OH radical reaction products

Reaction Kinetics. Formation of H0CgH4N02. Transients produced by hydrated electrons and OH radicals absorb in the same wavelength region, but p-nitrophenol reacts about one-tenth as fast with OH radicals (see below) than with hydrated electrons. Therefore, it is possible in deaerated 5 X 10 r,M p-nitrophenol solutions at pH 7 to observe the formation of the anion radical H0CGH4N02"- without the interference of the OH radical reaction product. From the exponential rate of formation of the anion radical H0CGH4N02 at 290 n.m., the rate constant kA was calculated to be (3.5 0.6) X 1010M 1 sec."1. 

The potentials associated with the redox reactions of R (aquo) span a wide range, table 24.8, and characteristic absorption spectra are obtained for the various oxidation states. As a consequence, this represents an interesting area for pulse radiolysis studies. Hart and Anbar (i960) give ° = 2.77 V for the reaction Caq -1- H 1/2 H2. Based on the reaction Cu II + OH Cu III (Baxendale et al., 1965) and the calculations of Henglein (1974), a standard oxidizing potential, E°, of the order of 2V may be associated with the OH radical. Reaction products with e or OH frequently have only a transient existence, but recent developments permit recording time-resolved spectra on the nanosecond time scale (Schmidt et al., 1976). 

Yim et al. (2002) studied the sonlytic degradation of diethyl phtahalate in aqueous solution. Degradation followed pseudo-first-order kinetics. Monoethyl phthalate, a hydrolysis product of diethyl phthalate, was approximately 3.3 times higher at pH 12 than at pH 7. The investigators concluded that the reaction was affected by pH of the solution. In the presence of ultrasound, the OH radical reaction, thermal reaction, and hydrolysis were all involved during the reaction. 

Moorthy PN, Hayon E (1975) Free-radical intermediates produced from the one-electron reduction of purine, adenine and guanine derivatives in water. J Am Chem Soc 97 3345-3350 Mori M, Teshima S-l, Yoshimoto H, Fujita S-l, Taniguchi R, Hatta H, Nishimoto S-l (2001) OH Radical reaction of 5-substituted uracils pulse radiolysis and product studies of a common redox-ambivalent radical produced by elimination of the 5-substituents. J Phys Chem B 105 2070-2078 Morin B, Cadet J (1995) Chemical aspects of the benzophenone-photosensitized formation of two lysine - 2 -deoxyguanosine cross-links. J Am Chem Soc 117 12408-12415 Morita H, Kwiatkowski JS,TempczykA(1981) Electronic structures of uracil and its anions. Bull Chem Soc Jpn 54 1797-1801... 

Vieira AJSC, Steenken S (1987a) Pattern of OH radical reaction with 6- and 9-substituted purines. Effect of substituents on the rates and activation parameters of unimolecular transformation reactions of two isomericOH adducts. J PhysChem 91 4138-4144 Vieira AJSC, Steenken S (1987b) Pattern of OH radical reaction with N6,N6-dimethyladenosine. Production of three isomeric OH adducts and their dehydration and ring opening reactions. J Am Chem Soc 109 7441-7448... 

The OH radical reactions with a number of nitrogen-, sulfur- and phosphorus-containing organic compounds appear to proceed, at least in part, by an initial addition reaction (Atkinson, 1989,1994 Kwok et al., 1996), although the products observed may in some cases be those expected from H-atom abstraction. Note that the recent study of Talukdar et al. (1997) indicates that the reactions of the OH radical with alkyl nitrates proceed only by H-atom abstraction, and Table 14.1 gives the applicable substituent group factors for alkyl nitrates. 

There are clearly several areas of significant uncertainty, including the role of photolysis in both the gas and particle phases and the identities and formation yields of the products of the gas-phase OH radical reactions. 

The implications of the versatile reaction mechanisms depicted in Figs. 7-1 to 7-4 are profound with respect to the complete understanding and hence to the kinetic modeling of AOPs. Despite the complexity of these photo-initiated reactions, it is possible to model AOPs with sufficient precision if all the rate constants of OH radical reactions involved and those of all other elementary reactions are known (Crittenden et al., 1999). Most importantly, the structures and the concentrations of all intermediary reaction products must be known. In addition, photoreactor specific parameters have to be included, such as the incident photon flow d>p and the dimensions of the irradiated volume. This task can be achieved for example... 

