1H Spin Research Articles

Dynamics in Flexible Pillar[n]arenes Probed by Solid-State NMR.

Pillar[n]arenes are supramolecular assemblies that can perform a range of technologically important molecular separations which are enabled by their molecular flexibility. Here, we probe dynamical behavior by performing a range of variable-temperature solid-state NMR experiments on microcrystalline perethylated pillar[n]arene (n = 5, 6) and the corresponding three pillar[6]arene xylene adducts in the 100–350 K range. This was achieved either by measuring site-selective motional averaged 13C 1H heteronuclear dipolar couplings and subsequently accessing order parameters or by determining 1H and 13C spin–lattice relaxation times and extracting correlation times based on dipolar and/or chemical shift anisotropy relaxation mechanisms. We demonstrate fast motional regimes at room temperature and highlight a significant difference in dynamics between the core of the pillar[n]arenes, the protruding flexible ethoxy groups, and the adsorbed xylene guest. Additionally, unexpected and sizable 13C 1H heteronuclear dipolar couplings for a quaternary carbon were observed for p-xylene adsorbed in pillar[6]arene only, indicating a strong host–guest interaction and establishing the p-xylene location inside the host, confirming structural refinements.

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.

Hyperpolarization of cis-15 N2 -Azobenzene by Parahydrogen at Ultralow Magnetic Fields*.

The development of nuclear spins hyperpolarization, and the search for molecules that can be efficiently hyperpolarized is an active area in nuclear magnetic resonance. In this work we present a detailed study of SABRE SHEATH (signal amplification by reversible exchange in shield enabled alignment transfer to heteronuclei) experiments on 15N2‐azobenzene. In SABRE SHEATH experiments the nuclear spins of the target are hyperpolarized through transfer of spin polarization from parahydrogen at ultralow fields during a reversible chemical process. Azobenzene exists in two isomers, trans and cis. We show that all nuclear spins in cis‐azobenzene can be efficiently hyperpolarized by SABRE at suitable magnetic fields. Enhancement factors (relative to 9.4 T) reach up to 3000 for 15N spins and up to 30 for the 1H spins. We compare two approaches to observe either hyperpolarized magnetization of 15N/1H spins, or hyperpolarized singlet order of the 15N spin pair. The results presented here will be useful for further experiments in which hyperpolarized cis‐15N2‐azobenzene is switched by light to trans‐15N2‐azobenzene for storing the produced hyperpolarization in the long‐lived spin state of the 15N pair of trans‐15N2‐azobenzene.

Effect of Methylene Chain Length on the Thermodynamic Properties, Ferroelastic Properties, and Molecular Dynamics of the Perovskite-type Layer Crystal [NH3(CH2) n NH3]MnCl4 (n = 2, 3, and 4).

The structures and phase transitions of [NH3(CH2)nNH3]MnCl4 (n = 2, 3, and 4) crystals were confirmed through X-ray diffraction and differential scanning calorimetry (DSC) experiments. Thermodynamic properties, ferroelastic properties, and molecular dynamics of three crystals were studied as a function of the number (n) of CH2 groups in the alkylene chains. The loss in molecular weight due to a decrease in n marked the onset of the partial thermal decomposition. The thermal decomposition temperature, Td, increased with increasing length of the CH2 chain. While the ferroelastic twin domain walls for n = 2 and 4 were in the same direction at all temperatures, the domain walls for n = 3 were rotated by 45°, and the direction of extinction in phase II was rotated by 45° with respect to phases I and III. The 1H and 13C MAS NMR spectra of the three crystals were recorded as a function of temperature. With increasing length of the CH2 chain, the 1H spin relaxation time decreased, indicating that molecular motions were activated. These results provide insights into the thermodynamic properties and structural dynamics of the [NH3(CH2)nNH3]MnCl4 crystals and are expected to facilitate their potential applications.

Benchtop In Situ Measurement of Full Adsorption Isotherms by NMR.

Physisorption using gas or vapor probe molecules is the most common characterization technique for porous materials. The method provides textural information on the adsorbent as well as the affinity for a specific adsorbate, typically through equilibrium pressure measurements. Here, we demonstrate how low-field NMR can be used to measure full adsorption isotherms, and how by selectively measuring 1H spins of the adsorbed probe molecules, rather than those in the vapor phase, this "NMR-relaxorption" technique provides insights about local dynamics beyond what can be learned from physisorption alone. The potential of this double-barreled approach was illustrated for a set of microporous metal-organic frameworks (MOFs). For methanol adsorption in ZIF-8, the method identifies multiple guest molecules populations assigned to MeOH clusters in the pore center, MeOH bound at cage windows and to MeOH adsorption on defect sites. For UiO-66(Zr), the sequential pore filling is demonstrated and accurate pore topologies are directly obtained, and for MIL-53(Al), structural phase transitions are accurately detected and linked with two populations of dimeric chemical species localized to specific positions in the framework.

