NMR Response Research Articles

Melting of the Na Layers in SolidNa0.8CoO2

Data of 23Na NMR spectra and relaxation measurements are interpreted as suggesting that, upon increasing temperature, the Na layers in Na0.8CoO2 adopt a 2D liquid state at T=291 K. The corresponding first order phase transition is preceded by a rapidly increasing mobility and diffusion of Na ions above 200 K. Above 291 K, the 23Na NMR response is similar to that previously observed in superionic conductors with planar Na layers.

Direct Correlation between the 31P MAS NMR Response and the Electronic Structure of Some Transition Metal Phosphides

A series of binary transition metal phosphides (Ni3P, Ni12P5, Ni2P, Ni5P4, NiP, NiP2, FeP, FeP2, FeP4, VP2, CoP) were investigated by solid state 31P MAS NMR, leading to rather different lineshapes, shifts, relaxation times, and temperature dependences. The electronic structures of these compounds were computed using various DFT codes, based either on plane wave PAW potentials (VASP) or on all-electron basis sets in the FPLAPW formalism (Wien2K). Depending on the electronic features of the phosphide, self-interaction corrected formalisms (DFT+U or PBE0 hybrid functional) were also used to reach a better description of the electronic ground state and to establish a correlation with the shape and the nature of the NMR signals. As a result of the analysis, the main categories are diamagnetic compounds (FeP4, NiP2) and metallic ones, either real (VP2) or with some electronic localization in band tails (Ni12P5, Ni2P, Ni5P4, NiP) or with spin-polarized conduction bands (CoP, FeP). FeP2 appears somewhat ambiguous, both based on the various computational results and on the NMR characteristics. Besides, FeP4 is the only compound for which very clear J couplings resulting from P−P bonds were observed.

Applications of 2D-NMR maps and geometric pore scale modeling for petrophysical evaluation of a gas well

This paper presents the petrophysical evaluation of a gas reservoir from the Gulf of San Jorge basin, Argentina, by means of two-dimensional nuclear magnetic resonance (2D-NMR) maps and a geometric pore scale modeling of the NMR response of the wetting phase. The reservoir contains gas and irreducible water saturation at the top with a sharp transition into the water zone. The well has been logged with conventional tools such as gamma ray, neutron, density, and also the MRExplorerSM (MREXSM) to acquire the NMR data for determining the irreducible water saturation. Data from special core analysis, scanning electron microscopy, thin sections, and capillary pressure tests have also been acquired. The core-log evaluation shows a very good agreement between the laboratory and log field data, especially in terms of irreducible water saturation, porosity, and permeability, which was also modeled using the Timur–Coates equation for purposes of field delivery. The 2D-NMR maps as T1 vs. T2 apparent (T2app) and diffusivity vs. T2 intrinsic (T2int) from both hydrocarbon and water zone lead to characterize the clay-bound and capillary-bound water and the under-called porosity due to the low hydrogen index of the gas. We use a pore scale model to quantify the effect of pendular water on T2 distribution. The methodology is based on constructing physically representative model rocks numerically, which allows precise geometric description of pore space. Unlike many other approaches to pore-level modeling, this approach introduces no adjustable parameters and can be used to produce quantitative, a priori predictions of the rock macroscopic behavior.

Effect of Cr(III) Acetate Concentration on the 1H NMR Behavior of HPAm/Cr(III) Acetate Gels

Partly hydrolyzed polyacrylamide-Cr(III) acetate (HPAm/Cr(III) acetate) aqueous gel systems formulated with HPAm to Cr(III) acetate weight ratios ranging from 5/1 to 60/1 and 5000 mg/L of HPAm were monitored using 1H nuclear magnetic resonance (1H NMR), bottle testing, and oscillatory rheology. The chemistry of the Cr(III) acetate complex dominated the 1H NMR response of the formulations that did not form a continuous network structure. 1H NMR detected gelation time and syneresis evolution in the formulations that rendered a continuous gel network structure.

