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Results for “Dipolar Recoupling”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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14 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

t 1 -noise elimination by continuous chemical shift anisotropy refocusing

We report that due to their high gyromagnetic ratio, there is considerable interest in measuring distances and correlations involving protons, but such measurements are compounded by the simultaneous recoupling of chemical shift anisotropy (CSA). This secondary recoupling adds additional modulations to the signal intensities that ultimately lead to t 1 -noise and signal decay. Recently, Venkatesh et al. demonstrated that the addition of CSA refocusing periods during 1 H-X dipolar recoupling led to sequences with far higher stability and performance. Herein, we describe a related effort and develop a symmetry-based recoupling sequence that continually refocuses the 1 H CSA. This sequence shows superior performance to the regular and t1-noise eliminated D-HMQC sequences in the case of spin-1/2 nuclei and comparable performance to the later for half-integer quadrupoles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Phase-sensitive γ-encoded recoupling of heteronuclear dipolar interactions and 1 H chemical shift anisotropy

γ-encoded recoupling sequences are known to produce strong amplitude modulations that lead to sharp doublets when Fourier transformed. These doublets depend very little on the recoupled tensor asymmetry and thus enable for the straightforward determination of dynamic order parameters. It can, however, be difficult to measure small anisotropies, or small order parameters, using such sequences; the resonances from the doublet may overlap with each other, or with the zero-frequency glitch. This limitation has prevented the widespread use of 1 H chemical shift anisotropy (CSA) for the measurement of dynamics, particularly for CH protons which typically have CSAs of only a few ppm when immobile. In this work, we introduce a simple modification to the traditional 1 H CSA and proton-detected local field pulse sequences that enables the acquisition of a hypercomplex dataset and the removal of the uncorrelated magnetization that results in the zero-frequency glitch. These new sequences then yield a frequency shift in the indirect dimension, rather than a splitting, which is easily identifiable even in cases of weak interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Highly Sensitive Measurements of Methylene Dynamics With a Frequency-Selective Double-Quantum Sideband Method

Probing the fast dynamics of surface sites using NMR spectroscopy is highly challenging owing to the sites’ high dilution and the difficulties often associated with isotopic enrichment. Intra-CH 2 1 H- 1 H dipolar couplings are ideal probes of motions given that they only involve 1 H’s and the tensor has a well-defined size and orientation. Here, we introduce a frequency-selective variant of the double-quantum sideband method to measure like-spin 1 H- 1 H dipolar coupling constants. The experiment dramatically reduces the instrument time required to measure dynamically-averaged intra-CH 2 dipolar couplings. We demonstrate the performance of the sequence using silica-supported silanes as model highly-mobile surface species.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Size matters: altering the metal-surface coordination in micropores via structural confinement effects

Solid-state NMR experiments were used to investigate the dynamics of supported complexes grafted to a series of silica gel materials of varied pore sizes. Through dipolar recoupling measurements, we found that ligand dynamics were impeded in the more confined environments, as would be expected. A new form of motion involving the complex as a whole, however, appeared in the most restricted environment consisting of 22 Å diameter pores. These motions persisted down to –100 °C at which point the ligands were frozen on the NMR timescale. The newly observed dynamics could only result from the breaking of secondary dative metal–siloxane interactions that otherwise lock the complex in a preferred orientation on the surface. Crucially, these results show that confinement effects alone can be sufficient to reduce a grafted metal's effective coordination number in direct analogy to the synthesis of undercoordinated complexes using bulky ligands. Finally, this finding could have important implications in the synthesis of more active heterogeneous catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

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. Here, 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.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Elucidating molecular level interfacial interactions between a de novo protein and nucleated calcite with solid-state NMR