Photochemically Induced Dynamic Nuclear Polarization of Heteronuclear Singlet Order.

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is a method to hyperpolarize nuclear spins using light. In most cases, CIDNP experiments are performed in high magnetic fields and the sample is irradiated by light inside a nuclear magnetic resonance (NMR) spectrometer. Here we demonstrate photo-CIDNP hyperpolarization generated in the Earth's magnetic field and under zero- to ultralow-field (ZULF) conditions. Irradiating a sample containing tetraphenylporphyrin and para-benzoquinone for several seconds with light-emitting diodes produces strong hyperpolarization of 1H and 13C nuclear spins, enhancing the NMR signals more than 200 times. The hyperpolarized spin states at the Earth's field and in ZULF are different. In the latter case, the state corresponds to the singlet order between scalar-coupled 1H-13C nuclear spins. This state has a longer lifetime than the state hyperpolarized at Earth's field. The method is simple and cost-efficient and should be applicable to many molecular systems known to exhibit photo-CIDNP, including amino acids and nucleotides.

Flow MAS NMR for In Situ Monitoring of Carbon Dioxide Capture and Hydrogenation Using Nanoporous Solids

The capability of 1H and 13C flow magic-angle spinning NMR is demonstrated for monitoring the capture of CO2 on the surface of mesoporous amine-functionalized silica and the progress of heterogeneo...

Glassy and Polymer Dynamics of Elastomers by 1H-Field-Cycling NMR Relaxometry: Effects of Fillers.

1H spin–lattice relaxation rate (R1) dispersions were acquired by field-cycling (FC) NMR relaxometry between 0.01 and 35 MHz over a wide temperature range on polyisoprene rubber (IR), either unfilled or filled with different amounts of carbon black, silica, or a combination of both, and sulfur cured. By exploiting the frequency–temperature superposition principle and constructing master curves for the total FC NMR susceptibility, χ″(ω) = ωR1(ω), the correlation times for glassy dynamics, τs, were determined. Moreover, the contribution of polymer dynamics, χpol″(ω), to χ″(ω) was singled out by subtracting the contribution of glassy dynamics, χglass″(ω), well represented by the Cole–Davidson spectral density. Glassy dynamics resulted moderately modified by the presence of fillers, τs values determined for the filled rubbers being slightly different from those of the unfilled one. Polymer dynamics was affected by the presence of fillers in the Rouse regime. A change in the frequency dependence of χpol″(ω) at low frequencies was observed for all filled rubbers, more pronounced for those reinforced with silica, which suggests that the presence of the filler particles can affect chain conformations, resulting in a different Rouse mode distribution, and/or interchain interactions modulated by translational motions.

Physicochemical properties and structural dynamics of organic\u2013inorganic hybrid [NH3(CH2)3NH3]ZnX4 (X\u2009=\u2009Cl and Br) crystals

The physical properties of the organic–inorganic hybrid crystals having the formula [NH3(CH2)3NH3]ZnX4 (X = Cl, Br) were investigated. The phase transition temperatures (TC; 268K for Cl and 272K for Br) of the two crystals bearing different halogen atoms in their skeletons were determined through differential scanning calorimetry. The thermodynamic properties of the two crystals were investigated through thermogravimetric analysis. The structural dynamics, particularly the role of the [NH3(CH2)3NH3] cation, were probed through 1H and 13C magic-angle spinning nuclear magnetic resonance spectroscopy as a function of temperature. The 1H and 13C NMR chemical shifts did not show any changes near TC. In addition, the 1H spin–lattice relaxation time (T1ρ) varied with temperature, whereas the 13C T1ρ values remained nearly constant at different temperatures. The T1ρ values of the atoms in [NH3(CH2)3NH3]ZnCl4 were higher than those in [NH3(CH2)3NH3]ZnBr4. The observed differences in the structural dynamics obtained from the chemical shifts and T1ρ values of the two compounds can be attributed to the differences in the bond lengths and halogen atoms. These findings can provide important insights or potential applications of these crystals.

