NMR Response Research Articles

Enantiospecificity in NMR enabled by chirality-induced spin selectivity

Spin polarization in chiral molecules is a magnetic molecular response associated with electron transport and enantioselective bond polarization that occurs even in the absence of an external magnetic field. An unexpected finding by Santos and co-workers reported enantiospecific NMR responses in solid-state cross-polarization (CP) experiments, suggesting a possible additional contribution to the indirect nuclear spin-spin coupling in chiral molecules induced by bond polarization in the presence of spin-orbit coupling. Herein we provide a theoretical treatment for this phenomenon, presenting an effective spin-Hamiltonian for helical molecules like DNA and density functional theory (DFT) results on amino acids that confirm the dependence of J-couplings on the choice of enantiomer. The connection between nuclear spin dynamics and chirality could offer insights for molecular sensing and quantum information sciences. These results establish NMR as a potential tool for chiral discrimination without external agents.

The mechanism of the two-dimensional NMR response in depleted oil reservoirs during supercritical CO2 injection

Depleted oil reservoirs are an important site for geological CO2 sequestration. In depleted oil reservoirs, water and hydrocarbons may overlap in the transverse relaxation time (T2) distribution, and two-dimensional (2D) nuclear magnetic resonance (NMR) has a great advantage for pore fluid evaluation. However, the mechanism of the 2D NMR response has not been well explained for CO2-saturated rocks, which is not conducive to the development of an NMR-based method for CO2-flooded enhancing oil recovery potential evaluation and CO2 sequestration assessment and safety monitoring. In this study, we first simulate the 2D NMR diffusion-T2 responses (D‒T2) of carbonate rocks, sand packs, and tight sandstones during supercritical CO2 injection using digital core technology. Based on the digital core models from micro-CT, we use mathematical morphology to construct the spatial distributions of the three-phase fluid of immiscible supercritical CO2, water, and oil at different saturations. We modify the random-walk method to simulate D‒T2 acquisitions, based on the constructed digital core models, and analyze the corresponding D‒T2 spectra of three types of rocks saturated with CO2. The results show that the T2 lifetime of water in the D‒T2 spectra gradually shortens, the oil peak remains constant as the CO2 saturation increases and the oil and water saturations gradually decrease. The water peak in the D‒T2 spectra gradually deviates from the water diffusion coefficient line as the water saturation decreases, indicating that the restricted diffusion of water is enhanced. One can determine water and oil contents from the D‒T2 spectra of CO2-, water- and oil-saturated rocks. One can also calculate the CO2 content in the rock if the porosity of the rock is known. These simulations provide a theoretical basis for explaining the D‒T2 response of CO2-saturated rock, which can be useful for evaluating CO2-flooded enhancing oil recovery potential in oil reservoirs and assessing CO2 sequestration and safety monitoring in depleted oil reservoirs.

Understanding polymer-colloid gels: a solvent perspective using low-field NMR.

The present work emphasizes the relevance of low-field NMR relaxometry to investigate colloid-polymer hydrogels by probing water dynamics across a wide range of formulations between 10 °C and 80 °C. By examining the temperature dependence of the transverse relaxation time T2, we demonstrate a clear link between the NMR response and the rheological behavior of the hydrogels. In particular, we show that NMR relaxometry targeting the solvent provides reliable insights into the hydrogel microstructure and allows the detection of phase transitions and aging processes. Our findings suggest that this solvent-focused technique could greatly benefit the soft matter community, complementing other experimental methods in the study of gels.

Pfeiffer effect on configurationally labile dyes within ternary complexes with metal ions and enantiopure macrocycles.

A configurationally-labile helical dye, 2,4,5,7-tetranitrofluorenone oximate, is used to probe complexes made of enantiopure macrocycles and mono/divalent metal ions. Induced electronic circular dichroism (ECD) and 1H NMR responses are amplified at room temperature only in the presence of K+ and Na+ ions despite larger binding efficiency with alkaline earth metal ions.

