This chapter describes the radiolytic spin labeling technique for analyzing products of catalytic reactions in zeolites. Radiolytic spin labeling of molecules adsorbed in zeolites occurs by ionization to form radical cations and by formation of H-adduct radicals by H atom addition. The chapter discusses the proton- and Na+-exchanged zeolites. Proton-exchanged zeolites derive catalytic activity from bridging hydroxyl groups that are strong Bronsted acid sites and Na+-exchanged zeolites are inert adsorbents. The advantages of the radiolysis/electron paramagnetic resonance (EPR) method for studying mechanisms of zeolite catalysis are due to the sensitivity and structural specificity of EPR, surpassing that of other in situ spectroscopies, such as fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR), and the ability to identify products at low temperature. It is often the case that at high temperatures needed to evolve products from the zeolite for ex situ analysis, a complex sequence of reactions has already occurred. Therefore, elucidation of the elementary reactions of the sequence necessitates in situ analysis at low temperatures. In addition, product selectivity can be more pronounced for reactions carried out under mild conditions.
This chapter focuses on the positive muon (μ+) as a probe in chemistry and introduces some basic aspects including typical radiation chemical effects, spectroscopic observations, and applications in investigations of kinetics and dynamics. With a single positive charge and a mass one-ninth of the proton mass, the muon mimics a light proton in matter. Muonium (Mu) is simply a light hydrogen isotope. The muon is a spin-l/2 particle with a magnetic moment 3.18 times that of the proton and it is obtained at the ports of suitable accelerators with a spin polarization close to 100%. This makes it an ideal magnetic probe. The signature of its chemical nature is based on its magnetic properties, and the experimental techniques for it are closely related to magnetic resonance. Mu addition to double bonds places the muon two bonds away from the radical center. Thus, Mu is not normally directly involved in reactions of the radical, and any kinetic isotope effects are secondary and small. This makes the muon a non-perturbing radical kinetics probe. Its advantage is the extraordinary sensitivity of the technique, which requires only a single muon in the sample at a given time.
This chapter discusses the chemistry behind the application of ionizing radiation in water-pollution abatement. Ionizing radiation possesses several attractive features as a means to induce the destruction, that is, in the limit, the mineralization of aqueous organic pollutants. The chapter emphasizes on the chemical-mechanistic aspects of the radiolytic decomposition of certain classes of environmentally noxious chemicals in the aquosphere. The chemistry at the basis of the technology consists in the reactions of the free radicals OH, H, and hydrated electron eaq-, created in the proportion of about 5/1/5 upon the radiolysis of the polluted water, with the pollutant solutes. In the abatement of aromatic pollutants, the main initial transient species is the substrate-OH-radical adduct, hydroxycyclohexadienyl. Depending on the nature of the substituents in the ring, the addition of dioxygen to these radicals may be reversible. Treatment of aromatics with ionizing radiation in order to degrade them is not necessarily always entirely beneficial. Thus, with an effluent that produces NO2, traces of nitro derivatives can be formed by free-radical recombination. These compounds are usually very resistant to biodegradation. Hydroxylated aromatics are among the intermediates in the deep degradation of aromatic hydrocarbons. In addition, they form a class of pollutants of their own. Phenoxyl radicals are important intermediates in their oxidative oxidation.
Publisher Summary This chapter focuses on the radiation chemistry of concentrated inorganic aqueous solutions. When the aqueous solution is irradiated with ionizing radiation, ionization and excitation take place. Radiation energy is mainly absorbed by water molecules and it can be said that the radiation induced reactions start from the decomposition of the water molecules. Much attention has been paid to the radiolysis of the concentrated aqueous solution because of the practical demands especially in nuclear technology. Highly concentrated nitric acid, of more than 3 M, is used under the strong radiation field as an aqueous phase during the reprocessing of nuclear spent fuels in the so called plutonium uranium reduction oxidation (Purex) process. When nitric acid is irradiated with an electron pulse of 10 ns duration, two formation processes, fast and slow, are observed. The fast process is completed after the duration of the pulse, but the slower one grows up to 200 ns in 3 M nitric acid. If an •OH scavenger is introduced, the slow component is easily reduced without the reduction of the fast component. This finding strongly suggests that the slow process is attributable to the •OH radical reaction with nitric acid or nitrate ion. When the sample is replaced with sodium nitrate solution, only the fast component is observable.
This chapter discusses the spin coherence effect in relation to radiation chemistry under magnetic fields. An in-depth study of spin coherence effects provides information related to the information available from magnetic resonance studies. The frequencies of quantum beats and the positions of lines in magnetically affected reaction yield curve (MARY)-spectra are determined by the same parameters as the electron spin resonance (ESR) spectrum structure. The methods discussed in the chapter can be considered as the variants of ESR-spectroscopy of spincorrelated radical ion pairs without mw-pumping. The main advantage of these methods as well as the optically detected (OD) ESR technique is their extremely high sensitivity. Recording luminescence, it is possible to study the spin-correlated pairs at stationary concentrations down to 100 particles per sample. Therefore, the weak radioactive sources can be used for radical ion pairs generation. Compared with the OD ESR technique, spin coherence effect, however, has an essential advantage. The spin coherence effect can be used to study the spin-correlated pairs with shorter lifetimes. In OD ESR, this time is limited by the time of electron spin flip around microwave magnetic field of spectrometer, whereas in the case of quantum beats or MARY spectroscopy, this limit depends on hyperfine fields or the difference in radical g-factors.
