One Scavengeable Species

The simplest reaction scheme used to test Smoluchowski s time dependent rate constant takes the form 

Agarwal, Simulation Studies of Recombination Kinetics and Spin Dynamics in Radiation Chemistty, Springer Theses, DOI 10.1007/978-3-319-06272-3 7, Springer International Publishing Switzerland 2014 

of realisation (rate constant) 1X10 (random flights and IRT) 

In the random flights simulation, scavengers were treated explicitly, such that the e has to diffuse towards a stationary S to react. The IRT algorithm made use of Smoluchowski s time dependent rate constant, in which reaction times were generated from the probability distribution 

It was hypothesised that the extra scavenging effect should diminish with increasing separation of the radical ion-pair. This is because as the ion-pair separation increases the initial drift velocity decreases (as shown in Eq. 7.10). If the drift velocity remains small whilst the e is scavenged then the kinetics should be accurately described using Smoluchowski s time dependent rate constant. As seen from the results presented in Fig. 7.2, this hypothesis is indeed found to be true. 

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Other means are available to study highly reactive intermediates that may be present at trace and ultra-trace levels. One may attempt to divert the intermediate from its normal course. We call this a trapping or scavenging experiment. If T symbolizes a species that rapidly reacts with intermediate I, one possible scheme is then... 

Irradiation of dilute aqueous solutions results in the interaction ofthe ionizing radiation with water molecules. The radiolysis of water produces hydrated electrons (eaq ", G = 2.8), hydrogen atoms (G = 0.6) and hydroxyl radicals (G = 2.8) which react with the molecules of the solutes. The use of special scavengers can convert one species to another, e.g. 

HO-oxidation of an individual NMHCj produces H02 radicals with a yield aj, and oxidation of the NMHC oxidation product produces H02 in stoichiometric amount The lumped coefficients or yields a and p need not be integers, and represent the effectiveness of a particular NMHCj in producing RO2. and H02 radicals (lumped together as HO2) that will then oxidize NO. to N02 in processes such as R6 and R13, producing one net ozone molecule each. Alternatively, when the NO. concentration is low, peroxyl radicals may form PAN (as in R22) or hydrogen peroxide (as in R33) which are other oxidant species. In this formulation, transport is expressed by an overall dilution rate of the polluted air mass into unpolluted air with a rate constant (units = reciprocal time dilution lifetime=1// ). This rate constant includes scavenging processes such as precipitation removal as well as mixing with clean air. 

If an aromatic compound reacts with an OH radical to form a specific set of hydroxylated products that can be accurately identified and quantified in biological samples, and one or more of these products are not identical to naturally occurring hydroxylated species, i.e. not produced by normal metabolic processes, then the identification of these unnatural products can be used to monitor OH radical activity therein. This is likely to be the case if the aromatic detector molecule is present at the sites of OH radical generation at concentrations sufficient to compete with any other molecules that might scavenge OH radical. 

Figure 5.1 shows that function 1 is therefore the oxidation of NO to N02, this last one being subsequently delivered to function 2 to oxidize HC to CxHyOz which will be delivered to function 3 to scavenge the adsorbed oxygen species left by the (NO)2 -adsorbed dinitrosyl species - decomposition. 

One other aspect of the photolysis of coordinate spin labeled derivatives is of interest. Nitroxides are good free radical scavengers (123). As a result, when methyl-cobalamin is photolyzed in the presence of a nitroxide, the methyl radical generated will react with the free nitroxide and cause disappearance of the ESR spectrum (123). However, once the nitroxide is coordinated it is no longer susceptible to attack by free radicals. Thus the nitroxyl function is quite well protected from approach by other species. 

Using deterministic kinetics, one can force-fit the time evolution of one species—for example, eh but then those of other yields (e.g., OH) will be inconsistent. Stochastic kinetics can predict the evolutions of radicals correctly and relate these to scavenging yields via Laplace transforms. 

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