Efficient determination of scalar coupling networks by band selective decoupled 2D NMR spectroscopy

Scalar (J) couplings constitute one of vital features observed in NMR spectroscopy and show valuable information for molecular structure elucidation and conformation analysis. However, existing J coupling measurement techniques are generally confined by the concerns of resolution, SNR, and experimental efficiency. Herein, we exploit an efficient 2D NMR protocol to deal with the above concerns by enabling rapid, sensitive, and high-resolution J coupling extraction. This protocol delivers full-resolved pure shift 2D absorption-mode spectroscopy to gain great convenience for efficient coupling measurements on overcrowded NMR signals. Resulting from band selective signal evolution, this protocol ensures high signal intensity with full magnetization preservation to meet the demand on probing low-concentration samples. This protocol focuses on accessing coupling information between specific two coupled spin families, and it is not applicable to all possible spin systems. Besides, it adopts echo-train selective refocusing acquisition to accelerate pure shift 2D J-edited implementations into pseudo-2D acquisition, and thus holding the experimental efficiency similar to conventional SERF experiments. Therefore, this study presents a promising tool for efficient extraction of J coupling networks, and takes an important step for coupling measurement techniques with wide applications on molecular conformation elucidation and stereochemical configuration analysis.

Pure shift edited NMR methodologies for the extraction of Homo- and heteronuclear couplings with ultra-high resolution

The scalar couplings that result in the splitting of the signals in the NMR spectrum arise due to the interaction of the nuclear spins, whereby the spin polarization is transmitted through chemical bonds. The interaction strengths depend inter alia on the number of consecutive chemical bonds intervening between the two interacting spins and on the molecular conformation. The pairwise interaction of many spins in a molecule resulting in a complex spectrum poses a severe challenge to analyse the spectrum and hence the determination of magnitudes and signs of homo- and heteronuclear couplings. The problem is more severe in the analysis of 1H spectra than the spectra of most of the other nuclei due to the often very small chemical shift dispersion. As a consequence, the straightforward analysis and the accurate extraction of the coupling constants from the 1H spectrum of a complex spin system continues to remain a challenge, and often may be a formidable task. Over the years, the several pure shift-based one-dimensional and two-dimensional methodologies have been developed by workers in the field, which provide broadband homonuclear decoupling of proton spectra, removing the complexity but at the cost of the very informative scalar couplings. To circumvent this problem, several one-dimensional and two-dimensional NMR experiments have been developed for the determination of homonuclear and heteronuclear couplings (nJHX, where n = 1,2,3) while retaining the high resolution obtained by implementing pure shift strategies. This review attempts to summarize the extensive work reported by a large number of researchers over the years for the accurate determination of homo- and heteronuclear scalar couplings.

Slice selective absorption-mode J-resolved NMR spectroscopy

Limited chemical shift dispersion and broad multiplet patterns limit resolution in ¹H NMR spectra. J-Resolved spectroscopy overcomes this problem to a great extent. However, the phase-twist line shape in J-Resolved spectroscopy allows only the magnitude mode of the experiment to be practical, which degrades resolution. Recently, various pure shift or broadband homonuclear decoupling approaches have been integrated with J-Resolved spectroscopy to eliminate the broad dispersive contribution. In the present work, we demonstrate a broadband ¹H-¹H J-Resolved spectrum with a greatly reduced dispersive contribution using the concept of slice selection. We show that slice selective excitation, t₁ encoding, storage, and detection of the in-phase absorptive signals can be executed, while a gradient-based suppression of the dispersive antiphase signals can be performed during the storage period. In more than two spin systems, a small part of the doubly antiphase absorptive signal may also contribute to the spectrum in addition to the inphase absorptive signals. The overall effect is a reduced multiplet pattern similar to a regular J-Resolved case as the passive spins remain unflipped due to slice selective pulses. However, the effect is broadband for a fraction of the spins when all slices are considered analogous to Zangger-Sterk (ZS) broadband homo-decoupling. Further, the fresh magnetization from neighboring slices can be accessed in different scans by frequency shifting of the slice selective pulses without a recycle delay-an elegant aspect of the ZS pulse element. This allows faster signal averaging, improving sensitivity which depends on the T₁ relaxation time of the signals. This method displays sensitivity up to 4-20 percent of the regular J-RES ¹H signals.

