Advanced ESR Methods in Polymer Research - Hardcover

Hardcover

ISBN 10: 0471731897 ISBN 13: 9780471731894

Verlag: Wiley, 2006

Inhaltsangabe

A definitive work on ESR and polymer science by today's leading authorities



The past twenty years have seen extraordinary advances in electron spin resonance (ESR) techniques, particularly as they apply to polymeric materials. With contributions from over a dozen of the world's top polymer scientists, Advanced ESR Methods in Polymer Research is the first book to bring together all the current trends in this exciting field into one comprehensive reference.

Part I establishes the fundamentals of ESR, from experimental techniques to data analysis, and serves as a valuable overview for the beginning ESR student. Part II introduces the broad range of ESR applications to polymeric systems, including living radical polymerization, block copoly-mers, polymer solutions, ion-containing polymers, polymer lattices, membranes in fuel cells, degradation, polymer coatings, dendrimers, and conductive polymers. By exposing readers to the great potential of ESR, the authors hope to encourage more extensive application of these methods.

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Über die Autorin bzw. den Autor

SHULAMITH SCHLICK, DSc, is a Professor of Physical and Polymer Chemistry in the Department of Chemistry and Biochemistry, University of Detroit Mercy. One of the foremost authorities in the field of polymer research, and the editor of one previous book, Dr. Schlick has held visiting professorships and appointments worldwide and has authored over 200 scientific articles and book chapters.

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A definitive work on ESR and polymer science by today's leading authorities

The past twenty years have seen extraordinary advances in electron spin resonance (ESR) techniques, particularly as they apply to polymeric materials. With contributions from over a dozen of the world's top polymer scientists, Advanced ESR Methods in Polymer Research is the first book to bring together all the current trends in this exciting field into one comprehensive reference.

Part I establishes the fundamentals of ESR, from experimental techniques to data analysis, and serves as a valuable overview for the beginning ESR student. Part II introduces the broad range of ESR applications to polymeric systems, including living radical polymerization, block copoly-mers, polymer solutions, ion-containing polymers, polymer lattices, membranes in fuel cells, degradation, polymer coatings, dendrimers, and conductive polymers. By exposing readers to the great potential of ESR, the authors hope to encourage more extensive application of these methods.

The past twenty years have seen extraordinary advances in electron spin resonance (ESR) techniques, particularly as they apply to polymeric materials. With contributions from over a dozen of the world's top polymer scientists, Advanced ESR Methods in Polymer Research is the first book to bring together all the current trends in this exciting field into one comprehensive reference.

Part I establishes the fundamentals of ESR, from experimental techniques to data analysis, and serves as a valuable overview for the beginning ESR student. Part II introduces the broad range of ESR applications to polymeric systems, including living radical polymerization, block copoly-mers, polymer solutions, ion-containing polymers, polymer lattices, membranes in fuel cells, degradation, polymer coatings, dendrimers, and conductive polymers. By exposing readers to the great potential of ESR, the authors hope to encourage more extensive application of these methods.

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Advanced ESR Methods in Polymer Research

John Wiley & Sons

Copyright © 2006 John Wiley & Sons, Inc.
All right reserved.
ISBN: 978-0-471-73189-4

Chapter One

CONTINUOUS-WAVE AND PULSED ESR METHODS

Gunnar Jeschke Max Planck Institute for Polymer Research, Mainz, Germany

Shulamith Schlick University of Detroit Mercy, Detroit, Michigan

Contents

1. Introduction 3 2. Fundamentals of Electron Spin Resonance Spectroscopy 4 2.1. Basic Principles 4 2.2. Anisotropic Hyperfine Interaction and g-Tensor 10 2.3. Isotropic Hyperfine Analysis 12 2.4. Environmental Effects on g- and Hyperfine Interaction 12 2.5. Accessibility to Paramagnetic Quenchers 13 2.6. Line Shape Analysis for Tumbling Nitroxide Radicals 15 3. Multifrequency and High-Field ESR 16 4. Pulsed ESR Methods 18 Acknowledgments 22 References 22

1. INTRODUCTION

Electron spin resonance (ESR) is a spectroscopic technique that detects the transitions induced by electromagnetic radiation between the energy levels of electron spins in the presence of a static magnetic field. The method can be applied to the study of species containing one or more unpaired electron spins; examples include organic and inorganic radicals, triplet states, and complexes of paramagnetic ions. Spectral features, such as resonance frequencies, splittings, line shapes, and line widths, are sensitive to the electronic distribution, molecular orientations, nature of the environment, and molecular motions. Theoretical and experimental aspects of ESR have been covered in a number of books, and reviewed regularly.

Currently available textbooks and monographs are written for students and scientists that specialize in the development of ESR technique and its application to a broad range of samples. Nowadays, however, research groups are interested in a specific field of applications, such as polymer science, and apply more than one characterization method to the materials of interest. An introduction to ESR that targets such an audience needs to be shorter, less mathematical, and focused on application rather than methodological issues. This chapter is an attempt to provide such a short introduction on the application of ESR spectroscopy to problems in polymer science.

