Homonuclear Dipolar Coupling Research Articles

Comparison of methods for the NMR measurement of motionally averaged dipolar couplings

Motionally averaged dipolar couplings are an important tool for understanding the complex dynamics of catalysts, polymers, and biomolecules. While there is a plethora of solid-state NMR pulse sequences available for their measurement, in can be difficult to gauge the methods’ strengths and weaknesses. In particular, there has not been a comprehensive comparison of their performance in natural abundance samples, where 1H homonuclear dipolar couplings are important and the use of large MAS rotors may be required for sensitivity reasons. In this work, we directly compared some of the more common methods for measuring C–H dipolar couplings in natural abundance samples using L-alanine (L-Ala) and the N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLF) tripeptide as model systems. We evaluated their performance in terms of accuracy, resolution, sensitivity, and ease of implementation. We found that, despite the presence of 1H homonuclear dipolar interactions, all methods, with the exception of REDOR, were able to yield the reasonable dipolar coupling strengths for both mobile and static moieties. Of these methods, PDLF provides the most convenient workflow and precision at the expense of low sensitivity. In low-sensitivity cases, MAS-PISEMA and DIPSHIFT appear to be the better options.

Serial Polarization Transfer by Combination of Cross-Relaxation and Rotational Resonance for Sensitivity-Enhanced Solid-State NMR.

Methods which induce site-specificity and sensitivity enhancement in solid-state magic-angle spinning NMR spectroscopy become more important for structural biology due to the increasing size of molecules under investigation. Recently, several strategies have been developed to increase site specificity and thus reduce signal overlap. Under dynamic nuclear polarization (DNP) for NMR signal enhancement, it is possible to use cross-relaxation transfer induced by select dynamic groups within the molecules which is exploited by SCREAM-DNP (Specific Cross Relaxation Enhancement by Active Motions under DNP). Here, we present an approach where we additionally reintroduce the homonuclear dipolar coupling with rotational resonance (R2) during SCREAM-DNP to further boost the selectivity of the experiment. Detailed analysis of the polarization buildup dynamics of 13C-methyl polarization source and 13C-carbonyl target in 2-13C-ethyl 1-13C-acetate provides information about the sought-after and spurious transfer pathways. We show that dipolar-recoupled transfer rates greatly exceed the DNP buildup dynamics in our model system, indicating that significantly larger distances can be selectively and efficiently hyperpolarized.

Supercycled R-symmetry sequences for robust heteronuclear polarization transfer in solid-state NMR

Herein, we introduce supercycle of R-symmetry sequences (SR-sequences) and incomplete supercycle schemes of R-symmetry sequences (iSR-I- and iSR-II-sequences) to improve the robustness of PRESTO for heteronuclear polarization transfer in MAS NMR. The constructions of SR- and iSR-I/II- sequences are based on the different phase-inverted supercycles of R-symmetry sequences, and such supercycles can suppress the influence of CSA, resonance offset and RF mismatch when incorporated into the PRESTO method. Moreover, the SR- and iSR-II-sequences are more efficient in suppressing the interference of homonuclear dipolar coupling. The improved robustness of SR-, iSR-I- and iSR-II-PRESTO over the original R-PRESTO has been verified by numerical simulations and NMR experiments on NH4H2PO4 and gamma-alumina at fast MAS conditions. It is also important to note that the SR- and iSR-II-PRESTO can greatly lengthen the transverse relaxation times and lead to much higher polarization transfer efficiency compared to R-PRESTO, thanks to their superior tolerance to RF inhomogeneity and homonuclear dipolar coupling.

Reducing the effects of weak homonuclear dipolar coupling with CPMG pulse sequences for static and spinning solids