This chapter reviews the radiation chemical studies that are concerned with the radical-induced redox, excitation, and alkylation reaction of fullerenes. In particular, the reactivity of fullerenes and functionalized fullerene derivatives are compared in homogeneous as well as heterogeneous systems, including micelles, vesicles, and guest-host complexes. The combination of a high degree of electron delocalization within the fullerene's π-system and their effective sizes prompts to the application of this carbon material as new electron accepting moieties. More importantly, the total reorganization energy upon reduction has been shown to be relative small. Hence, fullerenes became very appealing spheres for inter and intramolecular electron transfer processes under the aspect of energy conversion and energy storage. The chapter discusses two examples to demonstrate the participation of fullerenes in reductive and oxidative electron transfer reactions with a variety of electron donor (one-electron reduced metalloporhyrins) and electron acceptor moieties (one electron oxidized arenes), respectively.
This chapter discusses free radicals derived from xenobiotic molecules, especially drugs of interest in cancer therapy. The chapter focuses on chemical properties and chemical mechanisms in anti-cancer drug research, with an overview of redox characteristics of drugs and radicals. A compilation of reduction potentials of one-electron couples involving free radicals in aqueous solution includes an introductory outline of the quantitative basis for expressing redox properties by means of reduction potentials. In studies of free radicals of drugs, the desire is to produce selected drug radicals, usually involving one-electron oxidation or reduction, which arc putative intermediates in drug activation or metabolism. The redox properties of likely activating enzymes are thus of interest, but selectivity of reaction is the most important. Thus oxidizing drugs by •OH radicals is seldom useful, because the sites of reaction with typical drugs will be multiple. More selective oxidants are used, such as the secondary radicals derived from halide or pseudohalide oxidation by •OH. Reducing radicals are less of a problem, because the radicals from scavenging •OH by formate or 2-propanol usually reduce oxidants to produce the same radicals as cellular reduction systems.
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An overview of the fundamental early processes discussed in this section is summarized schematically in Figure 10. From this time onwards, thermal processes proceed.
This chapter discusses radiation pasteurization and sterilization of food. The complex phenomena associated with the interaction of ionizing radiation with diverse liquids and solids are relevant and applicable to irradiated chilled or frozen foods. Specific influences of the food composition and structure directly affect the nature and reaction of the resulting free radicals formed. Irradiation parameters and conditions further affect the reaction pathways of the radicals and the yield of stable products derived from them. The radiolysis of a complex food matrix can be considered the sum of the radiolysis of its major constituents, which essentially represent distinct and immiscible phases. In muscle foods, the composition is primarily water (65%), protein (20%), and lipids (15%). The water phase contains minor constituents, including certain proteins, small peptides, vitamins, and salts. The radiation chemical considerations would imply that suitable foods properly irradiated should be wholesome, that is, safe to consume and nutritionally adequate. Moreover, a judicious choice of readily controllable irradiation conditions ensures that chemical changes affecting the major quality attributes of taste, color, and texture can be minimized.
This chapter reviews the achievements of radiation chemistry and particularly focuses on the contributions from radiation chemistry to the broader area of physical chemistry and other areas of general contemporary interest in chemistry. Whereas much of the underlying mechanisms for the effects of radiation on materials were outlined using steady state radiation sources, the advent of pulse radiolysis on the heels of flash photolysis opened a window into direct observation of the intermediates. A major contribution from radiation chemistry to general physical chemistry was the experimental confirmation of the Marcus theory for electron transfer by Miller and Closs. The advantage of the radiolytic approach over the analogous laser flash photolysis technique, in this context, is the ability to generate only the reduced radical on one donor-acceptor molecule. Thus, a charge-shift reaction could be studied rather than an electron-hole recombination that is commonly generated photolytically. Nonetheless, the implications of the theory, which gain credence from the experimental verification, are far reaching especially in photoinduced electron transfer.