Biomineralization is the process by which organisms use biomolecules to produce hierarchically structured organic–inorganic composites. Using biology as inspiration, a protein construct (FD31) was previously designed to accelerate formation of nano-calcite with an unconventional {110} face. Here, to understand the molecular interactions essential for protein aided calcite nucleation, solid-state nuclear magnetic resonance (ssNMR) spectroscopy was used in this work to characterize the FD31–calcite interface at the atomic level. Glutamic acid side chains designed to interact directly with calcium ions on the surface were found to have dynamics on the sub-millisecond timescale, indicating possible interactions between the protein and surface waters that were not included in the original model. Dipolar ssNMR recoupling techniques also showed that the protein backbone is ∼2 Å closer to the surface than in the original docking model. Refined molecular simulations were done in the presence of explicit waters, which resulted in the protein backbone closer to the surface than in the original docking structure, providing better agreement with experiment and highlighting the important role played by water in FD31–calcite interactions. While this work provides the first experimental confirmation that FD31 interactions with calcite are localized to the surface of the protein designed to serve as a template, these studies do indicate a more dynamic binding and closer binding mode between FD31 and the nucleated surface than originally proposed. In all, this enhanced molecular insight into the FD31–calcite interface has advanced our fundamental understanding of the atomic interactions at the organic–inorganic interface and will aid in the design of biological templates for the nucleation of inorganic crystals.

Close, Emily G. S. [Pacific Northwest National Lab↗
DOE OSTI2025

DataSet for Elucidating molecular level interfacial interactions between a de novo protein and nucleated calcite with solid-state NMR

Biomineralization is the process by which organisms use biomolecules to produce hierarchically structured organic-inorganic composites. Using biology as inspiration, a protein construct (FD31) was previously designed to accelerate formation of nano-calcite with an unconventional {110} face. To understand the molecular interactions essential for protein aided calcite nucleation, solid-state nuclear magnetic resonance (ssNMR) spectroscopy was used in this work to characterize the FD31-calcite interface at the atomic level. Glutamic acid side chains designed to interact directly with calcium ions on the surface were found to have dynamics on the sub-millisecond timescale, indicating possible interactions between the protein and surface waters that were not included in the original model. Dipolar ssNMR recoupling techniques also showed that the protein backbone is ~2 Å closer to the surface than in the original docking model. Refined molecular simulations were done in the presence of explicit waters, which resulted in the protein backbone closer to the surface than in the original docking structure, providing better agreement with experiment and highlighting the important role played by water in FD31-calcite interactions. These studies provide the first experimental evidence to confirm that FD31 interactions with calcite are localized to the surface of the protein designed to serve as a template. However, these studies do indicate a more dynamic binding and closer binding mode between FD31 and the nucleated surface than originally proposed. In all, this enhanced molecular insight into the FD31-calcite interface has advanced our fundamental understanding of the atomic interactions at the organic-inorganic interface and will aid in the design of biological templates for the nucleation of inorganic crystals.

Saccuzzo Close, Emily Grace [Pacific Northwest Nat↗
DOE OSTI2022

Dipolar Heteronuclear Correlation Solid-State NMR Experiments between Half-Integer Quadrupolar Nuclei: The Case of 11 B– 17 O

With 73% of all NMR-active nuclei being quadrupolar, there is great interest in the development of NMR experiments that can probe the proximity of quadrupolar spins. Here, pulse sequences for magic-angle spinning (MAS) 11 B– 17 O resonance-echo saturation-pulse double-resonance (RESPDOR) and dipolar heteronuclear multiple quantum correlation (D-HMQC) solid-state NMR experiments were investigated. In these pulse sequences, rotational-echo double-resonance (REDOR) recoupling was used with central transition (CT)-selective π-pulses applied to either the 11 B or 17 O spins to recouple 11 B– 17 O dipolar interactions. 11 B{ 17 O} RESPDOR experiments on 17 O-enriched boric acid and benzene diboronic acid showed that application of dipolar recoupling on the 11 B channel yielded more signal dephasing than when recoupling is applied on the 17 O channel; however, short effective 11 B transverse relaxation time constants (T 2 ') hinder the acquisition of dephasing curves out to long recoupling durations. Application of REDOR recoupling to 17 O spins was found to produce significant dephasing without compromising the 11 B T 2 '. Comparison of experimental 11 B{ 17 O} RESPDOR curves to those of numerical simulations enabled the 17 O isotopic enrichment to be estimated. 2D 11 B{ 17 O} D-HMQC spectra were recorded with either 11 B or 17 O REDOR recoupling under a variety of radio frequency field conditions. Lastly, 2D 11 B{ 17 O} and 23 Na{ 17 O} D-HMQC spectra of an 17 O-enriched sodium borate glass were acquired to demonstrate the practical application of these heteronuclear correlation experiments to probe structural connectivity between two quadrupolar spins. Importantly, the high-field 2D 11 B– 17 O D-HMQC NMR spectrum revealed two unique 17 O sites correlating to 4-coordinate BO 4 ( [4] B), which were attributed to the [3] B–O– [4] B and [4] B–O– [4] B bridging O atoms. Furthermore, the heteronuclear correlation experiments outlined here should be applicable to a variety of quadrupolar spin pairs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Proton-detected 15 N- 1 H dipolar coupling/ 1 H chemical shift correlation experiment for the measurement of NH distances in biological solids under fast MAS solid-state NMR

