The Influence of the Magnetic Impurity Content on the Pore Size Distribution Determination via the DDIF Technique

The pore size distribution of porous media can be determined in a completely non-invasive manner using a new nuclear magnetic resonance (NMR) technique which monitors the magnetization decay due to diffusion in internal fields (DDIF). However, using of the DDIF technique is restricted to the low-phase encoding limit when only the relaxation mode and the first-order diffusion mode are excited. In the present work the fulfillment of such a limit is verified for a progressive increase of the magnetic impurity content of the porous media. If the higher order diffusion modes are excited they lead both to a stronger attenuation of the echo signal and to the appearance of ripples in the DDIF spectra which cannot be related to a pore size distribution. The samples used in this study are porous ceramics prepared using the replication technique and the magnetic impurity is iron (III) oxide which is introduced in an increasing concentration inside the porous matrix. All NMR experiments were done on water filling such porous ceramics using a low-field instrument operating at a proton resonance frequency of 20 MHz. The average pore dimension obtained with the DDIF technique in the weak encoding limit indicates a satisfactory agreement with that observed in optical microscopy images.     

Cycle time as a variable in CRAMPS NMR. Detection of minor impurities in solids to levels of ≤0.01 mol%

The use of varying multiple-pulse decoupling cycle times in homonuclear dipolar decoupling experiments with combined rotation and multipole-pulse spectroscopy proton nuclear magnetic resonance (NMR) is shown to be capable of quantitatively and qualitatively analyzing amounts of occluded residual liquid solvent, present as saturated solution, in some organic solids crystallized from solution. Lower limits of detection can be 0.01 mol%, as illustrated for the cases of durene,p-hydroxy benzoic acid, and adipic acid crystallized from ethanol, and alanine crystallized from water. Quantitative detection at this level depends upon the phase diagram of the system in question, and the ability to obtain a high-resolution proton NMR spectrum from that portion of the sample consisting of the occluded solvent impurity in the presence of a relatively large proton background from the probe. Determinations of spin diffusion may be used to infer the average size of the mobile domains containing the impurity. The variation of longitudinal relaxation time with temperature may be used as a check on the uniformity of saturated solvent occluded in the system studied. The classical case of using a cooling curve to detect the total amount of liquid protion remaining in the crystal after crystallization is discussed.     

Determination of moisture fraction in wood by mobile NMR device

A mobile NMR probe has been used as a non-destructive and non-invasive tool for water content analysis on wood samples. The porosity index, express as the fraction of the sensitivity volume of the probe occupied by water, is here proposed as an alternative to the moisture content index, namely the amount of water mass with respect to the mass of dried sample. In principle the method can be applied to any kind of porous media that has not detectable proton signal from the rigid matrix as, for instance, in building materials. In wood, where proton signal can be detected also from cellulose and others macromolecular components, some considerations and artifices are here proposed for eliminating this contribution. The method has allowed performing moisture volume fraction analysis on wood samples characterized by different wood species, cutting and moisture contents. The NMR data of moisture detection as volume fraction have successfully been compared with those obtained by the gravimetric method.     

NMR detection of thermal damage in carbon fiber reinforced epoxy resins

Composite materials of epoxy resins reinforced by carbon fibers are increasingly being used in the construction of aircraft. In these applications, the material may be thermally damaged and weakened by jet blast and accidental fires. The feasibility of using proton NMR relaxation times T1, T1rho, and T2 to detect and quantify the thermal damage is investigated. In conventional spectrometers with homogeneous static magnetic fields, T1rho is readily measured and is found to be well correlated with thermal damage. This suggests that NMR measurements of proton T1rho may be used for non-destructive evaluation of carbon fiber-epoxy composites. Results from T1rho measurements in the inhomogeneous static and RF magnetic fields of an NMR-MOUSE are also discussed.     

