Multipurpose Broadband NMR Inversion Sequences.

Solution-state 2D correlation experiments increase signal-to-noise, provide improved resolution, and inform about molecular connectivity. NMR experiments are compromised when the nuclei have broad chemical shift ranges that exceed the bandwidth of the experiment. Spectra acquired under these conditions are unphasable and artifact-prone, and peaks may disappear from the spectrum altogether. Existing remedies provide usable spectra only in specific experimental contexts. Here, we introduce a general broadband strategy that leads to a library of high performing NMR experiments. We achieve arbitrary and independent evolution of NMR interactions by only changing delays in our pulse block, letting the block replace inversion elements in any NMR experiment. The experiments improve the experimental bandwidth for both nuclei by an order of magnitude over conventional sequences, covering chemical shift ranges of most molecules, even at ultrahigh field. This library enables robust spectroscopy of molecules such as perfluorinated oils (19F{13C}) and fluorophosphorous compounds in battery electrolytes (19F{31P}).

Weighted AnX multiplets.

That the NMR transition of a spin-1/2 nucleus is split into n evenly spaced lines by indirect dipole-dipole (J) coupling to n magnetically equivalent nuclei, whose successive amplitudes follow the nth row of Pascal's triangle, is an elementary result in NMR. Described here are a family of less well known multiplet structures with different amplitudes for the evenly spaced lines. The amplitudes can be derived from the nth row of Pascal's triangle by weighting the corresponding value of the row by z or z2, where z is related to the summed magnetic quantum number of the magnetically equivalent spins. z1-multiplets have been described in INEPT experiments. A z2-multiplet can be indirectly observed in HSQC experiments when the decoupling pulse during t1 is removed, i.e., an F1-coupled HSQC. While not difficult to generate and despite some reported usefulness, to the best of our knowledge, z2-multiplets have not been rigorously described in previous literature.

NMR spectroscopy of coin cell batteries with metal casings.

Battery cells with metal casings are commonly considered incompatible with nuclear magnetic resonance (NMR) spectroscopy because the oscillating radio-frequency magnetic fields ("rf fields") responsible for excitation and detection of NMR active nuclei do not penetrate metals. Here, we show that rf fields can still efficiently penetrate nonmetallic layers of coin cells with metal casings provided "B1 damming" configurations are avoided. With this understanding, we demonstrate noninvasive high-field in situ 7Li and 19F NMR of coin cells with metal casings using a traditional external NMR coil. This includes the first NMR measurements of an unmodified commercial off-the-shelf rechargeable battery in operando, from which we detect, resolve, and separate 7Li NMR signals from elemental Li, anodic ß-LiAl, and cathodic LixMnO2 compounds. Real-time changes of ß-LiAl lithium diffusion rates and variable ß-LiAl 7Li NMR Knight shifts are observed and tied to electrochemically driven changes of the ß-LiAl defect structure.

Modes of Disorder in Poly(carbon monofluoride).

Poly(carbon monofluoride), or (CF)n, is a layered fluorinated graphite material consisting of nanosized platelets. Here, we present experimental multidimensional solid-state NMR spectra of (CF)n, supported by density functional theory (DFT) calculations of NMR parameters, which overhauls our understanding of structure and bonding in the material by elucidating many ways in which disorder manifests. We observe strong 19F NMR signals conventionally assigned to elongated or "semi-ionic" C-F bonds and find that these signals are in fact due to domains where the framework locally adopts boat-like cyclohexane conformations. We calculate that C-F bonds are weakened but are not elongated by this conformational disorder. Exchange NMR suggests that conformational disorder avoids platelet edges. We also use a new J-resolved NMR method for disordered solids, which provides molecular-level resolution of highly fluorinated edge states. The strings of consecutive difluoromethylene groups at edges are relatively mobile. Topologically distinct edge features, including zigzag edges, crenellated edges, and coves, are resolved in our samples by solid-state NMR. Disorder should be controllable in a manner dependent on synthesis, affording new opportunities for tuning the properties of graphite fluorides.

