Pulse radiolysis Subject

These solutions of Cd are powerful reducing agents, and after tens of microseconds all Cd has disappeared. If Cd complexes of such ligands as glycine, en, EDTA and NT A are subjected to pulse radiolysis, the Cd produced is present as a complex ... 

Measurements of absolute rate constants for the reduction and oxidation of metal ions by e, H- and OH- has been a prominent achievement of the technique of pulse radiolysis. This subject is too broad to be included in this review and is to be dealt with later in the series. A key reference is given, however, to help cover the interim period. 

Dopamine (10) has also been the subject of some study. Maity and coworkers17 have studied the pulse radiolysis or /-irradiation induced reduction of the protonated form. In this instance the addition of an electron affords the radical anion 11 with a bimolecular rate constant for the reaction of 2.5 x 108 M-1 s-1. 

Other important aromatic amines such as chlorpromazine (26) have also been subjected to oxidation studies using oxidants produced by pulse radiolysis. Typical among these is the use of chloroalkylperoxyl radicals formed by pulse radiolysis in a variety of solvents. These oxidants yield the corresponding radical cation. The rate constants (Table 3) for these reactions were determined42. Other studies have determined the reactivity between chlorpromazine and BiV- in H2O/DMSO in varying proportions. The rate constants for the formation of the radical cation of chlorpromazine were similar in value to those obtained from the peroxy radical reactions4. 

Redox reactions involving the nickel(IV) complex are also subject to divalent metal ion catalysis (170, 171). Oxidations of the two-electron reductant ascorbate (40) and the one-electron reductant [Fe(CN)6]4-(172) have been examined in some detail. Both reactions have as the rate-determining step the transfer of one electron from the reductant to nickel(IV) in an outer-sphere process to give an undetected nickel(III) transient. Spectroscopic properties of the nickel(III) species have been determined by pulse radiolysis (41). 

The pulse radiolysis method has been described in detail in some of the early papers (22, 22), in a brief review of the subject (23), and in a current comprehensive review (14). It is, in brief, a fast reaction method in which the external perturbation applied to the system is a microsecond pulse of electrons. The current is sufficiently high to produce an instantaneous concentration of transient species high enough to be observed by fast measurement of the optical absorption. Spectra may be recorded either photographically or spectrophotometrically. The kinetics are studied by fast spectrophotometry. Since a perturbing pulse as short as 0.4 /xsec. has been used, the time resolution has approached 10-7 sec. The flash photolysis method used in some of the other studies (27, 15) has been reviewed in detail (24). 

Specifically we wished to measure the rate of reaction of OH with MSA to enable modelling calculations of the stability of MSA in aerosol droplets. The one reported measurement of this rate (2), using pulse radiolysis techniques, 3.2 x 109 M 1 s 1, is fast enough to suggest that this reaction pathway could be an important sink for MSA. This is of interest in explaining an apparent discrepancy that exists between laboratory and field studies of tne oxidation of dimethyl sulfide. Although a number of laboratory studies (6-9 ) show that MSA is the major stable product, and SO2 a minor one, field observation suggest MSA is only a minor (10%) fraction (2) of total non-sea-salt sulfur in marine aerosols. Two possible rationalizations of this are that i) MSA is subject to further reaction in marine aerosols and ii) other reaction pathways of dimethyl sulfide, or perhaps other non-methylated sulfur compounds should be considered. 

Like that of the hydrated electron and the hydrogen atom, the potential of the hydroxyl radical has long been the subject of estimates based on thermochemical cycles involving the free energy of hydration of OH the results of these calculations appear, for example, in Standard Potentials (pp. 59-64). Recently, however, there have been two direct determinations of E° for the OH/OH- couple. In the first, Schwarz and Dodson (279) used pulse radiolysis to measure the equilibrium constants for... 

Radiation chemistry, and pulse radiolysis in particular, is now a mature subject that is available as a very valuable and a powerful tool by which fundamental problems in free radical reaction mechanisms can be addressed. This chapter is restricted to studies concerning sulfur-centered radicals and radical-ions performed by radiation chemistry techniques in the first eight years of XXI century (2001-2008). SuMur-centered radicals represent a very interesting class of radicals since they exhibit very interesting redox chemistry, including biological redox processes, and different spectral and kinetic properties as... 

In many cases the product S is itself a free radical (S ), or a hyper-reduced metal ion, which in turn reacts in one-electron gain or loss processes. It is not surprising, then, that radiation-chemical methods are widely used in the study of electron-transfer processes. Of particular value is the technique of pulse radiolysis which permits reactions to be studied on timescales ranging from seconds down to picoseconds, so that even the most reaetive speeies ean be studied. It is this technique and its applications that form the subject matter of this chapter which begins with an outline of the radiation chemistry of water and other solvents. Next there is a historical view of pulse radiolysis, some of the landmark discoveries are discussed, followed by a description of the principal features of a pulse radiolysis facility and the various methods of detecting and measuring transient speeies. The chapter ends with some examples of data capture and analysis, and methods of sample preparation. 

Combination of pulse radiolysis and two-laser flash photolysis will be another interesting subject of three-beam excitation chemistry to be investigated in the near future. In this case, radical ions generated during pulse radiolysis will be excited by the successive two lasers. That is, one can introduce additional pathways to the reaction scheme indicated in Scheme 2.3. Utilization of a dyad or triad molecular system will realize fast intramolecular reaction systems applicable to the multibit molecular memory. 

However, hydroxyl radicals are very reactive and known to react with aromatic compounds not only by electron abstraction but also by adding to the ring. Well-established, one-electron oxidizing radicals such as Br2 and N3, formed by pulsing a nitrous oxide saturated solution of potassium bromide or sodium azide, are used to produce cation radicals of the drug molecules. For example, the reactions that take place when a nitrous oxide aqueous solution of lO2 M potassium bromide in the presence of 10 4 M chlorpromazine (C1P) is subjected to pulse radiolysis (Asmus et al., 1979 Davies et al., 1979) are given below ... 

Wild-type metalloproteins were the subject of numerous studies by pulse radiolysis. For instance, one should mention the pioneering work on Cytochrome c (177, 178). Using secondary radicals, it appeared that the reactions with the metal centers almost always involved LRET. Thus several groups tried to modify these proteins in order to bring elements to the determination of electron pathways. 

That the hydrated electron is a separate chemical entity has been demonstrated by the technique of pulse radiolysis. This consists of subjecting a sample of pure water to a very short pulse of accelerated electrons. The energetic electrons have the same effect upon water as a beam of y-ray photons. Shortly after the pulse of electrons has interacted with the water, a short flash of radiation (ultraviolet and visible radiation from a discharge tube) is passed through the irradiated water sample at an angle of 90° to the direction of the pulse to detect the absorption spectra... 

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