Concepts in Magnetic Resonance Part A最新文献

The SPI/SPRITE class of techniques in magnetic resonance imaging are pure phase encode methods that are well established for systems with short transverse signal lifetimes. Applying a broadband radio-frequency pulse in the presence of a magnetic field gradient is unconventional in MRI but fundamental to these methods. Ordinarily, it is assumed that the excitation is instantaneous and any possible phase evolution during the RF pulse is ignored. High quality, quantitative imaging of a variety of samples over many years suggests that the off-resonance effects of the RF pulse, with consequent phase accumulation during the pulse, are not significant. However, a reconsideration of the RF pulse behavior in related work has shown that phase accumulation during the pulse may be non-negligible in some circumstances.

The effect of phase accumulation during the RF pulse is investigated through simulation of one-dimensional SPI experiments and is shown to manifest as a systematic scaling of the image field-of-view (FOV). The FOV scaling effect is also verified experimentally. One-dimensional profiles of a cylindrical elastomer sample were acquired employing a 2.4 T horizontal bore magnet. Experiments were undertaken with variation of the experimental RF pulse duration. Under typical experimental parameters, neglecting the phase accumulation during the RF pulse is acceptable.

The use of frequency-hopped pulse pairs and pulse pair trains for exciting NMR signals is introduced. Pairs of pulses, each pair lasting one nutation period about the effective field in the rotating frame, can be used to efficiently tip nuclear spin magnetization. In the limit where the frequency difference between the partners in a pair is large, the magnetization dynamics mimic those of an ordinary rectangular rf pulse with rf amplitude scaled down by a factor of π/2. When the amplitude of the applied rf begins to approach the offset frequency however, the excitation bandwidth broadens dramatically in a manner reminiscent of a composite pulse. The effects of the frequency-hopped pulse pairs is described with analytical calculations and numerical simulations, and are shown to agree well with experiment.

Solid-state nuclear magnetic resonance (SSNMR) spectroscopy has largely overtaken nuclear quadrupole resonance (NQR) spectroscopy for the study of quadrupolar nuclei. In addition to information on the electric field gradient, SSNMR spectra may offer additional information concerning other NMR interactions such as magnetic shielding. With continued technological advances contributing to developments such as higher magnetic fields, SSNMR boasts several practical advantages over NQR. However, NQR is still a relevant technique, as it may often be the most practical approach in cases of extremely large quadrupolar coupling constants. Here, we discuss the advantages and disadvantages of SSNMR and NQR spectroscopies, with the quadrupolar halogens serving as examples. The purpose of this article is to serve as a guide on using SSNMR and NQR as complementary tools, covering some of their practicalities, limitations, and experimental challenges.

Understanding the dynamics of electron or nuclear spins during a magnetic resonance experiment requires to solve the Schrödinger equation for the spin system considering all contributions to the Hamiltonian from interactions of the spins with each other and their surroundings. In general, this is a difficult task as these interaction terms can be both time-dependent and might not commute with each other. A powerful tool to analytically approximate the time evolution is average Hamiltonian theory, in which a time-independent effective Hamiltonian is taking the place of the time-dependent Hamiltonian. The effective Hamiltonian is subjected to the Magnus expansion, allowing to calculate the effective Hamiltonian to a certain order. The goal of this paper is to introduce average Hamiltonian theory in a rigorous but educational manner. The application to two composite pulses in NMR spectroscopy is used to demonstrate important aspects of average Hamiltonian theory.

Electron paramagnetic resonance spectra of nitroxide radicals are reporters of molecular tumbling correlation times. The concepts of electron paramagnetic resonance and molecular tumbling are demonstrated by examination of spectra of the radical tempol (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) in water:glycerol mixtures. Analysis of the spectral line shapes with the Kivelson model and computer simulation with EasySpin are discussed. Values of the tumbling correlation times obtained by the two methods are shown to be in good agreement. Comparison of the experimental tumbling correlation times with values calculated with the Stokes-Einstein model indicates a slip coefficient of 0.066, which is consistent with other reports for nitroxides in various solvents.

Alex D. Bain was an exceptional NMR spectroscopist who played an important role in the development of modern NMR methods and whose keen intellect and wonderful personal qualities endeared him to faculty and students alike who often sought him out for NMR advice. In this brief recollection, I will focus on a number of seminal contributions that Alex made that greatly influenced my research, illustrating how they changed the practice of modern NMR spectroscopy and laid the foundation for new experiments that are currently in widespread use.

In this article, we discuss the progress achieved with the use of evolutionary algorithms for the analysis of ¹H Nuclear Magnetic Resonance spectra of solutes in orientationally ordered liquids. With these tools the analysis of extremely complex spectra that were hitherto impossible to solve has now become eminently feasible. We discuss applications to 2 molecules of special interest: (a) hexamethylbenzene, which is a text book example of steric hindrance between adjacent rotating methyl groups; and (b) cyclohexane which is the standard example of interconversion between various molecular conformations. New interesting physics is obtained in both cases.

An unusually large geminal coupling has been observed in a stereochemically rigid system that permits a favorable coupling alignment between the methylene protons and an adjacent carbon-carbon double bond. Such couplings have been found to depend on both the alignment of a line connecting the geminal hydrogens and the nodal plane of the adjacent double bond, and the orientation of lone-pair electrons on adjacent heteroatoms. Electron withdrawal from symmetrical bonding orbitals results in more slightly positive geminal coupling constants while that from antisymmetrical bonding orbitals produces more negative coupling constants, which has been found to be very large in certain rigid systems.