Hyperpolarization transfer pathways in inorganic materials.

Dynamic nuclear polarization can be used to hyperpolarize the bulk of proton-free inorganic materials in magic angle spinning NMR experiments. The hyperpolarization is generated on the surface of the material with incipient wetness impregnation and from there it is propagated towards the bulk through homonuclear spin diffusion between weakly magnetic nuclei. This method can provide significant gains in sensitivity for MAS NMR spectra of bulk inorganic compounds, but the pathways of the magnetization transfer into the material have not previously been elucidated. Here we show how two-dimensional experiments can be used to study spin diffusion from the surface of a material towards the bulk. We find that hyperpolarization can be efficiently relayed from surface sites to multiple bulk sites simultaneously, and that the bulk sites also engage in rapid polarization exchange between themselves. We also show evidence that the surface peaks can exchange polarization between different sites in cases of disorder.

Quantification of Uncoupled Spin Domains in Spin-Abundant Disordered Solids.

Materials often contain minor heterogeneous phases that are difficult to characterize yet nonetheless significantly influence important properties. Here we describe a solid-state NMR strategy for quantifying minor heterogenous sample regions containing dilute, essentially uncoupled nuclei in materials where the remaining nuclei experience heteronuclear dipolar couplings. NMR signals from the coupled nuclei are dephased while NMR signals from the uncoupled nuclei can be amplified by one or two orders of magnitude using Carr-Meiboom-Purcell-Gill (CPMG) acquisition. The signal amplification by CPMG can be estimated allowing the concentration of the uncoupled spin regions to be determined even when direct observation of the uncoupled spin NMR signal in a single pulse experiment would require an impractically long duration of signal averaging. We use this method to quantify residual graphitic carbon using 13C CPMG NMR in poly(carbon monofluoride) samples synthesized by direct fluorination of carbon from various sources. Our detection limit for graphitic carbon in these materials is better than 0.05 mol%. The accuracy of the method is discussed and comparisons to other methods are drawn.

The Atomic-Level Structure of Cementitious Calcium Aluminate Silicate Hydrate.

Despite use of blended cements containing significant amounts of aluminum for over 30 years, the structural nature of aluminum in the main hydration product, calcium aluminate silicate hydrate (C-A-S-H), remains elusive. Using first-principles calculations, we predict that aluminum is incorporated into the bridging sites of the linear silicate chains and that at high Ca:Si and H2O ratios, the stable coordination number of aluminum is six. Specifically, we predict that silicate-bridging [AlO2(OH)4]5- complexes are favored, stabilized by hydroxyl ligands and charge balancing calcium ions in the interlayer space. This structure is then confirmed experimentally by one- and two-dimensional dynamic nuclear polarization enhanced 27Al and 29Si solid-state NMR experiments. We notably assign a narrow 27Al NMR signal at 5 ppm to the silicate-bridging [AlO2(OH)4]5- sites and show that this signal correlates to 29Si NMR signals from silicates in C-A-S-H, conflicting with its conventional assignment to a "third aluminate hydrate" (TAH) phase. We therefore conclude that TAH does not exist. This resolves a long-standing dilemma about the location and nature of the six-fold-coordinated aluminum observed by 27Al NMR in C-A-S-H samples.

Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS-PIETA NMR Spectroscopy.

