Extremely low-frequency spectroscopy in low-field nuclear magnetic resonance

We demonstrate a new phenomenon in nuclear magnetic resonance spectroscopy, in which nuclear spin transitions are induced by radio frequency irradiation at extremely low frequencies (of the order of a few Hz). Slow Rabi oscillations are observed between spin states of different exchange symmetry. These “forbidden” transitions are rendered weakly allowed by differential electronic shielding effects on the radio frequency field. We generate coherence between the singlet and triplet states of 15N-labeled nitrous oxide in solution, and estimate the scalar coupling between the two 15N nuclei with a precision of a few mHz

Symmetry-based pulse sequences in solid-state NMR and applications to biological systems

We present some applications of solid-state nuclear magnetic resonance to model compounds and biological systems. We highlight a class of pulse sequences that are designed based on symmetry properties of the internal spin interactions. Examples are given showing resonance assignments, determination of internuclear distances, and torsion angle determinations in representative model systems as well as true biological systems

Theory and applications of supercycled symmetry-based recoupling sequences in solid-state NMR

We present the theoretical principles of supercycled symmetry-based recoupling sequences in solid-state magic-angle-spinning NMR. We discuss the construction procedure of the SR26 pulse sequence, which is a particularly robust sequence for double-quantum homonuclear dipole-dipole recoupling. The supercycle removes destructive higher-order average Hamiltonian terms and renders the sequence robust over long time intervals. We demonstrate applications of the SR26 sequence to double-quantum spectroscopy, homonuclear spin counting, and determination of the relative orientations of chemical shift anisotropy tensors

Simulation and design of shaped pulses beyond the piecewise-constant approximation

Response functions of resonant circuits create ringing artefacts if their input changes rapidly. When physical limits of electromagnetic spectroscopies are explored, this creates two types of problems. Firstly, simulation: the system must be propagated accurately through every response transient, this may be computationally expensive. Secondly, optimal control: circuit response must be taken into account; it may be advantageous to design pulses that are resilient to such distortions. At the root of both problems is the popular piecewise-constant approximation for control sequences in the rotating frame; in magnetic resonance it has persisted since the earliest days and has become entrenched in the commercially available hardware. In this paper, we report an implementation and benchmarks of recent Lie-group methods that can efficiently simulate and optimise smooth control sequences

Unique Group 1 cations stabilised by homoleptic neutral phosphine coordination

Homoleptic coordination of the neutral diphosphines Me2P(CH2)2PMe2 and o-C6H4(PMe2)2 to the hard Li+ and Na+ cations is achieved using Li[Al{OC(CF3)3}4] and Na[B{3,5-(CF3)2-C6H3}4] as ‘naked’ cation sources. Crystallographic, solid state and solution multinuclear NMR studies confirm distorted octahedral coordination solely via three chelating diphosphines in these unique species

The long-lived nuclear singlet state of 15N-nitrous oxide in solution

A 15N nuclear singlet lifetime of over 26 min has been observed in a solution of 15N2O, by using a field-cycling NMR pulse sequence. This observation suggests applications of hyperpolarized 15N2O in medical imaging and for flow and diffusion studies

¹⁴N overtone transition in double rotation solid-state NMR

Solid-state NMR transitions involving outer energy levels of the spin-1 14N nucleus are immune, to first order in perturbation theory, to the broadening caused by the nuclear quadrupole interaction. The corresponding overtone spectra, when acquired in conjunction with magic-angle sample spinning, result in lines, which are just a few kHz wide, permitting the direct detection of nitrogen compounds without the need for labeling. Despite the success of this technique, “overtone” resonances are still broadened due to indirect, second order effects arising from the large quadrupolar interaction. Here we demonstrate that another order of magnitude in spectral resolution may be gained by using double rotation. This brings the width of the 14N solid-state NMR lines much closer to the region commonly associated with high-resolution solid-state NMR spectroscopy of 15N and demonstrates the improvements in resolution that may be possible through the development of pulsed methodologies to suppress these second order effects

14N overtone NMR under MAS: signal enhancement using symmetry-based sequences and novel simulation strategies

Overtone 14N NMR spectroscopy is a promising route for the direct detection of 14N signals with good spectral resolution. Its application is currently limited, however, by the absence of efficient polarization techniques for overtone signal enhancement and the lack of efficient numerical simulation techniques to aid in both the development of new methods and the analysis and interpretation of experimental data. In this paper we report a novel method for the transfer of polarization from 1H to the 14N overtone using symmetry-based R-sequences that overcome many of the limitations of adiabatic approaches that have worked successfully on static samples. Refinement of these sequences and the analysis of the resulting spectra have been facilitated through the development of an efficient simulation strategy for 14N overtone NMR spectroscopy of spinning samples, using effective Hamiltonians on top of Floquet and Fokker–Planck equation

Strategies for 1H-Detected Dynamic Nuclear Polarization Magic-Angle Spinning NMR Spectroscopy

© 2020 The Authors. Published by Wiley-VCH GmbH Combining dynamic nuclear polarization with proton detection significantly enhances the sensitivity of magic-angle spinning NMR spectroscopy. Herein, the feasibility of proton-detected experiments with slow (10 kHz) magic angle spinning was demonstrated. The improvement in sensitivity permits the acquisition of indirectly detected 14N NMR spectra allowing biomolecular structures to be characterized without recourse to isotope labelling. This provides a new tool for the structural characterization of environmental and medical samples, in which isotope labelling is frequently intractable

Sol–gel preparation of low oxygen content, high surface area silicon nitride and imidonitride materials

Reactions of Si(NHMe)4 with ammonia are effectively catalysed by small ammonium triflate concentrations, and can be used to produce free-standing silicon imide gels. Firing at various temperatures produces amorphous or partially crystallised silicon imidonitride/nitride samples with high surface areas and low oxygen contents. The crystalline phase is entirely ?-Si3N4 and structural similarities are observed between the amorphous and crystallised materials