Electron Paramagnetic Resonance - ACS Publications

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Jan 1, 2009 - Theory and Practical Applications,. Second Edition by John A. ... The detection of electron magnetic resonance by Zavoiskii in the mid 1940s (1) ...
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Book & Media Reviews Electron Paramagnetic Resonance: Elementary Theory and Practical Applications, Second Edition by John A. Weil and James R. Bolton Wiley Interscience, John Wiley & Sons, Inc.: Hoboken, New Jersey, 2007. 664 pp. ISBN 978-0471754961. $159.50 reviewed by Ffrancon Williams

The detection of electron magnetic resonance by Zavoiskii in the mid 1940s (1) ushered in a golden age of physical and chemical applications. Perhaps no single book did more to stimulate this development of EPR spectroscopy than the classic text by Wertz and Bolton (2), which appeared in 1972. A revised version, with John A. Weil added as a co-author, was published by Wiley in 1994. This 2007 text is formally described as the second edition of the 1994 version. Wertz died shortly after the publication of the 1994 edition leaving Weil and Bolton as authors. In noting that the senior author ( JAW) takes most of the responsibility for the content of this 2007 version, the Preface refers to it at one point as the “third edition”, which of course is precisely how older readers will regard it. The main thrust of the book is decidedly on the physical aspects of EPR, so that it nicely complements the more chemical emphasis provided in the recent comprehensive text by Gerson and Hüber (3). As the authors remark, the 2007 edition does not differ dramatically from the 1994 version. The titles of the 13 chapters remain the same except for chapter 11, which now refers to the “Noncontinuous” instead of the “Time-Dependent” Excitation of Spins. Recent developments are generally accommodated by a few extra pages in each chapter. Thus, chapter 1 on Basic Principles of Paramagnetic Resonance has been expanded from 31 to 36 pages to introduce the topics of parallel-field EPR, time-resolved EPR, “computerology”, and EPR imaging. Chapter 2 on Magnetic Interactions is essentially unchanged while chapter 3 on Isotropic Hyperfine Effects has been expanded to include new sections on Deviations from the Simple Multinomial Scheme (3.7) and Some Interesting π-Type Free Radicals (3.9). Section 3.9 provides a useful corrective to the notion that the EPR method can detect and characterize almost any type of radical species. This welcome touch of realism is nicely illustrated by mentioning some of the difficulties regarding the use of EPR in studying the OH and hydrated electron (eaq−) intermediates generated in liquid water by high energy radiation. The problems of detecting the OH radical due to its high reactivity and g-tensor anisotropy are well known and duly noted, but perhaps more striking is the fact that the EPR characterization of eaq− still remains uncertain even after more than 40 years of study. However, in suggesting the possibility that the uninformative singlet EPR spectrum assigned to eaq− could well be coming from the neutral hydronium (H3O) radical, the authors make the surprising assertion that the latter would be a π-type radi-

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Cheryl Baldwin Frech University of Central Oklahoma Edmond, OK 73034