Quantitative evaluation of sebum lipid components with nuclear magnetic resonance

A NMR spectroscopic method is described that enables the quantitation of specific lipid classes and components, independent of fatty acid composition. We demonstrate this method for measuring cholesterol, squalene, and pools of sterol esters, wax esters (WEs), and triglyceride (TG) components in sebum and meibum. When 600 MHz NMR equipment is used in conjunction with highly sensitive cryogenically cooled probes, this method has adequate sensitivity, and for some applications, advantages over commonly used HPLC-evaporative light-scattering detection and mass spectrometry-based approaches. This method is shown to be useful for preclinical and clinical monitoring of the efficacy of sebum-reducing agents in animals and humans. In Syrian hamsters, 3% topical flutamide and 20 mg/kg oral isotretinoin reduced sterol esters by 18.7% and 30.0%, respectively, and reduced WEs by 32.9% and 31.8%, respectively, as measured in a punch biopsy of the ear. In a 72 patient clinical methodology study, the assay delivered reproducible and noninvasive measurements of WEs, cholesteryl esters, TGs, and squalene from Sebutape skin blots. The quantitative results of sebum analysis obtained by the NMR method correlate well with those obtained with HPLC-based approaches. This approach may be broadly applicable to cases in which fatty acid-independent quantification of lipid classes is desired.

NMR relaxation measurements to quantify immiscible organic contaminants in sediments

Laboratory NMR relaxation time (T2) measurements at 2 MHz were used to quantify the amount of toluene and bis(2‐ethylhexyl)phthalate (BEHP) in the presence of water in sand‐clay mixtures. NMR measurements were made on water, toluene, or BEHP in each sand‐clay mixture to understand the role of clay and iron content on the T2 of each fluid; we then made NMR measurements on mixtures of water and the contaminants in each sand‐clay mixture. Our ability to resolve the NMR response of the contaminant from that of water and to quantify the amount of the contaminant varied significantly with the clay and iron content of the sand‐clay mixtures. Detection limits as low as 3% contaminant (i.e., percent of the pore fluid, by weight) were observed in some sand‐clay mixtures. Our results suggest that NMR measurements could quantify toluene if some clay is present but are unlikely to be useful for quantifying BEHP.

Precision and sensitivity optimization of quantitative measurements in solid state NMR

This work presents a methodology for optimizing the precision, accuracy and sensitivity of quantitative solid state NMR measurements based on the external reference method. It is shown that the sample must be exclusively located within and completely span the coil region where the NMR response is directly proportional to the sample amount. We describe two methods to determine this “quantitative” coil volume, based on whether the probe is equipped or not with a gradient coil. In addition, to improve the sensitivity and the accuracy, an optimum rotor packing design is described, which allows the sample volume of the rotor to be matched to the quantitative coil volume. Experiments conducted on adamantane and NaCl, which are representative of a soft and hard material, respectively, show that one order of magnitude increase in experimental precision can be achieved with this methodology. Interestingly, the precision can be further improved by using the ERETIC™ method in order to compensate for most instrumental instabilities.

Organo-modified mesoporous silicas for organic pollutant removal in water: Solid-state NMR study of the organic/silica interactions

The adsorption properties of various organo-modified mesoporous silicas towards a methoxy-modified chlorophenol in water have been studied. Phenyl, propyl, hexyl and hexadecyl groups were selected to study the possible interactions that the pollutant could preferentially develop with the surface sites. The best performing sample appears to be the hexyl-modified porous silica and one of the least performing the un-modified mesoporous silica. The physical state of the adsorbed chlorophenol in these two confined environments as well as the pollutant/surface interactions were characterized by a combination of solid-state NMR techniques. The 1H and 13C NMR responses show a high mobility of the molecules in both the modified and un-modified silica, suggesting the absence of strong interactions between the pollutant and the surface, despite the large difference in the chemical nature of the silica surface sites.