Bulk magnetization and 1H NMR spectra of magnetically heterogeneous model systems

Bulk magnetization and ¹H static and magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of two magnetically heterogeneous model systems based on laponite (LAP) layered silicate or polystyrene (PS) with low and high proton concentration, respectively, and ferrimagnetic Fe₂O₃ nano- or micro-particles have been studied. In LAP+Fe₂O₃, a major contribution to the NMR signal broadening is due to the dipolar coupling between the magnetic moments of protons and magnetic particles. In PS+Fe₂O₃, due to the higher proton concentration in polystyrene and stronger proton–proton dipolar coupling, an additional broadening is observed, i.e. ¹H MAS NMR spectra of magnetically heterogeneous systems are sensitive to both proton–magnetic particles and proton–proton dipolar couplings. An increase of the volume magnetization by ～1 emu/cm³ affects the ¹H NMR signal width in a way that is similar to an increase of the proton concentration by ～2×10²²/cm³. ¹H MAS NMR spectra, along with bulk magnetization measurements, allow the accurate determination of the hydrogen concentration in magnetically heterogeneous systems.     

A combined 14N/27Al nuclear magnetic resonance and powder X-ray diffraction study of impurity phases in beta-sialon ceramics

Beta-sialons are ceramic phases occurring in the SiO(2)-Si(3)N(4)-AlN-Al(2)O(3) system. A series of samples with differing compositions has been investigated by magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy and powder X-ray diffraction (XRD). Although the constituent nitrogen nuclei occupy positions of low symmetry in the beta-sialon structure, 14N NMR spectra could be recorded for the samples examined. The origin of the 14N signal could be traced to the presence of an aluminium nitride (AlN) impurity phase with the help of 27Al NMR and XRD results. Similarly, the existence of Al(2)O(3) grains could be readily detected for a number of samples. Thus, the combination of 14N and 27Al NMR is shown to be an especially effective tool in identifying and characterizing impurity phases in sialon ceramics, complementing the results obtained from standard XRD analysis.     

Multipoint mapping for imaging of semi-solid materials

Multipoint k-space mapping is a hybrid between constant-time (single-point mapping) and spin-warp imaging, involving sampling of a k-line segment of r points per TR cycle. In this work the method was implemented for NMR imaging of semi-solid materials on a 400 MHz micro-imaging system and two different k-space sampling strategies were investigated to minimize the adverse effects from relaxation-induced k-space signal modulation. Signal attenuation from T(2) decay results in artifacts whose nature depends on the k-space sampling strategy. The artifacts can be minimized by increasing the readout gradient amplitude, by PSF deconvolution or by oversampling in readout direction. Finally, implementation of a T(2) selective RF excitation demonstrates the feasibility of obtaining short-T(2) contrast even in the presence of tissues with long-T(2). The method's potential is illustrated with 3D proton images of short-T(2) materials such as synthetic polymers and bone.     

New proton conducting materials for technical applications: what can we learn from solid state NMR studies?

Many novel proton conducting materials are based on complex hydrogen bonding networks of amphoteric hydrogen bonded moieties. Solid state NMR provides unique methods to study the properties of such network and specific proton conduction mechanisms in detail. In particular 1H solid state NMR techniques under fast magic angle spinning are powerful tools in this area. Site specific studies of the dynamic behavior via variable temperature 1H MAS measurements provide insight in the thermodynamics of the hydrogen bonding as well as activation energies for the proton transfer between the amphoteric sites. On macroscopic length scales, pulsed field gradient NMR experiments are able to determine the proton mobility and the contribution of different conduction mechanisms. In this article, aspects of recent solid state NMR studies in the field are reviewed and typical experimental methods as well as their possible outcome are discussed.     

