Excitation of magnetization using a modulated radiation damping field

In this work, pulsed-field gradients are used to modulate the radiation damping field generated by the detection coil in an NMR experiment in order that spins with significantly different chemical shifts can affect one another via the radiation damping field. Experiments performed on solutions of acetone/water and acetone/DMSO/water demonstrate that spins with chemical shift differences much greater than the effective radiation damping field strength can still be coupled by modulating the radiation damping field. Implications for applications in high-field NMR and for developing sensitive magnetization detectors are discussed.     

Sizable concentration-dependent frequency shifts in solution NMR using sensitive probes

With the growing use of high fields and ultrasensitive probes, radiation damping emerges as a significant feedback interaction in modern solution NMR. Motivated by recent observations of mysterious concentration-dependent frequency shifts, experiments carried out on a cryoprobe at 600 MHz have revealed a time-averaged frequency shift of up to +83/-81 Hz. The sizable frequency shifts arise from deviations in the phase of the radiation damping field from perfect orthogonality relative to the net transverse magnetization. The frequency shift is shown to depend on the longitudinal magnetization and probe tuning conditions through experiments and numerical simulations. Such unexpected shifts in the solvent precession frequency provide a physical explanation for the empirical practice of adjusting the irradiation frequency of the saturating B1 field in solvent presaturation to achieve optimal suppression. Additional applications of the radiation damping induced frequency shift to solvent suppression and NMR methodology are discussed.     

Enricher for fraction of high-pressure liquid chromatography

A device is developed for concentrating a dilute solution without losing the components with boiling points slightly higher than the solvent. The device consists of an evaporator, receptor, and approximately 100 capillaries. A dilute solution is introduced into the evaporator and heated at a lower temperature than the boiling point of the solvent with the addition of a helium gas flow. As a result, mostly only the solvent evaporates, passes through the capillaries, and enters into the receptor. The low-boiling-point components in the solute, with boiling points slightly higher than the solvent, are trapped at the inlet of the capillaries. These components are then recovered by a small amount of solvent supplied from the receptor through the capillaries, with the main components of the solute concentrated at the bottom of the evaporator. A diesel fuel is separated into aliphatic and aromatic fractions by high-pressure liquid chromatography using a silica gel column. These fractions are then analyzed by low-resolution field ionization mass spectrometry, following concentration using the described device. The analytical results show that the final composition of the fractions is almost the same as that of the aliphatic or aromatic hydrocarbons in the original fuel.     

Numerical and experimental studies of long-range magnetic dipolar interactions

We describe several numerical methods developed to analyze the behavior of spin polarized liquids in the presence of long-range magnetic dipolar interactions and external field gradients. Two of the methods use a discrete lattice of spins. In the first we calculate the magnetic field from the lattice of spins directly, either in the rotating frame, or in the lab frame. In the second method we include the dipolar fields from linear magnetization gradients analytically and calculate the dipolar fields from higher order gradients in Fourier space, where they are a local function of the magnetization. In the third method the magnetization is expanded in a Taylor series and the dipolar fields are calculated analytically for each term. The results of these calculations are compared to experimental data, in which we use two superconducting quantum interference device magnetometers adjacent to a spherical sample of hyperpolarized liquid 129Xe to detect the evolution of magnetization gradients. In particular, we observe an increase by a factor of 100 of the spin dephasing time in a longitudinal magnetic field gradient due to dipolar interactions of the spins. While each of the numerical techniques has certain limitations, they are generally in agreement with each other and with experimental data.     

Measurement of carbon-proton dipolar couplings in liquid crystals using DAPT

Dipolar couplings provide valuable information on order and dynamics in liquid crystals. For measuring heteronuclear dipolar couplings in oriented systems, a new separated local field experiment is presented here. The method is based on the dipolar assisted polarization transfer (DAPT) pulse sequence proposed recently (Chem. Phys. Lett. 2007, 439, 407) for transfer of polarization between two spins I and S. DAPT utilizes the evolution of magnetization of the I and S spins under two blocks of phase shifted BLEW-12 pulses on the I spin separated by a 90 degree pulse on the S spin. Compared to the rotating frame techniques based on Hartmann-Hahn match, this approach is easy to implement and is independent of any matching conditions. DAPT can be utilized either as a proton encoded local field (PELF) technique or as a separated local field (SLF) technique, which means that the heteronuclear dipolar coupling can be obtained by following either the evolution of the abundant spin like proton (PELF) or that of the rare spin such as carbon (SLF). We have demonstrated the use of DAPT both as a PELF and as a SLF technique on an oriented liquid crystalline sample at room temperature and also have compared its performance with PISEMA. We have also incorporated modifications to the original DAPT pulse sequence for (i) improving its sensitivity and (ii) removing carrier offset dependence.     