A New Workflow for Assessment of Fluid Components and Pore Volumes From 2D NMR Measurements in Formations With Complex Mineralogy and Pore Structure

It is challenging to reliably identify fluid components and estimate their saturations in formations with complex lithology, complex pore structure, or varying wettability conditions. Common practices for assessing fluid saturations rely on the interpretation of resistivity measurements. These techniques require model calibration, which is time consuming/expensive and can only differentiate conductive and nonconductive fluids. Interpretation of 2D NMR maps provides a viable alternative for identifying fluid components and fluid volumes. However, conventional techniques for the interpretation of 2D NMR rely on cutoffs in the T1-T2 or D-T2 maps. The application of cutoffs is prone to inaccuracies when fluid-component relaxation responses overlap. To address these shortcomings, we introduce a new workflow for identifying/tracking fluid components and estimating their volumes from the interpretation of 2D NMR measurements. We developed a workflow that approximates 2D NMR maps with a superposition of 2D Gaussian distributions. The algorithm automatically determines the optimum number of Gaussian distributions and their corresponding properties (i.e., amplitudes, variances, and means). Next, a clustering technique is implemented to the dataspace containing the Gaussian distribution parameters obtained for the entire logged interval. Each Gaussian is assigned to a cluster corresponding to different pore/fluid components. We then calculate the volumes under the Gaussian distributions corresponding to each cluster at each depth. The volumes associated with each cluster translate directly into the pore volumes corresponding to the different fluid components (e.g., heavy/light hydrocarbon, bound/free water) at each depth. A highlighted contribution of this work is that, in contrast to the alternative petrophysical interpretation techniques for fluid characterization, the introduced workflow does not require calibration efforts, user-defined cutoffs, or proprietary data sets. Furthermore, approximating 2D NMR data with a superposition of Gaussian distributions improves the accuracy of estimated pore volumes of fluid components with overlapping NMR responses. The clustering using the Gaussian distribution parameters as inputs enables depth tracking of different fluid components without making use of user-defined 2D cutoffs. Finally, the multidimensional nature of the introduced clustering provides the unique capability of identifying different fluid components with 2D NMR response located in the same range of coordinates in a T1-T2 map. We successfully verified the reliability and robustness of the new workflow for enhancing petrophysical interpretation in two organic-rich mudrock formations with complex mineralogy and pore structure.

Fluorinated Mn(III)/(II)-Porphyrin with Redox-Responsive 1 H and 19 F Relaxation Properties.

A new fluorinated manganese porphyrin, (Mn-TPP-p-CF3 ) is reported capable of providing, based on the Mn(III)/Mn(II) equilibrium, dual 1 H relaxivity and 19 F NMR response to redox changes. The physical-chemical characterization of both redox states in DMSO-d6 /H2 O evidenced that the 1 H relaxometric and 19 F NMR properties are appropriate for differential redox MRI detection. The Mn(III)-F distance (dMn-F =9.7-10 Å), as assessed by DFT calculations, is well tailored to allow for adequate paramagnetic effect of Mn(III) on 19 F T1 and T2 relaxation times. Mn-TPP-p-CF3 has a reversible Mn(II)/Mn(III) redox potential of 0.574 V vs. NHE in deoxygenated aqueous HEPES/ THF solution. The reduction of Mn(III)-TPP-p-CF3 in the presence of ascorbic acid is slowly, but fully reversed in the presence of air oxygen, as monitored by UV-Vis spectrometry and 19 F NMR. The broad 1 H and 19 F NMR signals of Mn(III)-TPP-p-CF3 disappear in the presence of 1 equivalent ascorbate replaced by a shifted and broadened 19 F NMR signal from Mn(II)-TPP-p-CF3 . Phantom 19 F MR images in DMSO show a MRI signal intensity decrease upon reduction of Mn(III)-TPP-p-CF3 , retrieved upon complete reoxidation in air within ~24 h. 1 H NMRD curves of the Mn(III)/(II)-TPP-p-CF3 chelates in mixed DMSO/water solvent have the typical shape of Mn(II)/Mn(III) porphyrins.

