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Role of Water in Electron-Initiated Processes and Radical Chemistry:  Issues and Scientific Advances

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Role of Water in Electron-Initiated Processes and Radical Chemistry:  Issues and Scientific Advances
  Role of Water in Electron-Initiated Processes and Radical Chemistry:Issues and Scientific Advances Bruce C. Garrett,* ,† David A. Dixon,* ,‡ Donald M. Camaioni, † Daniel M. Chipman, § Mark A. Johnson, # Charles D. Jonah, ⊥ Gregory A. Kimmel, † John H. Miller, | Thomas N. Rescigno, X Peter J. Rossky, 3 Sotiris S. Xantheas, † Steven D. Colson, † Allan H. Laufer, [ Douglas Ray, † Paul F. Barbara, 3 David M. Bartels, § Kurt H. Becker, ∆ Kit H. Bowen, Jr., × Stephen E. Bradforth, ] Ian Carmichael, § James V. Coe, + L. Rene Corrales, † James P. Cowin, † Michel Dupuis, † Kenneth B. Eisenthal, ∞ James A. Franz, † Maciej S. Gutowski, † Kenneth D. Jordan,*Bruce D. Kay, † Jay A. LaVerne, § Sergei V. Lymar, £ Theodore E. Madey, b C. William McCurdy, X Dan Meisel, § Shaul Mukamel, | Anders R. Nilsson, f Thomas M. Orlando, ¢ Nikolay G. Petrik, † Simon M. Pimblott, § James R. Rustad, ¶ Gregory K. Schenter, † Sherwin J. Singer, + Andrei Tokmakoff, ∀ Lai-Sheng Wang, | Curt Wittig, ] and Timothy S. Zwier I Chemical Science Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352; Department of Chemistry, Shelby Hall, University of Alabama, Box 870336, Tuscaloosa, Alabama 35487-0336; Notre Dame Radiation Laboratory, University of Notre Dame,Notre Dame, Indiana 46556; Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 0520-8107; Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439; Department of Computer Science and Department of Physics, 2710 University Drive,Washington State University, Richland, Washington 99352-1671; Lawrence Berkeley National Laboratory, 1 Cyclotron Road Mailstop 1-0472,Berkeley, California 94720; Department of Chemistry and Biochemistry, University of Texas at Austin, 1 University Station A5300,Austin, Texas 78712; Office of Basic Energy Sciences, U.S. Department of Energy, SC-141/Germantown Building, 1000 Independence Avenue,S.W., Washington, D.C. 20585-1290; Department of Physics and Engineering Physics, Stevens Institute of Technology, Castle Point on Hudson,Hoboken, New Jersey 07030; Department of Chemistry, Johns Hopkins University, 34th and Charles Streets, Baltimore, Maryland 21218; Department of Chemistry, University of Southern California, Los Angeles, California 90089-1062; Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210-1185; Department of Chemistry, Columbia University, Box 3107, Havemeyer Hall,New York, New York 10027; Department of Chemistry, University of Pittsburgh, Parkman Avenue and University Drive,Pittsburgh, Pennsylvania 15260; Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000; Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Road, Piscataway, New Jersey 08854-8019; Department of Chemistry,516 Rowland Hall, University of California, Irvine, Irvine, California 92697-2025; Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, 2575 Sand Hill Road, Mail Stop 69, Menlo Park, California 94025; School of Chemistry and Biochemistry, Georgia Institute of Technology, 770 State Street, Atlanta, Georgia 30332-0400; Geology Department, University of California, Davis, One Shields Avenue,Davis, California 95616-8605; Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue,Cambridge, Massachusetts 02139-4307; Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084 Received July 23, 2004  Contents  1. Introduction 3561.1. Importance of Electron-Driven Processes inAqueous Systems3561.2. Challenge of Understanding Electron-DrivenProcesses in Aqueous Systems3571.3. Current State of Understanding of AqueousSystems Relevant to Aqueous RadiationChemistry3592. Initial Excitation and Relaxation Processes 3622.1. Critical Research Issues 3622.2. Current Research Advances 3642.2.1. Electronic Structure of Aqueous Phases 3642.2.2. Electron − Water Scattering Cross Sections 3652.2.3. Dynamics of Electronically Excited States 3682.2.4. Relaxation and Reaction ProcessesOccurring under Highly NonequilibriumConditions3693. Reactions of Radicals and Ions in AqueousSolutions3723.1. Critical Research Issues 3723.1.1. Radical Diffusion 3733.1.2. Activated Radical Reactions with StableMolecules3733.1.3. Radical − Radical Combination Reactions 3743.1.4. Hydrated Electron-Scavenging Reactions 3743.1.5. Recombination of Hydrated Electrons 3753.1.6. More Complex Reactions 3753.2. Current Research Advances 3763.2.1. Structure and Energetics of TransientRadical and Ionic Species in AqueousEnvironments3763.2.2. Dynamics of Radical and Ionic Speciesand Coupling to Solvent Dynamics inAqueous Systems3793.2.3. Interfacial Processes 3834. Summary 3845. Acknowledgment 3856. Glossary of Acronyms and Terms 3857. References 386 * Authors to whom correspondence should be addressed (e-mailbruce.garrett@pnl.gov or dadixon@bama.ua.edu). † Pacific Northwest National Laboratory. ‡ University of Alabama. § Notre Dame Radiational Laboratory. #  Yale University. ⊥  Argonne National Laboratory. | Washington State University. X Lawrence Berkeley National Laboratory. 