新药阿德福韦酯的核磁共振研究

采用2D NMR技术对阿德福韦酯的氢谱和碳谱进行了全归属。对其结构进行确定，用于质量标准研究。     

Proton nuclear magnetic resonance spectra of brain tumors

Proton nuclear magnetic resonance (NMR) spectra were successfully measured in human brain tumor tissues and experimental rat brain tumors. The investigation was performed on clinical materials which consisted of tissue from one normal brain and 36 brain tumors. Normal rat brain tissue and rat glioma implanted in the brain were also analysed. NMR measurements were carried out at the resonance frequency of 99.54 MHz. The proton NMR spectrum of the normal brain consisted of one broad component and eight superimposed sharp peaks. The sharp peaks obtained from the brain tumors varied from those of the normal brain. A decrease in the signal intensity from N-acetyl aspartate was the most common finding in all tumors. Spectral patterns were similar within the same histological types, but varied among the different types. Therefore, ¹H-NMR spectra might indicate the metabolism characteristic of each tumor type which would be invaluable for clinical differential dagnosis of brain tumors.     

Measurement of the relaxation rate of the magnetization in Mn12O12-acetate using proton NMR echo

We present a novel method to measure the relaxation rate W of the magnetization of Mn 12O (12)-acetate (Mn12) magnetic molecular cluster in its S = 10 ground state at low T. It is based on the observation of an exponential growth in time of the proton NMR signal during the thermal equilibration of the magnetization of the molecules. We can explain the novel effect with a simple model which relates the intensity of the proton echo signal to the microscopic reversal of the magnetization of each individual Mn12 molecule during the equilibration process. The method should find wide application in the study of magnetic molecular clusters in off-equilibrium conditions.     

Reorientation phenomena in imidazolium methyl sulfonate as probed by advanced solid-state NMR

Evidence for reorientation of imidazolium rings in imidazolium methylsulfonate is demonstrated using solid-state NMR. This material is a model system for exciting new proton-conducting materials based on imidazole. Two advanced NMR methods, including 1H-13C and 1H-15N recoupled polarization transfer with dipolar sideband pattern analysis and analysis of the coalescence of 13C lineshapes are used to characterize the ring reorientation. The process is found to occur at temperatures well below the melting point of the salt, between 240 and 380 K, and is described by a single activation energy, of 38+/-5 kJ/mol. This material is considered as a model system for quantifying the ring reorientation process, which is often proposed to be the rate-limiting step in proton transport in imidazole-based proton conducting materials.     

Microstructure and texture of cementitious porous materials

We have characterized the microstructure of different cementitious materials (white and Portland cement pastes, mortars, concretes) by different magnetic resonance techniques. In particular, we show how the measurement of proton nuclear magnetic spin-lattice relaxation as a function of magnetic field strength (and hence nuclear Larmor frequency) can provide reliable information on the dynamics of proton species at the surface of CSH, the specific surface area and the pore size distribution throughout the progressive hydration of cement-based materials. The measurement does not require any drying temperature modification and is sufficiently fast to be applied continuously during the progressive hydration of the material. Coupling this method with the standard proton nuclear spin relaxation and high-resolution NMR allows us to follow the development of microscale texture within the material.     

Modulation effects in non-drilling NMR in the Earth’s field

The paper describes the variant of nuclear magnetic resonance (NMR) in the Earth’s field using no prepolarization. This method can be employed to record the NMR signal of underground proton-containing liquids, such as water, located at the depth of more than 100 m without drilling. The non-drilling NMR in the Earth’s field is recorded by locating on the surface the circular wire with a diameter of about 100 m. This wire serves as an antenna for the exciting field source and the NMR signal receiver. The radiofrequency pulse with the carrier frequency equal to that of proton resonance in the Earth’s field is passed through the wire. When the exciting pulse is switched off, the induction e.m.f. signal caused by the free Larmor nuclear precession in the geomagnetic fields is observed. An important case when in the non-drilling NMR in the Earth’s field the signal is excited and observed at different frequencies is studied both from theoretical and practical viewpoints. The resulting modulation effects are considered. Such situations arise e.g. either in magnetic rocks or upon magnetic storms.     

