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Selective 1D COSY can unambiguously identify coupled spins but is often limited both by lack of selectivity, and by unfavourable multiplet lineshapes. Here, ultra selective GEMSTONE excitation is employed with CLIP COSY to provide through-bond correlations for nuclei whose NMR signals overlap. The new method is illustrated using the coccidiostat lasalocid and the immunosuppressant cyclosporin.

This chapter introduces pure shift NMR spectroscopy and shows how it can be combined with 2D methods to simplify spectra and aid interpretation. Typically, pure shift NMR is used to produce a spectrum that contains only chemical shift information in the 1H dimension, with signal overlap caused by wide multiplets removed or greatly reduced. This simplification is achieved by removing the effects of homonuclear scalar coupling. Singlet peaks, reporting only chemical shifts, are present in the 1H dimension of the final spectrum. When incorporated into 2D NMR methods, pure shift NMR can provide unparalleled spectral simplicity, removing signal overlap from 2D traces that are already the gold standard for resolution. Pure shift 2D NMR spectroscopy will be introduced through the building blocks of the commonest experiments. Pure shift implementations that exist for variants of almost all standard 2D NMR experiments will be deconstructed. The resulting components will then be used to provide a description of the concepts, theory, and practical aspects required for setting up, acquiring and processing these advanced and useful methods. There are several pure shift approaches available to the spectroscopist, each of which will be introduced and their strengths and weaknesses discussed in the context of 2D NMR. Pure shift NMR methods are not intrinsically quick to acquire. However, as pure shift methods provide direct access to spectral information that might otherwise require much more time-consuming methods they warrant a position alongside fast 2D NMR methods.

Human milk oligosaccharides belong to an important class of bioactive molecules with diverse effects on the development of infants. NMR is capable of providing vital structural information about oligosaccharides which can aid in determining structure-function relationships. However, this information is often concealed by signal overlap in 1H spectra, due to the narrow chemical shift range and signal multiplicity. Signal overlap in oligosaccharide spectra can be greatly reduced, and resolution improved, by utilising pure shift methods. Here the benefits of combining pure shift methods with the CASPER computational approach to resonance assignment in oligosaccharides are demonstrated.

An ultra-selective 1D NMR experiment – GEMSTONE-ROESY – enables clear, unambiguous assignment of ROE signals in the not uncommon situation that traditional selective methods fail. Its usefulness is demonstrated in the analysis of the natural products cyclosporin and lacto-N-difucohexaose I, providing detailed insight into the structure and conformation of these molecules.

We report the synthesis and characterization of uranium(IV) and thorium(IV) mesoionic carbene complexes [An{N(SiMe

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.

Most interesting problems in chemistry, biology, and pharmacy involve mixtures. However, analysis of such mixtures by NMR remains a challenge, often requiring the mixture components to be physically separated before analysis. A variety of methods have been proposed that exploit species-specific properties such as diffusion and relaxation to distinguish between the signals of different components in a mixture without the need for laborious separation. However, these methods can struggle to distinguish between components when signals overlap. Here, we exploit the relaxation properties of selected nuclei to distinguish between different components of a mixture while using pure shift methods to increase spectral resolution by up to an order of magnitude, greatly reducing signal overlap. The advantages of the new method are demonstrated in a mixture of d-xylose and l-arabinose, distinguishing unambiguously between the five major species present.

We report that the agile eight-membered cycloalkane can be stabilized by fusing two rigid benzene rings, substituted with proper functional groups. The conformational change of dibenzocycloocta-1,5-diene (DBCOD), a rigid-flexible-rigid organic moiety, from Boat to Chair conformation requires an activation energy of 42 kJ/mol that is substantially lower than that of existing submolecular shape-changing unit. Experimental data corroborated by theory calculations demonstrate that intramo-lecular hydrogen bonding can stabilize Boat whereas electron repulsive interaction from opposing ester substituents favors Chair. Intramolecular hydrogen bonding, formed by 1,10-diamide substitution stabilizes Boat, spiking the temperature at which Boat and Chair can readily interchange from –60 °C to 60 °C. Concomitantly this intramolecular attraction raises the energy barrier from 42 kJ/mol of unsubstituted DBCOD to 68 kJ/mol of diamide-substituted DBCOD. Remarkably, this value falls within the range of the activation energy of highly efficient enzyme catalyzed biological reactions. With shape changes once considered only possible with high-energy, our work reveals a potential pathway exemplified by a specific submolecu-lar structure to achieve low-energy driven shape changes for the first time. Together with intrinsic cycle stability and high energy output systems that would have incurred damage under high-energy stimuli, could particularly benefit from this new kind of low-energy driven shape-changing mechanism. This work has laid the basis to construct systems for low-energy driv-en stimuli-responsive applications, hitherto a challenge to overcome.