3 University of Texas. [ Office of Basic Energy Sciences, DOE. ∆ Stevens Institute of Technology. × Johns Hopkins University. ] University of Southern California. + The Ohio State University. ∞ Columbia University. O University of Pittsburgh. £ Brookhaven National Laboratory. b The State University of New Jersey. | University of California, Irvine. f Stanford Synchrotron Radiation Laboratory. ¢ Georgia Institute of Technology. ¶ University of California, Davis. ∀ Massachusetts Institute of Technology. I Purdue University. 355 Chem. Rev.  2005,  105,  355 − 38910.1021/cr030453x CCC: $53.50 © 2005 American Chemical SocietyPublished on Web 12/09/2004  1. Introduction   An understanding of electron-initiated processes inaqueous systems and the subsequent radical chem-istry these processes induce is significant in suchdiverse fields as waste remediation and environmen-tal cleanup, radiation processing, nuclear reactors,and medical diagnosis and therapy. We review thestate of the art in the physical chemistry and chemi-cal physics of electron-initiated processes in aqueoussystems and raise critical research issues and fun-damental questions that remain unanswered. 1.1. Importance of Electron-Driven Processes inAqueous Systems The study of the radiolysis of water has been anactive field for over 50 years 1 because of its impor-tance in nuclear reactors, storage of transuranic andhigh-level mixed wastes, industrial applications, biol-ogy, and medicine. Research to develop an under-standing of the consequences of electrons is at thecore of many research programs, especially thosesponsoredbytheU.S.DepartmentofEnergy,becausethe interactions of electrons strongly affect thefeasibility and possibilities for the best use of presentand future energy technologies.The importance of electron-driven processes inaqueous systems is exemplified by radiolytic molec-ular hydrogen generation in some of the 177 wastetanks at the Hanford Site in Washington state, aDepartment of Energy nuclear weapons productionfacility. 2 It is known that some of the hydrogen gasis the product of hydrogen atom recombination reac-tions and reactions between organic radiolysis prod-ucts. Some of the hydrogen also seems to be formedin fast processes following the primary energy depo-sition process, and these processes have not beenfully explained.Electron-driven chemistry is also important inwater-cooled nuclear reactors, where radicals andions that are created by ionizing radiation (e.g.,hydroxyl radicals) lead to corrosion of the reactorinfrastructure. An understanding of the reactionmechanisms for these radical and ionic species isneeded to devise approaches to mitigate their cor-rosive effects. For example, hydrogen is injected intothe reactor cooling system to inhibit the formationof H 2 O 2 . However, the amount of hydrogen that mustbe added to effectively quench these reactions ismuch higher than that which has been calculated bycurrentlyavailablemodels. 3 Recentevidencesuggeststhat this discrepancy may arise from an insufficientknowledge of the chemical reaction rates at highertemperatures. 4 Electron-driven processes in aqueous environmentsare also important because they generate radical andionic species that interact with macromolecules.Knowledge in this area is important for the under-standing of the impact of radiation exposure onbiological systems as well as to advancing the fieldsof nuclear medicine and radiation therapy. Recentstudies, which measured single- and double-strandbreaks induced in DNA by electrons with energieswell below molecular ionization thresholds, bring adifferent perspective to the source of damage inbiologically important molecules. 5 Water plays anobvious role in these reactions as it can lead toconformational changes in the macromolecules, for Bruce C. Garrett was born in Fort Knox, KY, in 1951 and received a B.S.in chemistry from the University of California, Irvine (1973) and a Ph.D.from the University of California, Berkeley (1977). He was a postdoctoralresearch specialist at the University of Minnesota from 1977 to 1979. Hewas a member of the scientific staff at Battelle Columbus Laboratoriesfrom 1979 to 1980 and cofounded a small contract research company,Chemical Dynamics Corp. (Columbus, OH), where he conducted basicresearch from 1980 to 1989. He joined Pacific Northwest NationalLaboratory in 1989 and is currently Laboratory Fellow and