Ligand exchange and CdS shell growth onto colloidal CdSe nanoplatelets (NPLs) using colloidal atomic layer deposition (c-ALD) were investigated by solid-state nuclear magnetic resonance (NMR) experiments, in particular, dynamic nuclear polarization (DNP) enhanced phase adjusted spinning sidebands-phase incremented echo-train acquisition (PASS-PIETA). The improved sensitivity and resolution of DNP enhanced PASS-PIETA permits the identification and study of the core, shell, and surface species of CdSe and CdSe/CdS core/shell NPLs heterostructures at all stages of c-ALD. The cadmium chemical shielding was found to be proportionally dependent on the number and nature of coordinating chalcogen-based ligands. DFT calculations permitted the separation of the the 111/113Cd chemical shielding into its different components, revealing that the varying strength of paramagnetic and spin-orbit shielding contributions are responsible for the chemical shielding trend of cadmium chalcogenides. Overall, this study points to the roughening and increased chemical disorder at the surface during the shell growth process, which is not readily captured by the conventional characterization tools such as electron microscopy.

Measurement of Proton Spin Diffusivity in Hydrated Cementitious Solids.

The study of hydration and crystallization processes involving inorganic oxides is often complicated by poor long-range order and the formation of heterogeneous domains or surface layers. In solid-state NMR, 1H-1H spin diffusion analyses can provide information on spatial composition distributions, domain sizes, or miscibility in both ordered and disordered solids. Such analyses have been implemented in organic solids but crucially rely on separate measurements of the 1H spin diffusion coefficients in closely related systems. We demonstrate that an experimental NMR method, in which "holes" of well-defined dimensions are created in proton magnetization, can be applied to determine spin diffusion coefficients in cementitious solids hydrated with 17O-enriched water. We determine proton spin diffusion coefficients of 240 ± 40 nm2/s for hydrated tricalcium aluminate and 140 ± 20 nm2/s for hydrated tricalcium silicate under quasistatic conditions.

Atomic-level passivation mechanism of ammonium salts enabling highly efficient perovskite solar cells.

The high conversion efficiency has made metal halide perovskite solar cells a real breakthrough in thin film photovoltaic technology in recent years. Here, we introduce a straightforward strategy to reduce the level of electronic defects present at the interface between the perovskite film and the hole transport layer by treating the perovskite surface with different types of ammonium salts, namely ethylammonium, imidazolium and guanidinium iodide. We use a triple cation perovskite formulation containing primarily formamidinium and small amounts of cesium and methylammonium. We find that this treatment boosts the power conversion efficiency from 20.5% for the control to 22.3%, 22.1%, and 21.0% for the devices treated with ethylammonium, imidazolium and guanidinium iodide, respectively. Best performing devices showed a loss in efficiency of only 5% under full sunlight intensity with maximum power tracking for 550 h. We apply 2D- solid-state NMR to unravel the atomic-level mechanism of this passivation effect.

Line narrowing in 1H NMR of powdered organic solids with TOP-CT-MAS experiments at ultra-fast MAS.

The residual broadening observed in 1H spectra of rigid organic solids at natural abundance under 111 kHz magic angle spinning (MAS) is typically a few hundred Hertz. Here we show that refocusable and non-refocusable interactions contribute roughly equally to this residual at high-fields (21.14 T), and suggest that the removal of the non-refocusable part will produce significant increase in spectral resolution. To this end, we demonstrate an experiment for the indirect acquisition of constant-time experiments at ultra-fast MAS (CT-MAS) which verifies this hypothesis. The combination of this experiment with the two-dimensional one pulse (TOP) transformation reduces the experimental time to a fraction of the original cost while retaining the narrowing effects. Results obtained with TOP-CT-MAS at 111 kHz MAS on a sample of ß-AspAla yield up to 30% higher resolution spectra than the equivalent one-pulse experiment, in less than 10 min.

One- and Two-Dimensional High-Resolution NMR from Flat Surfaces.