A family of heterometallic rings [Me

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.

The interfacial adsorption of single-walled carbon nanotubes and few-layer graphene flakes, prepared by solution phase exfoliation, is compared. Strong adsorption of carbon nanotubes was observed at the water/1,2-dichloroethane interface, while a weaker adsorption of the graphene dispersion was seen. Addition of electrolyte to the organic phase was found to have a strong effect on the adsorption of graphene. A simple surface energy model does not fully explain these observations, rather residual charges and their distribution appears to be the key factor behind this difference in adsorptive behaviour. Carbon nanomaterials adsorbed at the liquid-liquid interface can function as bipolar electrodes: a preliminary investigation of the oxidation of the 1,2-dichlorobenzene by metal-modified graphene particles is performed.

Selective 1D TOCSY is a powerful tool in the assignment of NMR spectra of organic molecules. Here an order of magnitude improvement in selectivity, allowing overlapping multiplets to be excited separately, is achieved in a single scan using the very recent GEMSTONE method. The new experiment is illustrated using an antibiotic and a mixture of diastereomers.

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.

[2] and [3] hybrid rotaxanes are reported based on {Ti7M} rings (M is a trivalent metal such as FeIII or GaIII). NMR studies show that [2]rotaxanes can act as molecular shuttles, while EPR studies of [3]rotaxanes show weak interactions between the paramagnetic components of the supramolecular assemblies.

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.

This paper discusses the living nature of the ring opening metathesis polymerisation (ROMP) of alkoxy and alkyl substituted [2.2] paracyclophane-1,9-dienes (M1 and M2), initiated with Grubbs’ second and third generation catalysts (G2 and G3). The active ruthenium carbene chain ends present during the ROMP of these monomers have been identified by in situ1H and 31P NMR spectroscopy and the relative kinetics of the initiation and propagation reactions in the polymerisation determined for both G2 and G3 complexes. The apparent rate constants for initiation of both M1 and M2 using G3 are at least one order of magnitude larger than those determined for polymerisation using G2 and this leads to lower dispersity (Đm) polymers using the G3 catalyst as initiator. Complexation of the ruthenium centre in the active chain end by the oxygen of the alkoxy substituents in M1 is observed in the NMR spectra of the reaction and this leads to significantly slower rates of propagation than those observed for the alkyl derivative M2 using both G2 and G3 complexes. Although the polymerisation requires longer to reach full conversion for monomer M1 the slower propagation generally results in better control of the polymer molecular weight (lower Đm) as the ratio of apparent rate constants for initiation and propagation are larger than those determined for the ROMP of M2.

Progress reaction profiles are affected by both catalyst activation and deactivation processes occurring alongside the main reaction. These processes complicate the kinetic analysis of reactions, often directing researchers toward incorrect conclusions. We report the application of two kinetic treatments, based on variable time normalization analysis, to reactions involving catalyst activation and deactivation processes. The first kinetic treatment allows the removal of induction periods or the effect of rate perturbations associated with catalyst deactivation from kinetic profiles when the quantity of active catalyst can be measured. The second treatment allows the estimation of the activation or deactivation profile of the catalyst when the order of the reactants for the main reaction is known. Both treatments facilitate kinetic analysis of reactions suffering catalyst activation or deactivation processes.

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.

This work presents a cyclodextrin-enhanced organocatalytic method from molecular to process design. Cinchona-thiourea and -squaramide catalysts were covalently anchored to inherently large, stable and well-defined permethyl-β-cyclodextrins. The asymmetric catalysis was successfully demonstrated on the Michael reaction of 1,3-diketones and trans-β-nitrostyrene. Both emerging green and conventional solvents were screened for the asymmetric addition (up to 99% ee), and the Kamlet–Taft solvent parameters were correlated to the enantioselectivity. Quantum chemical modelling revealed that the catalyst anchoring resulted in favorable structural changes, and stronger intermolecular interactions between the catalyst and the reagents. Continuous organocatalysis was performed in coiled tube flow reactor coupled with a membrane separation unit, which allowed complete recovery of the catalyst and 50% solvent (2-MeTHF) recycling. The 100% conversion, 98% purity, 99% ee, 100% in-line catalyst recovery, and 80 g L−1 h−1 productivity makes it an attractive catalytic platform.