The Carr-Purcell/Meiboom-Gill (CPMG) pulse sequence, initially introduced for measuring transverse relaxation time constants (T2), can provide significant signal enhancements for solid-state NMR (SSNMR) spectra. The proper implementation of CPMG for acquiring spectra influenced by chemical shift anisotropies (CSAs), first and/or second order quadrupolar interactions, or paramagnetic broadening has been well documented to date, as have the effects of heteronuclear dipolar coupling on CPMG echo trains and T2 lifetimes. Homonuclear dipolar coupling can also impact T2 lifetimes and CPMG echo trains; these effects have been thoroughly investigated for spectra of homonuclear dipolar coupled spin-1/2 nuclei typically acquired under static conditions that are predominantly influenced by dipolar broadening (e.g., 1H, 19F, etc.). In particular, it has been shown that short refocusing pulses with small flip angles can extend the effective T2 (T2eff, the observed T2 constant as impacted by experimental conditions) measured by CPMG sequences for strong homonuclear dipolar coupled spin-1/2 pairs under static conditions. To date, these effects have not been explored for (i) spin-1/2 nuclei that have significant CSAs and simultaneously feature weak homonuclear dipolar couplings, (ii) for quadrupolar nuclei that are also weakly homonuclear dipolar coupled, and (iii) for either of these cases under magic-angle spinning (MAS) conditions. Herein, we demonstrate that short refocusing pulses that cause small flip angles can reduce the attenuation of signal in CPMG echo trains resulting from dipolar dephasing caused by the weak homonuclear dipolar couplings. For both spin-1/2 and quadrupolar nuclei, this can lead to significant extensions in T2eff and signal enhancements of up to three times compared to conventional CPMG in favourable cases. These phenomena can occur under both static and magic-angle spinning (MAS) conditions, in the latter of which homonuclear couplings are reintroduced by rotational resonance (R2) recoupling. Experimental examples of 13C (I = 1/2), 2H (I = 1), 87Rb (I = 3/2), 23Na (I = 3/2), and 35Cl (I = 3/2) NMR under static and MAS conditions, as well as simulations of these phenomena, are shown and discussed.

Silicon-29 echo train coherence lifetimes and geminal [formula omitted]-couplings in network modified silicate glasses

The natural abundance 29Si echo-train coherence lifetimes in network-modified silicate glasses were examined under static and magic-angle spinning (MAS) conditions. The nuclear magnetic properties of modifier cations were found to play a major role in determining 29Si coherence lifetimes, leading to differences as large as three orders of magnitude. In compositions with abundant NMR active nuclei, such as alkali silicates, the 29Si coherence lifetimes are dominated by coherent dephasing due to residual heteronuclear dipolar couplings, whereas in compositions dilute in NMR active nuclei, such as alkaline earth silicates, the 29Si coherence lifetimes are dominated by incoherent dephasing due to paramagnetic impurities. Expressing the inverse of the coherence lifetime as a residual full width at half maximum (FWHM), we found that increasing rates of both MAS and a π-pulse train are effective in removing the residual 29Si heteronuclear broadenings, with a near-linear relationship between FWHM and MAS rotor period and π-pulse spacing. Based on these results, we conclude that accurate 29Si J coupling measurements will be the most challenging in lithium silicate glasses due to strong homonuclear dipolar couplings among 7Li nuclei, requiring MAS speeds up to 100 kHz, and be the least challenging in the alkaline earth silicate glasses. At a modest MAS speed of 14kHz, distributions of geminal J couplings across Si-O-Si linkages were measured in alkali and alkaline earth silicate glasses giving mean values of 4.2Hz and 5.1Hz in 0.4 CaO·0.6 SiO2 and 0.33 Ba2O·0.67 SiO2 glasses, respectively, and 5.2Hz and 5.3Hz in 0.33 Na2O·0.67 SiO2 and 0.33 K2O·0.67 SiO2 glasses, respectively. We also observe greater variance in the J distributions of alkaline earth silicate glasses consistent with greater structural disorder due to increased modifier cation potential, i.e., the charge-to-radius ratio, Z/r of the cation.

Theory and simulations of homonuclear three-spin systems in rotating solids.

The homonuclear dipolar coupling is the internal spin interaction that contributes the most to the line shapes in magic-angle-spinning (MAS) 1H NMR spectra of solids, and linewidths typically extend over several hundred Hertz, limiting the 1H resolution. Understanding and reducing this contribution could provide rich structural information for organic solids. Here, we use average Hamiltonian theory to study two- and three-spin systems in the fast MAS regime. Specifically, we develop analytical expressions to third order in the case of two and three inequivalent spins (I = ½). The results show that the full third-order expression of the Hamiltonian, without secular approximations or truncation to second order, is the description that agrees the best, by far, with full numerical calculations. We determine the effect on the NMR spectrum of the different Hamiltonian terms, which are shown to produce both residual shifts and splittings in the three-spin systems. Both the shifts and splittings have a fairly complex dependence on the spinning rate with the eigenstates having a polynomial ωr dependence. The effect on powder line shapes is also shown, and we find that the anisotropic residual shift does not have zero average so that the powder line shape is broadened and shifted from the isotropic position. This suggests that in 1H MAS spectra, even at the fastest MAS rates attainable today, the positions observed are not exactly the isotropic shifts.