Measurement of distances from dipolar couplings is essential for structural characterization, refinement and validation using the solid-state nuclear magnetic resonance (ssNMR) spectroscopy. Particularly, knowledge about NH dipolar interactions in biological solids is important for understanding the hydrogen (H)-bonding interactions, molecular geometry and spin dynamics. In this regard, we have proposed a proton-detected two-dimensional (2D) 15 N- 1 H dipolar coupling/ 1 H chemical shift correlation experiment using the C-symmetry based windowless recoupling of chemical shift anisotropy (ROCSA) in combination with the DIPSHIFT pulse-based method for the measurement of short NH distances in the isotopically labeled and naturally abundant biological solids at fast magic angle spinning (MAS) rates (40–70 kHz). Our proposed method results in undistorted recoupled 15 N- 1 H dipolar coupling powder lineshapes that are free from the recoupled 1 H CSA contributions under the 15 N evolution, a feature that is essential for the measurement of NH distances with improved accuracy (± 500 Hz in terms of the NH dipolar couplings). The pulse sequence developed in the present study is also insensitive to the 1H–1H homonuclear dipolar interactions, relaxation effects owing to its constant-time implementation, and t1-noise from the fluctuations in the MAS.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

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 (δ aniso ), 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 1 H's δ aniso in a densely-coupled network. Here, we demonstrate the CSA recoupling of strongly dipolar coupled 1 H spins using the $Cn^1_n (90_{0}360_{180}540_{0}360_{180}90_{0})$ 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 δ aniso but is not influenced by homonuclear dipolar interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Determination of the relative orientation between 15 N- 1 H dipolar coupling and 1 H chemical shift anisotropy tensors under fast MAS solid-state NMR

Here, in this work, we have proposed a proton-detected three-dimensional (3D) 15 N- 1 H dipolar coupling (DIP)/ 1 H chemical shift anisotropy (CSA)/ 1 H chemical shift (CS) correlation experiment to measure the relative orientation between the 15 N- 1 H dipolar coupling and the 1 H CSA tensors under fast magic angle spinning (MAS) solid-state NMR. In the 3D correlation experiment, the 15 N- 1 H dipolar coupling and 1 H CSA tensors are recoupled using our recently developed windowless C-symmetry-based $C3^{1}_{3}$-ROCSA (recoupling of chemical shift anisotropy) DIPSHIFT and $C3^{1}_{3}$-ROCSA pulse-based methods, respectively. The 2D 15 N- 1 H DIP/ 1 H CSA powder lineshapes extracted using the proposed 3D correlation method are shown to be sensitive to the sign and asymmetry of the 1 H CSA tensor, a feature that allows the determination of the relative orientation between the two correlating tensors with improved accuracy. The experimental method developed in this study is demonstrated on a powdered U- 15 N L-Histidine.HCl·H 2 O sample.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Double echo symmetry-based REDOR and RESPDOR pulse sequences for proton detected measurements of heteronuclear dipolar coupling constants

1 H{X} symmetry-based rotational echo double resonance pulse sequences (S-REDOR) and symmetry-based rotational echo saturation pulse double resonance (S-RESPDOR) solid-state NMR experiments have found widespread application for 1 H detected measurements of difference NMR spectra, dipolar coupling constants, and internuclear distances under conditions of fast magic angle spinning (MAS). In these experiments the supercycled $\text{R4}^2_1 (\text{SR4}^2_1)$ symmetry-based recoupling pulse sequence is typically applied to the 1 H spins to reintroduce heteronuclear dipolar couplings. However, the timing of $\text{SR4}^2_1$ and other symmetry-based pulse sequences must be precisely synchronized with the rotation of the sample, otherwise, the evolution of 1 H CSA and other interactions will not be properly refocused. For this reason, significant distortions are often observed in experimental dipolar dephasing difference curves obtained with S-REDOR or S-RESPDOR pulse sequences. Here we introduce a family of double echo (DE) S-REDOR/S-RESPDOR pulse sequences that function in an analogous manner to the recently introduced $t_1$-noise eliminated (TONE) family of dipolar heteronuclear multiple quantum coherence (D-HMQC) pulse sequences. Through numerical simulations and experiments the DE S-REDOR/S-RESPDOR sequences are shown to provide dephasing difference curves similar to those obtained with S-REDOR/S-RESPDOR. However, the DE sequences are more robust to the deviations of the MAS frequency from the ideal value that occurs during typical solid-state NMR experiments. In this work, the DE sequences are shown to provide more reliable 1 H detected dipolar dephasing difference curves for nuclei such as 15 N (with isotopic labelling), 183 W and 35 Cl. The double echo sequences are therefore recommended to be used in place of conventional S-REDOR/S-RESPDOR sequences for measurement of weak dipolar coupling constants and long-range distances.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

