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Chapter

Cover Electron Paramagnetic Resonance

1Advanced EPR techniques  

This chapter explains the basic theory of continuous wave (CW) electron paramagnetic resonance (EPR), illustrating the power of the technique to study a wide range of paramagnetic systems. It cites several experiments based on pulsed techniques similar to those routinely employed in nuclear magnetic resonance (NMR) spectroscopy. It also talks about how Pulse EPR can offer significant advantages over CW methods, such as direct detection of relaxation times and access to longer distances between paramagnetic centres. The chapter talks about the independent control of the electron and nuclear spins via the application of short microwave (MW) and radiofrequency (RF) pulses. It presents the vector model and product operator formalism used in pulse techniques.

Chapter

Cover Electron Paramagnetic Resonance

Anisotropic EPR spectra in the solid state  

This chapter explores the origins of the anisotropies in g and A for a spin. It explains how symmetry derived anisotropies in the solid state are manifested through g and how the interpretation of this tensor provides valuable information on the symmetry of the paramagnetic centre. It also concentrates on the lineshapes for powder spectra and the origins of the hyperfine A tensor. The chapter considers the electron paramagnetic resonance (EPR) spectra of a paramagnetic vanadyl and presents the theory explaining the origins of anisotropies. It describes a tensor as a mathematical object that illustrates a physical property and outlines the rank of the tensor that depends on the number of directions needed to describe that property.

Chapter

Cover Electron Paramagnetic Resonance

A brief overview of Electron Paramagnetic Resonance spectroscopy  

This chapter provides a background on electron paramagnetic resonance (EPR) spectroscopy, which is a magnetic resonance technique used for the study of systems containing unpaired electrons. It explains how EPR systems are paramagnetic and attracted by magnetic fields. It also reviews the applications of EPR in a wide variety of gaseous, liquid, and solid samples and confined to systems bearing unpaired electrons. The chapter outlines basic principles and the underlying physics of EPR. These are similar to those encountered in nuclear magnetic resonance (NMR). It points out how EPR and NMR techniques deal with the interaction of electromagnetic radiation with inherent magnetic moments within the sample.

Chapter

Cover Nuclear Magnetic Resonance

Chemical exchange  

P.J Hore

This chapter evaluates the theory of chemical exchange, which is straightforward compared to the complex computations required to obtain chemical shifts and J-couplings. Chemical exchange effects in nuclear magnetic resonance (NMR) arise from dynamic chemical and conformational equilibria. The chapter studies the cases of symmetrical two-site exchange and unsymmetrical two-site exchange. NMR lines are broadened by slow exchange. Meanwhile, differences in the NMR frequencies of exchanging spins (δν) can be averaged by fast exchange. The magnitude of δν relative to the exchange rate constant(s) determines whether the exchange is 'slow' or 'fast'. Ultimately, chemical exchange effects give information on the rates and mechanisms of chemical reactions, molecular rearrangements, and internal motions.

Chapter

Cover Nuclear Magnetic Resonance

Chemical shifts  

P.J Hore

This chapter discusses chemical shifts. These give information on molecular identity and structure. The nuclear magnetic resonance (NMR) frequency of a nucleus in a molecule is determined principally by its magnetogyric ratio and the strength of the magnetic field it experiences. Chemical shifts arise because the field actually experienced by a nucleus in an atom or molecule differs slightly from the external field produced by the magnet. A magnetic field can induce two kinds of electronic current in a molecule: diamagnetic and paramagnetic. Diamagnetic and paramagnetic currents flow in opposite directions and give rise to nuclear shielding and deshielding, respectively. Chemical shifts can often be understood by considering the effects of electron donating and withdrawing groups, induced currents in neighbouring groups, charged or polar groups, hydrogen bonds, and unpaired electrons.

Chapter

Cover NMR: THE TOOLKIT

Density matrices  

Introduction The introduction of operator exponentials provides a huge simplification in describing the evolution of NMR spin states, but the solution is still given as a vector of coefficients, which has to be further interpreted to obtain information about the observable NMR signal. In this...