Dipolar dephasing for structure determination in a paramagnetic environment

We demonstrate the efficacy of the REDOR-type sequences in determining dipolar coupling strength in a paramagnetic environment. Utilizing paramagnetic effects of enhanced relaxation rates and rapid electronic fluctuations in Cu(II)-(DL-Ala)2.H2O, the dipolar coupling for the methyl C–H that is 4.20 ​Å (methyl carbon) away from the Cu2+ ion, was estimated to be 8.8 ​± ​0.6 ​kHz. This coupling is scaled by a factor of ~0.3 in comparison to the rigid limit value of ~32 ​kHz, in line with partial averaging of the dipolar interaction by rotational motion of the methyl group. Limited variation in the scaling factor of the dipolar coupling strength at different temperatures is observed. The C–H internuclear distance derived from the size of the dipolar coupling is similar to that observed in the crystal structure. The errors in the dipolar coupling strength observed in the REDOR-type experiments are similar to those reported for diamagnetic systems. Increase in resolution due to the Fermi contact shifts, coupled with MAS frequencies of 30–35 ​kHz allowed to estimate the hyperfine coupling strengths for protons and carbons from the temperature dependence of the chemical shift and obtain a high resolution 1H–1H spin diffusion spectrum. This study shows the utility of REDOR-type sequences in obtaining reliable structural and dynamical information from a paramagnetic complex. We believe that this can help in studying the active site of paramagnetic metalloproteins at high resolution.

Spin diffusion transfer difference (SDTD) NMR: An advanced method for the characterisation of water structuration within particle networks

HypothesisThe classical STD NMR protocol to monitor solvent interactions in gels is strongly dependent on gelator and solvent concentrations and does not report on the degree of structuration of the solvent at the particle/solvent interface. We hypothesised that, for suspensions of large gelator particles, solvent structuration could be characterised by STD NMR when taking into account the particle-to-solvent 1H–1H spin diffusion transfer using the 1D diffusion equation. ExperimentsWe have carried out a systematic study on effect of gelator and solvent concentrations, and gelator surface charge, affecting the behaviour of the classical STD NMR build-up curves. To do so, we have characterised solvent interactions in dispersions of starch and cellulose-like particles prepared in deuterated water and alcohol/D2O mixtures. FindingsThe Spin Diffusion Transfer Difference (SDTD) NMR protocol is independent of the gelator and solvent concentrations, hence allowing the estimation of the degree of solvent structuration within different particle networks. In addition, the simulation of SDTD build-up curves using the general one–dimensional diffusion equation allows the determination of minimum distances (r) and spin diffusion rates (D) at the particle/solvent interface. This novel NMR protocol can be readily extended to characterise the solvent(s) organisation in any type of colloidal systems constituted by large particles.

Structural Role and Spatial Distribution of Carbonate Ions in Amorphous Calcium Phosphate

The local structures of a series of amorphous calcium phosphate (ACP) phases with increasing carbonate contents (2–14 wt %) were studied by multinuclear 1H, 13C, 23Na, and 31P magic-angle spinning ...

Structural characterization, thermal properties, and molecular motions near the phase transition in hybrid perovskite [(CH2)3(NH3)2]CuCl4 crystals: 1H, 13C, and 14N nuclear magnetic resonance

The structural characterization of the [(CH2]3(NH3)2]+ cation in the perovskite [(CH2)3(NH3)2]CuCl4 crystal was performed by solid-state 1H nuclear magnetic resonance (NMR) spectroscopy. The 1H NMR chemical shifts for NH3 changed more significantly with temperature than those for CH2. This change in cationic motion is enhanced at the N-end of the organic cation, which is fixed to the inorganic layer by N–H···Cl hydrogen bonds. The 13C chemical shifts for CH2-1 increase slowly without any anomalous change, while those for CH2-2 move abruptly compared to CH2-1 with increasing temperature. The four peaks of two groups in the 14N NMR spectra, indicating the presence of a ferroelastic multidomain, were reduced to two peaks of one group near TC2 (= 333 K); the 14N NMR data clearly indicated changes in atomic configuration at this temperature. In addition, 1H and 13C spin–lattice have shorter relaxation times (T1ρ), in the order of milliseconds because T1ρ is inversely proportional to the square of the magnetic moment of paramagnetic ions. The T1ρ values for CH2 and NH3 protons were almost independent of temperature, but the CH2 moiety located in the middle of the N–C–C–C–N bond undergoes tumbling motion according to the Bloembergen–Purcell–Pound theory. Ferroelasticity is the main cause for the phase transition near TC2.