Random-walk technique for simulating NMR measurements and 2D NMR maps of porous media with relaxing and permeable boundaries

We revisit random-walk methods to simulate the NMR response of fluids in porous media. Simulations reproduce the effects of diffusion within external inhomogeneous background magnetic fields, imperfect and finite-duration B 1 pulses, T 1/ T 2 contrasts, and relaxing or permeable boundaries. The simulation approach consolidates existing NMR numerical methods used in biology and engineering into a single formulation that expands on the magnetic-dipole equivalent of spin packets. When fluids exhibit low T 1/ T 2 contrasts and when CPMG pulse sequences are used to acquire NMR measurements, we verify that classical NMR numerical models that neglect T 1 effects accurately reproduce surface magnetization decays of saturated granular porous media regardless of the diffusion/relaxation regime. Currently, analytical expressions exist only for the case of arbitrary pore shapes within the fast-diffusion limit. However, when fluids include several components or when magnetic fields are strongly inhomogeneous, we show that simulations results obtained using the complete set of Bloch’s equations differ substantially from those of classical NMR models. In addition, our random-walk formulation accurately reproduces magnetization echoes stemming from coherent-pathway calculations. We show that the random-walk approach is especially suited to generate parametric multi-dimensional T 1/ T 2/ D NMR maps to improve the characterization of pore structures and saturating fluids.

Solid-State NMR Characterization of the Surfactant−Silica Interface in Templated Silicas: Acidic versus Basic Conditions

A combination of one-dimensional and two-dimensional solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) experiments has been used to investigate the hybrid organic−inorganic interfaces in surfactant templated silicas. Samples prepared with cetyltrimethylammonium bromide (CTAB) under acidic (HCl) and basic (NaOH) conditions have been compared. The use of sequences based on the 29Si-1H heteronuclear dipolar interactions allows us to selectively filter the NMR response of the protons close to the Si surface sites showing directly the clear difference between the two systems. The basic sample is characterized by a small amount of Si−OH groups and a short distance between the Si−O- surface groups and the surfactant polar head group, while the acidic sample exhibits a silanol-rich surface with a longer distance between the Si surface sites and the polar head groups. The nature of the interface induces consequent differences in the structure of the adsorbed water layers present at the interfac...

Polymer gel dosimeters with reduced toxicity: a preliminary investigation of the NMR and optical dose–response using different monomers

In this work, three new polymer gel dosimeter recipes were investigated that may be more suitable for widespread applications than polyacrylamide gel dosimeters, since the extremely toxic acrylamide has been replaced with the less harmful monomers N-isopropylacrylamide (NIPAM), diacetone acrylamide and N-vinylformamide. The new gel dosimeters studied contained gelatin (5 wt%), monomer (3 wt%), N,N′-methylene-bis-acrylamide crosslinker (3 wt%) and tetrakis (hydroxymethyl) phosphonium chloride antioxidant (10 mM). The NMR response (R 2) of the dosimeters was analysed for conditions of varying dose, dose rate, time post-irradiation, and temperature during irradiation and scanning. It was shown that the dose–response behaviour of the NIPAM/Bis gel dosimeter is comparable to that of normoxic polyacrylamide gel (PAGAT) in terms of high dose-sensitivity and low dependence on dose rate and irradiation temperature, within the ranges considered. The dose–response (R 2) of NIPAM/Bis appears to be linear over a greater dose range than the PAGAT gel dosimeter. The effects of time post-irradiation (temporal instability) and temperature during NMR scanning on the R 2 response were more significant for NIPAM/Bis dosimeters. Diacetone acrylamide and N-vinylformamide gel dosimeters possessed considerably lower dose-sensitivities. The optical dose–response, measured in terms of the attenuation coefficient for each polymer gel dosimeter, showed potential for the use of optical imaging techniques in future studies.