Determining atomic-level characteristics of molecules on two-dimensional surfaces is one of the fundamental challenges in chemistry. High-resolution nuclear magnetic resonance (NMR) could deliver rich structural information, but its application to two-dimensional materials has been prevented by intrinsically low sensitivity. Here we obtain high-resolution one- and two-dimensional 31P NMR spectra from as little as 160 picomoles of oligonucleotide functionalities deposited onto silicate glass and sapphire wafers. This is enabled by a factor >105 improvement in sensitivity compared to typical NMR approaches from combining dynamic nuclear polarization methods, multiple-echo acquisition, and optimized sample formulation. We demonstrate that, with this ultrahigh NMR sensitivity, 31P NMR can be used to observe DNA bound to miRNA, to sense conformational changes due to ion binding, and to follow photochemical degradation reactions.

Maximizing nuclear hyperpolarization in pulse cooling under MAS.

It has recently been shown how dynamic nuclear polarization can be used to hyperpolarize the bulk of proton-free solids. This is achieved by generating the polarization in a wetting phase, transferring it to nuclei near the surface and relaying it towards the bulk through homonuclear spin diffusion between weakly magnetic nuclei. Pulse cooling is a strategy to achieve this that uses a multiple contact cross-polarization sequence for bulk hyperpolarization. Here, we show how to maximize sensitivity using the pulse cooling method by experimentally optimizing pulse parameters and delays on a sample of powdered SnO2. To maximize sensitivity we introduce an approach where the magic angle spinning rate is modulated during the experiment: the CP contacts are carried out at a slow spin rate to benefit from faster spin diffusion, and the spin rate is then accelerated before detection to improve line narrowing. This method can improve the sensitivity of pulse cooling for 119Sn spectra of SnO2 by an additional factor of 3.5.

Chemical exchange at the ferroelectric phase transition of lead germanate revealed by solid state 207Pb nuclear magnetic resonance.

The nature of the dynamics and structural changes that take place at the ferroelectric phase transition in lead oxides is a rich field of study. Solid-state nuclear magnetic resonance (NMR) of 207Pb is well suited to study the local structure and disorder in lead oxide ferroelectric transitions at the atomic level. However, very large 207Pb shielding anisotropy results in poor resolution in 1D static and magic angle spinning (MAS) NMR spectra. We address this problem by using short high-power adiabatic pulses (SHAPs) with magic-angle-turning sequences to correlate the isotropic and anisotropic parts of the 207Pb chemical shift tensor in a 2D NMR experiment, yielding resolved 207Pb NMR spectra of the nine distinct lead sites in uniaxial ferroelectric lead germanate (Pb5Ge3O11). Using this technique we detect the magnetic environments of displaced Pb2+ ions and unambiguously identify the nature of the phase transition as mixed displacive and order-disorder. We also observe that the atomic-level process responsible for the phase transition in ferroelectric lead germanate is chemical exchange on the kilohertz timescale. We derive an activation energy of 103.4 ± 1.7 kJ mol-1 and compare it to dielectric spectroscopy studies on similar materials. These results show that this method can be used to characterize ferroelectric phase transitions of complex materials with high resolution using nuclei that are typically inaccessible due to their large shielding anisotropy.

Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy.

Understanding the surface of semiconductor nanocrystals (NCs) prepared using colloidal methods is a long-standing goal of paramount importance for all their potential optoelectronic applications, which remains unsolved largely because of the lack of site-specific physical techniques. Here, we show that multidimensional 113Cd dynamic nuclear polarization (DNP) enhanced NMR spectroscopy allows the resolution of signals originating from different atomic and magnetic surroundings in the NC cores and at the surfaces. This enables the determination of the structural perfection, and differentiation between the surface and core atoms in all major forms of size- and shape-engineered CdSe NCs: irregularly faceted quantum dots (QDs) and atomically flat nanoplatelets, including both dominant polymorphs (zinc-blende and wurtzite) and their epitaxial nanoheterostructures (CdSe/CdS core/shell quantum dots and CdSe/CdS core/crown nanoplatelets), as well as magic-sized CdSe clusters. Assignments of the NMR signals to specific crystal facets of oleate-terminated ZB structured CdSe NCs are proposed. Significantly, we discover far greater atomistic complexity of the surface structure and the species distribution in wurtzite as compared to zinc-blende CdSe QDs, despite an apparently identical optical quality of both QD polymorphs.

