Pulse-radiolysis

The main differences between pulse radiolysis and flash photolysis arise from the use in the former of ionizing radiation instead of light to initiate the reaction. Thus a pulse of electrons from a linear accelerator (or, less commonly, an X-ray pulse) 

Several relevant articles reviewing the area of pulse radiolysis and the related technique of flash photolysis have appeared. 

155 Radiation Effects in Liquids and Solids , Faraday Discuss. Chem. Soc., 1977, No. 63. 

The expected order for electrophilic attack by OH on co-ordinated and unco-ordinated pyridine derivatives is py [Co(NH3)spy] + Hpy+. For pyridine and isonicotinamide (ina) this order is observed and similarities in the spectra of the unstable addition products suggest that OH attacks at the same place in both bound and unbound ligands. With nicotinamide (na), reaction of the cobalt(iii) complex is faster (2.1 x 10 1 moh s at 22 C) and produces a transient spectrum which is red-shifted compared with that of the unbound ring adduct. It is suggested that in this case attack occurs not on the aromatic system but on an amide group. This behaviour may explain why the europium(ii) reduction of [Co(NH3)5(ina)] + is autocatalytic whereas the corresponding reduction of the nicotinamide complex is not. In the pulse radiolysis studies, the transient intermediate complexes decay by a second-order pathway with rate constants 1.3 x 10, 3.0 x 10 , and 6.0 x 10 1 mol for [CofNHa) sPy] , [Co(NH3) s(na)] +, and [Co(NH3)5(ina)] +respectively 

Reducing radicals such as CO2 and MejCOH react with complexes of the type fCo(NH3)j(02CArN02)] +, where O.CArNOa is a mono- or di-nitrobenzoate ligand, to produce a co-ordinated nitrobenzoate ligand radical  

The subsequent decay of these species corresponds to intramolecular electron transfer from the aromatic ring to the metal centre, a process which is slow because of the low electron permeability of the carboxylate group  

The importance of this technique to chemistry and biology has been far less widely accepted than it deserves. The essential technical problem involves the generation of short pulses of ionizing radiation followed generally by optical detection of transient species (Swallow, 1973 von Sonntag, 1987 Kiefer, 1990). 

The basis of the method is shown in Fig. 3.8. Pulsed radiation is usually generated by linear electron accelerators (Linacs) or by Van de Graaf accelerators, each having certain advantages. Both give nan- 

TABLE 5.5 Some Typical Sources of Reactive Species in FP - RF Systems 

The reaction cell has a White cell optical system (see Chapter ll.A.lc) with a pulsed xenon lamp light source. Once the radicals are formed, they are detected by their absorptions in the UV using the Xe lamp and a monochromator-photomultiplier or photodiode array detector. Thus the absorption spectra of the free radicals generated in the system can be measured and the absorption at a particular wavelength used to follow their reaction kinetics. 

This technique has similarities to photolysis in that a large perturbation is involved and reactive transients can be produced and examined. accelerators or, more 

The mixture of radicals would be difficult to examine or use. Fortunately, by the careful choice of an added substrate, some of the radicals produced in (3.16) are rapidly removed so 

Loss of 1-5 gM (initial concentrations) all conform to the solid curve. There is no evidence of any significant first-order component even at [Ny = 0.5 gM. Reproduced with permission from Z. B. Al-fassi and R. H. Schuler, J. Phys. Chem. 89, 3359 (1985). (1985) American Chemical Society. 

Radical Production (added substances underlined) Remarks 

Thble 3.6 Examples of Use of Radicals Generated by Pulse Radiolysis 

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Smaller B, Remko J R and Avery E C 1968 Electron paramagnetic resonance studies of transient free radicals produced by pulse radiolysis J. Chem. Rhys. 48 5174-81... 

Many experimental methods may be distinguished by whether and how they achieve time resolution—directly or indirectly. Indirect methods avoid the requirement for fast detection methods, either by detemiining relative rates from product yields or by transfonuing from the time axis to another coordinate, for example the distance or flow rate in flow tubes. Direct methods include (laser-) flash photolysis [27], pulse radiolysis [28]... 

