Zero-Field Nuclear Magnetic Resonance of Chemically Exchanging Systems

Nuclear magnetic resonance (NMR) spectra and its action on characterization of pH dependent systems are among the most useful tools in spectroscopic techniques. Nevertheless, practical considerations often limit the application of High field NMR owing to its huge dimensions, expense and time-consuming property. On the other hand, benchtop devices could be another option with portable function and permanent magnet mechanism (1–2 T) but with several disadvantages such as reduced sensitivity and low resolution. In this study, proton exchange rates depending on pH of a selected molecule, were monitored by both 800 MHz High-Field and 60 MHz Bench-top H1 NMR to be compared. L-Ascorbic acid (AA) which is an important substrate for body and biological reac-tions was chosen as a target model molecule. This study supported the use of NMR for pH dependent chemical changes due to protonation. According to spectrum results and previous literature studies, shifting behaviour of Peak 1 (assigned to C4-H13) towards lower frequencies upon pH increase, gave us its feasibility for pKa determi-nation. While High-field NMR spectroscopy demonstrated the compositional analysis of AA and proton exchanges showing couplings between protons, Benchtop NMR was displayed as a limited tool owing to broadened lines of splitting peaks. Even so, shifting tendency of Peak 1 was still obtained quantitatively and promoted the feasibility of Benchtop NMR for characterization of a model molecule which was L-ascorbic acid. In brief, this study sup-ported the effect of magnetic field strength on characterization of pH dependent chemical exchange regimes for vitamins.

This work describes the development and applications to solids and liquid crystals of zero field nuclear magnetic resonance (NMR) experiments with pulsed dc magnetic fields. Zero field NMR experiments are one approach for obtaining high resolution spectra of amorphous and polycrystalline materials which normally (in high field) display broad featureless spectra. The behavior of the spin system can be coherently manipulated and probed in zero field with dc magnetic field pulses which are employed in a similar manner to radiofrequency pulses in high field NMR experiments. Nematic phases of liquid crystalline systems are studied in order to observe the effects of the removal of an applied magnetic field on sample alignment and molecular order parameters. In nematic phases with positive and negative magnetic susceptibility anisotropies, a comparison between the forms of the spin interactions in high and low fields is made. High resolution zero field NMR spectra of unaligned smectic samples are also obtained and reflect the symmetry of the liquid crystalline environment. These experiments are a sensitive measure of the motionally induced asymmetry in biaxial phases. Homonuclear and heteronuclear solute spin systems are compared in the nematic and smectic phases. Nonaxially symmetric dipolar couplings are reported for several systems. The effects of residual fields in the presence of a non-zero asymmetry parameter are discussed theoretically and presented experimentally. Computer programs for simulations of these and other experimental results are also reported. 179 refs., 75 figs.

Paul Christian Lauterbur

Nuclear magnetic resonance (NMR) spectroscopy is a valuable and complementary tool in environmental research, but it is underutilized due to the cost, size, and maintenance requirements of standard "high-field" NMR spectrometers. "Low-field" NMR spectrometers are a financially and physically accessible alternative, but their lower sensitivity and increased spectral overlap limit the analysis of heterogeneous environmental/biological media, especially with fast-relaxing samples that produce broad, low-intensity spectra. This study therefore investigates the potential of the steady-state free precession (SSFP) experiment to enhance signal-to-noise ratios (SNRs) of fast-relaxing, complex samples at both high- and low-field. SSFP works by obtaining steady-state transverse signal using a train of equally spaced radiofrequency pulses with the same flip angle and a time between pulses less than the transverse relaxation time, allowing for thousands of scans to be summed in a short time period. Here, 13C-SSFP is applied to samples of varying complexity (egg white, dissolved organic matter, and crude oil) at low-field and at high-field for testing and comparison. The potential of in vivo SSFP NMR is additionally investigated by applying 31P-SSFP to live Eisenia fetida at high-field. In some samples, SSFP increased 13C SNR by over 2000% at both high-field and low-field compared to standard 13C NMR and enabled detection of peaks that were not observable by standard 13C NMR. Ultimately, SSFP holds great potential for improving analysis of fast-relaxing, complex samples, which could in turn make low-field NMR spectroscopy a more effective tool not only in environmental/biological research but also in numerous other disciplines.