A pH-responsive sensor based on intramolecular internal standard for reproducible detection of strong acids and bases via 19F NMR spectroscopy

Numerous methods, including pH meters and optical sensors, have been developed for the detection of pH, which is an important indicator in various fields. However, those methods are susceptible to errors in strongly acidic and basic ranges and inaccurate pH measurement due to sample turbidity, hindering their application such as photographic industries and wastewater treatment facilities. Eco-friendly and non-invasive 19F NMR spectroscopy is a promising technique for measurement of strong acids and bases owing to its high sensitivity and little interference; nevertheless, inconsistencies in reproducibility impede its widespread adoption. Herein, we developed a19F NMR-based pH sensor by introducing an intramolecular internal standard strategy into a pH-responsive fluorinated material. Based on the acceptable deviation (ΔδF = 17−19 ppb) in the evaluation of the internal standard signal, this pH-sensing platform enabled reproducible pH measurements in strongly acidic and basic environments. Moreover, its 19F NMR response showed reversibility and high stability to potential interfering factors, and the low absolute difference (0.026−0.086 in pH) for real samples such as diet Coke suggests its potential suitability for various acidic beverages.

Simulation of NMR response of microfractures based on digital rock technology

The NMR response of microfractures in sandstone reservoirs is important for evaluating and predicting their performance. However, current methods have limitations, including a lack of high-resolution pore structure information and the inability to accurately model the complex geometry of microfractures. In this study, we aimed to overcome these limitations by using digital rock technology to simulate and analyze the NMR response of microfractures in a systematic way. The high-resolution pore structure information of sandstone samples is obtained and segmented by machine learning algorithm, and the 3D digital rock of sandstone with dual media of fracture and pore is reconstructed by using discrete fracture network model. Then, the NMR T2 spectrum of various saturation under various fracture parameters are simulated by using random walk method based on the constructed digital rock. The results show that the aperture, length, dip angle, density and water saturation of microfractures have different effects on the NMR response. An increase in the aperture, length, density, and water saturation increases the T2 relaxation time of the saturated water. At the same time, the closer the fracture dip angle is to 45°, the greater the response to NMR is. The density of microfractures has little effect on NMR response, mainly reflected in the backward shift of the front end of T2 spectrum. When the pore contains oil, the T2 spectrum mainly reflects the relaxation characteristics of the fluid itself, not the pore structure of the core. By using digital rock technology, we were able to accurately model the complex geometry of microfractures and incorporate high-resolution pore structure information, leading to a more comprehensive understanding of the NMR response of these reservoirs.

Molecular Influences on the Quantification of Lewis Acidity with Phosphine Oxide Probes.

Gutmann-Beckett-type measurements with phosphine oxide probes can be used to estimate effective Lewis acidity with 31P nuclear magnetic resonance spectroscopy, but the influence of the molecular structure of a given probe on the quantification of Lewis acidity remains poorly documented in experimental work. Here, a quantitative comparison of triethyl (E), trioctyl (O), and triphenyl (P) phosphine oxides as molecular probes of Lewis acidity has been carried out via titration studies in MeCN with a test set of six mono- and divalent metal triflate salts. In comparison to E, the bulkier O displays a similar range of chemical shift values and binding affinities for the various test metal ions. Spectral linewidths and speciation properties vary for individual cation-to-probe ratios, however, confirming probe-specific properties that can impact the data quality. Importantly, P displays a consistently narrower dynamic range than both E and O, illustrating how electronic changes at phosphorus can influence the NMR response. Comparative parametrizations of the effective Lewis acidities of a broader range of metal ions, including the trivalent rare earth ions Y3+, Lu3+, and Sc3+ as well as the uranyl ion (UO22+), can be understood in light of these results, providing insight into the fundamental chemical processes underlying the useful approach of single-point measurements for quantification of effective Lewis acidity. Together with a study of counteranion effects reported here, these data clarify the diverse ensemble of factors that can influence the measurement of Lewis acid/base interactions.