AssociateDirector for Molecular Interactions and Transformation in the ChemicalSciences Division. His research interests include theories of chemicalreactions, with focus on the effects of molecular environment (liquids,solids, and interfaces) on chemical reaction rates, molecular theories ofgas-to-particle nucleation, and molecular simulations of molecular pro-cesses at aqueous interfaces. He is an American Physical Society Fellow.David A. Dixon was born in Houston, TX, in 1949 and received his B.S.in chemistry from Caltech (1971) and his Ph.D. in physical chemistry fromHarvard University in 1976. He was a Junior Fellow, Society of Fellows,Harvard University, 1975 − 1977. After 6 years on the faculty at theUniversity of Minnesota, he joined DuPont Central Research andDevelopment at the Experimental Station, where he spent 12 years endingas a Research Fellow. In 1995, he became the Associate Director forTheory, Modeling and Simulation in the William R. Wiley EnvironmentalMolecular Sciences Laboratory, Pacific Northwest National Laboratory.In January 2004, he joined the Department Chemistry, University ofAlabama, where he is currently the Robert Ramsay Chair. His mainresearch interest is the use of numerical simulation techniques combiningelectronic structure theory and high-performance computing to solvechemical problems in the areas of catalysis, environmental chemistry,fluorine chemistry, and actinide chemistry. He has received a number ofawards including a Sloan Fellowship, a Camille and Henry DreyfusTeacher − Scholar fellowship, the 1989 ACS Leo Hendrik Baekeland Award,and the 2003 ACS Award for Creative Work in Fluorine Chemistry. He isa Fellow of the American Association for the Advancement of Scienceand the American Physical Society. 356  Chemical Reviews, 2005, Vol. 105, No. 1 Garrett et al.  example, A- to B-DNA. In addition, rapid relaxationprocesses in the solvent layers of DNA determine theelectronic states leading to stable damage products,such as strand scission and oxidized bases.Electron-driven chemistry at interfaces betweenaqueous solutions and solids is important in anumber of areas. For example, an understanding of this type of interfacial chemistry is crucial to under-standing corrosion, one of the major problems in thenuclear power industry. There is currently a lack of knowledge about the mechanisms for the transfer of energy, which is deposited by ionizing radiation intosolids, to interfaces where it can create radicals andions in solution. One example arises in the generationof H 2  in solid nuclear wastes such as those made fromcement. It was found that the H 2  was formed in thewater in the waste; however, it was clear that muchof the energy that generated this hydrogen came fromenergy deposition in the solid. 6 Utilization of thesetypes of reactions may make it possible to generatehydrogen for the hydrogen economy using radiationfrom waste fuels. Also, it may be possible to makeuse of the energy deposited in particles, in so-calledwide band-gap semiconductors, to efficiently decom-pose waste. This may be a particularly advantageousprocess, because charge separation can be enhanced.There have been considerable efforts recently todevelop new methods to destroy hazardous organicwastes. Techniques such as the irradiation of wastesor the use of supercritical water oxidation have beenproposed. 7 The possible enhancement of such de-struction by interfacial chemistry was discussedabove. In all destruction studies, one needs to un-derstand the chemistry that occurs in these processesbecause one must verify that the destruction of onehazardous material does not lead to the formation of a product that is equally as or more toxic than itspredecessor. For example, one must ensure that thedestruction of polychlorinated biphenyls (PCBs) doesnot lead to the formation of phosgene. Simulationsof supercritical water oxidation have assumed thatreactions proceed similarly to the reactions in the gasphase. 8 However, recent experimental data show thatsome of the fundamental reactions occur with greatlydiffering rates in the liquid phase (even in super-critical liquid) and in the gas phase. 