Search

Determining the nature and extent of covalency of early actinide chemical bonding is a fundamentally important challenge. Recently, X-ray absorption, electron paramagnetic, and nuclear magnetic resonance spectroscopic studies have probed actinide-ligand covalency, largely confirming the paradigm of early actinide bonding varying from ionic to polarised-covalent, with this range sitting on the continuum between ionic lanthanide and more covalent d transition metal analogues. Here, we report measurement of the covalency of a terminal uranium(VI)-nitride by 15N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the 15N nuclear magnetic resonance spectroscopy parameter space, and experimentally confirms a prior computational prediction that the uranium(VI)-nitride triple bond is not only highly covalent, but, more so than d transition metal analogues. These results enable construction of general, predictive metal-ligand 15N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies. Determining the covalency of actinide chemical bonding is a fundamentally important challenge. Here, the authors report a 15N nuclear magnetic resonance spectroscopy study of a terminal uranium-nitride, revealing exceptional NMR properties and covalency that redefine 15N NMR parameter space and actinide chemical bonding.

Pure shift nuclear magnetic resonance (NMR) methods suppress the effect of homonuclear scalar couplings to produce NMR spectra consisting solely of a single signal for each chemically distinct site. They are increasingly relied upon for analysis of complex molecules and mixtures as they overcome the extensive signal overlap that complicates proton NMR spectra of all but the simplest species. Current broadband pure shift methodologies for 1D proton spectra suffer from reduced sensitivity compared with their conventional counterparts and typically require a large amount of instrument time for low concentration samples. In this study, we demonstrate how the sensitivity limitation may be overcome by transiently increasing the bulk polarization using signal amplification by reversible exchange (SABRE) hyperpolarization. We utilize para-enriched dihydrogen to enhance the pure shift NMR resonances of pyridine by up to a factor of 60 in a single-scan experiment and extend this to propose a method to unambiguously determine mixture components based on the enhancement of their pure shift NMR signals.

2D NMR is an immensely powerful structural tool but it is time‐consuming. Targeting individual chemical groups by selective excitation in a 1D experiment can give the information required far more quickly. A major problem, however, is that proton NMR spectra are often extensively overlapped, so that in practice only a minority of sites can be selectively excited. Here we overcome that problem using a fast, single‐scan method that allows selective excitation of the signals of a single proton multiplet even where it is severely overlapped by other multiplets. The advantages of the method are illustrated in a selective 1D NOESY experiment, the most efficient way to determine relative configuration unambiguously by NMR. The new approach presented here has the potential to broaden significantly the applicability of selective excitation and unlock its real potential for many other experiments., Selective excitation of overlapped multiplets in a single scan broadens the applicability of 1D selective NMR experiments.

Selectively main chain 13C-labeled poly(p-phenylenevinylene)s (PPVs) were synthesized by ring-opening metathesis polymerization (ROMP) of 13C-labeled dialkoxy-substituted [2,2] paracyclophane-1,9-dienes (M) using the Grubbs second generation ruthenium carbene complex (G2). Analysis of the natural abundance and 13C-labeled PPVs by NMR spectroscopy showed no main chain structural defects for these polymers. Comparison of the in situ 1H and 13C NMR spectroscopy during the ROMP of labeled and unlabeled monomer enabled the active ruthenium carbene chain ends present during the initiation and propagation reaction to be definitively characterized. Using 13C NMR spectroscopy, the regiochemistry of the propagation of the asymmetric monomer M with G2 was found to be essentially regiorandom.

Efficient, practical and non-destructive analysis of complex mixtures is vital in many branches of chemistry. Here we present a new type of NMR experiment that allows the study of very challenging intact mixtures, in which subspectra of individual components can be extracted when other NMR means fail, for the case of a single, intact, static (constant composition) sample. We demonstrate the new approach, SCALPEL (Spectral Component Acquisition by Localized PARAFAC Extraction of Linear components), on a natural fermented beverage, beer, and other carbohydrate mixtures, obtaining individual carbohydrate component sub-spectra. This new class of NMR experiment is based on dissecting the spectrum rather than the sample, using pulse sequences tailored to generate data suitable for powerful tensor decomposition methods to allow highly complex spectra to be analyzed stepwise, one small section at a time. It has the clear potential to attack problems beyond the reach of current methods.