To what extent do bond length and angle govern the 13C and 1H NMR response to weak CH⋯O hydrogen bonds? A case study of caffeine and theophylline cocrystals

Weak hydrogen bonds are important structure-directing elements in supramolecular chemistry and biochemistry. We consider here weak CH⋯O hydrogen bonds in a series of cocrystals of theophylline and caffeine and assess to what extent the CH⋯O distance and angle govern the observed 13C and 1H isotropic chemical shifts. Gauge-including projector-augmented wave density functional theory (GIPAW DFT) calculations consistently predict a decrease in the 13C and 1H magnetic shielding constants upon hydrogen bond formation on the order of 2–5 ppm (13C) and 1–2 ppm (1H). These trends are reproduced using the machine-learning approach implemented in ShiftML. Experimental 13C and 1H chemical shifts obtained for powdered samples using one-dimensional NMR spectroscopy as well as heteronuclear correlation (HETCOR) spectroscopy correlate well with the GIPAW DFT results. However, the experimental 13C NMR response only correlates moderately well with the hydrogen bond length and angle, while the experimental 1H chemical shifts only show very weak correlations to these local structural elements. DFT computations on isolated imidazole-formaldehyde models show that the 13C and 1H chemical shifts generally decrease with the C⋯O distance but show no clear dependence on the CH⋯O angle. These results demonstrate that the 13C and 1H response to weak CH⋯O hydrogen bonding is influenced significantly by additional weak contacts within cocrystal heterodimeric units.

Evaluation of tight waterflooding reservoirs with complex wettability by NMR data: A case study from Chang 6 and 8 members, Ordos Basin, NW China

After long-term water injection, many developed oilfields in the Ordos Basin, China, have entered a period of the high water-cut stage. In waterflooded layers, formation water is a mixture of injected water and primary formation water. The variation of formation water salinity will significantly affect the evaluation of reservoirs using resistivity logging curves while having little effect on the NMR response. It lays a foundation for identifying fluid type and evaluation of waterflooding grade by NMR logging. However, in waterflooding layers with mixed wettability, the NMR T2 responses are still complicated and unclear. In this paper, the tight sandstone samples with different wettability of Chang 6 and Chang 8 members of Triassic Yanchang Formation in the Ordos Basin are used to investigate the NMR experiments under three saturation states: water-saturated state, oil displacing to bound water state (abbreviated as oil flooding) and water flooding to the residual oil state (abbreviated as water flooding). Results show that the distributions of standard T2 spectra, without an external gradient magnetic field, of the water-saturated state in mix-wetted samples is very close to that of after oil flooding. Thus, the standard T2 spectra will be hard to distinguish the oil layer, water layer, and waterflooded layer of mix-wetted reservoirs. The diffusion coefficients of water and oil in pores differ, resulting in different shift T2 spectra results under gradient magnetic field. Therefore, the shift T2 spectra with the external gradient magnetic field and different echo time TEs were calculated and measured. When the echo time TE = 3.6 ms is adopted, the T2 signal of water-saturated tight rock samples will be nearly distributed less than 100 ms with magnetic field gradient G = 18 G/cm, rather the T2 signal greater than 100 ms is the oil signal. This characteristic makes the shift T2 spectra outstanding in the fluid identification of tight reservoirs with complex wettability. Finally, based on this experiment, a residual oil index based on the shift T2 spectrum is constructed to evaluate the flooded grades of tight waterflooding reservoirs. The evaluation results are consistent with the core description and oil test results in the study infilled wells.

NMR Relaxation Modelling in Porous Media with Dual-Scale-Resolved Internal Magnetic Fields

A pore-scale forward modelling approach for NMR relaxation responses of sandstones incorporating their dual-scale nature is presented. The approach utilises X-ray micro-CT images to capture inter-granular porosity and scanning electron microscopy images to reconstruct clay regions via a resolved clay micro-structure model. A key to calculating the NMR response with resolved clay micro-structure is the development of a dual-scale internal magnetic field calculation. This is achieved by a separation of near- and far-field effects in a dipole approximation of the internal field with periodic clay micro-structures, the latter of which take local clay pocket porosity into account. Tri-linear interpolation of the micro-CT image before calculation of the internal magnetic field further reduces errors in the transition regions between coarse- and fine-scale structure, with final discretisation level matching the fine-scale clay micro-structure model across the whole domain. The method is validated against direct calculations of model media at full resolution and applied to Bentheimer sandstone. Measured and simulated NMR T_2 relaxation responses, including relaxation time distribution shape, are in excellent agreement and distributions of internal magnetic field gradients at the highest spatial resolution as well as diffusion-averaged effective gradients are reported.