Determination of the chemical shift tensor anisotropy and asymmetry of strongly dipolar coupled protons under fast MAS

Orientationally-dependent interactions such as dipolar coupling, quadrupolar coupling, and chemical shift anisotropy (CSA) contain a wealth of spatial information that can be used to elucidate molecular conformations and dynamics. To determine the sign of the chemical shift tensor anisotropy parameter (δaniso), both the |m| ​= ​1 and |m| ​= ​2 components of the CSA need to be symmetry allowed, while the recoupling of the |m| ​= ​1 term is accompanied with the reintroduction of homonuclear dipolar coupling components. Therefore, previously suggested sequences which solely recouple the |m| ​= ​2 term cannot determine the sign a 1H's δaniso in a densely-coupled network. In this study, we demonstrate the CSA recoupling of strongly dipolar coupled 1H spins using the Cnn1(9003601805400360180900) sequence. This pulse scheme recouples both the |m| ​= ​1 and |m| ​= ​2 CSA terms but the scaling factors for the homonuclear dipolar coupling terms are zeroed. Consequently, the sequence is sensitive to the sign of δaniso but is not influenced by homonuclear dipolar interactions.

Quantifying the Mechanisms Contributing to Nuclear-Magnetic-Resonance Surface Relaxation of Protons in Kerogen Pores of Organic-Rich Mudrocks

Summary The evaluation of nuclear–magnetic–resonance (NMR) measurements can be challenging in organic–rich mudrocks because of their heterogeneity, tight pores, presence of kerogen, and the lack of understanding regarding the relaxation mechanism on the kerogen surface. Numerical simulation of NMR responses in the pore–scale domain in such complex rocks is also not very useful because most of the inputs are derived from conventional surface–relaxivity models. The conventional grain/fluid–interaction models for quantifying surface relaxivity do not account for any dipolar coupling in kerogen pores. The objectives of this paper are to develop a new surface–relaxivity model that accurately accounts for homonuclear dipolar coupling in kerogen pores; to introduce a pore–scale simulation method for reliable modeling of NMR response; and to quantify the effects of applying the new relaxivity model on simulated NMR responses and phase saturations. We start by considering the generalized Langmuir adsorption (GLA) theory for the adsorption of hydrocarbons on the surfaces of organic pores in mudrock samples. We used this adsorption model and the anisotropic rotation of molecules to develop a new surface–relaxivity model that accurately quantifies both transverse (T2) and longitudinal (T1) relaxation of protons in kerogen pores. The new model was used to simulate NMR responses in ellipsoidal pores and segmented focused–ion–beam scanning–electron–microscope (FIB–SEM) images of organic–rich mudrock samples using a pore–scale finite–volume simulation technique. The inputs to the simulator are the previously discussed pore geometries and the bulk and surface properties of different fluids present in the pore space. The outputs from the simulator were T2 and T1 decay constants in the previously mentioned pore geometries. The results of NMR simulations are then used to quantify the sensitivity of NMR responses to surface relaxivities computed using different models and NMR–based estimates of adsorbed–hydrocarbon volume. The results obtained from the new model verified that intramolecular coupling dominates the T1 and T2 surface relaxivities at high correlation times (greater than 1×10–7 seconds), which are usually observed for hydrocarbons in kerogen pores. The new model also confirmed the observation that NMR responses in mudrocks are not a function of kerogen thermal maturity but strongly depend on kerogen type. The results of numerical simulations demonstrated that dominant T2 peaks, T1–T2 ratios, and estimated adsorbed fractions are functions of molecular correlation time. Numerical simulations of NMR responses in organic–rich mudrock demonstrated that misidentifying coupling mechanisms could cause errors of up to 40.9 and 57.3% in estimates of adsorbed–hydrocarbon volume calculated using T2 and T1–T2 measurements, respectively. The surface–relaxivity model developed in this paper is more reliable than the previously published relaxivity models because it includes the effects of different coupling mechanisms on surface relaxation in organic pores. The new model can be reliably extended for quantifying surface relaxivity at higher temperatures and for different fluids, which enables interpretation of NMR logs at in-situ conditions. Enhanced quantification of surface relaxivity also enhances NMR–based reservoir characterization and helps to improve the estimates of hydrocarbon reserves in organic–rich mudrocks.