Pore-scale modelling of NMR relaxation for the characterization of wettability

Several research groups are currently investigating the determination of wettability using NMR relaxation times. Although correlations with traditional wettability indices have been presented with some success, further effort is needed to relate the wettability at pore-scale to a core-scale measurement of NMR response. For example, a qualitative method using the arithmetic mean of relaxation times at various saturations has been presented [Guan, H., Brougham, D., Sorbie, K.S., Packer, K.J., 2002. Wettability effects in a sandstone reservoir and outcrop cores from NMR relaxation time distributions. J. Petroleum Sci. and Eng. 34, 35–54] and a wettability index that quantifies the amount of surface area that is wetted either by oil or by water, by using the T2 peak at four different saturations has been proposed [Fleury, M., Deflandre, F., 2003. Quantitative evaluation of porous media wettability using NMR relaxometry. Mag. Reson. Imaging 21, 385–387]. Our group at the Imperial College have previously shown experimentally that the T 2 distribution provides valuable information about wettability and overall fluid distribution within the pore-space, which is lost if only a single value from the T 2 distribution is considered [Al-Mahrooqi, S.H., Grattoni, C.A., Moss, A.K., Jing, X.D., 2003. An investigation of the effect of wettability on NMR characteristics of sandstone rock and fluid systems. J. Petroleum Sci. and Eng. 39, 389–398]. In this paper we use a simple pore-scale model to understand the effect of wetting and its relationship with NMR relaxation times. The model uses triangular capillary pores with a given pore size distribution. The oil/water distribution within the pores is obtained as a function of capillary pressure and wettability. At a given capillary pressure, the volumes and surface areas of water and oil are calculated for each individual pore. This allows us to calculate the theoretical T 2 distribution for that pore size distribution as a function of wettability and saturation. We have used the model to study the T 2 distribution for a range of wettabilities and saturations. Results from the model confirmed previous observations from experiments regarding the effect of wettability on NMR T 2 distributions. Based on these qualitative results, an improved index for characterising wettability from the T 2 distribution has been proposed. We tested the proposed index using NMR T 2 data from synthetic and real sandstone core plugs with different wettabilities, ranging from strongly water-wet to strongly oil-wet. Comparison between the proposed index and wettability for the synthetic samples and Amott–Harvey index for core plugs show good correlation.

NMR wettability indices: Effect of OBM on wettability and NMR responses

Wettability and NMR surface relaxation are related to each other. Wettability controls the fluid distribution in porous media. NMR surface relaxation dictates that the fluid in contact with the mineral surface has a relaxation time shorter than its bulk value. In this study, first, the nature of wettability effect on NMR responses was demonstrated by a parameter ρ 2, eff, the effective surface relaxivity. Quantitative changes of ρ 2, eff of water or oil for unconsolidated silica flour and calcite were shown to be consistent with the expected wettability alteration based on the contact angle measurements. Based on the concept of effective surface relaxivity, a novel NMR model was then proposed to quantify rock wettability by two NMR wettability indices from either water or oil responses. This model was tested with water/oil partially saturated Berea cores at different wettability conditions. Correlations show that both NMR indices agree well with the Amott–Harvey wettability index, suggesting that quantitative information of reservoir rock wettability can be gained from NMR measurements. Finally, the effect of oil base mud (OBM) surfactants on wettability alteration and NMR responses was systematically investigated with Berea cores. Results show that the originally strongly water-wet Berea cores are altered to be intermediate-wet or oil-wet by OBM surfactants. As a result, the irreducible water saturation from NMR interpretation assuming water-wetness when wettability alteration occurs generally underestimates the measured value. The magnitude of underestimation correlates well with the Amott–Harvey wettability index.

Coherent NMR responses in spin systems with complex spectral structure

We show that for nonresonant excitation of a quantum system, with an NMR spectrum consisting of two inhomogeneously broadened lines, by a combination of an extended and a short delta-shaped pulse, in the free-precession and spin-echo signals after the second pulse we observe the effect of doubling of the additional emission signals. We establish that the nature of this effect is due to zero beats arising when the variable oscillation frequency of the magnetic moments of the material matches the detuning of the pulse carrier frequency from the resonant frequencies of the spectral lines. Within the theory obtained, we propose a formula which we can use, if we know the times at which the additional emission signals arise, to very accurately determine the frequency shift of the lines in the complex spectrum of the material. The theoretical results agree well with experimental data on generation of multiple NMR signals in toluene, the spectrum of which consists of two non-overlapping lines.