Predictability of Chalcogen-Bond-Driven Crystal Engineering: An X-ray Diffraction and Selenium-77 Solid-State NMR Investigation of Benzylic Selenocyanate Cocrystals.

We describe a series of new chalcogen-bonded cocrystals featuring 1,2-bis(selenocyanatomethyl)benzene (DSN) and 1,2,4,5-tetrakis(selenocyanatomethyl)-benzene (TSN) as the donor moieties and a variety of Lewis bases such as onium halides, N-oxides, and pyridine-containing heterocycles as the acceptors. Single-crystal X-ray diffraction demonstrates that, in every case, the selenocyanates consistently interact with the acceptor molecules through strong and directional Se···X chalcogen-bonds (ChBs) (X = halides, oxygen, and nitrogen). 77Se solid-state nuclear magnetic resonance spectroscopy was applied to measure selenium chemical shift tensor magnitudes and to explore potential correlations between these tensor elements and the local ChB geometry. In every case, the isotropic 77Se chemical shift decreases, and the chemical shift tensor span increases upon cocrystallization of DSN with the various ChB acceptors. This work contributes to a growing body of knowledge concerning the predictability and robustness of chalcogen bonds in crystal engineering as well as the NMR response to the establishment of chalcogen bonds. In particular, among the systems studied here, highly linear chalcogen bonds are formed exclusively at the stronger σ-hole of each and every selenium atom regardless of the size, charge, or denticity of the electron donor moiety.

Modular Pulse Program Generation for NMR Supersequences.

NMR supersequences allow multiple 2D NMR data sets to be acquired in greatly reduced experiment durations through tailored detection of NMR responses within concatenated modules. In NOAH (NMR by Ordered Acquisition using 1H detection) experiments, up to five modules can be combined (or even more when parallel modules are employed), which in theory leads to thousands of plausible supersequences. However, constructing a pulse program for a supersequence is highly time-consuming, requires specialized knowledge, and is error-prone due to its complexity; this has prevented the true potential of the NOAH concept from being fully realized. We introduce here an online tool named GENESIS (GENEration of Supersequences In Silico), available via https://nmr-genesis.co.uk, which systematically generates pulse programs for arbitrary NOAH supersequences compatible with Bruker spectrometers. The GENESIS website provides a unified “one-stop” interface where users may obtain customized supersequences for specific applications, together with all associated acquisition and processing scripts, as well as detailed instructions for running NOAH experiments. Furthermore, it enables the rapid dissemination of new developments in NOAH sequences, such as new modules or improvements to existing modules. Here, we present several such enhancements, including options for solvent suppression, new modules based on pure shift NMR, and improved artifact reduction in HMBC and HMQC modules.

NMR Response of the Tetrel Bond Donor

NMR Response of the Tetrel Bond Donor

Stable Diarylamine-Substituted Tris(2,4,6-trichlorophenyl)methyl Radicals: One-Step Synthesis, Near-Infrared Emission, and Redox Chemistry

Stable Diarylamine-Substituted Tris(2,4,6-trichlorophenyl)methyl Radicals: One-Step Synthesis, Near-Infrared Emission, and Redox Chemistry

Temperature Dependence of Nuclear Magnetic Resonance Relaxation Time in Carbonate Reservoirs

