reaction with photo-chemically produced hydroxyl radicals 21.49 hr oxidation oxidation by chlorine and/or ozone could account for a small portion of the dissolved compound hydrolysis not an important process volatilization probably too slow to compete with adsorption as a transport process, evaporation may be important, but limited by adsorption, half-life 43 days sorption very strong adsorption onto suspended solids is the dominant transport process, adsorption in estuarine water 3 pg/L, 71% adsorbed on particles after 3 hr, after 3hr incubation in natural seawater, 75% of 2 pg/L adsorbed to suspended aggregates of dead photoplankton cells and bacteria biological processes bioaccumulation is short-term metabolization and microbial degradation are principal fates... 

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