Simultaneous determination of multiple coupling networks by high-resolution 2D J-edited NMR spectroscopy

J coupling constitutes an important NMR parameter for molecular-level composition analysis and conformation elucidation. Dozens of J-based approaches have been exploited for J coupling measurement and coupling network determination, however, they are generally imposed to insufficient spectral resolution to resolve crowded NMR resonances and low measurement efficiency that a single experiment records one J coupling network. Herein, we propose a general NMR method to collect high-resolution 2D J-edited NMR spectra, which are characterized with advantages of pure absorptive lineshapes, decoupled chemical shift dimension, as well as eliminated axial peaks, thus facilitating J coupling partner assignments and J coupling constant measurements. More meaningfully, this protocol allows simultaneous determination of multiple coupling networks for highly efficient multiplet analyses via addressing multiple protons within one single experiment. Additionally, another variant is proposed for high-resolution applications under adverse magnetic field conditions. Therefore, this study provides a useful NMR protocol for configurational and structural studies with extensive applications in chemistry, biology, and material science.

Unambiguous and accurate measurement of scalar coupling constants through a selective refocusing NMR experiment

Scalar coupling plays an important role in the analysis of molecular structure and dynamics. A great number of nuclear magnetic resonance (NMR) selective refocusing experiments, such as 2D G-SERF and PSYCHEDELIC, were developed to extract scalar coupling constants involving a selected proton from overlapped spectra. However, intense axial peaks occur in this type of experiments, leading to possible ambiguity in the assignment of spectral peaks and subsequent accurate measurement of ¹H-¹H scalar coupling constants. Here, a method based on selective coherence transfer and PSYCHEDELIC module is designed to acquire absorption-mode selective refocusing spectrum while suppressing intense axial peaks. Therefore, unambiguous and accurate measurement of scalar coupling constants involving the selectively excited proton can be achieved. The performances of the proposed method are demonstrated on several samples.

Fully Exploiting the Power of 2D NMR J-Resolved Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical tool that enables one to study molecular properties and interactions. Homonuclear couplings provide valuable structural information but are often difficult to disentangle in crowded ¹H NMR spectra where complex multiplets and signal overlap commonly exist. Multidimensional NMR experiments push the power of NMR to a new level by providing better signal dispersion. Among them, 2D J-resolved spectroscopy is widely used for multiplet analysis and the measurement of scalar coupling constants. Here, we present a new 2D J-resolved method, CASCADE, through which easier multiplet analysis and unambiguous measurement of specific coupling constants can be achieved at the same time, fully exploiting the power of 2D J-resolved spectroscopy. It is expected that this method may replace a conventional 2D J experiment in many cases, facilitating structural and configurational studies as well as chemical and biological analyses.

PE-SERF: A sensitivity-improved experiment to measure JHH in crowded spectra

Aiming at facilitating the analysis of molecular structure, the gradient-encoded selective refocusing methods (G-SERF) and a great number of its variants for measuring proton-proton coupling constants have been proposed. However, the sensitivity is an issue in the 2D gradient-encoded experiments, because the signal intensity is determined by the slice thickness of the sample that depends on encoding gradient and the bandwidth of selective pulses which is limited by the smallest chemical shift difference of any two coupled protons. Here, we present a method dubbed PE-SERF (perfect echo selective refocusing) which can determine all J_HH values involving a selected proton with improved sensitivity compared to original G-SERF experiment. The modules of perfect echo involving selective pulses and gradient-encoded selective refocusing are combined in the method, so that the unwanted J couplings arising from coupled spin pairs in the same sample slice would be nullified. In this way, instead of single proton, a pair of coupled protons is allowed to share a sample slice, and thus the slice thickness can be increased and the spectral sensitivity can be improved. The performance of the method is demonstrated by experiments on quinine and strychnine.

G-SERF Editing in Two-Dimensional Pure-Shift Total Correlation Spectroscopy: Scalar Coupling Measurements for a Group of Spins in Organic Molecules

A novel G-SERF-PSYCHE-TOCSY (gradient encoded selective refocusing in pure shift yielded by chirp excitation version of total correlation spectroscopy) NMR pulse scheme has been proposed, which produces TOCSY chemical shift correlations, on one hand, and scalar coupling values for the spins scalarly coupled to irradiated resonances, by showing them as doublets along the indirect dimension, on the other. Therefore, recording such an experiment, for a group of spins with overlapping chemical shifts, in organic molecules can adequately provide scalar coupling information in a G-SERF manner along the indirect dimensions, and they can be assigned to particular spin pairs. Such COSY chemical shift correlations (which appear as doublets for the scalarly coupled spins) can be readily discriminated from the TOCSY peaks (which do not show such splitting) in the G-SERF-PSYCHE-TOCSY spectrum.