A fine example of the use of pulse radiolysis technique in combination with steady-state product distribution is the recent study by Schuler and co-workers to probe charge distribution in aromatics. Their initial studies on biphenyf which were extended recently to phenof have been concerned with the determination of relative yields at o-, m- and -positions to examine the substituent effects on charge distribution. For example, Fig. 2 shows the contour plot of HPLC chromatographic data obtained in the OH radical reaction following the irradiation of phenol in the presence of the oxidant ferricyanide. Figure 3 shows the transient absorption spectra recorded in the reactions of OH and Ng radicals with phenol. The absorption... 

Figure 1 Three-dimensional HPLC chromatogram showing the separation of degradation products in N.O saturated dye (Apollofix-Red AR-28 at 0.25 mmol /" Isolation irradiated with 0.6 kGy dose. Destruction ofAR-28 molecules and formation of degradation products in OH radical reactions as measured by HPLC with diode array detection. AR i and AR it are used for AR-28 and for its hydrolysed form, respectively, Pi and PII are products.

To study the reaction products flic OH oxidation of NMP in the aqueous phase, die continuous photolysis of H2O2 was used to produce OH radicals (reaction 8). 

Table III summarizes OH rate constants for these oxygenated hydrocarbons, and products of the OH radical reaction are listed in Table IV.

Products of OH Radical Reaction with Oxygenated Hydrocarbons That Are Possible Gasoune Components... 

Both the steady state (20) and the pulse radiolysis results can be accounted for on the assumption that the primary OH radical addition product H0C6H4N03-- undergoes the intramolecular rearrangement Reaction 6. It produces the OH benzene ring addition product... 

The question raised by Dainton and Wiseall (6) with respect to absolute values of the rate constant of OH radical reaction with the basic form of RNO must await clarification of product spectra and yields. These will resolve whether or not G( — RNO)440 is indeed numerically equal to G(—RNO). It is also not clear in acidic RNO solutions what kind of species are causing the residual yields with ethyl alcohol and the unusual kinetic behavior arising from changes in oxygen concentration. Clearly, more studies of these effects are called for and are underway at the present time. 

Formaldehyde is a first-generation product that reacts further, by photolysis by reactions 5.32a and 5.32b and with the OH radical, reaction 5.33. Formaldehyde is the first major product of CH4 oxidation with a lifetime longer than a few seconds. The lifetimes of HCHO resulting from photolysis and OH radical reaction are 4 hours and 1.5 days, respectively, leading to an overall lifetime of 3 hours for overhead sun conditions. 

Laboratory kinetic studies of gas-phase elementary reactions of importance to tropospheric chemistry were initiated as part of the LACTOZ programme at the Swiss Federal Institute of Environmental Science and Technology (EAWAG) in October 1989. Two types of experiments have been carried out (i) studies of the kinetics of competitive OH radical reactions with VOC in an atmospheric flow reactor and (ii) detailed end-product analyses of the OH radical initiated photooxidations of VOCs carried out in a static Teflon bag reactor. 

After a long discussion [107 to 111, 116, 117, 121] NH2 + NO N2 +H20 (a) is accepted as the main reaction pathway, as already pointed out in an earlier mass-spectrometric investigation [43], contributing about 85 to 90%. The water obtained in this highly exothermic reaction was found to be vibrationally excited [43, 111]. The production of OH radicals (reaction pathway (b)) contributes about 10 to 15% of the total reaction. This is in agreement with earlier determinations of a quantum yield of c )(N2) = 1 for N2 molecules [58, 59, 94, 95, 125]. A more pronounced contribution of the reaction channel (b) would lead to a significantly higher quantum yield of c )(N2) = 2 due to secondary NH2 formation, since OH is an efficient chain carrier at any temperature via OH + NH3 NH2 + H20 (NH3 is available in excess in the photolysis systems). The problem that is not yet completely solved is whether channel (b) should be written... 

Pulse radiolysis results (74) have led other workers to conclude that adsorbed OH radicals (surface trapped holes) are the principal oxidants, whereas free hydroxyl radicals probably play a minor role, if any. Because the OH radical reacts with HO2 at a diffusion controlled rate, the reverse reaction, that is desorption of OH to the solution, seems highly unlikely. The surface trapped hole, as defined by equation 18, accounts for most of the observations which had previously led to the suggestion of OH radical oxidation. The formation of H2O2 and the observations of hydroxylated intermediate products could all occur via... 

The effect of increasing pressure is to move the average hydrocarbon content towards the heavier species, but increasing temperature seems to favour the production of lighter species. The final proportions are also determined by the state of the catalyst, and the physical anangement of tire reactor. The formation of the oxygenated compounds could also involve reactions between the H2O content of tire gas in the form of adsorbed OH radicals and hydrocarbon radicals since the production of these molecules is also well beyond the thermodynamic expectation. 

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