cal. In fact, H3O with its 9 valence electrons is a hypervalent radical and can be represented as an excess electron bound to a closed-shell H3O+ structure, such that the unpaired electron occupies a Rydberg 3s-type orbital rather than a 2p-orbital on oxygen. This description is confirmed by high-level theoretical calculations (4, 5) which also indicate that H3O would only have marginal stability, its dissociation into a hydrogen atom and H2O being exothermic with an estimated barrier (0.004 eV) that is much less than kT at room temperature (0.026 eV). On the other hand, the eaq− species is relatively stable in purified water at pH 8 and quite long lived (t½ = 500 µs) with respect to its transformation to the hydrogen atom (6). Moreover, eaq− has been shown to possess a unit negative charge (7), so its assignment to H3O or even a solvated H3O (8) must be regarded as extremely questionable. Chapters 4 and 5 deal with the basic anisotropic Zeeman energy (g) and hyperfine (A) interactions that must be considered in the analysis of solid-state EPR spectra. Although these chapters are again much the same as in the earlier editions, Section 4.9, Comparative Overview, has been added and briefly discusses the pros and cons surrounding the methodology and information derived from studies of single crystal and powder spectra. In chapter 5, the important topic of anisotropic 1H hyperfine coupling is again introduced via the VOH center in MgO but a much clearer example could have been used in this new edition. As it is, the illustrations in Fig. 5.1 are marred by the presence of additional lines in the spectra that are said to originate from other (related) defect centers. Although an effort has been made to mask these extra lines in the presented spectra, the general effect is inelegant and provides a needless distraction on a first reading of the subject. Chapters 6 through 8 contain only minor additions to the 1994 work. In chapter 6 on systems with S > ½, there are two brief new sections. Section 6.4 addresses the subject of Interacting Radical Pairs while Section 6.9 refers to the burgeoning literature on Modeling the Spin-Hamiltonian Parameters, with examples taken primarily from defect centers in crystalline quartz. Chapter 7 on Paramagnetic Species in the Gas Phase has likewise been expanded by Sections 7.8 on Reaction Kinetics and 7.9 on Astro-EPR, each consisting of a single paragraph. The diversity of radicals in outer space is illustrated by mentioning that in addition to the ubiquitous presence of H atoms that are readily detected by their well-known 1420.4 MHz signature emission, there exist exotic “coronium” ions in the solar corona. These are typified by Fe13+ with an aluminum-like 3s23p1 electron configuration and a 2P½ ground state that should be detectable by EPR. In chapter 8 on Transition-Group Ions, Section 8.6 on A Ferroelectric System has been added; it describes in a few lines how the V2+ ion can be used to probe the ferroelectric transition in potassium ferrocyanide trihydrate at 247 K. Chapter 9 on The Interpretation of EPR Parameters is essentially the same as that in the 1994 edition. An opportunity was clearly lost here to bring this material up to date. For example, in discussing the use of molecular-orbital (MO) theory

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Book & Media Reviews for estimating the spin distribution in σ-type organic radicals, reference is only made to the 1968 INDO program although it was supplanted in the 1990s by more accurate DFT (density functional theory) methods (9). Moreover, in suggestions for further reading on the relations between hyperfine splittings and spin densities, the preponderance of the references are to works in the 1960s except for the 1993 Atherton book (10) and the 2003 text by Gerson and Hüber (3). Appendix 9A on the use of Hückel MO calculations in π-electron systems is retained, but in Section 9.2 on π-type organic radicals there is no mention of the cases of symmetry-enhanced and symmetry-forbidden hyperconjugation in cyclic systems. Such effects are exemplified by the β-hydrogen couplings in the cyclohexadienyl and cyclobutenyl radicals where the symmetry of the SOMO is the determining factor and the usual McConnell–Heller equation (11) for the proportionality between the α-carbon spin density and β-hydrogen hyperfine coupling does not apply (12, 13). In fact, these EPR results that depend on the amplitude of the wave function provide a direct illustration of the operation of the superposition principle in quantum mechanics (14). The above omissions bring to mind that at least one major area of EPR applications is not covered here in any depth. This pertains to work carried out over the last four decades in the field of physical organic chemistry. There are, for example, no references to the extensive studies of G. A. Russell, K. U. Ingold, and J. K. Kochi that have contributed greatly to our detailed understanding of both the structure and the kinetic transformations of organic radicals. Another field that is completely neglected is that of radical cations derived from saturated molecules, despite a considerable level of activity during the 1980s and 1990s as represented particularly by the innovative work of M. Iwasaki on alkanes and of L. B. Knight, Jr. on water, methane, and other small molecules. Even more surprisingly, given the book’s emphasis on solid-state applications, there is hardly any mention of the considerable body of work on radiation damage to biological molecules such as DNA. Chapters 10 through 13 cover Relaxation Times, Noncontinuous Excitation, Double Resonance Techniques, and Other Topics, respectively. New sections include those on Longitudinal Detection (10.6), Dynamic Nuclear Polarization (10.9), BioOxygen Measurements (10.10), and Spin Coherence and Correlation (11.8). As before, chapter 13 lists comprehensive sets of literature references on a wide variety of 17 EPR-related topics, new additions being the introductory three-line Apologia (13.1) and almost three pages on Geologic/Mineralogic Systems and Selected Gems (13.10), the latter perhaps reflecting the particular interests of the senior author in EPR studies of minerals. Nine appendices (A through I) round out the book and these cover a wide range of background reference material. Discussions of mathematical operations (A) and quantum mechanics (B) are followed by an exact solution of the energy eigenvalues as well as the derivation of selection rules for the transitions of the hydrogen atom and related species (C). Appendix D is new and deals with the physical properties and magnetic resonance aspects of photons. This is followed by discussions of instrumentation (E) and experimental consid-