Guldi D M 1997 Capped fullerenes stabilization of water-soluble fullerene monomers as studied by flash photolysis and pulse radiolysis J. Phys. Chem. A 101 3895-900... 

Guldi D M, Hungerbuhler H, Janata E and Asmus K-D 1993 Radieal-indueed redox and addition reaetions with Cgg studied by pulse radiolysis J. Chem. See., Chem. Commun. 6 84... 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

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 polarographic half-wave reduction potential of 4-nitroisothiazole is -0.45 V (pH 7, vs. saturated calomel electrode). This potential is related to the electron affinity of the molecule and it provides a measure of the energy of the LUMO. Pulse radiolysis and ESR studies have been carried out on the radical anions arising from one-electron reduction of 4-nitroisothiazole and other nitro-heterocycles (76MI41704). 

An interesting example of the N(9)-C(8) prototropic tautomerism has been reported for the caffeine radical by pulse radiolysis studies in aqueous solution the transformation of the heteroatom-protonated electron adduct 25 into the carbon-protonated tautomer 26 occurred spontaneously in neutral media [95JCS(F)615]. 

This dissolution process takes place in many solvents to an extent governed by Eq. (3). Solvated electrons can be formed in all solvents by many means. Their kinetics is best studied with the use of pulse radiolysis. 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

Complexes of cations in unstable oxidation states in aqueous solutions as studied by pulse radiolysis. D. Meyerstein, Acc. Chem, Res., 1978,11,43-48 (95). 

Some of the most important questions one can ask in the study of kinetics concern the rates of reactions of the intermediates. In some cases, values can be obtained by direct experiments. For example, one might generate the intermediate by an independent method capable of producing it much more rapidly than it reacts. Then it can be examined in its own right. Chapter 11 presents methods for doing so, such as flash photolysis and pulse radiolysis. 

The reader can show that, with the steady-state approximation for [Tl2+], this scheme agrees with Eq. (6-14), with the constants k = k i and k = k j/k g. Of course, as is usual with steady-state kinetics, only the ratio of the rate constants for the intermediate can be determined. Subsequent to this work, however, Tl2+ has been generated by pulse radiolysis (Chapter 11), and direct determinations of k- and k g have been made.5... 

Some of the most conclusive studies of the mechanisms of chain reactions come from experiments in which some of the propagating steps have been independently measured directly. This measurement can sometimes be done by the use of flash photolysis and pulse radiolysis (Chapter 11). Such determinations can verify the occurrence of a certain reaction in the sequence and provide its rate constant. 

The absorption of radiation produces unstable species. Flash photolysis does so by interaction of light with a solute. The transient may be a photoexcited state or a molecular fragment. Pulse radiolysis starts with highly reactive entities formed by dissociation of the solvent (e.g., H, eaq, and HO from H20) and consists of a study of their reactions or of reactive transients derived from them. In either case one monitors the ensuing reactions by luminescence (for excited states), light absorption, or conductivity changes. 

Photolytic methods are used to generate atoms, radicals, or other highly reactive molecules and ions for the purpose of studying their chemical reactivity. Along with pulse radiolysis, described in the next section, laser flash photolysis is capable of generating electronically excited molecules in an instant, although there are of course a few chemical reactions that do so at ordinary rates. To illustrate but a fraction of the capabilities, consider the following photochemical processes ... 

Reactions such as these are of interest in themselves. Beyond that, one can use the pulse radiolysis experiment as a preparative technique for other species. Thus, the reactions of numerous aliphatic, carbon-centered radicals have been evaluated.22 If one employs a reasonably high concentration of solute, say 0.1-1 M CH3OH, the formation of CH2OH is complete within the electron pulse. Following that, reactions such as the following can be studied ... 

Pulse radiolysis requires access to an electron accelerator or similar device. This requirement usually restricts work to specialized laboratories. Thorough descriptions of the experimental apparatus and protocols have been given.23,24... 