Nuclear spin relaxation driven by intermolecular dipolar interactions: the role of solute-solvent pair correlations in the modeling of spectral density functions

Nuclear spin relaxation provides useful information related to the dynamics of molecular systems. When relaxation is driven by intermolecular dipolar interactions, the relevant spectral density functions (SDFs) also have significant contributions, in principle, from distant spins all over the dynamic range typically probed by NMR experiments such as NOESY. In this work, we investigate the intermolecular dipolar spin relaxation as driven by the relative diffusion of solvent and solute molecules taking place under a central force field, and we examine the relevant implications for (preferential) solvation studies. For this purpose, we evaluate the SDFs by employing a numerical approach based on spatial discretization of the time-propagation equation, and we supply an analytical solution for the simplest case of a steplike mean-field potential. Several situations related to different solute-solvent pair correlation functions are examined in terms of static/dynamic effects and relaxation modes, and some conclusions are drawn about the interpretation of NOE measurements. While we confirm previous results concerning the spoiling effect of long-range spins (Halle, B. J. Chem. Phys. 2003, 119, 12372), we also show that SDFs are sufficiently sensitive to pair correlation functions that useful, yet rather complicated, inferences can be made on the nature of the solvation shell.     

Nature of the ferromagnetic behavior and possible occurrence of the ferrimagnetic phase transition in genuinely organic molecule-based assemblages with an S = 1 and S = 1/2 antiferromagnetic alternating spin chain: a Green's function approach

The temperature dependence of magnetic susceptibility and sublattice magnetizations were calculated for a Heisenberg Hamiltonian of an S = 1 and S = 1/2 antiferromagnetic alternating spin chain by means of the many-body Green's function theory to show the possible occurrence of a ferrimagnetic phase transition for genuinely organic molecule-based magnets. The S = 1 site in the chain is composed of two S = 1/2 spins coupled by a finite ferromagnetic interaction. From the calculated results, it is found that the sublattice magnetization at low-spin S = 1/2 sites changes its sign from negative to positive with increasing temperature, giving rise to the spin alignments along the chain changing from antiferromagnetic to ferromagnetic ones, which indicates that there is a magnetic phase transition occurring. Because of the weak intermolecular antiferromagnetic interactions, the curves of the magnetic susceptibility multiplied by temperature (chiT) against temperature show a round peak at low temperatures, which is well consistent with recent experimental observations, and the ferrimagnetic phase transition could only be detected at an ultralow-temperature region and under very weak external magnetic fields in practical organic materials. From the analysis of the sublattice magnetizations, it is uncovered that the appearance of the low-temperature peak in the curves of the chiT originates from the ferromagnetic spin alignments for all the spins along the chain, and the intermolecular antiferromagnetic interactions play a pivotal role in ferrimagnetic spin alignments of the magnetic systems. It is also found that the higher spatial symmetry of the intermolecular antiferromagnetic interactions have contributions to stabilize the ferrimagnetic ordering state in the molecule-based magnetic materials.     

Fourier synthesis techniques for NMR spectroscopy in inhomogeneous fields

This paper describes a method for synthesizing spin rotations with arbitrary space dependence on a sample of noninteracting spin 12 by using nonselective radio frequency pulses and pulsed field gradients. This method is used to map out spatial distribution of inhomogeneous B(0) field and to engineer a space dependent evolution of spins that cancels the space dependent phase spins acquire when precessing in an inhomogeneous magnetic field. The technique allows one to record high resolution spectra in inhomogeneous magnetic field by using only time varying linear gradients and rf fields and is expected to find applications in ex situ NMR.     

Non‐Linear Signal Detection Improvement by Radiation Damping in Single‐Pulse NMR Spectra

When NMR lines overlap and at least one of them is affected by radiation damping, the resonance line shapes of all lines are no longer Lorentzian. We report the appearance of narrow signal distortions, which resemble hole‐burnt spectra. This new experimental phenomenon facilitates the detection of tiny signals hidden below the main resonance. Theoretical analysis based on modified Maxwell–Bloch equations shows that the presence of strong transverse magnetization creates a feedback through the coil, which influences the magnetization of all spins with overlapping resonance lines. In the time domain this leads to cross‐precession terms between magnetization densities, which ultimately cause non‐linear behavior. Numerical simulations corroborate this interpretation.     