Evaluation of wettabilities and pores in tight oil reservoirs by a new experimental design

Understanding different types of pores and their wettability conditions in tight oil reservoirs is a crucial step in evaluating oil accumulation and migration process of these energy resources. Nuclear magnetic resonance (NMR) technique has been widely used in conventional reservoirs to study rock properties including porosity, permeability, pore structures, and wettability. However, the existence of high contents of organic matter and complicated NMR relaxation mechanisms pose considerable challenges on interpreting the NMR results of tight oil reservoirs. In this study, we performed NMR measurements on rock samples under different fluid saturation states and combined the results with imbibition methods as the novel experimental procedure to characterize wettability and pores of tight oil reservoirs. 1D and 2D NMR experiments were first conducted on core samples at four different fluid saturation states including water-saturated state, after-centrifuge state, 80 °C oven-dried state, and 140 °C oven-dried state. Then, we investigated the unmovable hydrogen-bearing matters by monitoring the loss of NMR signal in the four saturation states. In the next step, pores having different wettability conditions were investigated based on the NMR results of oil imbibition and water imbibition experiments. The results showed that while the transverse relaxation behavior of the tight oil rock samples were different from each other, hydrocarbon-wet pores, mix-wet pores and water-wet pores coexisted in these samples. NMR relaxation of oil in hydrocarbon-wet organic pores happened to be the fastest relaxation mechanism compared to other types of pores, and the peak transverse relaxation time was about 0.13 ms. In addition, our experimental results revealed the existence of intermolecular homonuclear dipolar coupling between organic matters and fluids.

A robust NMR method to measure porosity of low porosity rocks

Rocks in low porosity reservoirs contain a considerable number of micro-pores which make the accurate measurement of total porosity more and more difficult. Nuclear magnetic resonance (NMR) is wildly used in petrophysical analyses; however, when using traditional NMR porosity measurement method, it is hard to detect signals of micro-pores due to the limitation of echo spacing. In the current research study, we used a robust NMR method, i.e. magic-sandwich echo (MSE) pulse sequence, to measure the porosities of various types of hydrocarbon-bearing rocks, and compared the results with those of the conventional measurement methods. It is worth mentioning that clay minerals have a great influence on NMR porosity measurement results. Therefore, it was needed to measure the magnetic resonance signal of dry rock to reduce the influences of clay minerals on the rock porosity measurement by NMR approach. After conducting experiments on different low porosity rocks, the results showed that the relative error of our method was 13.50%, which was more accurate than traditional NMR porosity measurement method. In addition, based on porosity deviations of shales and homonuclear dipolar coupling, we put forward a novel method to estimate shale total organic carbon (TOC) content which is a helpful tool for evaluation of unconventional shale reservoirs.

Determining shale organic porosity and total organic carbon by combining spin echo, solid echo and magic echo

Organic matter in shale is very important for shale evaluation. However, current methods for evaluating the structure and porosity of organic matter are time consuming and expensive. Previous low-field NMR work using magic-echo methods provided relationships between magic echo T1−T2 data and total organic carbon (TOC) of shales using partial least-squares regression (PLSR), but uncertainty in estimation of organic pores indicated further study was required. The magic echo and solid echo measurements produce additional signal in the oil shale samples compared to the standard methodologies. The difference between these signals is mainly caused by the homonuclear dipolar coupling in the organic matter; therefore, it implies the content of the organic matter. Compared to the spin echo method, the solid echo and magic echo measurements are able to access additional information of the quantity and porosity of the organic matter in shale samples. Based on this, we suggest a new method using the amplitude of magic echo, solid echo and spin echo to obtain information about quantity and porosity of the organic matter in oil shale samples. The results demonstrate the feasibility of the new method for organic matter estimation.