Spectroscopic characterization of the polycrystalline copper(I) di- n-butyldithiophosphate cluster – Cu 8[S 2P(O- n-Bu) 2] 6(μ 8-S): Solid-state 31P CP/MAS and static 65Cu NMR studies

A polycrystalline copper(I) O, O′-di- n-butyldithiophosphate cluster compound, Cu 8[S 2P(O- n-Bu) 2] 6(μ 8-S), was synthesized and characterized by 31P CP/MAS NMR at 8.46 T and static 65Cu NMR at multiple magnetic field strengths (7.05, 9.4 and 14.1 T). The principal values of the 31P chemical shift tensor and the 65Cu chemical shift and quadrupolar splitting parameters are presented. The data are compared to those for the analogous octa-nuclear cluster compounds [Cu 8(S 2P(OEt) 2) 6(μ 8-S)], [Cu 8(S 2P(O- n-Pr) 2) 6(μ 8-S)] and [Cu 8(S 2P(O- i-Bu) 2) 6(μ 8-S)]. The transverse relaxation time constant, T 2, for the [Cu 8(S 2P(O- n-Bu) 2) 6(μ 8-S)] cluster compound was found to be 160 ± 8 μs. Possible intra-molecular motions in the cluster structures in terms of size and branching of the hydrocarbon chains are discussed as reasons for the different 65Cu NMR responses of the systems.

Formula omitted]: An unusual low-dimensional quantum magnet

Results of present and previous measurements of the 23Na NMR response, dc- and ac-magnetic susceptibilities and the specific heat of Na 2 V 3 O 7 at low temperatures suggest that this material is close to a quantum critical point (QCP) at μ 0 H = 0 T . The experimental data can be explained by assuming that below 100 K the localized V magnetic moments ( S = 1 2 ) form a collection of dimers, with a broad distribution of singlet–triplet gaps. Most of the dimers adopt a singlet ground state with gaps between 0 and 350 K. A small fraction of them forms triplet ground states with gaps between 0 and 15 K. The degeneracy of the triplet ground states is lifted by a phase transition at an unusually low temperature of 0.086 K. Modest magnetic fields effectively quench this low-temperature state and the system is driven away from the QCP as the applied fields are enhanced to above 1 T.

Synthesis and Characterization of Novel lacZ Gene Reporter Molecules: Detection of β-Galactosidase Activity by 19F Nuclear Magnetic Resonance of Polyglycosylated Fluorinated Vitamin B6

Gene therapy has emerged as a promising strategy for treatment of various diseases. However, widespread implementation is hampered by difficulties in assessing the success of transfection, in particular, the spatial extent of expression in the target tissue and the longevity of expression. Thus, the development of noninvasive reporter techniques based on appropriate molecules and imaging modalities may help to assay gene expression. We have previously demonstrated the ability to detect beta-galactosidase (beta-gal) activity on the basis of 19F NMR chemical shift associated with release of fluorophenyl aglycons from galactopyranoside conjugates. Use of fluoropyridoxol as the aglycon provides a potential less toxic alternative and we now report the design, synthesis, and structural analysis of a series of novel polyglycosylated fluorinated vitamin B6 derivatives as 19F NMR-sensitive aglycons for detection of lacZ gene expression. In particular, we report the activity of 3,alpha4,alpha5-tri-O-(beta-D-galactopyranosyl)-6-fluoropyridoxol 4, 3-O-(beta-D-galactopyranosyl)-alpha4,alpha5-di-O-(beta-D-glucopyranosyl)-6-fluoropyridoxol 12, and 3-O-(beta-D-galactopyranosyl)-alpha4,alpha5-di-O-(alpha-D-mannopyranosyl)-6-fluoropyridoxol 13. Compounds 4, 12, and 13 all show promising characteristics including highly sensitive 19F NMR response to beta-gal activity (Deltadelta=9.0 approximately 9.4 ppm), minimal toxicity for substrate or aglycon, and good water solubility. However, the differential glycosylation of 12 and 13 appears more advantageous for assessing lacZ gene expression in vivo.