Summary Nuclear magnetic resonance (NMR)-based interpretation models are commonly calibrated using laboratory ambient core NMR measurements. For applying the core calibrated models to downhole NMR logging interpretation, the difference between the NMR responses measured at ambient and reservoir temperature needs to be evaluated. The temperature dependence of NMR relaxation time in high-quality (HQ) carbonate reservoirs has been studied, and NMR temperature dependence models were established using data analytic methods. In this paper, we extend our early studies on temperature dependence of NMR relaxation time to low-quality (LQ) carbonate formations. For more than 95% of the LQ samples investigated, NMR relaxation time shows a positive correlation with temperature. The correlation is similar to that observed in HQ carbonate rocks but slightly less significant. Temperature-dependent correlations for predicting the geometric mean of NMR transverse relaxation time (T2,GM) from a measured temperature to any other temperature were derived from HQ to LQ carbonate rocks independently first, then a unified T2,GM correlation was derived including both the HQ and LQ carbonate reservoirs. Predicting NMR transverse relaxation time T2 distribution from one temperature to other temperatures was achieved using a dimension reduction approach involving the principal component analysis (PCA) technique. It was found that the T2 distributions at any given temperature for both HQ and LQ carbonate reservoirs can be predicted robustly from the T2 distributions at the ambient temperature by representing the T2 distributions with principal components (PCs) at the ambient temperature and then using these PCs to predict the PCs at a different temperature. The optimal number of PC components depends on the multimodality of the T2 distribution. This work extends the validity range of the data analytic methods, in particular parameter and dimension reduction methods, that quantify the temperature dependence of carbonate NMR properties. The new NMR temperature model enables the integration of NMR laboratory studies and downhole measurements for advanced petrophysical analyses in a wide range of carbonate reservoirs.

Highly Selective and Sensitive Colorimetric and Fluorescent Chemosensors for Rapid Detection of Cyanide Anions in Aqueous Medium: Investigation on Supramolecular Recognition of Tweezer‑shaped Salophenes.

Three tweezer‑shaped salophenes having catechols (1), phenols (2) and anisoles (3) units in conjunction to the dipodal Schiff bases have been applied for the optical sensing of cyanide (CN¯) ions in CH3CN-H2O (7:3) as solvent of choice. Among them, compounds 1 and 2 recognized CN¯, relying on distinct color and spectral changes. They are easy-to-use probes that exhibit extremely high sensitivity (limit of detection = 1-10 nM), rapid response (5s) and excellent selectivity. Moreover, the visual detection and concentration determination of CN¯ by solution test kits of both sensors are the advantages for the practical applications. Based on the fluorescence and NMR spectroscopy, as well as the OH¯ and reversibility experiments, the explicit effect of hydroxyl groups on sensing and as well the different recognition of 1 and 2 toward CN¯ ions was proved. While probe 1 senses CN¯ via deprotonation, probe 2 recognizes it through an intramolecular aldimine condensation cyclization, leading to formation of anions of dihydroxyquinoxaline 4. This chemodosimetry is being reported for the first time in a Schiff's base. Furthermore, the similarity of fluorescence and NMR responses of 2 and 4 toward CN¯ supports the proposed process.

NMR response of maturity parameters and biodegradation grade of heavy oil

The NMR of the Trinity reservoir indicate that changes in crude oil compositions result in differences in NMR characteristics. On this basis, the chemical properties of crude oil are analyzed, and two criteria affecting the NMR characteristics of crude oil with different physical properties are presented. The research shows that the chromatographic characteristics of crude oils with different maturity and biodegradability correspond well with NMR characteristics. Lateral relaxation time of heavy oil at low maturity stage and experiencing strong biodegradability is shortened. Two parameters, maturity index and biodegradation level, can be reliably applied to the evaluation and production of heavy oil reservoir.

DFT Simulations as Valuable Tool to Support NMR Characterization of Halide Perovskites: the Case of Pure and Mixed Halide Perovskites

AbstractSolid state NMR spectroscopy is swiftly emerging as useful tool to characterize the structure, composition and dynamic properties of lead halide perovskites. On the other hand, interpretation of solid state NMR signatures is often challenging, because of the potential presence of many overlapping signals in small range of chemical shifts, hence complicating the extraction of detailed structural features. Here, we demonstrate the reliability of periodic Density Functional Theory in providing theoretical support for the NMR characterization of halide perovskite compounds, considering nuclei with spin I=1/2. For light 1H and 13C nuclei, we predict NMR chemical shifts in good agreement with experiment, further highlighting the effects of motional narrowing. Accurate prediction of the NMR response of 207Pb nuclei is comparably more challenging, but we successfully reproduce the downshift in frequency when changing the halide composition from pure iodine to pure bromine. Furthermore, we confirm NMR as ideal tool to study mixed halide perovskite compounds, currently at the limelight for tandem solar cells and color‐tunable light emission.