Nuclear magnetic resonance surface relaxation mechanisms of kerogen

Nuclear magnetic resonance (NMR) relaxometry is an excellent tool for probing the interactions between solid pore surface and pore fluids in porous media. Surface relaxation is a key component of NMR relaxation. It is well-known that in conventional rocks, paramagnetic centers contribute most to the surface relaxation phenomenon. However, the interactions between organic pore surfaces and pore fluids, and the mechanism of surface relaxation in organic shale pores, are not well-understood. We tackle the issue using deuterated compounds to adjust the proton density in the liquid phase and monitoring the transverse relaxation rate changes of kerogen-fluid mixtures. With the Barnett and Eagle Ford kerogen isolates, we found that for alkanes, it is intramolecular dipolar coupling that dominates among the magnetic interactions. As a result, the transverse relaxation rate of alkane proton spins is more likely to be dependent on the concentration of active adsorption sites on the kerogen surface, rather than the kerogen proton density. For water inside organic pores, surface relaxation most likely originates from hydrogen bonding and intermolecular dipolar coupling. We also examined the temperature effect on kerogen surface relaxation and found temperature-dependent behavior that is consistent with surface relaxation by hydrogen bonding and homonuclear dipolar coupling interactions.

Magic Echo for Nuclear Magnetic Resonance Characterization of Shales

Low-field nuclear magnetic resonance (NMR) has been widely applied in the oil industry as a noninvasive technique for rock analysis. There are many organic compounds, such as kerogen, in unconventional shale resources. However, it is difficult to evaluate solid organic matter in shales using common NMR techniques. Previous work has shown the use of a solid echo on shale samples provides more information than the standard T1–T2 method with spin echo. But for multiple homonuclear dipolar coupling present in shales, solid echo is less efficient. We propose a new multimagic-echo sequence for T1 and T1–T2 measurements. The magic echo and standard T1 and T2 measurement technologies are combined to provide a more complete evaluation of hydrogen-containing shale sample components. Compared with other measurements, magic-echo measurements provide more signal of organic matters in shale samples. The additional signal is contributed by homonuclear dipolar coupling in organic matter. The partial least-squares regress...

Significance of symmetry in the nuclear spin Hamiltonian for efficient heteronuclear dipolar decoupling in solid-state NMR: A Floquet description of supercycled rCW schemes.

Symmetry plays an important role in the retention or annihilation of a desired interaction Hamiltonian in NMR experiments. Here, we explore the role of symmetry in the radio-frequency interaction frame Hamiltonian of the refocused-continuous-wave (rCW) pulse scheme that leads to efficient 1H heteronuclear decoupling in solid-state NMR. It is demonstrated that anti-periodic symmetry of single-spin operators (Ix, Iy, Iz) in the interaction frame can lead to complete annihilation of the 1H-1H homonuclear dipolar coupling effects that induce line broadening in solid-state NMR experiments. This symmetry also plays a critical role in cancelling or minimizing the effect of 1H chemical-shift anisotropy in the effective Hamiltonian. An analytical description based on Floquet theory is presented here along with experimental evidences to understand the decoupling efficiency of supercycled (concatenated) rCW scheme.

Detection of intermolecular homonuclear dipolar coupling in organic rich shale by transverse relaxation exchange

The mechanism behind surface relaxivity within organic porosity in shales has been an unanswered question. Here, we present results that confirm the existence of intermolecular homonuclear dipolar coupling between solid and liquid phases in sedimentary organic matter. Transverse magnetization exchange measurements were performed on an organic-rich shale saturated with liquid hydrocarbon. Liquid and solid constituents were identified through both sample resaturation and through their T1/T2 ratios. Extensive cross peaks are observed in the T2-T2 exchange spectra between the solid and liquid constituents, indicating an exchange of magnetization between the two phases. This result cannot arise from physical molecular diffusion, and the dissolution energies are too high for chemical exchange, such that the magnetization exchange must arise from intermolecular homonuclear dipolar coupling. These results both confirm a possible source of surface relaxivity in organic matter and emphasize caution in the use of standard porous media interpretations of relaxation results in shales because of coupling between different magnetization environments.