Pore-Scale Characterization of Carbonates Using X-Ray Microtomography

Summary A reservoir carbonate core plug has been imaged in 3D across a range of length scales using high resolution X-ray microtomography (μ-CT). Data from the original 40-mm diameter plug was obtained at the vug scale (42 μm resolution) and allows the size, shape and spatial distribution of the disconnected vuggy porosity, ϕvug = 3.5% to be measured. Within the imaged volume over 32,000 separate vugs are identified and a broad vug size distribution is measured. Higher resolution images, down to 1.1 μm resolution, on subsets of the plug exhibit interconnected porosity and allow one to measure characteristic, intergranular pore size. Pore scale structure and petrophysical properties (permeability, drainage capillary pressure, formation factor, and NMR response) are derived directly on the highest resolution tomographic dataset. We show that data over a range of porosity can be computed from a single plug fragment. Data for the carbonate core is compared to results derived from 3D images of clastic cores and strong differences noted. Computations of permeability are compared to conventional laboratory measurements on the same core material with good agreement. This demonstrates the feasibility of combining digitized images with numerical calculations to predict properties and derive cross-correlations for carbonate lithologies.

Synthesis of novel 1-(2,3-dihydro-5 H-4,1-benzoxathiepin-3-yl)-uracil and -thymine, and their corresponding S-oxidized derivatives

On the basis of molecular variations on isosteric replacements from the prototype 1-(2,3-dihydro-5 H-1,4-benzodioxepin-3-yl)-5-fluorouracil a series of 3-(2,3-dihydro-5 H-4,1-benzoxathiepin-3-yl)-uracil or -thymine O, N-acetals was prepared. The nature of the cis- and trans-sulfoxide isomers was established by means of their conformational analyses carried out with Sybyl and after comparing the theoretical results with the 1H NMR responses of the target molecules. ( RS)-3-(1,1-Dioxo-2,3-dihydro-5 H-4,1-benzoxathiepin-3-yl)thymine and (1 S*,3 S*)-1-(1-oxo-3,5-dihydro-2 H-4,1-benzoxathiepin-3-yl)thymine were found to be inhibitors of the MCF-7 cell growth.

95Mo Magic Angle Spinning NMR at High Field: Improved Measurements and Structural Analysis of the Quadrupole Interaction in Monomolybdates and Isopolymolybdates

In this study, 95Mo quadrupole couplings in various molydbates were measured easily and accurately with magic angle spinning (MAS) NMR under a directing field of 19.6 T. The resonance frequency of 54 MHz was sufficiently high to remove acoustic ringing artifacts, and the spectra could be analyzed in the usual terms of chemical shift and quadrupolar line shapes. For monomolybdates and molybdite, the quadrupole coupling dominated the NMR response, and the quadrupole parameters could be measured with better accuracy than in previous lower field studies. Moreover, despite the low symmetry of the molybdenum coordination, the usefulness of such measurements to probe molybdenum environments was established by ab initio density functional theory (DFT) calculations of the electric field gradient from known structures. The experimental NMR data correlated perfectly with the refined structures. In isopolymolybdates, the resonances were shapeless and DFT calculations were impossible because of the large and low symmetry unit cells. Nevertheless, empirical but clear NMR signatures were obtained from the spinning sidebands analysis or the MQMAS spectra. This was possible for the first time thanks to the improved baseline and sensitivity at high fields. With the generalization of NMR spectrometers operating above 17 T, it was predicted that 95Mo MAS NMR could evolve as a routine characterization tool for ill-defined structures such as supported molybdates in catalysis.