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Radiolytic reduction of chlororhodium(III) tetramesitylporphyrin ((ClRhP)-P-III) in alcohol solutions forms a transient (RhP)-P-II, which reacts to yield different products under different conditions. In alkaline 2-propanol the product is (RhP)-P-I-, in weakly acidic conditions (HRhP)-P-III is formed, and under strongly acidic conditions the main products of radiolysis are the alkylated rhodium complexes, R-(RhP)-P-III. The latter products are formed by reaction of (RhP)-P-II with alkyl radicals (R(.)) that are produced in the irradiated solvent (R(.) = (CH3)-C-. and (CH3)2(C)(.)OH in 2-propanol). UV photolysis of (ClRhP)-P-III in acetone/2-propanol solutions led to formation of HO(CH3)(2)C-(RhP)-P-III. One-electron reduction of CH3-(RhP)-P-III occurs at the porphyrin ligand to produce a transient pi-radical anion, CH3-(RhP.-)-P-III. In alkaline solution, this transient eliminates (.)CH3 to form the stable (RhP-)-P-I, but in neutral or acidic solutions, it undergoes disproportionation, promoted by protonation on the macrocycle, to form CH3-Rh-III-chlorin and then CH3-Rh-III-isobacteriochlorin upon further reduction. In the presence of CO2, the initial radiolytic reduction yields are increased. After extensive irradiation, however, the yields of reduction are decreased and a very low yield of CO was found. No reaction was detected between (RhP-)-P-I or (HRhP)-P-III and CO2, even under visible light illumination. Although this system is found to catalyze homogeneous photochemical formation of H-2, no catalytic activity for CO2 reduction was found under the current experimental conditions.
AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
One of the new aspects of radiation chemistry at present and that will be in the near future is the use of new radiation sources. The character and potential advantages of ion beam compared to conventional radiation would be large and localized energy deposition in materials with high spatial resolution; the materiality of the beam that can implant atoms into the target and can endow the product with novel function; wide variety of secondarily produced radiation that enables sophisticated analysis of material bulk and surface; and transmutability of material including nuclear reaction. The peculiarity related to large and localized energy deposition is sometimes called linear energy transfer (LET) effects. The chapter focuses on LET, research activities, and technical aspects of ion beam radiation chemistry. An ion beam loses energy through interaction with target material. The energy deposition or energy loss in depth direction per unit thickness -dE/dx, where E is energy and x is depth, is defined as the stopping power. LET is different from stopping power. LET excludes contribution of energetic secondary electrons, whereas stopping power considers every contribution. However, they are very close to each other in most cases of radiation chemistry. The chapter describes two aspects of ion beam radiation chemistry in polymers—namely, fundamental-oriented and application-oriented.
This chapter describes the technology that enables the investigation of radiation chemical phenomena at picosecond and femtosecond timescales. It particularly focuses on ultrafast studies using ionizing radiation, primarily electron beams, as the excitation source. Particle accelerators of every size and type consist of two major components, which are a particle source which is sometimes called an injector, and an acceleration stage. The job of the injector is to generate a particle beam with the desired position momentum relationship for acceptance into the acceleration section. Ultrafast accelerators require an injection system that deposits the entire electron bunch in a single RF period. The electron pulse is generated by gating the thermionic cathode of an electrostatic electron gun on and off within a few nanoseconds. Another type of injector developed in recent years for ultrafast accelerators is the radio-frequency photocathode electron gun. Radio-frequency (RF) photocathode electron guns consist of one or more conducting resonant cavities that are filled with several megawatts of microwave power to create transient electric field gradients of 80 to 100 MV/m.
In this review (124 refs), several problems in radiolysis of saturated hydrocarbons are examined. Special attention is paid to the chemistry of radical cations, high-mobility holes, excited state and spur dynamics, magnetic field and spin effects, and optically detected magnetic resonance spectroscopy.
This chapter reviews most recent data on radiation chemistry of proteins and provides an overview of the unknown aspects in protein radiation chemistry as well as in some of the expected biological consequences of protein radiolytic modifications. Under irradiation, proteins are affected by direct and indirect effects of ionizing radiations. When these macromolecules are in liquid solution, the indirect effects are predominant and the direct effects can be neglected. On the contrary, in a solid state, proteins are ionized mainly by direct interaction. Proteins in a solid state can be found in different forms, lyophilized or in frozen aqueous solution. Under irradiation, lyophilized proteins mostly aggregate. On the contrary, irradiation of frozen protein solutions gives rise to fragmentation. In an aqueous solution, the first step is the reaction of free radicals. This reaction proceeds with a rate constant that varies with the nature of the free radical but very little with the protein. The chemical nature of the resulting odd-electron site(s) can be hypothesized, but prediction of its location is still difficult. The reactivity of residues depends on many factors, including accessibility, neighboring residues, and electrostatic guidance.
AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
The radiation chemistry of deoxyribose nucleic acid (DNA) has generally been studied to gain a detailed understanding of the chemical modifications induced in DNA by ionizing radiation. This chapter focuses on the chemistry of DNA damage, how the DNA structure may alter the chemistry and distribution of products, and how this damage may influence the biological effects of ionizing radiation. To date, the majority of the information on radiation-induced DNA damage comes from the use of low linear energy transfer (LET) radiations, such as γ- radiation, hard X-rays, and high energy electrons. The latter type of radiation is generated by accelerators and is generally used in the technique of pulse radiolysis, which has provided most of the quantitative information on the reactivity and types of DNA radicals. Pulse radiolysis has provided the majority of information on the interactions of water radicals with DNA. Other techniques and in particular electron spin resonance (ESR) have provided the majority of information on the types of damage produced by direct energy deposition in DNA. Ionizing radiation randomly induces a variety of damages to cellular DNA. The most frequent types of DNA damage produced are single (ssb) and double (dsb) strand breaks, base and sugar modifications, and DNA-protein crosslinks.