4 Many biological systems and waste treatmentsystems are biphasic, with micelles, colloidal par-ticles, or liposomes encapsulating much of the im-portant chemistry. It is critical to understand howthese interactions occur. One example where electron-driven reactions are important is the potential useof TiO 2  for waste destruction. Irradiation of TiO 2 particles by light leads to the formation of holes andelectrons in the semiconductor particles. Electron-transfer reactions then can occur at the interfacebetween the particle and aqueous solution to reactwith solutes in aqueous solution. The goal is to applythis electron-driven reaction to destroy waste byusing the TiO 2  particles as photocatalysts.The one common thread that ties this researchtogether is that all processes are controlled by theinteractions of electrons with aqueous solutions andthe chemistry and physics that evolve from theseprocesses. Some of the important research questionsand technological challenges are as follows: •  How can we control the chemistry in nuclearreactors where reactions take place under extremeconditions, under high linear energy transfer (LET)radiolysis, and at interfaces? •  How is hydrogen gas generated in nuclear wastewhere radical reactions, interfacial chemistry, andprimary radiolytic processes dominate? •  How can we perform hazardous waste destructionusing supercritical water oxidation and radiolytictreatments in which radical, ionic, and interfacialchemistries are important? •  What is the impact of radicals produced in waterradiolysisandinterfacialchemistryonenvironmentalmonitoring and remediation? •  What are the biological effects of radiation towhich radical and ion chemistries contribute signifi-cantly? •  What are the roles of radical and ion chemistriesand interfacial processes in radiation therapy? •  What is the role of radical and ion chemistries inthe processing of radioactive materials? •  What are the roles of radicals and ions in thedeterioration of materials due to radiation damage?The processes initiated by electrons can also beused as tools in tackling other complex chemicalquestions, including  •  catalysis where interfacial chemistry induced byelectron-driven processes can be used to understandreactive sites and •  energy conversion and storage, for example, thedesign of hydrogen storage materials for the hydro-gen economy, where the well-controlled generationof ions can simplify the fundamental chemical stud-ies. 1.2. Challenge of Understanding Electron-DrivenProcesses in Aqueous Systems Radiolysisofwaterfromnaturalandanthropogenicsources leads to the formation of aqueous electrons.Because of the complexity inherent in treating elec-tron-driven processes in water, important questionsregarding the primary chemical events remain evenafter decades of inquiry. The excitation, relaxation,and reaction processes driven by electrons in aqueoussystems span a wide range of energies and timescales s from thermal energies up to tens of electron- volts and from femtoseconds to microseconds orlonger.Much of our current knowledge about the processesdriven by electrons in aqueous systems comes fromstudies of the radiolysis of water. The general mech-anism of the effects of high-energy radiation on liquidwater was known by the early 1970s. 9,10 Briefly, high-energy particles (such as 1 MeV electrons) create asparse track of ionization events in liquid water (i.e.,the track structure), which generate lower energysecondary electrons (i.e., a distribution of electronenergies with mean below 100 eV). The secondaryelectrons have sufficient energy to cause additionalionization events, which create copious quantities of low-energy electrons with mean energy below 10 eV.The secondary events occur in close proximity to the Role of Water on Electron-Initiated Processes and Radical Chemistry Chemical Reviews, 2005, Vol. 105, No. 1  357  primary event, resulting in nonhomogeneous distri-butions of ionizations and excitations called “spurs”.The electrons become hydrated, and H 2 O + dispropor-tionates to form H 3 O + and OH on time scales fasterthan 1 ps. The thermalized radical and ionic speciesgo on to react with water and other species in thesolution to form a variety of species (e.g., H 2  H 2 O 2 ,and OH - ). These results are summarized in Figure1.The difficulty in understanding electron-drivenreactions in aqueous media (and in fact all reactionsin aqueous media) begins with the media. Water isa highly polar, strongly hydrogen-bonding material,which means that one cannot easily treat the solventas a continuum for some properties. 11 In fact, anunderstanding of the electronically excited states of a water molecule in liquid water is still very frag-mentary.Understandingthedynamicsofhigh-energyprocesses in water is complicated by the breakdownof linear response theory, so the solvent response toa perturbation cannot be approximated by the rateof solvent fluctuations s an approximation that doeshold for many solvents. The importance of theseproblems was further highlighted with the efforts tocalculate the structure and spectrum of the hydratedelectron in liquid water. Early quantum calcula-tions, 12 which used an electron, several water mol-ecules, and a dielectric continuum, predicted that thespectrum was narrow and that the water moleculesnearest the electron were aligned with the moleculardipoles toward the charge. This result was in conflictwith results on similar hydrated electrons deter-mined by electron spin resonance (ESR) measure-ments. 