Hydrogen motional disorder in crystalline iron group chloride dihydrates.

The principal components and the relative orientation of the 2H paramagnetic shift and quadrupolar coupling tensors have been measured for the MCl2·2D2O family of compounds, M = Mn, Fe, Co, Ni, and Cu, using the two-dimensional shifting-d echo nuclear magnetic resonance experiment in order to determine (1) the degree of unpaired electron delocalization and (2) the number and location of crystallographically distinct hydrogen sites around oxygen and their fractional occupancies. Expressions for the molecular susceptibility of 3d ion systems, where the spin-orbit coupling is a weak perturbation onto the crystal field, are derived using the generalized Van Vleck equation and used to predict molecular susceptibilities. These predicted molecular susceptibilities are combined with various point dipole source configurations modeling unpaired electron delocalization to predict 2H paramagnetic shift tensors at potential deuterium sites. The instantaneous deuterium quadrupolar coupling and shift tensors are then combined with parameterized motional models, developed for trigonally (M = Mn, Fe, Co, and Cu) and pyramidally (M = Ni) coordinated D2O ligands, to obtain the best fit of the experimental 2D spectra. Dipole sources placed onto metal nuclei with a small degree of delocalization onto the chlorine ligands yield good agreement with the experiment for M = Mn, Fe, Co, and Ni, while good agreement for CuCl2·2D2O is obtained with additional delocalization onto the oxygen. Our analysis of the salts with trigonally coordinated water ligands (M = Mn, Fe, Co, and Cu) confirms the presence of bisector flipping and the conclusions from neutron scattering measurements that hydrogen bonding to chlorine on two adjacent chains leads to the water molecule in the [M(D2O)2Cl4] cluster being nearly coplanar with O-M-Cl involving the shortest metal-chlorine bonds of the cluster. In the case of NiCl2·2D2O, the experimental parameters were found to be consistent with a motional model where the D2O ligands are pyramidally coordinated to the metal and undergo bisector flipping while the water ligand additionally hops between two orientations related by a 120° rotation about the Ni-O bond axis. The position of the three crystallographically distinct hydrogen sites in the unit cell was determined along with fractional occupancies. This restricted water ligand motion is likely due to van der Waals interactions and is concerted with the motion of neighboring ligands.

Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface.

NMR is a method of choice to determine structural and electronic features in inorganic materials, and has been widely used in the past, but its application is severely limited by its low relative sensitivity. We show how the bulk of proton-free inorganic solids can be hyperpolarized with a general strategy using impregnation dynamic nuclear polarization through homonuclear spin diffusion between low-γ nuclei. This is achieved either through direct hyperpolarization or with a pulse cooling cross-polarization method, transferring hyperpolarization from protons to heteronuclei at particle surfaces. We demonstrate a factor of 50 gain in overall sensitivity for the 119Sn spectrum of powdered SnO2, corresponding to an acceleration of a factor >2500 in acquisition times. The method is also shown for 31P spectra of GaP, 113Cd spectra of CdTe, and 29Si spectra of α-quartz.

DNP enhanced NMR with flip-back recovery.

DNP methods can provide significant sensitivity enhancements in magic angle spinning solid-state NMR, but in systems with long polarization build up times long recycling periods are required to optimize sensitivity. We show how the sensitivity of such experiments can be improved by the classic flip-back method to recover bulk proton magnetization following continuous wave proton heteronuclear decoupling. Experiments were performed on formulations with characteristic build-up times spanning two orders of magnitude: a bulk BDPA radical doped o-terphenyl glass and microcrystalline samples of theophylline, l-histidine monohydrochloride monohydrate, and salicylic acid impregnated by incipient wetness. For these systems, addition of flip-back is simple, improves the sensitivity beyond that provided by modern heteronuclear decoupling methods such as SPINAL-64, and provides optimal sensitivity at shorter recycle delays. We show how to acquire DNP enhanced 2D refocused CP-INADEQUATE spectra with flip-back recovery, and demonstrate that the flip-back recovery method is particularly useful in rapid recycling regimes. We also report Overhauser effect DNP enhancements of over 70 at 592.6 GHz/900 MHz.