Several advanced solid-state nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) techniques have been applied to inorganic materials of commercial interest. The specific techniques applied to each system were chosen to solve specific obstacles which impeded the flow of industrial science. First, the dispersion of brominated flame retardants in polymers is monitored using pulsed 81Br NQR (nuclear quadrupole resonance) techniques. The NQR spectrometer consists of a homemade NMR console and an automatically tuned loop-gap resonator probe. The two factors which affect the 81Br NQR transition frequencies of brominated aromatics are: electron-donating and withdrawing substituents on the ring and bromine intermolecular contacts. The crystal structure of isobutyl aluminoxane and high field 27Al MAS NMR spectra, acquired at spinning speeds of 35 kHz, are presented. The structure contains both bridging hydrides and three-coordinate aluminum sites. Geometry optimized restricted Hartree-Fock (RHF) calculations of an idealized structure yield Cq and h in close agreement with experiment. Very high field, field swept 27Al NMR spectra of two forms of MAO, MAO(solid) and MAO(gel) were recorded. In the field swept experiment, Bo is swept while B1 remains constant. Using a sweep technique facilitates the observation of exceedingly large line-widths. Simulations of the MAO(gel) line-shape indicate the upper limits of Cq to be 22.5 MHz (calculated using h = 0). Additionally, the spectra of MAO and related compounds were studied by 27Al MAS NMR spectroscopy using a high homogeneity 19.6 T magnet and spinning speed up to 35 kHz. Successful determination of spectral parameters in these cases awaits the availability of even higher magnetic fields and spinning rates in excess of 35 kHz. Last, to measure the abundance of bridging methyl groups in MAO, 1H NMR spin-lattice relaxations in the rotating frame, T1ñ, investigations of methyl group motions were observed. MAO is found to exhibit a biexponential decay. The T1ñ method serves as a qualitative measure of bridging methyl group abundance and MAO(gel) is shown to have nearly twice as many bridging methyl sites as MAO(solid).

Applications of NMR in the Analysis of Agrochemicals and Pesticides

The development of high field superconducting magnets using high temperature superconductors (HTSs) is progressing for high energy physics, nuclear magnetic resonance and energy storage applications. Yet the key issue of quench protection remains unresolved, primarily due to the slow normal zone propagation velocity (NZPV) in HTS magnets. High magnetic field may affect the quench behavior through two opposing effects: increased NZPV may result due to reduced critical temperature and current sharing temperature, but decreased NZPV may result due to reduced critical current density and thus operating current. At present it is unclear which effect dominates. Here, a series of quench experiments at high magnetic field on multilayer wind-and-react Bi2Sr2CaCu2Ox (Bi2212) coils addresses this question. The two- and three-dimensional quench behavior is investigated in a magnetic field up to 20 T at 4.2 K. With increasing magnetic field, the minimum quench energy decreases significantly. The NZPV also decreases with magnetic field, but only up to about 8 T. For magnetic fields above 8 T, the NZPV is independent of magnetic field up to at least 20 T. Thus, at low field the NZPV is dominated by the decreasing critical current density, whereas at higher magnetic field the competing effects of decreasing critical current density and decreasing temperature margin offset each other.

High magnetic field is an important research condition which can provide great opportunities for the development of many subjects. Since 1913, there are 19 Nobel Prizes which are related to high magnetic field. Scientific research under high magnetic field involves a large amount of subjects and various scientific issues. In many research areas such as the quantum transport in semiconductors, Fermi surface measurements, the study of the upper critical field of high temperature superconductors, nuclear magnetic resonance study etc., high magnetic field is an essential condition. Therefore, the scientific research under high magnetic field is always the frontier. In this review, the cutting-edge scientific issues under high magnetic field are presented. The issues in physics include the fractional quantum Hall effect in graphene, the quantum oscillations in black phosphorus, the topological insulator Bi2Se3, the Weyl semimetal NbP, the quantum ordering in non-conventional superconductor, the magnetic field induced new superconducting phase, the magnetic field induced metamagnetism transition, the quantum effect of low dimensional materials, the spin frustration, etc. In chemistry and material science, the focuses are magnetic field induced new chemical reaction, control of reaction routes, the reaction kinetics, synthesis of single molecular magnet and chiral materials, nano-catalysis, etc. Issues related to nuclear magnetic resonance are also mentioned, including the protein structure determination, magnetic resonance imaging for medical science, the structure of melt, geosciences and environmental sciences, etc. Moreover, the present condition and achievement of the high magnetic field facility in China are shown in the following part. Three water-cooled magnets have created three world records. The water-cooled magnet WM1 with a 32 mm bore can generate a steady magnetic field of 38.5 Tesla, which has exceeded the previous record 37.5 Tesla. The water-cooled magnet WM5 with a 50 mm bore has produced a magnetic field of 35 Tesla, which is also the highest for the same type of magnets. The water-cooled magnet WM4 with a 32 mm bore has generated a magnetic field of 27.5 Tesla when inputting 10 MW power. All the magnets have corresponding experimental systems. For example, WM1 has a 3He extreme low temperature experiment dewar installed providing a temperature region from 325 mK to 70 K, which is mainly used for transport measurement. The high-homogeneity water-cooled magnet WM2 has a high field high frequency (82 to 690 GHz) electron magnetic resonance continuous wave system installed. WM5 can cooperate with a Fourier transform infrared spectroscopy (18000 to 4000 cm - 1), a vibrating sample magnetometer for magnetization measurement, and a high pressure experiment system (0–200 GPa). The service of the high magnetic field facilities in China will provide strong support for various cutting-edge research and create advantage for significant original achievement.