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.

The frequency range of quantitative NMR is increased from tens to hundreds of kHz by a new pulse sequence, CHORUS. It uses chirp pulses to excite uniformly over very large bandwidths, yielding accurate integrals even for nuclei such as (19)F that have very wide spectra.

The combination of NHC–BH3/I2 represents a simple method for the reductive α-borylation of α,β-unsaturated esters to form useful α-boryl esters., Useful α-boryl esters can be synthesized in one step from α,β-unsaturated esters using just a simple to access NHC–BH3 (NHC = N-heterocyclic carbene) and catalytic I2. The scope of this reductive α-borylation methodology is excellent and includes a range of alkyl, aryl substituted and cyclic and acyclic α,β-unsaturated esters. Mechanistic studies involving reductive borylation of a cyclic α,β-unsaturated ester with NHC–BD3/I2 indicated that concerted hydroboration of the alkene moiety in the α,β-unsaturated ester proceeds instead of a stepwise process involving initial 1,4-hydroboration; this is in contrast to the recently reported reductive α-silylation. The BH2(NHC) unit can be transformed into electrophilic BX2(NHC) moieties (X = halide) and the ester moiety can be reduced to the alcohol with the borane unit remaining intact to form β-boryl alcohols. The use of a chiral auxiliary, 8-phenylmenthyl ester, also enables effective stereo-control of the newly formed C–B bond. Combined two step ester reduction/borane oxidation forms diols, including excellent e.e. (97%) for the formation of S-3-phenylpropane-1,2-diol. This work represents a simple transition metal free route to form bench stable α-boryl esters from inexpensive starting materials.

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.

Biocatalytic retrosynthetic analysis of dibenz[c,e]azepines has highlighted the use of imine reductase (IRED) and ω-transaminase (ω-TA) biocatalysts to establish the key stereocentres of these molecules. Several enantiocomplementary IREDs were identified for the synthesis of (R)- and (S)-5-methyl-6,7-dihydro-5H-dibenz[c,e]azepine with excellent enantioselectivity, by reduction of the parent imines. Crystallographic evidence suggests that IREDs may be able to bind one conformer of the imine substrate such that, upon reduction, the major product conformer is generated directly. ω-TA biocatalysts were also successfully employed for the production of enantiopure 1-(2-bromophenyl)ethan-1-amine, thus enabling an orthogonal route for the installation of chirality into dibenz[c,e]azepine framework.

We report broadband proton-decoupled CLIP/CLAP-HSQC experiments for the accurate determination of one-bond heteronuclear couplings and, by extension, for the reliable measurement of small residual dipolar coupling constants. The combination of an isotope-selective BIRD(d) filter module with a nonselective H-1 inversion pulse is employed to refocus proton-proton coupling evolution prior to the acquisition of brief chunks of free induction decay that are subsequently assembled to reconstruct the fully-decoupled signal evolution. As a result, the cross-peaks obtained are split only by the heteronuclear one-bond coupling along the F-2 dimension, allowing coupling constants to be extracted by measuring simple frequency differences between singlet maxima. The proton decoupling scheme presented has also been utilized in standard HSQC experiments, resulting in a fully-decoupled pure shift correlation map with significantly improved resolution. (C) 2013 Elsevier Inc. All rights reserved.