Book

Cover Electron Paramagnetic Resonance

Victor Chechik, Emma Carter, and Damien Murphy

Electron Paramagnetic Resonance starts off with an overview of electron paramagnetic resonance (EPR) spectroscopy. The first chapter presents the theory of continuous wave (CW) EPR spectroscopy. The text then goes on to look at experimental methods in CW EPR and isotropic EPR spectra of organic radicals. It also examines anisotropic EPR spectra in the solid state and transition metal ions and inorganic radicals. The last few chapters look at systems with multiple unpaired electrons, linewidth of EPR spectra, and advanced EPR techniques.

Chapter

Cover Electron Paramagnetic Resonance

Experimental methods in CW EPR  

This chapter reviews the fundamental basis of electron paramagnetic resonance (EPR) spectroscopy involving the observation of electron spin transitions in the presence of a magnetic field. It discusses the continuous wave (CW) technique, wherein a continuous source of microwave (MW) radiation of fixed frequency is applied to the sample mounted in a cavity or resonator to induce spin transitions. It also shows the detection of the absorbed MW energy, which is a function of the magnetic field resulting in the observed EPR signal. The chapter focuses on the essential hardware components required to perform a CW EPR experiment, including the microwave bridge, the resonant cavity, the external magnet, and the console. It looks at different functions of hardware components and explains how to optimize the EPR signal through judicious selection of the instrumental parameters.

Chapter

Cover NMR: THE TOOLKIT

Fourier transform NMR  

Introduction This chapter reviews some of the basic elements of Fourier transform NMR spectroscopy, including a brief introduction to two-dimensional NMR. We shall not refer to specific experiments, but will attempt to lay the foundations for the more detailed discussions that follow. As with the...

Chapter

Cover Mass Spectrometry

Interpretation of mass spectra  

This chapter describes the resulting fragmentation pattern seen in the mass spectrum or MS/MS spectrum, which can be used to provide information about the structure of the molecule. It explains how fragmentation data can be interpreted and outlines the fragmentation of small organic molecules using EI-MS. It also introduces the basic types of fragmentation process and presents the diagnostic ions and neutral losses seen for different organic structures and functional groups. The chapter summarize the dissociation of protonated and deprotonated molecules ([M+H]+/[M–H]-), such as those formed by ESI, MALDI, or CI. It discusses the fragmentation of small molecules, including peptides and other large biomolecules.

Chapter

Cover Mass Spectrometry

Introduction  

This chapter explains that mass spectrometry (MS) is an analytical technique that forms ions from atoms or molecules and measures their mass-to-charge ratios. It points out how MS provides information about molecular and elemental composition and quantifies the abundance of individual chemical components. It also deals with ions which can be accelerated, deflected, and deaccelerated in electric and magnetic fields. These are crucial features for the operation of a mass spectrometer. The chapter emphasizes the formation of a molecule into a gas-phase ion which should be stable long enough to reach the detector of the mass spectrometer. It discusses the important part of the history of MS, which is the development of instrumentation that is capable of measuring compounds with a wider range of chemical and physical properties.

Chapter

Cover Nuclear Magnetic Resonance

Introduction  

P.J Hore

This introductory chapter provides an overview of nuclear magnetic resonance (NMR). Molecules are inconveniently small and difficult to observe directly. To learn about their structures, motions, reactions, and interactions we need microscopic spies able to relay information on their molecular hosts without significantly perturbing them. The spies that form the subject of this book are atomic nuclei, and the attribute that makes them successful at espionage is their magnetism. The chapter then looks at spin angular momentum and nuclear magnetism. In a magnetic field, the energy levels are split apart by an amount proportional to the size of the nuclear magnetic moment and the strength of the field. NMR spectroscopy uses electromagnetic radiation to cause transitions between the energy levels.

Chapter

Cover Electron Paramagnetic Resonance

Isotropic EPR spectra of organic radicals  

This chapter explains how to build splitting diagrams, measure hyperfine constant values and simulate the spectra, and what structural information can be obtained from these data. It discusses strategies for the generation and detection of short-lived organic radicals. It also recounts the anisotropic interaction that averages out to zero for rapidly tumbling radicals, such as small organic radicals in low viscosity solvents. The chapter describes the commonly used microwave frequencies, wherein the energy of the hyperfine interaction for organic radicals which is much smaller than the energy of the electron Zeeman interaction. It highlights the hyperfine interaction with several equivalent nuclei which is considered in the same way as the interaction with nonequivalent nuclei.