A comparative evaluation of low-field and high-field NMR untargeted analysis: Authentication of virgin coconut oil adulterated with refined coconut oil as a case study

High-field Nuclear Magnetic Resonance (NMR) spectroscopy and Gas Chromatography (GC) were used to analyze 16 monovarietal olive oils obtained from few matched Mediterranean cultivars grown in experimental fields located in Italy and in the Catamarca region of Argentina. The Catamarca region is characterized by extreme pedoclimatic conditions and by a wild spontaneous vegetation. The proposed sampling allows to study the effect of different pedoclimatic conditions on olive oil composition. GC gives the fatty acid profile of olive oil samples. 1H and 13C NMR techniques provide different information: the 1H NMR spectrum allows the measurement of minor components of olive oils such as b-sytosterol, hexanal, trans-2-hexenal, formaldehyde, squalene, cycloartenol and linolenic acid; the 1C NMR spectrum allows to obtain information about glycerol tri-esters of olive oils, i.e., about their acyl composition and positional distribution on glycerol moiety. All the NMR and GC results have been submitted to Linear Discriminant Analysis (LDA) and Tree Cluster Analysis (TCA). A careful analysis of the statistical results allows to select the Mediterranean cultivars less affected by the climatic conditions present in the Catamarca region. The selected cultivars produce olive oils which keep their Mediterranean characteristics and which can be proposed as colonizing plants in this wild Argentine region.

Understanding the kinetics of proteins interacting with their ligands is important for characterizing molecular mechanism. However, it can be difficult to determine the extent and nature of these interactions for weakly formed protein-ligand complexes that have lifetimes of micro- to milliseconds. Nuclear magnetic resonance (NMR) spectroscopy is a powerful solution-based method for the atomic-level analysis of molecular interactions on a wide range of timescales, including micro- to milliseconds. Recently the combination of thermodynamic experiments using isothermal titration calorimetry (ITC) with kinetic measurements using ZZ-exchange and CPMG relaxation dispersion NMR spectroscopy have been used to determine the kinetics of weakly interacting protein systems. This chapter describes the application of ITC and NMR to understand the differences in the kinetics of carbohydrate binding by the β1- and β2-carbohydrate-binding modules of AMP-activated protein kinase.

Review: 194 refs.