Hydrogen incorporation mechanisms in forsterite: New insights from1H and29Si NMR spectroscopy and first-principles calculation

The presence of water (hydrogen) in nominally anhydrous mantle minerals may have profound effects on their physical properties (e.g., electrical conductivity, diffusivity, rheology), and these effects are expected to depend on how water is incorporated in the crystal structure. For olivine, the most abundant upper mantle mineral, despite extensive studies, mostly using vibrational spectroscopy, the interpretations are still not well constrained. To provide better understanding on this issue, we carried out a comprehensive 1 H and 29 Si NMR study on an Mg 2 SiO 4 forsterite sample containing about 0.5 wt% H 2 O synthesized at 12 GPa and 1200 °C, complemented by Raman measurement and first-principles calculation of the geometry, stability, and NMR parameters of model structures. The Raman spectra contain relatively sharp O-H stretching bands near 3612, 3579, and 3567 cm −1 and a broader band near 3547 cm −1 , similar to previous reports. The 1 H static and MAS NMR data revealed that there are two main populations of protons in the hydrous forsterite structure, one experiencing strong 1 H- 1 H homonuclear dipolar couplings and contributing to a broad peak near 2.4 ppm, and another with weaker dipolar couplings and contributing to a narrower peak near 1.2 ppm in the MAS NMR spectrum at 30 kHz. Two-dimensional 1 H CRAMPS-MAS NMR measurements confirmed that the two proton components belong to the same phase and the contrast in MAS NMR peak width is largely due to difference in the strength of 1 H- 1 H homonuclear dipolar couplings. In addition, there is also a very weak, narrow 1 H MAS NMR peak near 7.3 ppm (contributing to Si ] in a tetrahedral site of olivine, but unlike hydrogarnet with one of the protons pointing away from the tetrahedral center and located in an adjacent interstitial site, thus experiencing weaker dipolar couplings than those in the vicinity of the vacant tetrahedron; the very weak narrow peak near 7.3 ppm can be attributed to the substitution mechanism of two H ions for one Mg in an M1 site [(2H) M1 ] of forsterite. The 1 H- 29 Si CP-MAS NMR spectra revealed both a broad peak encompassing the position for OH defect-free forsterite (−61.7 ppm) and a narrower peak at higher frequency (−60.9 ppm). First-principles calculation indicates that these peaks are accountable by the same models as for the 1 H NMR data. Thus, this study has provided unambiguous evidence supporting that hydrogen is incorporated in forsterite at relatively high-pressure dominantly as (4H) Si defects, with (2H) M1 defects playing only a very minor role. The much larger 1 H chemical shift for protons associated with the latter (than the former) is correlated with stronger hydrogen bonding for the latter, which in turn reflects difference in bonding environments of the OH groups (with the latter bonded to a Si, and the former only bonded to Mg). Similar correlation applies to the O-H stretching frequency. The (4H) Si defects are responsible for the observed high-frequency O-H stretching bands (>3450 cm −1 ), and the (2H) M1 defects give lower frequencies (undetected here due to low abundance, but most likely near 3160–3220 cm −1 as previously reported) in vibrational spectra. These results can serve as a guide for (re-)interpretation of infrared and Raman spectroscopic data on hydrous olivine produced under different pressure and silica activity conditions, and require reconsideration of any models for the effects of water on physical properties of olivine based on different interpretations of such data. This study also demonstrated the usefulness of the combined solid-state NMR and first-principles calculation approach in unraveling the hydrogen incorporation mechanisms in nominally anhydrous minerals.

Exciting Wide NMR Spectra of Static Solid Samples with Weak Radiofrequency Fields

Abstract Trains of short pulses in the manner of ‘delays alternating with nutations for tailored excitation’ (DANTE) have been applied to the Pake patterns of protons of water molecules trapped in a static powdered sample of barium chlorate monohydrate. The spin dynamics in the course of such experiments have been investigated by means of numerical simulations and compared with the ideal refocusing that can be achieved under magic-angle spinning (MAS). Solid echoes yield essentially undistorted lineshapes, in contrast to direct excitation without refocusing that leads to severe dispersions of the phases because of inhomogeneous interactions such as homonuclear dipolar couplings and anisotropic chemical shifts. The selectivity of DANTE sequences allows one to access ‘slices’ of the Pake pattern that can be related to particular crystallite orientations. Single-crystal spectra can therefore be extracted from powder spectra. A similar behavior is expected for both dipolar and quadrupolar echoes.