内压与Henry常数

建立了一个气体溶解度的新模型,它实际上是Pierotti理论的修正,按照这个模型,稀溶液中的溶质被视为虚拟的完全气体,Henry常数则是1mol完全气体的压力与一个Boltzmann因子的乘积,这个因子取决于溶质分子周围溶剂的内压。对若干气体在有机溶剂和聚合物中溶解度数据检验结果表明,这个模型能满意地用来描述Henry常数随温度的变化规律。计算得到的稀溶液形成的偏摩尔热力学函数也与实验值吻合。     

Signal irreproducibility in high-field solution magnetic resonance experiments caused by spin turbulence

Turbulent spin dynamics arising from the joint action of radiation damping and the distant dipolar field are shown to generate irreproducible measurements in popular high-field, gradient-based magnetic resonance (MR) experiments, undermining the prevailing assumption of essentially predictable observables in MR. Sizeable fluctuations in echo amplitudes are reported and numerically simulated for pulsed gradient spin echo and stimulated echo diffusion measurements. The underlying microscopic dynamical instability is characterized by analysis of the finite-time Lyapunov exponents. Perturbations to the modulated magnetization are shown to render magic-angle gradients ineffective in suppressing signal fluctuations. Alternative approaches are suggested for cancelling out the feedback interactions leading to spin turbulence.     

Background-Free Detection of Spin-Exchange Dynamics at Ultra-Low Magnetic Field

Ultra-low field nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI) inherently suffer from a low signal-to-noise ratio due to the small thermal polarization of nuclear spins. Transfer of polarization from a pre-polarized spin system to a thermally polarized spin system via the Spin Polarization Induced Nuclear Overhauser Effect (SPINOE) could potentially be used to overcome this limitation. SPINOE is particularly advantageous at ultra-low magnetic field, where the transferred polarization can be several orders of magnitude higher than thermal polarization. Here we demonstrate direct detection of polarization transfer from highly polarized ¹²⁹Xe gas spins to ¹H spins in solution via SPINOE. At ultra-low field, where thermal nuclear spin polarization is close to background noise levels and where different nuclei can be simultaneously detected in a single spectrum, the dynamics of the polarization transfer can be observed in real time. We show that by simply bubbling hyperpolarized ¹²⁹Xe into solution, we can enhance ¹H polarization levels by a factor of up to 151-fold. While our protocol leads to lower enhancements than those previously reported under extreme Xe gas pressures, the methodology is easily repeatable and allows for on-demand enhanced spectroscopy. SPINOE at ultra-low magnetic field could also be employed to study ¹²⁹Xe interactions in solutions.     

Optical pumping by excitation of molecular vibrations

A new optical pumping mechanism is observed for dilute Yb³⁺ impurities in a single crystal of yttrium ethyl sulphate at low temperatures. The polarization of Yb³⁺ spins is reduced by illumination with near infrared radiation. This reduction in spin polarization is caused by excitation of molecular vibrations of the ligand molecules surrounding the paramagnetic impurity.     

Solute-solvent interactions probed by intermolecular NOEs

Nuclear Overhauser effects arising from the interactions of spins of solvent molecules with spins of a solute should reveal the "exposure" of solute spins to collisions with solvent. Such intermolecular NOEs could, therefore, provide information regarding conformation or structure of the solute. Determinations of solute-solvent NOEs of 1,3-di-tert-butylbenzene in solvents composed of perfluoro-tert-butyl alcohol, tetramethylsilane, and carbon tetrachloride have been carried out. A crude, but apparently reliable, method for prediction of intermolecular solvent-solute NOEs based on hard (noninteracting) spheres was developed. Comparison of experimental to predicted NOEs indicates that tetramethylsilane interacts with the solute according to the model. By contrast, intermolecular NOE data indicate attractive interactions between the solute and perfluoro-tert-butyl alcohol. All NOE results and the corresponding predictions confirm that proton H2 of the solute is protected by the flanking tert-butyl groups from interactions with solvent molecules.     