Chapter

Cover Electron Paramagnetic Resonance

Linewidth of EPR spectra  

This chapter highlights the importance of linewidth as a parameter in electron paramagnetic resonance (EPR) spectroscopy, describing narrow lines that are saturated or distorted if incorrect spectrometer parameters are chosen. It describes broad lines as difficult to distinguish from the background, particularly if the intensity is not very high. It also analyses linewidth measurements that provide important information about the structure and dynamics of the spin system. The chapter looks at a few important relaxation mechanisms that contribute to the linewidth. It illustrates how it is not possible for a complex spectra, such as anisotropic patterns, to extract the linewidth directly and notes that information about linewidth can be obtained through spectral simulations.

Book

Cover Mass Spectrometry

James McCullagh and Neil Oldham

Mass Spectrometry firstly introduces this topic. The next chapter looks at methods of ionization. Chapter 3 covers methods of mass analysis. The next chapter looks at resolution, accurate mass, and sensitivity. Then the text turns to look at tandem mass spectrometry. It also offers an interpretation of mass spectra and separation techniques and qualifications. Finally, the text looks at mass spectrometry applications.

Chapter

Cover Mass Spectrometry

Mass spectrometry applications  

This chapter reviews the ability of mass spectrometry (MS) for identifying and quantifying individual compounds from complex samples with high sensitivity. This has led to a wide range of physical, chemical, biological, medical, and industrial applications. It cites examples that have benefited from technical advances in sensitivity, selectivity, and the ability to process mass spectrometric data more rapidly and at a larger scale. It also points out how developments in MS have enabled the identification and quantification of proteins and metabolites in biological samples and metabolomics. The chapter deals with small molecule applications, particularly those related to the environment, sports, biology, and medicine. It focuses on the characterization of larger molecules with an emphasis on the analysis of proteins and mass spectrometry imaging.

Chapter

Cover Mass Spectrometry

Methods of ionization  

This chapter examines the most common ionization techniques in mass spectrometry (MS), demonstrating the process of forming positively or negatively charged ions from analyte molecules. It mentions how ions can be electrically or magnetically manipulated inside the high vacuum of a mass spectrometer to facilitate mass measurement. It also highlights different mechanisms of ion formation, basic ion-source designs, their relative performances, and areas of application. The chapter explores the development of MS as a pre-eminent analytical technique, including its applications in modern physical, biological, and medical sciences. These have been driven by the development of new ionization methods. It covers ionization methods that are distinguished by the physical characteristics of the ionization process, such as desorption ionization and atmospheric pressure ionization.

Chapter

Cover Mass Spectrometry

Methods of mass analysis  

This chapter looks at the basic principles of operation of the most common mass analyzer types, which require physical and mathematical treatment while maintaining sufficient detail to understand how they function. It focuses on analysers that require an analyzer-dependent high vacuum and span the range of range 10-3 to 10-10 mbar. It also considers different analyzer types which possess different properties and performance characteristics, such as size, sensitivity, resolving power, m/z range, scan time, duty cycle, and cost. The chapter discusses beam analysers and trapping analyzers, which are the two main classes of mass analyzers. It mentions modern commercial mass spectrometers that output the mass spectrum automatically on the m/z scale.

Chapter

Cover Nuclear Magnetic Resonance

NMR experiments  

P.J Hore

This chapter explains how nuclear magnetic resonance (NMR) experiments work. Modern NMR spectroscopy is much more than simply recording a spectrum and interpreting the positions, widths, and intensities of the lines. The spins can be manipulated to tailor the information that appears in the spectrum. The chapter begins by introducing the 'vector model'. Although it has its origin in the quantum mechanics of spin angular momentum, it has the distinct advantage of being pictorial and essentially non-mathematical. The disadvantage is that it only helps us to understand the simplest NMR experiments. The chapter then uses the vector model to discuss two techniques for measuring spin relaxation times and an important method for sensitivity enhancement (Insensitive Nuclei Enhanced by Polarization Transfer or INEPT).

Book

Cover NMR: THE TOOLKIT

P. J. Hore, J. A. Jones, and S. Wimperis