Electrochemical Control and Understanding of the Electronic and Optical Properties of ZnO Formed in Rechargeable Zn-Alkaline Batteries

Rechargeable alkaline batteries using zinc metal (Zn) electrodes are of significant interest for next-generation energy storage because of their high energy density, low cost, environmental friendliness, and inherent safety. Rechargeable Zn-alkaline batteries, however, have not been widely commercialized in part due to the low cyclability of Zn electrodes. At high depths of discharge, zinc oxide (ZnO) discharge product accumulates in the electrode, passivating the zinc-electrolyte interface and ultimately leading to cell failure. While the ZnO discharge product has been studied for decades, there is no general consensus on its local structure, composition, and electrochemical properties, nor on how these properties are affected by the electrode potential or cycling conditions.In this work, we used in operando analytical techniques including X-Ray diffraction (XRD), confocal Raman spectroscopy, electrochemical impedance spectroscopy (EIS), and ultraviolet-visible (UV-vis) spectroscopy to demonstrate that the discharge product is proton-doped ZnO with oxygen vacancies, and that the electrical conductivity, band gap, and color vary systematically as a function of electrode potential. To investigate local hydrogen and zinc environments within the ZnO discharge product, we performed ex situ solid-state 1H, 2H, and 67Zn magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Solid-state 1H and 2H NMR measurements establish that hydrogen environments change as a function of potential, while solid-state 67Zn NMR spectra indicate that the local zinc environments are distorted compared to ZnO generated via conventional routes. We also performed 2D 1H-1H exchange (EXSY) NMR measurements to investigate proton mobility between defect environments and acquired 1D 1H-1H dipolar-mediated double quantum-filtered MAS NMR spectra to establish through-space molecular-level proximities between proton sites. This work provides insights into the dynamics of zinc electrodes in alkaline environments, as well as how to electrochemically control the defect structures and electronic and optical properties of ZnO in alkaline media, which are expected to aid the development of next-generation Zn alkaline batteries and electronic devices utilizing ZnO.

Dynamic Nuclear Polarization with Vanadium(IV) Metal Centers

Summary Dynamic nuclear polarization (DNP) harnesses the large polarization of electron spins to dramatically increase nuclear magnetic resonance (NMR) sensitivity. This study expands the scope of DNP beyond its traditional focus on hyperpolarizing the solvent network using exogenous polarizing agents (PAs). We introduce 1H DNP with endogenous V4+ centers positioned in a set of vanadyl complexes with tunable V4+-1H distances. We traced the polarization transfer from V4+ to 1H spins, specifically differentiating between direct V4+-1Hs polarization transfer and the 1H spin-diffusion-mediated bulk solvent 1H polarization buildup and illuminated the effect of the V4+-1H distance on these processes. These results deepen our understanding of polarization pathways and expand the catalog of PAs to broad-line transition metals. This study establishes crucial first steps toward employing strategically positioned endogenous paramagnetic metal centers for DNP and the conceptual framework of hyperfine DNP spectroscopy that merges both spatial and chemical diagnosis of target nuclear spins.

Glassy and Polymer Dynamics of Elastomers by 1H Field-Cycling NMR Relaxometry: Effects of Cross-Linking.

1H spin lattice relaxation rate (R1) dispersions were acquired by field-cycling (FC) NMR relaxometry between 0.01 and 35 MHz over a wide temperature range on polyisoprene (IR), polybutadiene (BR), and poly(styrene-co-butadiene) (SBR) rubbers, obtained by vulcanization under different conditions, and on the corresponding uncured elastomers. By exploiting the frequency–temperature superposition principle, χ″(ωτs) master curves were constructed by shifting the total FC NMR susceptibility, χ″(ω) = ωR1(ω), curves along the frequency axis by the correlation times for glassy dynamics, τs. Longer τs values and, correspondingly, higher glass transition temperatures were determined for the sulfur-cured elastomers with respect to the uncured ones, which increased by increasing the cross-link density, whereas no significant changes were found for fragility. The contribution of polymer dynamics, χpol″(ω), to χ″(ω) was singled out by subtracting the contribution of glassy dynamics, χglass″(ω), well represented using a Cole–Davidson spectral density. For all elastomers, χpol″(ω) was found to represent a small fraction, on the order of 0.05–0.14, of the total χ″(ω), which did not show a significant dependence on cross-link density. In the investigated temperature and frequency ranges, polymer dynamics was found to encompass regimes I (Rouse dynamics) and II (constrained Rouse dynamics) of the tube reptation model for the uncured elastomers and only regime I for the vulcanized ones. This is clear evidence that chemical cross-links impose constraints on chain dynamics on a larger space and time scale than free Rouse modes.