13 The application of quantum path integraltechniques showed that the consideration of theentire solvent led to bond - dipole alignment andsuggested that the width of the absorption spectrumof the hydrated electron was due to multiple waterstructures around the electron. 14 Finally, results fromsophisticated, time-dependent quantum moleculardynamics calculations of the electron in water havebeen able to predict the spectrum and dynamics of the electron. 15 The physical and chemical processes and theirrespective (approximate) time scales are shown inFigure 2. The ultrafast physical processes include theionization and relaxation of the ions that are formedby the ionization process. The physiochemical stagecontinues through the evolution of highly excitedstates and their interactions with the solvent (sol- vation). Reaction can overlap these relaxation pro-cesses. The chemical processes occur both when theions and radicals are distributed nonhomogeneouslyin “clusters” of ionization and excitations (i.e., spurs)and homogeneously after the reactive species havediffused. 10 Electrons with kinetic energies of the order of  e 100eV play a pivotal role in the absorption of ionizing radiation by materials. The predominance of elec-trons in this energy range results from the long rangeof Coulomb interactions and the distribution of oscillator strengths of molecules. Even if the primaryradiation is not charged (neutrons and photons, forexample), high-velocity charged particles will begenerated as the primary radiation penetrates theabsorber. The swiftly moving ions transfer theirkinetic energy to electronic excitations through Cou-lomb interactions with large impact parameters. Inglancing collisions, the most loosely bound electronsof the stopping material receive an impulse that,through Fourier analysis, can be viewed as broad-spectrum photoexcitation. The oscillator strength of molecules peaks at ∼ 20 - 30 eV and decreases to verylow values of  ∼ 100 eV. Hence, for all radiation fields,the initial response of the stopping material is largelydetermined by the oscillator strength distribution of its valence electronic structure, which leads primarilyto the formation of electrons with energies of   < 100eV.The copious number of low-energy electrons gener-ated in the slowing of high-energy charged particlesmakes them the focus of studies to understand theearly stages of radiation chemistry. Electrons in therange of 30 - 100 eV will ionize more than onemolecule so that there will be localized regions of highconcentration of ionized species. For these reasonsit is crucial to understand the scattering processes,both elastic and inelastic, that such electrons un-dergo. These processes will determine the spatialdistribution of radicals and ions and thus determinethe nonhomogeneous chemistry that will occur.The determination of scattering cross sections withsub-100-eV electrons is difficult because the Bornapproximation, which relates the scattering crosssection to the optical absorption, no longer holds atthe lower energies. 16 The use of different approxima-tions for the scattering parameters leads to differ- Figure 1.  Initial processes in the decomposition of waterby ionizing radiation. Figure 2.  Approximate time scales of processes initiatedby ionizing radiation. 358  Chemical Reviews, 2005, Vol. 105, No. 1 Garrett et al.  ences in simulations of track structure. 17 Condensed-phase electron-scattering cross sections have beendirectly measured only for amorphous solid waterfilms, although total and momentum-transfer crosssections are available for liquid water. 18,19 In water vapor, the collision processes that remove energyfrom low-energy electrons have been studied by a variety of experimental techniques. These results,when translated to the liquid phase, provide qualita-tive agreement with experimental data (as discussedbelow). Femtosecond laser studies of electron local-ization, solvation, and reaction provide importantinsights into the fundamental processes of low-energyelectrons. 20 However, the energy distribution of elec-trons from lasers is almost certainly not the same asfrom ionizing radiation. Consequently, a substantialgap exists in our understanding of the processes thatinitiate radiation chemistry in condensed matter.Scattering data have been assembled to allowsimulations that predict the yield and spatial distri-bution of species such as the hydrated electron (e - aq ),the hydrated proton (H + aq ), OH, H, and H 2  in pureliquid water. 21,22 These simulations are based on amapping of inelastic collisions to primary specieswithout explicit consideration of relaxation processesand intermediate states. The accuracy of these mod-els is judged primarily on their ability to lay thegroundwork for understanding radiolysis experi-ments in the picosecond to microsecond time scales.These models can be expanded to radiation sourcessuch as neutrons or  R   particles, where the primaryionization events (of   ∼ 100 eV) are within a nano-meter of each other. 