1H–1H Double Quantum NMR Investigation of Proton Dynamics in Solid Acids

Currently, the most popular proton exchange membrane (PEM) for fuel cell applications is Nafion. However, Nafion does not retain its high conductivity at high temperatures due to its dependence on water for proton transport. Because operational temperatures higher than the evaporation point of water are desirable, a family of solid acids was investigated. Cations known to transport protons were paired with anions to make acidic salts. Solid acids discussed here include imidazole paired with trifluoromethanesulfuric acid as well as imidazole, benzimidazole and adenine paired with methanesulfonic acid. Solid-state NMR was utilized to show the relative mobility of protons through double-quantum filter (DQF) experiments. The POST-C7 homonuclear dipolar-recoupling scheme was paired with DUMBO homonuclear decoupling to produce 1H double-quantum coherence buildup curves for the hydrogen-bonded protons of interest. Experimental buildup curves, which reflect both local structure as well as dynamics, are compared to theoretical curves of the static system. The SPINEVOLUTION-simulated curves utilized up to eight pairs of homonuclear dipolar couplings within a sphere of 7 Å diameter centered on the proton of interest. Steep buildup of the DQ curve and maxima at short recoupling times in the buildup curves indicate strong dipole–dipole coupling and are interpreted to indicate limited dynamics of the H-bonded protons. In contrast, shallower buildup curves and maxima at longer recoupling times imply that H-bonded protons (in an otherwise similar structure) are associated with local mobility, which reduces their local dipolar coupling and may facilitate proton transport. Bulk proton conductivities measured via electrochemical impedance spectroscopy were compared to DQF measurements to understand proton conduction within these materials.

Proton chemical shift tensors determined by 3D ultrafast MAS double-quantum NMR spectroscopy.

Proton NMR spectroscopy in the solid state has recently attracted much attention owing to the significant enhancement in spectral resolution afforded by the remarkable advances in ultrafast magic angle spinning (MAS) capabilities. In particular, proton chemical shift anisotropy (CSA) has become an important tool for obtaining specific insights into inter/intra-molecular hydrogen bonding. However, even at the highest currently feasible spinning frequencies (110-120 kHz), (1)H MAS NMR spectra of rigid solids still suffer from poor resolution and severe peak overlap caused by the strong (1)H-(1)H homonuclear dipolar couplings and narrow (1)H chemical shift (CS) ranges, which render it difficult to determine the CSA of specific proton sites in the standard CSA/single-quantum (SQ) chemical shift correlation experiment. Herein, we propose a three-dimensional (3D) (1)H double-quantum (DQ) chemical shift/CSA/SQ chemical shift correlation experiment to extract the CS tensors of proton sites whose signals are not well resolved along the single-quantum chemical shift dimension. As extracted from the 3D spectrum, the F1/F3 (DQ/SQ) projection provides valuable information about (1)H-(1)H proximities, which might also reveal the hydrogen-bonding connectivities. In addition, the F2/F3 (CSA/SQ) correlation spectrum, which is similar to the regular 2D CSA/SQ correlation experiment, yields chemical shift anisotropic line shapes at different isotropic chemical shifts. More importantly, since the F2/F1 (CSA/DQ) spectrum correlates the CSA with the DQ signal induced by two neighboring proton sites, the CSA spectrum sliced at a specific DQ chemical shift position contains the CSA information of two neighboring spins indicated by the DQ chemical shift. If these two spins have different CS tensors, both tensors can be extracted by numerical fitting. We believe that this robust and elegant single-channel proton-based 3D experiment provides useful atomistic-level structural and dynamical information for a variety of solid systems that possess high proton density.

A unified heteronuclear decoupling strategy for magic-angle-spinning solid-state NMR spectroscopy.

A unified strategy of two-pulse based heteronuclear decoupling for solid-state magic-angle spinning nuclear magnetic resonance is presented. The analysis presented here shows that different decoupling sequences like two-pulse phase-modulation (TPPM), X-inverse-X (XiX), and finite pulse refocused continuous wave (rCW(A)) are basically specific solutions of a more generalized decoupling scheme which incorporates the concept of time-modulation along with phase-modulation. A plethora of other good decoupling conditions apart from the standard, TPPM, XiX, and rCW(A) decoupling conditions are available from the unified decoupling approach. The importance of combined time- and phase-modulation in order to achieve the best decoupling conditions is delineated. The consequences of different indirect dipolar interactions arising from cross terms comprising of heteronuclear and homonuclear dipolar coupling terms and also those between heteronuclear dipolar coupling and chemical-shift anisotropy terms are presented in order to unfold the effects of anisotropic interactions under different decoupling conditions. Extensive numerical simulation results are corroborated with experiments on standard amino acids.