The Dynamics of Nuclear Spins

The dynamics of nuclear spins can be treated by a time-dependent Schrodinger equation, where the Hamiltonian contains terms for each constituting relaxation mechanism ... 

In-depth treatments of the topic are available in several books [1-6] and in a large number of review articles. The monograph by Dong [6], for example, focuses on aspects like the dynamics of nuclear spins, orientational order, molecular field theories, nuclear spin relaxation theory, director fluctuation and spin relaxation, rotational and translational dynamics, internal dynamics of flexible mesogens, and multiple-quantum and two-dimensional NMR, topics that will be touched upon very briefly here. Re-... 

The dynamics of nuclear spins in magnetic fields and their manipulation are the underlying common theme in many new developments in NMR. We hope that the reader will enjoy the range of aspects covered in our book. Furthermore, we would like to thank all the contributing authors for their valuable contributions. 

NMR spectroscopy is a powerful technique to study molecular structure, order, and dynamics. Because of the anisotropy of the interactions of nuclear spins with each other and with their environment via dipolar, chemical shift, and quadrupolar interactions, the NMR frequencies depend on the orientation of a given molecular unit relative to the external magnetic field. NMR spectroscopy is thus quite valuable to characterize partially oriented systems. Solid-state NMR... 

Nuclear magnetic resonance (NMR) is a widely utilized technique, which detects the reorientation of nuclear spins in a magnetic field. It can potentially be used to determine the 3-D structure of the protein itself, as well as supplying information on kinetics and dynamics, ligand binding, determination of pK- values of individual amino acid residues, on electronic structure and magnetic properties, to mention only some of the applications. In addition, it can be selectively applied to specific nuclei—1H, 13C, 15N, 19F (often substituted for H as a... 

Nuclear magnetic resonance (NMR) spectroscopy is a nonin-vasive and nondestructive spectroscopic technique that allows determination of the constitution and relative configuration of molecules, the characterization of the dynamic three-dimensional (3D) conformation of molecules, and their interaction with other molecules. NMR spectroscopy detects the characteristics of nuclear spins the most commonly studied nuclei are the spin-i/z-particles H, N, and NMR observables... 

A well-known and important phenomenon in the area of nuclear-spin resonance (NMR) in gases, liquids, or solid samples is dynamic nuclear-spin polarisation (DNP) (see e.g. [M6]). This term refers to deviations of the nuclear magnetisation from its thermal-equilibrium value, thus a deviation from the Boltzmann distribution of the populations of the nuclear Zeeman terms, which is produced by optical pumping (Kastler [31]), by the Overhauser effect [32], or by the effet solide or solid-state effect [33]. In all these cases, the primary effect is a disturbance of the Boltzmann distribution in the electronic-spin system. In the Overhauser effect and the effet solide, this disturbance is caused for example by saturation of an ESR transition. Owing to the hyperfine coupling, a nuclear polarisation then results from coupled nuclear-electronic spin relaxation processes, whereby the polarisation of the electronic spins is transferred to the nuclear spins. 

Chemically Induced Dynamic Nuclear Polarization (CIDNP) This term has been used to describe the enhancement of nuclear spin polarization observed In the NMR spectra of compounds undergoing radical reactions. Some exciting applications are described In Chapter X. 

Nuclear magnetic resonance (NMR) is the responses of nuclear spins to external radiofrequency (RF) stimulations, or the absorption and reemission of RF pulses by nuclear spins in a magnetic field. NMR techniques are developed to utilize this NMR phenomenon for the characterization of structures and dynamic properties of molecular systems, and for the identification and visualization of molecules and distributions. In the field of PEMFCs, the NMR techniques are frequently applied to the development and improvement of essential materials, proton exchange membrane and electrocatalysts, and the water management of PEMFCs. 

Li and Be work has gained from the application of the stimulated-echo spectroscopy to study the ultra-slow dynamics of nuclear spin-3/2 probes. Apart from the dominant first-order quadrupolar interaction, the impact of the homonuclear dipolar interactions was also considered. Explicit analytical expressions describing various aspects of a coupled quadupolar pair subjected to a Jeener-Broekaert pulse sequence have been derived. Extensions to larger spin systems are also briefly discussed. These results are compared with experimental data on a single-crystalline Li ion conductor. 

Chapter 2 focused on the evolution of a nuclear spin system without examining how it achieves thermal equilibrium with the lattice by energy exchange. The lattice consists of all degrees of freedom, except those of the nuclear spins, associated with molecular rotations and translations in physical systems such as liquid crystals. Spin-lattice relaxation describes how the system of nuclear spins evolves towards thermal equilibrium with the large heat reservoir, the lattice. The spin relaxation rates with which the nuclei arrive at their equilibrium magnetization may be experimentally determined. There is a well-defined connection between the relaxation rates and the dynamics of the lattice provided that the coupling interactions between the nuclear spin system and the lattice are known. Thus, nuclear spin relaxation may be used to study motional processes in molecular systems. 

The phenomenon of chemically induced dynamic nuclear polarization (CIDNP) consists of the manifestation of unusual line intensities and/or phases of signals of radical reaction products in the NMR spectrum when reaction takes place directly in the probe of the spectrometer. These anomalous lines (enhanced absorption or emission of NMR signals), which reflect the populations of nuclear spin states deviating from the Boltzmann condition, are observed within the time range of nuclear relaxation times of the diamagnetic molecules (T, ), which are as a rule, several seconds to tens of seconds. Subsequently, the NMR spectrum re-acquires its usual form. In 1967, two research groups in Europe (J. Bargon, H. Fischer, and U. Johnson) and the USA (H. Ward and R. Lawler) discovered independently that this phenomenon is directly associated with the free radicals involved in the process. Later on, it was shown that this also pertains to radical ions and triplet excited states of molecules. 

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