23 However, these models do notinclude many of the important fundamental pro-cesses, such as reactions of hydrated electron precur-sors and dynamics of excited states, that may wellbe more important when ionization density is higher.Traditional experimental approaches to studying electron-driven processes in aqueous systems (e.g.,electron-pulse radiolysis) have been instrumental inefforts to understand the initial processes in electron-driven reactions in liquid water. 24 However, theseapproaches do not provide a direct probe of many of the processes that occur on time scales of less thantens of picoseconds. Stochastic simulations have beenuseful for inferring more detailed information aboutthis mechanism; 21,22 however, these simulations re-quire fundamental information about the physicaland chemical processes, which is incomplete orcompletely lacking in some cases.There are almost no data that directly probe thedynamical processes that occur with highly excitedstates. In addition to the primary steps in waterradiolysis, including ionization and the production of highly excited states in water, it is clear that complexrelaxation takes place. For example, in water radi-olysis, the production of H - and H 2  and the interfer-ence with the production of the hydrated electron,which has been seen in both photolytic and radiolyticformations of electrons, show that it is important tounderstand the dynamics of the relaxation of highlyexcited states.Electron-driven reactions are important in systemsother than pure water. In biological systems, proteinsand DNA are solvated by water molecules thatappear to be differently structured (e.g., differenthydrogen bonding patterns) from water in the bulkof the solution. For example, these solvation effectscan affect the transfer of electrons in DNA, whichcan alter the denaturation of DNA. The structurecould be similar to that found in thin amorphouswater systems or similar to those in water - electronclusters. 25 These additional complexities can con-found the understanding of how electron reactionsoccur in water.Heterogeneous systems add to the complexity of understanding electron-driven reactions in water. 26 The presence of a surface will alter the structure of water near the surface. This change will occur formultiple reasons. For example, simply the existenceof an interface provides a nonisotropic environment,orients water molecules, and will break up thestructure of the water near the interface. In addition,strong interactions that can exist between the surfaceand water molecules will also change the waterstructure.Finally, water has a wide range of propertiesdepending on temperature and pressure. As has beenpreviously mentioned, almost all electron-scattering measurements have been done in the gas phase orin amorphous solid water. Chemistry in water nearroom temperature is clearly important, particularlyfor biological systems, and increasing focus is being placed on chemistry at high temperatures and pres-sures of water, including supercritical water. 27  As isdiscussed later in section 3, radical chemistry changesat high temperature and in supercritical water.Clearly, even minor changes in water structure/ internalenergycanbeimportantindefiningchemicalpathways.To summarize, the challenges to understanding electron-driven processes in water include •  the complexities of the aqueous medium itself; •  the unknown structure of excited and ionizedstates in liquid water; •  the highly excited states and the relaxation of these states that can occur in water; •  the complexities that arise from the existence of largepolymericmolecules,suchasproteinsandDNA,or heterogeneous materials, such as particles, cata-lysts, and surfaces; and • the effect on reactions due to the change of waterproperties that can occur by changing temperatureand pressure.However, as discussed in sections 2 and 3, newtheoretical and experimental techniques make itpossible to attack these problems. New theoreticaltechniques provide frameworks for understanding processes in aqueous systems, and new experimentaltechniques make it possible to probe these frame-works in even more exacting fashion. 1.3. Current State of Understanding of AqueousSystems Relevant to Aqueous RadiationChemistry Water can act in the reactions of interest both asachemicalparticipantandasamediumthatmodifiesthe chemical processes that occur due to its large Role of Water on Electron-Initiated Processes and Radical Chemistry Chemical Reviews, 2005, Vol. 105, No. 1  359
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