Determining the Surface Structure of Silicated Alumina Catalysts via Isotopic Enrichment and Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy.

Isotopic enrichment of 29Si and DNP-enhanced NMR spectroscopy are combined to determine the detailed surface structure of a silicated alumina catalyst. The significant sensitivity enhancement provided by DNP is vital to the acquisition of multinuclear and multidimensional experiments that provide information on the atomic-level structure of the species present at the surface. Isotopic enrichment not only facilitates spectral acquisition, particularly given the low (1.5 wt %) Si loading, but also enables spectra with higher resolution than those acquired using DNP to be obtained. The unexpected similarity of conventional, CP, and DNP NMR spectra is attributed to the presence of adventitious surface water that forms a sufficiently dense 1H network at the silica surface so as to mediate efficient polarization transfer to all Si species regardless of their chemical nature. Spectra reveal the presence of Si-O-Si linkages at the surface (identified as Q4(3Al)-Q4(3Al)) and confirm that the anchoring of the surface overlayer with the alumina occurs through AlIV and AlV species only. This suggests the presence of Q3/Q4 Si at the surface affects the neighboring Al species, modifying the surface structure and making it less likely AlVI environments are in close spatial proximity. In contrast, Q1/Q2 species, bonded to the surface by fewer covalent bonds, have less of an effect on the surface, and more AlVI species are consequently found nearby. The combination of isotropic enrichment and DNP provides a definitive and fully quantitative description of the Si-modified alumina surface, and we demonstrate that almost one-third of the silicon at the surface is connected to another Si species, even at the low level of coverage used, lowering the propensity for the formation of Brønsted acid sites. This suggests that a variation in the synthetic procedure might be required to obtain a more even coverage for optimum performance. The work here will allow for more rigorous future investigations of structure-function relationships in these complex materials.

Two-dimensional NMR measurement and point dipole model prediction of paramagnetic shift tensors in solids.

A new two-dimensional Nuclear Magnetic Resonance (NMR) experiment to separate and correlate the first-order quadrupolar and chemical/paramagnetic shift interactions is described. This experiment, which we call the shifting-d echo experiment, allows a more precise determination of tensor principal components values and their relative orientation. It is designed using the recently introduced symmetry pathway concept. A comparison of the shifting-d experiment with earlier proposed methods is presented and experimentally illustrated in the case of (2)H (I = 1) paramagnetic shift and quadrupolar tensors of CuCl2⋅2D2O. The benefits of the shifting-d echo experiment over other methods are a factor of two improvement in sensitivity and the suppression of major artifacts. From the 2D lineshape analysis of the shifting-d spectrum, the (2)H quadrupolar coupling parameters are 〈Cq〉 = 118.1 kHz and 〈ηq〉 = 0.88, and the (2)H paramagnetic shift tensor anisotropy parameters are 〈ζP〉 = - 152.5 ppm and 〈ηP〉 = 0.91. The orientation of the quadrupolar coupling principal axis system (PAS) relative to the paramagnetic shift anisotropy principal axis system is given by (α,ß,γ)=(π2,π2,0). Using a simple ligand hopping model, the tensor parameters in the absence of exchange are estimated. On the basis of this analysis, the instantaneous principal components and orientation of the quadrupolar coupling are found to be in excellent agreement with previous measurements. A new point dipole model for predicting the paramagnetic shift tensor is proposed yielding significantly better agreement than previously used models. In the new model, the dipoles are displaced from nuclei at positions associated with high electron density in the singly occupied molecular orbital predicted from ligand field theory.