Organic radicals occur in polymers as intermediates in chain-growth and depolymerization reactions, or as a result of high-energy irradiation ([gamma], electron beams). Paramagnetic transition metal ions are present in a number of functional polymer materials, such as catalysts and photovoltaic devices. However, much of the modern ESR work in polymer science focuses on diamagnetic materials that are either doped with stable radicals as "spin probes", or labeled by covalent attachment of such radicals as "spin labels" to polymer chains. This chapter therefore treats the basic concepts that are required to understand ESR spectra of a broad range of organic radicals and transition metal ions, and describes more advanced concepts as applied to the most popular class of spin probes and labels: nitroxide radicals.

2. FUNDAMENTALS OF ELECTRON SPIN RESONANCE SPECTROSCOPY

2.1. Basic Principles

Spins are magnetic moments that are associated with angular momentum; they interact with external magnetic fields (Zeeman interaction) and with each other (couplings). In most cases, the Zeeman interaction of the electron spin is the largest interaction in the spin system (high-field limit). The electron Zeeman (EZ) interaction can generally be described by the Hamiltonian below,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where S is the spin vector operator, [B.sub.0] is the transposed magnetic field vector in gauss (G) or tesla (1 T = [10.sup.4] G), [.sub.e] is the Bohr magneton equal to 9.274 x [10.sup.-21] erg[G.sup.-1] (or 9.274 x [10.sup.-24] [JT.sup.-1]), and g is the g tensor. For a free electron, g is simply the number [g.sub.e] = 2.002319. The transition energy is then [DELTA]E = h]v.sub.mw] = [g.sub.e] [.sub.e][B.sub.0], where [B.sub.0] is the magnitude of the magnetic field. Typical values are [B.sub.0] [approximately equal to] 0.34 T (3400 G) corresponding to microwave (mw) frequencies of [approximately equal to] 9.6 GHz (X band), or [B.sub.0] [approximately equal to] 3.35 T corresponding to mw frequencies of [approximately equal to] 94 GHz (W band).

The g-value of a bound electron generally exhibits some deviation from [g.sub.e] that is mainly due to interaction of the spin with orbital angular momentum of the unpaired electron (spin-orbit coupling). Spin-orbit coupling is a relativistic effect that tends to increase with increasing atomic number of the nuclei that contribute atomic orbitals to the singly occupied molecular orbital. Therefore, g-values deviate more strongly from [g.sub.e] for transition metal complexes than for organic radicals. As the orbital angular momentum is quenched in the ground state of molecules, spin-orbit coupling comes about only by admixture of excited orbitals. Such admixture is stronger for low-lying excited states, which are relevant, for example, if the unpaired electron has high density at an oxygen atom. Oxygen-centered organic radicals thus tend to have higher g-values than carbon-centered ones.

As the orbital angular momentum relates to a molecular coordinate frame and the spin is quantized along the magnetic field (z axis of the laboratory frame), the g-value depends on the orientation of the molecule with respect to the field. This anisotropy can be described by a second rank tensor with three principal values, [g.sub.x], [g.sub.y], and [g.sub.z]. The corresponding principal axes define the molecular frame. In fluid solutions, molecules tumble with a rotational diffusion rate that is much higher than the differences of the electron Zeeman frequencies between different orientations. In this situation, the g-value is orientationally averaged and only its isotropic value [g.sub.iso] = ([g.sub.x] + [g.sub.y] + [g.sub.z])/3 can be measured. A good overview of isotropic g-values of organic radicals can be found in Ref. 23; Ref. 5 collects information on g tensors for transition metal complexes.

The real power of ESR spectroscopy for structural studies is based on the interaction of the unpaired electron spin with nuclear spins. This hyperfine interaction splits each energy level into sublevels and often allows the determination of the atomic or molecular structure of species containing unpaired electrons, and of the ligation scheme around paramagnetic transition metal ions. For a system with m nuclear spins (identified by index k) and a single electron spin, which may be larger than one-half as explained below, the hyperfine Hamiltonian is given in Eq. 2,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where the [I.sub.k] are nuclear spin vector operators and the [A.sub.k] are hyperfine tensors in frequency units (Hz). Each hyperfine tensor...

Details zur Ausgabe

• Verlag: Wiley
• Erscheinungsdatum: 2006
• Sprache: Englisch
• ISBN 10: 0471731897
• ISBN 13: 9780471731894
• Einband: Gebundene Ausgabe
• Auflage: 1
• Anzahl der Seiten: 368
• Herausgeber: Schlick Shulamith
• Kontakt zum Hersteller: Wiley-VCH GmbH; product_safety@wiley.com; ; Boschstrasse 12; Weinheim; 69469; Deutschland