Diffusion-ordered spectroscopy (DOSY) is an effective method for the analysis of intact mixtures, but the quality of results is critically limited by resolution in the NMR dimension. A new experiment integrating diffusion weighting into the PSYCHE method for pure shift NMR spectroscopy allows DOSY spectra to be measured with ultrahigh NMR resolution at improved sensitivity.

A broadband proton-proton-decoupled CPMG-HSQMBC method for the precise and direct measurement of long-range heteronuclear coupling constants is presented. The Zangger-Sterk-based homodecoupling scheme reported herein efficiently removes unwanted proton-proton splittings from the heteronuclear multiplets, so that the desired heteronuclear couplings can be determined simply by measuring frequency differences between singlet maxima in the resulting spectra. The proposed pseudo-1D/2D pulse sequences were tested on nucleotides, a metal complex incorporating P heterocycles, and diglycosyl (di)selenides, as well as on other carbohydrate derivatives, for the extraction of (n) J((1) H,(31) P), (n) J((1) H,(77) Se), and (n) J((1) H,(13) C) values, respectively.

BIRD-Perspektive: Die Verwendung von periodischer BIRD-J-Refokussierung (BIRD=bilineare Rotationsentkopplung) bei der Datenerhebung eines HSQC-Experiments führt zu homonuclearer Breitbandentkopplung, sodass nur ein Signal für jede 1H-chemische Verschiebung erhalten wird. Diese reine Verschiebungsmethode verbessert sowohl die Auflösung als auch das Signal-Rausch-Verhältnis, ohne dass eine besondere Datenverarbeitung notwendig wäre.

A new pulse sequence, CHORUS Oneshot, allows measurements of diffusion-ordered spectroscopy (DOSY) spectra over the full chemical shift range of (19)F for the first time. Swept-frequency pulses are used to give very broadband excitation; the sequence is a prototype for a large family of very broadband liquid state NMR methods.

The characterization of materials by the inherently insensitive method of NMR spectroscopy plays a vital role in chemistry. Increasingly, hyperpolarization is being used to address the sensitivity limitation. Here, by reference to quinoline, we illustrate that the SABRE hyperpolarization technique, which uses para-hydrogen as the source of polarization, enables the rapid completion of a range of NMR measurements. These include the collection of (13)C, (13)C{(1)H}, and NOE data in addition to more complex 2D COSY, ultrafast 2D COSY and 2D HMBC spectra. The observations are made possible by the use of a flow probe and external sample preparation cell to re-hyperpolarize the substrate between transients, allowing repeat measurements to be made within seconds. The potential benefit of the combination of SABRE and 2D NMR methods for rapid characterization of low-concentration analytes is therefore established.

The sensitivity of both nuclear magnetic resonance spectroscopy and magnetic resonance imaging is very low because the detected signal strength depends on the small population difference between spin states even in high magnetic fields. Hyperpolarization methods can be used to increase this difference and thereby enhance signal strength. This has been achieved previously by incorporating the molecular spin singlet para-hydrogen into hydrogenation reaction products. We show here that a metal complex can facilitate the reversible interaction of para-hydrogen with a suitable organic substrate such that up to an 800-fold increase in proton, carbon, and nitrogen signal strengths are seen for the substrate without its hydrogenation. These polarized signals can be selectively detected when combined with methods that suppress background signals.