Contrast enhancement by feedback fields in magnetic resonance imaging

A conceptually new approach giving rise to contrast enhancement by feedback fields in magnetic resonance imaging is proposed, and the detailed mechanism is described. Nonlinear spin dynamics under the feedback fields of the distant dipolar field and/or radiation damping are examined and shown to amplify contrast due to small variations in spin density and precession frequency. Feedback-based contrast enhancement depends on the instability of the initial magnetization configuration and is propagated by positive feedback, as shown through numerical simulations and experimental results on simple phantom samples. On the basis of a theoretical understanding of contrast enhancement, insight into pulse sequence design and optimal contrast attainable under the individual and joint feedback fields is provided.     

Analytical formulas for the magnetic field produced by a spin or a paramagnetic current density in the case of Gaussian basis functions

We present a derivation of simple formulas for the evaluation at any point of space of the magnetic field produced by a spin or a paramagnetic orbital current when Cartesian Gaussian basis functions are used, as is often the case in quantum chemistry. These formulas can be useful to plot the magnetic field vector density obtained from ab initio calculations or from a density operator fitted on experimental data. The magnetic field density is the observable probed in polarized neutron diffraction (PND) experiment, for it is, in fact, with this quantity that the neutron spins interact and not with the spin or magnetization density. The formulas make extensive use of the confluent hypergeometric function. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 11–15, 2001     

低密度溶液中溶剂的重组织性质

分析了溶液的微观结构,结果表明,单个溶质粒子影响其周围的溶剂的结构,溶质粒子间的相互作用也将影响溶剂的结构,溶质对溶剂结构的影响称作溶剂的重组织.提出了二阶重组织能及二阶重组织熵等概念,可以描述在两个溶质粒子发生碰撞时对其周围溶剂结构的影响.利用二元系的集团展开理论,给出了溶剂的一阶、二阶重组织能和重组织熵的表达式.统计热力学分析给出了溶剂-溶剂径向分布函数与溶质和溶剂化学势之间的关系,给出了无限稀溶液模型是否成立的宏观判据.提出的理论可用于低密度的二元溶液.     

Preparation of sterically congested diphenyldiazomethanes having a pyridine ligand and magnetic characterization of photoproducts of their 2:1 copper complexes

[structure: see text] To confirm whether high-spin species can be generated as a result of ferromagnetic interaction between the 3d spin of metal ions and the 2p spins of triplet carbene through the pyridyl group located remote from the carbene center, [2,6-dibromo-4-(3- and 4-pyridyl)phenyl](4-tert-butyl-2,6-dimethylphenyl)diazomethanes were prepared and the corresponding carbenes were generated either in the absence or presence of Cu(hfac)2. These were characterized by ESR and UV/vis in a matrix at low temperature, and by laser flash photolysis in solution at room temperature. These studies indicated that although both carbenes generated a fairly stable complex with copper ions, the 4-pyridyl isomer formed a high-spin species as a result of ferromagnetic interaction between the 3d spin of metal ions and the 2p spins of triplet carbene. Such an interaction in the corresponding 3-isomer is likely to be antiferromagnetic. This is further confirmed by magnetic measurements using a Superconducting Quantum Interference Device (SQUID). The results demonstrate that extension of this method will enable stable high-spin polycarbenes to be obtained.     

Large Magnetization and Frustration Switching of Magnetoresistance in the Double‐Perovskite Ferrimagnet Mn2FeReO6

Ferrimagnetic A₂BB′O₆ double perovskites, such as Sr₂FeMoO₆, are important spin‐polarized conductors. Introducing transition metals at the A‐sites offers new possibilities to increase magnetization and tune magnetoresistance. Herein we report a ferrimagnetic double perovskite, Mn₂FeReO₆, synthesized at high pressure which has a high Curie temperature of 520 K and magnetizations of up to 5.0 μ_B which greatly exceed those for other double perovskite ferrimagnets. A novel switching transition is discovered at 75 K where magnetoresistance changes from conventional negative tunneling behavior to large positive values, up to 265 % at 7 T and 20 K. Neutron diffraction shows that the switch is driven by magnetic frustration from antiferromagnetic Mn²⁺ spin ordering which cants Fe³⁺ and Re⁵⁺ spins and reduces spin‐polarization. Ferrimagnetic double perovskites based on A‐site Mn²⁺ thus offer new opportunities to enhance magnetization and control magnetoresistance in spintronic materials.