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erations (F). Finally, there are listings of the main EPR-related books and chapters (G); fundamental constants, conversion factors, and tables of atomic hyperfine constants (H); and a comprehensive 11-page glossary of symbols (I). Altogether these appendices provide an invaluable summary of the theoretical foundations of EPR as well as a mine of readily accessible information and key data. On the back cover, the authors claim to have brought this popular text up to date and that it provides a basic understanding of the underlying theory, fundamentals, and applications of EPR. It is therefore useful to ask how well they have succeeded in this daunting task. The text has certainly been brought to a higher level of thoroughness and topicality through the addition of 96 extra pages relative to the 1994 edition. On the other hand, one can point to instances where newer material with clearer spectra would have been preferable to the continuation of the original version with its strong 1960s vintage identification. Also, although the authors take pains to emphasize that the text is not meant to be comprehensive, the omission of some recent areas of application where there have been striking advances is regrettable. Even with these imperfections, however, it still remains the text of choice for beginning students of EPR spectroscopy, and the demanding problem sets make the book eminently suitable for graduate courses specializing in solid-state physics and chemistry. It can also be highly recommended to researchers seeking applications that would harness the potential of EPR spectroscopy to related fields of study. The authors are to be congratulated on producing a truly formidable EPR bible that lays claim once again to being the standard work on the subject. Despite the ever-growing list of titles (15, 16), it is unlikely to be seriously challenged by competitors in the foreseeable future. Literature Cited 1. Zavoiskii, E. J. Phys. U.S.S.R. 1945, 9, 211, 245; ibid. 1946, 10, 170. 2. Wertz, J. E.; Bolton, J. R. Electron Spin Resonance: Elementary Theory and Practical Applications, McGraw-Hill; New York, 1972. 3. Gerson, F.; Hüber, W. Electron Spin Resonance Spectroscopy of Organic Radicals; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2003. 4. Chen, F.; Davidson E. R. J. Phys. Chem. A 2001, 105, 10915– 10921. 5. Tachikawa, H.; Yamano T. Chem. Phys. Lett. 2001, 335, 305– 310. 6. Hart, E. J.; Gordon, S.; Fielden, E. M. J. Phys. Chem. 1966, 70, 150. 7. Czapski, G.; Schwarz, H. A. J. Phys. Chem. 1962, 66, 471. 8. Sobolewski, A. L.; Domcke, W. Phys. Chem. Chem. Phys. 2002, 4, 4–10. 9. (a) Batra, R.; Giese, B.; Spichty, M.; Gescheidt, G.; Houk, K. N. J. Phys. Chem. 1996, 100, 18371–18379. (b) Adamo, C.; Barone, V.; Fortunelli, A. J. Phys. Chem. 1994, 98, 8648. (c) Barone, V. Theor. Chim. Acta 1995, 91, 113. (d) Barone, V. J. Phys. Chem. 1995, 99, 11659. (e) Hermosilla, L.; Calle, P.; Garcia de la Vega,

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10. 11. 12. 13. 14. 15.

J. M.; Sieiro, C. J. Phys. Chem. A 2005, 109, 1114; J. Phys. Chem. A 2005, 109, 7626. Atherton, N. M. Principles of Electron Spin Resonance; PrenticeHall: New York, 1993. Heller, C.; McConnell, H. M. J. Chem. Phys. 1960, 32, 1535. Whiffen, D. H. Mol. Phys. 1963, 6, 223. Davies, A. G. J. Chem. Soc., Perkin Trans. 2 1999, 2461. Atkins, P. W. Quanta: A Handbook of Concepts; Clarendon Press: Oxford, 1974; p 231. Brustolon, M. R. Principles and Applications of Electron Paramagnetic Resonance Spectroscopy; Blackwell Publishers: Oxford, 2008.

16. Lund, A.; Shiotani, M. Principles and Applications of Electron Spin Resonance; Springer-Netherland: Dordrecht, 2009.

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Ffrancon Williams is a member of the Department of Chemistry, University of Tennessee, Knoxville, TN 37996; williams@ ion.chem.utk.edu.

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