Pulse radiolysis. Consider the reaction shown in Eq. (11-58). Show that H and e q are related as conjugate acid and base. Calculate the pKa of H. ... 

Pulse radiolysis. If one wishes to determine the rate constant for the reaction of e q with a substrate A, the approximate value of which is 107 LmoP1 s l, what is the lowest pH that can be used ... 

Proton inventory technique. 21.9-220 Pseudo-first-order kinetics, 16 Pulse-accelerated-flow method. 255 Pulse radiolysis, 266-268 Pump-probe technique. 266... 

Hayon23 studied the yields of ions and excited states in pulse radiolysis of liquid DMSO using anthracene as a solute to determine the yield of free ions and naphthalene as a solute to measure the yield of triplet excited states. Anthracene is known to react with solvated electrons to give the anthracene radical anion, A T... 

From the pulse radiolysis of DMSO solutions of naphthalene and the absorption of the naphthalene triplets (e = 2.26 x 104 M "1 cm 1) Hayon23 found that the yield of triplet excited states in irradiated DMSO is G = 0.57 and G = 0.36 for DMSO saturated with argon gas or N20 gas, respectively. 

FIGURE 4. Transient absorption spectrum immediately after pulse radiolysis of dimethyl sulphoxide alone. Pulse length 50 ns, dose 1000-2000rad , path length 2.5m, time resolution 3ns +, path length 5 cm, time resolution 10 ns. The dashed line represents the spectrum of the short-lived component (t1/2,14 ns), subtracted from the overlapping longer-lived component which is unaffected by N20. Reproduced by permission of the authors from Reference 29. 

Meissner and coworkers36 studied the pulse radiolysis of aqueous solutions of dimethyl sulfoxide. It was found that hydrated electrons react with DMSO with a rate constant of... 

Veltwisch and colleagues45 studied the reaction of OH with several sulfoxides by pulse radiolysis using electrical conductivity for the detection of formation or disappearance of ions. Pulse radio lysis of N20-saturated aqueous solution of DMSO (10-3m) leads to a decrease in conductivity at basic pH (pH = 9.0) and an increase in conductivity at acidic pH (pH = 4.4). This is explained by the reactions... 

The branching of the adduct decomposition was studied by measuring the pKa of the mixtures. Thus for pulse radiolysis of N20-saturated methyl phenyl sulfoxide the results yield a p/Ca obs value of 1.50 while the values for methane sulfinic and benzene sulfinic acids are 2.28 and 1.29, respectively. The fraction of each branch can be calculated from the equation,... 

They measured the formation of ionic species in the pulse radiolysis of a 10"3 m MTMSO aqueous solution saturated by N20 gas at pH 5.6 by conductivity detection and found an increase in conductivity. Assuming the molar conductivity of CH3SCH2S02 to be the same as that measured for CH3S02- 45, it is found that G(CH3SCH2S02") = 2.7. 

Nelson54 studied the products of radiolysis of aqueous solution by variable-field CIDNP pulse radiolysis. On the basis of the chemical shifts the following products were identified methyl methanesulfinate, methanol, l,2-bis(methylsulfinylethane) [CH3S(0)CH2CH2S(0)CH3], dimethyl sulfone, dimethyl sulfide, methane and ethane. The high field polarization was used to study the mechanism of formation of polarized products. 

This review is concerned with the formation of cation radicals and anion radicals from sulfoxides and sulfones. First the clear-cut evidence for this formation is summarized (ESR spectroscopy, pulse radiolysis in particular) followed by a discussion of the mechanisms of reactions with chemical oxidants and reductants in which such intermediates are proposed. In this section, the reactions of a-sulfonyl and oc-sulfinyl carbanions in which the electron transfer process has been proposed are also dealt with. The last section describes photochemical reactions involving anion and cation radicals of sulfoxides and sulfones. The electrochemistry of this class of compounds is covered in the chapter written by Simonet1 and is not discussed here some electrochemical data will however be used during the discussion of mechanisms (some reduction potential values are given in Table 1). 

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