Broadband radio-frequency transmitter for fast nuclear spin control.

The active manipulation of nuclear spins with radio-frequency (RF) coils is at the heart of nuclear magnetic resonance (NMR) spectroscopy and spin-based quantum devices. Here, we present a miniature RF transmitter designed to generate strong RF pulses over a broad bandwidth, allowing for fast spin rotations on arbitrary nuclear species. Our design incorporates (i) a planar multilayer geometry that generates a large field of 4.35 mT per unit current, (ii) a 50 Ω transmission circuit with a broad excitation bandwidth of ∼20 MHz, and (iii) an optimized thermal management leading to minimal heating at the sample location. Using individual 13C nuclear spins in the vicinity of a diamond nitrogen-vacancy center as a test system, we demonstrate Rabi frequencies exceeding 70 kHz and nuclear π/2 rotations within 3.4 μs. The extrapolated values for 1H spins are about 240 kHz and 1 μs, respectively. Beyond enabling fast nuclear spin manipulations, our transmitter system is ideally suited for the incorporation of advanced pulse sequences into micro- and nanoscale NMR detectors operating at a low (<1 T) magnetic field.

Decoherence optimized tilted-angle cross polarization: A novel concept for sensitivity-enhanced solid-state NMR using ultra-fast magic angle spinning

Ultra-fast magic-angle spinning (UFMAS) at a MAS rate (ωR/2π) of 60 kHz or higher has dramatically improved the resolution and sensitivity of solid-state NMR (SSNMR). However, limited polarization transfer efficiency using cross-polarization (CP) between 1H and rare spins such as 13C still restricts the sensitivity and multi-dimensional applications of SSNMR using UFMAS. We propose a novel approach, which we call decoherence-optimized tilted-angle CP (DOTA CP), to improve CP efficiency with prolonged lifetime of 1H coherence in the spin-locked condition and efficient band-selective polarization transfer by incorporating off-resonance irradiation to 1H spins. 13C CP-MAS at ωR/2π of 70–90 kHz suggested that DOTA CP notably outperformed traditional adiabatic CP, a de-facto-standard CP scheme over the past decade, in sensitivity for the aliphatic-region spectra of 13C-labeled GB1 protein and N-formyl-Met-Leu-Phe samples by up to 1.4- and 1.2-fold, respectively. 1H-detected 2D 1H/13C SSNMR for the GB1 sample indicated the effectiveness of this approach in various multidimensional applications.

Dynamics of Ionic Liquids in Confinement by Means of NMR Relaxometry-EMIM-FSI in a Silica Matrix as an Example.

1H and 19F spin–lattice relaxation studies for 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide in bulk and mesoporous MCM-41 silica matrix confinement were performed under varying temperatures in a broad range of magnetic fields, corresponding to 1H resonance frequency from 5Hz to 30MHz.A thorough analysis of the relaxation data revealed a three-dimensional translation diffusion of the ions in the bulk liquid and two-dimensional diffusion in the vicinity of the confining walls in the confinement. Parameters describing the translation dynamics were determined and compared. The rotational motion of both kinds of ions in the confinement was described by two correlation times that might be attributed to anisotropic reorientation of these species.

Dynamics in nanoporous materials probed by 2H solid state NMR: estimation of self-diffusion coefficients

2H solid-state NMR represents a reliable method for probing the dynamics of the molecules in confined geometries, including microporous and mesoporous materials such as zeolites or metal–organic frameworks (MOFs). Dynamic characteristics derived from analyses of 2H NMR line shape and spin relaxation for adsorbed deuterated molecules can be related with translational motions along the porous framework. This opens up the possibility to use 2H NMR for probing self-diffusion within porous materials. Herein we discuss the basic principles and approaches that can be used to derive the information on diffusion coefficients based on analysis of the dynamics of confined molecules by 2H NMR. We discuss the corresponding possibilities and limitations of 2H NMR compared to other experimental methods, such QENS and PFG NMR. Several examples for estimation of diffusion coefficients by 2H NMR for alkanes, olefins, alcohol, benzene, pyridine in zeolites and MOFs are provided.