An N-terminal L-α-methylvaline dimer induces complete conformational control over the screw sense of an otherwise achiral helical peptide foldamer formed from the achiral quaternary amino acids Aib and Ac6c. The persistent right-handed screw-sense preference of the helix enables remote reactive sites to fall under the influence of the terminal chiral residues, and permits diastereoselective reactions such as alkene hydrogenation or iminium ion addition to take place with 1,16-, 1,31-, 1,46- and even 1,61-asymmetric induction. Stereochemical information may be communicated in this way over distances of up to 4 nm. Reaction at a distance: By inducing a quantitative, persistent preference for right-handed helicity, chirality at one end of an otherwise achiral helical molecule is able to control the stereoselectivity of reactions at sites located up to 60 bonds away, shattering previous records for remote stereochemical control. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

Atropisomeric biaryl pyridine and isoquinoline N-oxides were synthesized enantioselectively by dynamic kinetic resolution (DKR) of rapidly racemizing precursors exhibiting free bond rotation. The DKR was achieved by ketoreductase (KRED) catalyzed reduction of an aldehyde to form a configurationally stable atropisomeric alcohol, with the substantial increase in rotational barrier arising from the loss of a bonding interaction between the N-oxide and the aldehyde. Use of different KREDs allowed either the M or P enantiomer to be synthesized in excellent enantiopurity. The enantioenriched biaryl N-oxide compounds catalyze the asymmetric allylation of benzaldehyde derivatives with allyltrichlorosilane.

NMR techniques incorporating pure shift methods to improve signal resolution have recently attracted much attention, owing to their potential use in studies of increasingly complex molecular systems. Extraction of frequencies from these simplified spectra enables easier structure determination, but only a few of the methods presented provide structural parameters derived from signal integral measurements. In particular, for quantification of the nuclear Overhauser effect (NOE) it is highly desirable to utilize pure shift techniques where signal overlap normally prevents accurate signal integration, to enable measurement of a larger number of interatomic distances. However, robust methods for the measurement of interatomic distances using the recently developed pure shift techniques have not been reported to date. In this work we discuss some of the factors determining the accuracy of measurements of signal integrals in interferogram-based Zangger-Sterk (ZS) pure shift NMR experiments. The ZS broadband homodecoupling technique is used in different experiments designed for quantitative NOE determination from pure shift spectra. It is shown that the techniques studied can be used for quantitative extraction of NOE-derived distance restraints, as exemplified for the test case of strychnine.

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.

Lappert's original Th(iii) complex, [Th{C5H3(SiMe3)2-1,3}3], reduces white phosphorus to give a cyclo-P4 dianion, which exhibits an unprecedented μ–η1:η1-binding mode in the dinuclear Th(iv) product.

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.

The PSYCHE method for pure shift NMR is exploited to generate 2D J spectra with full decoupling in one dimension and multiplet structure in the other, allowing spin–spin coupling constants to be measured even in very crowded spectra. Significant improvements over existing techniques are demonstrated for the hormones estradiol and androstenedione.

Here we show an elegant and general route to the assembly of a giant {M12C24} cage from 12 palladium ions (M) and 24 heterometallic octanuclear coordination cages (C = {Cr7Ni-Py2}). The molecule is 8 nm in size, and the methods for its synthesis and characterization provide a basis for future developments at this scale.

Spectral resolution in proton NMR spectroscopy is reduced by the splitting of resonances into multiplets due to the effect of homonuclear scalar couplings. Although these effects are often hidden in protein NMR spectroscopy by low digital resolution and routine apodization, behind the scenes homonuclear scalar couplings increase spectral overcrowding. The possibilities for biomolecular NMR offered by new pure shift NMR methods are illustrated here. Both resolution and sensitivity are improved, without any increase in experiment time. In these experiments, free induction decays are collected in short bursts of data acquisition, with durations short on the timescale of J-evolution, interspersed with suitable refocusing elements. The net effect is real-time (t2) broadband homodecoupling, suppressing the multiplet structure caused by proton–proton interactions. The key feature of the refocusing elements is that they discriminate between the resonances of active (observed) and passive (coupling partner) spins. This can be achieved either by using band-selective refocusing or by the BIRD element, in both cases accompanied by a nonselective 180° proton pulse. The latter method selects the active spins based on their one-bond heteronuclear J-coupling to 15N, while the former selects a region of the 1H spectrum. Several novel pure shift experiments are presented, and the improvements in resolution and sensitivity they provide are evaluated for representative samples: the N-terminal domain of PGK; ubiquitin; and two mutants of the small antifungal protein PAF. These new experiments, delivering improved sensitivity and resolution, have the potential to replace the current standard HSQC experiments. Electronic supplementary material The online version of this article (doi:10.1007/s10858-015-9913-z) contains supplementary material, which is available to authorized users.