$t_1$-Noise eliminated dipolar heteronuclear multiple-quantum coherence solid-state NMR spectroscopy

Heteronuclear correlation (HETCOR) spectroscopy is one of the key tools in the arsenal of the solid-state NMR spectroscopist to probe chemical and spatial proximities between two different nuclei and enhance spectral resolution. Dipolar heteronuclear multiple-quantum coherence (D-HMQC) is a powerful technique that can be potentially utilized to obtain 1 H detected 2D HETCOR solid-state NMR spectra of any NMR active nucleus. A long-standing problem in 1 H detected D-HMQC solid-state NMR experiments is the presence of t 1 -noise which reduces sensitivity and impedes spectral interpretation. In this contribution, we describe novel pulse sequences, termed t 1 -noise eliminated (TONE) D-HMQC, that minimize t 1 -noise and can provide higher sensitivity and resolution than conventional D-HMQC. Monte-Carlo and numerical simulations confirm that t 1 -noise in conventional D-HMQC primarily occurs because random MAS frequency fluctuations cause variations in the NMR signal amplitude from scan to scan, leading to imperfect cancellation of uncorrelated signals by phase cycling. The TONE D-HMQC sequence uses 1 H π-pulses to refocus the evolution of 1 H CSA across each SR4 2 1 recoupling block, improving the stability of the pulse sequence to random MAS frequency fluctuations. The 1 H refocusing pulses also restore the orthogonality of in-phase and anti-phase magnetization for all crystallite orientations at the end of each recoupling block, enabling the use of 90° flip-back or LG spin-lock trim pulses to reduce the intensity of uncorrelated signals. We demonstrate the application of these methods to acquire 1 H detected 2D 1 H{ 35 Cl} and 1 H{ 13 C} HETCOR spectra of histidine·HCl·H 2 O with reduced t 1 -noise. To show generality, we also apply these methods to obtain 2D 1 H{ 17 O} spectra of 20%- 17 O fmoc-alanine and for the first time at natural abundance, 2D 1 H{ 25 Mg} HETCOR spectra of magnesium hydroxide. The TONE D-HMQC sequences are also used to probe 1 H– 25 Mg and 1 H– 27 Al proximities in Mg–Al layered double hydroxides and confirm the even mixing of Mg and Al in these materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

1 H chemical shift anisotropy: a high sensitivity solid-state NMR dynamics probe for surface studies?

We report dynamics play significant roles in chemistry and biochemistry-molecular motions impact both large- and small-scale chemical reactions in addition to biochemical processes. In many systems, including heterogeneous catalysts, the characterization of dynamics remains a challenge. The most common approaches involve the solid-state NMR measurement of anisotropic interactions, in particular 2 H quadrupolar coupling and 1 H–X dipolar coupling, which generally require isotope enrichment. Due to the high sensitivity of 1 H NMR, 1 H chemical shift anisotropy (CSA) is a particularly enticing, and underexplored, dynamics probe. We carried out 1 H CSA and 1 H– 13 C dipolar coupling measurements in a series of model supported complexes to understand how 1 H CSA can be leveraged to gain dynamic information for heterogeneous catalysts. Mathematical descriptions are given for the dynamic averaging of the CSA tensor, and its dependence on orientation and asymmetry. The variability of the orientation of the tensor in the molecular frame, in addition to its magnitude and asymmetry, negatively impacts attempts to extract quantitative dynamic information. Nevertheless, 1 H CSA measurements can reveal useful qualitative insights into the motions of a particularly dilute site, such as from a surface species.

1H chemical shift anisotropy↗