Pure shift NMR spectroscopy, or broadband homonuclear decoupled NMR spectroscopy, produces NMR spectra that contain only chemical shift information. In a pure shift NMR spectrum the effects of J-coupling are hidden so that all multiplets are collapsed to singlets. Pure shift methods can deliver a significant improvement in resolution of NMR signals and greatly reduce signal overlap. Pure shift NMR can be achieved using direct or indirect acquisition; for direct acquisition, real-time- or interferogram-based FID collection can be used. The range of pure shift methods includes those that refocus the effects of J-coupling (BIRD, Zangger–Sterk, band selective decoupling, and anti z-COSY); manipulate the evolution of chemical shift within a constant time delay to keep the J-evolution in an indirect acquisition constant; or extract a 1-D spectrum from a 2-D J dataset. The methods and their mechanisms in weakly coupled systems are described using the product operator formalism. Examples are provided for a range of applications. Keywords: pure shift NMR spectroscopy; broadband homonuclear decoupling; BIRD; Zangger–Sterk; anti z-COSY; constant time

Resolution and sensitivity are paramount for extracting detailed structural information using NMR spectroscopy. Recently developed "pure shift" techniques have greatly improved the resolution attainable in one- and two-dimensional NMR, but at a considerable cost in sensitivity. A newly introduced method, PSYCHE, greatly reduces this loss. It produces pure shift spectra with significantly improved sensitivity, spectral purity, and tolerance of strong coupling compared to previous methods. Here PSYCHE is applied to the TOCSY experiment. In combination with covariance processing, the result is a high-quality, high-resolution TOCSY spectrum with singlets in both dimensions: a pure chemical shift correlation map. Such spectra should greatly simplify both manual spectral analysis and automated structure elucidation.

All psyched up: A flexible and general pure shift experiment (PSYCHE) has been developed that offers superior sensitivity, spectral purity, and tolerance of strong coupling over existing methods for broadband homonuclear decoupling. The partial spectra of estradiol in [D6]DMSO obtained by normal 1H NMR spectroscopy and PSYCHE are shown for comparison.

An NMR method is reported that allows diastereomeric ratios to be determined even in crowded spectra or where chemical shift differences are small compared to multiplet widths. Band-selective pure shift NMR collapses multiplets to singlets, greatly improving spectral resolution while largely retaining, or even enhancing, signal-to-noise ratio.

In diffusion-ordered spectroscopy (DOSY) the aim is to separate signals from different molecular species according to their different diffusion coefficients. Each species has its individual diffusion coefficient (that may accidentally coincide with that of another species, e.g. if they are of very similar size). In exchanging systems, however, there is a serious complication in that the apparent diffusion coefficient of an exchanging signal will be a compromise that depends, among other factors, on the diffusion coefficients of the exchange partners and the rate of exchange between them. The DOSY spectrum will be much harder to interpret and can often give the appearance of extra (spurious) components in the mixture. Here a new and surprisingly simple experiment is described that suppresses the effects of exchange on apparent diffusion coefficients, restoring the simplicity of interpretation enjoyed by non-exchanging systems. (C) 2013 The Authors. Published by Elsevier Inc. All rights reserved.

A simple modification of the WATERGATE solvent suppression method greatly improves the quality of 1H NMR spectra obtainable from samples in H2O. The new method allows 1H signals to be measured even when close in chemical shift to the signal of water, as for example in the NMR spectra of carbohydrates. This journal is © 2013 The Royal Society of Chemistry.

A new family of NMR pulse sequences is reported for the recording of para-hydrogen enhanced NMR spectra. This Only Para-hydrogen SpectroscopY (OPSY) approach uses coherence selection to separate hyperpolarized signals from those of fully relaxed and thermally equilibrated protons. Sequence design, performance, practical aspects and applicability to other hyperpolarization techniques are discussed.

High resolution diffusion-ordered NMR spectroscopy allows the separation of signals from different species based on their diffusion coefficients. In general this requires that the NMR spectra of the components do not have overlapping signals, and that the diffusion coefficients are significantly different. Modifying the solvent matrix in which a sample is dissolved can change the diffusion coefficients observed, allowing resolution ("matrix-assisted DOSY"). We show here that dissolving the two naturally-occurring epimers of naringin in an aqueous solution of β-cyclodextrin causes both shift and diffusion changes, allowing the signals of the epimers to be distinguished. Chiral matrix-assisted DOSY has the potential to allow simple resolution and assignment of the spectra of epimers and enantiomers, without the need for derivatisation or for titration with a shift reagent.