$J$-coupled nuclear magnetic resonance (NMR) spectroscopy in the strong coupling regime at low magnetic field ($10{}^{\ensuremath{-}7}$ T $lBl{10}^{\ensuremath{-}3}$ T) is more complex than at high field ($Bg{10}^{\ensuremath{-}3}$ T) and at ultralow field ($Bl{10}^{\ensuremath{-}7}$ T). We show that several upper and lower boundaries ${B}_{i}^{\mathrm{up}}$ and ${B}_{i}^{\mathrm{low}}$ of the magnetic field $B$ exist, where the complexity of $J$-coupled NMR spectra changes in terms of the number of lines. The index $i=1,2,\dots{}$ for ${B}_{i}^{\mathrm{up}}$ at high field specifies the perturbation order of the dominating Zeeman interaction and for ${B}_{i}^{\mathrm{low}}$ at ultralow field the perturbation order of the dominating $J$-coupling interaction. Mathematical expressions for these boundaries are derived for the case of a $J$-coupled $S$-$I$${}_{N}$ group where $S$ and $I$ are rare and abundant spins $\frac{1}{2}$ and $N$ counts the abundant spins $I$. The entire $B$-field range can further be delineated into two weak coupling regimes, one at high field with ${B}_{2}^{\mathrm{up}}lBl{B}_{1}^{\mathrm{up}}$ ($10{}^{\ensuremath{-}3}$ T $lBl{10}^{2}$ T), one at low field with ${B}_{1}^{\mathrm{low}}lBl{B}_{2}^{\mathrm{low}}({10}^{\ensuremath{-}8}$ T $lBl{10}^{\ensuremath{-}7}$ T), and a strong coupling regime with ${B}_{2}^{\mathrm{low}}lBl{B}_{2}^{\mathrm{up}}$ ($10{}^{\ensuremath{-}7}$ T $lBl{10}^{\ensuremath{-}3}$ T). The corresponding NMR spectra for the $S\ensuremath{-}{I}_{N}$ group are investigated by experiment and by simulation. In the strong coupling regime, the maximum number of lines is $(N+1){}^{2}$. In the weak coupling regime ${B}_{1}^{\mathrm{low}}lBl{B}_{2}^{\mathrm{low}}$ at low field, symmetric multiplet structures group around the frequencies $0$, $J$, ($3/2$)$J$, $2J$, ($5/2$)$J$, etc. These spectra determine the structure of the $S\ensuremath{-}{I}_{N}$ group unambiguously and are in dual correspondence to the weakly coupled spectra at high field. High-resolution NMR spectroscopy at ultralow field may open up new ways for chemical analysis by small and mobile instruments with many applications in science and technology.

This article concerns the application of nuclear magnetic resonance (NMR) spectroscopic techniques to the analysis of geological materials and glasses. It includes inorganic minerals and glasses, but does not cover soils and clay minerals to any extent. The first part presents an overview of solid‐state NMR experiments applicable to inorganic solids. The range of nuclei accessible by NMR and the particular requirements for obtaining good spectra of more difficult nuclei, including quadrupolar nuclei, are discussed. The experimental conditions under which solid‐state NMR can provide quantitative analytical measurements are also considered.NMR spectroscopy provides element‐specific speciation and structural information including coordination environment and bond distances. It is particularly valuable for the analysis of amorphous and partially crystalline materials not amenable to structure determination by X‐ray diffraction. NMR can detect and quantify materials containing multiple phases and can be used for in situ monitoring of phase transitions. It provides qualitative and quantitative speciation information for framework nuclei in glasses and is also capable of characterizing the dynamic behavior of mobile ions and solvent through relaxation, diffusion and chemical exchange experiments. Thus it has become an important technique for the study of hydrous minerals and gels and for ion‐conducting glasses.The sensitivity of NMR varies considerably, depending on the nucleus observed, its abundance and its chemical environment. NMR sensitivities and detection limits cannot compete with elemental analytical techniques such as X‐ray fluorescence. In addition, quantitation in solids requires careful control of experimental parameters and careful calibration to avoid artifacts. However, the selectivity of NMR as an analytical technique is unsurpassed and the development of specialist pulse sequences to measure specific local interactions in solids makes it particularly well suited to the analysis of geological materials and glasses.NMR imaging enables structural information to be resolved in three dimensions, although resolution in solid‐state imaging is limited to tens of micrometers at best owing to the broader line widths and lower signal‐to‐noise ratios compared with liquids. However, capabilities in this field are being extended all the time and the application of NMR imaging techniques in materials analysis is expanding steadily. An alternative to conventional imaging is NMR force microscopy, which shows potential for surface mapping at higher resolutions.The applications section is organized on the basis of the materials being analyzed. Within each section, NMR techniques are classified according to the observed nuclei. The emphasis is on more recent applications and on developments leading to enhanced resolution and sensitivity. Silicates and aluminosilicates represent the largest class of applications, followed by phosphate, borate and other oxide glasses. Sol–gel materials and the reactions leading to gel formation are also discussed, as are hydrous materials more generally. The other important areas of application reviewed are minerals, ceramics, high‐temperature melts and ion‐conducting glasses. Finally, the applications of NMR imaging in this field, although sparse at present, are potentially very great. This is illustrated by two major areas of application to date, namely the imaging of bone minerals and cement and concrete.

In situ measurement of liquid-liquid equilibria by medium field nuclear magnetic resonance