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.

We report pump-probe experiments employing laser-synchronized reactions of para-hydrogen (para-H2) with transition metal dihydride complexes in conjunction with nuclear magnetic resonance (NMR) detection. The pump-probe experiment consists of a single nanosecond laser pump pulse followed, after a precisely defined delay, by a single radio frequency (rf) probe pulse. Laser irradiation eliminates H2 from either Ru(PPh3)3(CO)(H)2 1 or cis-Ru(dppe)2(H)2 2 in C6D6 solution. Reaction with para-H2 then regenerates 1 and 2 in a well-defined nuclear spin state. The rf probe pulse produces a high-resolution, single-scan (1)H NMR spectrum that can be recorded after a pump-probe delay of just 10 μs. The evolution of the spectra can be followed as the pump-probe delay is increased by micro- or millisecond increments. Due to the sensitivity of this para-H2 experiment, the resulting NMR spectra can have hydride signal-to-noise ratios exceeding 750:1. The spectra of 1 oscillate in amplitude with frequency 1101 ± 3 Hz, the chemical shift difference between the chemically inequivalent hydrides. The corresponding hydride signals of 2 oscillate with frequency 83 ± 5 Hz, which matches the difference between couplings of the hydrides to the equatorial (31)P nuclei. We use the product operator formalism to show that this oscillatory behavior arises from a magnetic coherence in the plane orthogonal to the magnetic field that is generated by use of the laser pulse without rf initialization. In addition, we demonstrate how chemical shift imaging can differentiate the region of laser irradiation thereby distinguishing between thermal and photochemical reactivity within the NMR tube.

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.

Signal amplification by reversible exchange (SABRE) of a substrate and parahydrogen at a catalytic center promises to overcome the inherent insensitivity of magnetic resonance. In order to apply the new approach to biomedical applications, there is a need to develop experimental equipment, in situ quantification methods, and a biocompatible solvent. We present results detailing a low-field SABRE polarizer which provides well-controlled experimental conditions, defined spins manipulations, and which allows in situ detection of thermally polarized and hyperpolarized samples. We introduce a method for absolute quantification of hyperpolarization yield in situ by means of a thermally polarized reference. A maximum signal-to-noise ratio of ∼10(3) for 148 μmol of substance, a signal enhancement of 10(6) with respect to polarization transfer field of SABRE, or an absolute (1)H-polarization level of ≈10(-2) is achieved. In an important step toward biomedical application, we demonstrate (1)H in situ NMR as well as (1)H and (13)C high-field MRI using hyperpolarized pyridine (d3) and (13)C nicotinamide in pure and 11% ethanol in aqueous solution. Further increase of hyperpolarization yield, implications of in situ detection, and in vivo application are discussed.

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.

Fully homodecoupled HSQC spectra can be obtained through the use of a new pulse sequence element, "perfectBIRD". By way of illustration, we show that perfectBIRD decoupling allows one-bond residual dipolar couplings (RDCs), which provide important NMR restraints for structure elucidation, to be measured with outstanding precision, even in methylene groups.

An NMR method is reported for the efficient removal of signals derived from nuclei with thermally equilibrated spin state populations whilst leaving, intact, signals derived from para-hydrogen induced polarisation (PHIP) through gradient assisted coherence selection.

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.

Mastocarpus stellatus seaweeds were subjected to different alkali pre-treatments to investigate the effect of alkali type, concentration and treatment duration on the chemical structure and the gelling properties of extracted kappa/iota-hybrid carrageenan (KI). Increasing the concentration in KOH and the pretreatment time gives KI with lower amounts of nu-carrageenan units, which have a direct impact on the improved gelling properties. However, excessive KOH concentration and pre-treatment duration give KI with smaller molecular mass and depressed gel properties. NaOH is more efficient in converting nucarrageenan to iota-carrageenan, but no correlation between the chemical structure and gel properties was found. An interaction between the alkali concentration and the pre-treatment duration complicates the rationalization of NaOH efficiency. A set of KI with 60e75 mol% kappa-carrageenan and tailored gelling properties for a wide range of applications is delivered, FEDER funds through the program COMPETE (project PTDC/CTM/100076/2008) and by the UK Engineering and Physical Sciences Research Council (Grant Numbers EP/ H024336/1 and EP/E05899X/1)