Synthesis and proton conductivity studies of azole functional organic electrolytes

As an attempt to produce azole functional proton conductors, organic electrolytes with triazole and tetrazole functional groups were synthesized via substitution reaction of 1,3,5-benzenetricarbonyl trichloride with aminotriazole and aminotetrazole. The samples were doped with triflic acid with molar ratios of 0.25 and 0.50. FTIR, nuclear magnetic resonance (NMR), and elemental analysis were used to characterize the resulting materials. Thermogravimetric analysis showed that the samples are thermally stable up to 150?°C. The effect of acid doping on proton conductivity was investigated with impedance spectrometer. Both pure samples and the doped ones revealed high proton conductivity. In anhydrous conditions (TMA)-TriTA_0.50 and TMA-TetTA_0.50 have proton conductivities of 1.8 and 19?mS/cm at 150?°C, respectively. Solid-state NMR studies revealed that there are three different types of hydrogen-bonded acidic proton in the systems. Moreover, these different types of acidic protons present at different ratio in triazole and tetrazole systems.     

Through-bond heteronuclear single-quantum correlation spectroscopy in solid-state NMR, and comparison to other through-bond and through-space experiments

A new through-bond carbon-proton correlation technique, the MAS-J-HSQC experiment, is described for solid-state NMR. This new pulse scheme is compared experimentally with the previously proposed MAS-J-HMQC experiment in terms of proton resolution on a model sample of powdered L-alanine. We show that for natural abundance compounds, the MAS-J-HMQC and MAS-J-HSQC experiments give about the same proton resolution, whereas, for (13)C-labeled materials, narrower proton linewidths are obtained with the MAS-J-HSQC experiment. In addition we show that in scalar as well as in dipolar heteronuclear shift correlation experiments, when the proton chemical shift is encoded by the evolution of a single-quantum coherence, the proton resolution can be enhanced by simply adding a 180 degrees carbon pulse in the middle of the t(1) evolution time.     

Dipolar induced Para-Hydrogen-Induced Polarization

Analytical expressions for the signal enhancement in solid-state PHIP NMR spectroscopy mediated by homonuclear dipolar interactions and single pulse or spin-echo excitation are developed and simulated numerically. It is shown that an efficient enhancement of the proton NMR signal in solid-state NMR studies of chemisorbed hydrogen on surfaces is possible. Employing typical reaction efficacy, enhancement-factors of ca. 30–40 can be expected both under ALTADENA and under PASADENA conditions. This result has important consequences for the practical application of the method, since it potentially allows the design of an in-situ flow setup, where the para-hydrogen is adsorbed and desorbed from catalyst surfaces inside the NMR magnet.     

Influence of the Structure of an Inhomogeneously Broadened NMR Line on the Shape of the Free Precession Signal

We have obtained theoretically an expression for the nuclear free precession signal in an inhomogeneously broadened spin system with a complex structure of the NMR spectrum. It is shown that in this case in the nuclear precession signal there appear maxima the moments of whose formation depend on the detuning of the pulse carrier frequency from the central frequencies of the NMR lines, their widths, and the Rabi frequency. The theoretical results are in good agreement with the experimental data on the observation of proton free precession in toluene. We have constructed a physical model explaining the appearance of maxima in the free precession signal of the spin system.     

New method of 1H NMR diffusion measurements in chiral liquid crystals

A novel method for diffusion measurements in chiral liquid crystals by means of ¹H NMR is proposed. The proton NMR signal caused by a special preparation pulse sequence is computed. We determine the component of the diffusion tensor in the direction of the cholesteric helix by fitting the calculated to corresponding experimental lineshapes. A time-resolved study is possible.     

NMR with Hyperpolarised Protons in Metals

Proton pulse NMR, established as a versatile method in Solid State Physics, Chemistry, Biology and Medical Science, requires on the order of 10¹⁸ nuclei to detect an electromagnetic signal in a free induction decay (FID). The main cause for this small sensitivity is the low polarisation in the order of a few ppm due to the Boltzmann distribution in the magnetic field. Thus, NMR experiments on hydrogen are limited to metals with extremely high hydrogen solubility like Pd near room temperature. Using a polarised proton beam, a NMR signal is possible with as few as 10¹³ implanted nuclei. For the first time spin–spin and spin–lattice relaxation times were measured in Au and W with this technique at the Bonn cyclotron.