Pushing resolution limits for extracting 1H-1H scalar coupling constants by a resolution-enhanced selective refocusing method

Nuclear magnetic resonance (NMR) spectroscopy enables one to study molecular structure and dynamics in a noninvasive manner and has long served as a versatile and indispensable analytical tool in physics, chemistry, and biology. Scalar coupling, an essential feature in NMR spectroscopy, provides rich information regarding molecular structure and conformation. The measurement of scalar coupling constants, therefore, constitutes an important issue in NMR spectroscopy. Homonuclear 2D J-resolved spectroscopy is a powerful tool for multiplet analysis and coupling measurement. Recently, a number of phase-sensitive J-resolved methods and selective measuring methods have been developed to facilitate the extraction of coupling constants. However, resolution remains a crucial challenge when extracting small coupling constants or under inhomogeneous fields. In this paper, we present a resolution-enhanced selective refocusing (RESERF) method for the extraction of coupling constants. The effect of magnetic field inhomogeneity can be eliminated, resulting in very narrow linewidths. Therefore, samples with small coupling constants or under inhomogeneous fields can be well analyzed. The RESERF method may be of great value for structural and conformational studies in chemistry and biology.

Real-time pure shift measurements for uniformly isotope-labeled molecules using X-selective BIRD homonuclear decoupling

We introduce a novel selective inversion element for chunked homonuclear decoupling that combines isotope selection via BIRD-filtering with band-selective inversion on the X-heteronucleus and allows efficient real-time decoupling of homonuclear and heteronuclear couplings. It is especially suitable for uniformly isotope-labeled compounds. We discuss in detail the inversion element based on band-selective refocusing on the X-nuclei (BASEREX), highlighting in particular the role of appropriate band-selective shaped refocusing pulses and the application of broadband X-pulses for an effective BIRD^d element during homodecoupling. The approach is experimentally verified and studied in detail using uniformly ¹³C-labeled glucose and a uniformly ¹⁵N,¹³C-labeled amino acid mixture.

High-resolution methods for the measurement of scalar coupling constants

Scalar couplings provide important information regarding molecular structure and dynamics. The measurement of scalar coupling constants constitutes a topic of interest and significance in NMR spectroscopy. However, the measurement of J values is often not straightforward because of complex signal splitting patterns and signal overlap. Many methods have been proposed for the measurement of scalar coupling constants, both for homonuclear and heteronuclear cases. Different approaches to the measurement of scalar coupling constants are reviewed here with several applications presented. The accurate measurement of scalar coupling constants can greatly facilitate molecular structure elucidation and the study of molecule dynamics.

Spatial encoding and spatial selection methods in high-resolution NMR spectroscopy

A family of high-resolution NMR methods share the common concept of acquiring in parallel different sub-experiments in different spatial regions of the NMR tube. These spatial encoding and spatial selection methods were for the most part introduced independently from each other and serve different purposes, but they share common ingredients, often derived from magnetic resonance imaging, and they all benefit from a greatly improved time-efficiency. This review article provides a description of several spatial encoding and spatial selection methods, including single-scan multidimensional experiments (ultrafast 2D NMR, DOSY, Z spectroscopy, inversion recovery and Laplace NMR), pure shift and selective refocusing experiments (including Zangger-Sterk decoupling, G-SERF and PSYCHE), a Z filter, and fast-pulsing slice-selective experiments. Some key elements for spatial parallelisation are introduced and when possible a common framework is used for the analysis of each method. Sensitivity considerations are discussed, and a selection of applications is analysed to illustrate which questions can be answered thanks to spatial encoding and spatial selection methods, and discuss the perspectives for future developments and applications.

High Resolution for Chemical Shifts and Scalar Coupling Constants: The 2D Real-Time J-Upscaled PSYCHE-DIAG

The facile determination of chemical shift and scalar coupling constants in NMR spectra is often prevented by spectral overlap and limited resolution. Here, we present a high-resolution NMR experiment for the simultaneous detection of both resonance frequencies and coupling patterns even with small J-values. A PSYCHE-decoupled DIAG (Pure Shift Yielded by Chirp Excitation- DIAGonal) experiment, which resolves chemical shift in the indirect dimension of a 2D experiment is combined with real-time J-upscaling in order to visualize small coupling constants that would otherwise be hidden in the linewidth of a regular proton or DIAG spectrum.

Orchestrated approaches using pure shift NMR: Extraction of spectral parameters, ultra-high resolution, and sensitivity enhancement

The limited chemical shift range of protons and pairwise interaction among all the abundant nuclear spins of a molecule makes ¹ H spectrum too complicated. As a consequence, the straightforward analysis and the accurate extraction of their interaction strengths from the ¹ H spectrum of a complex spin system are formidably difficult or often impossible. This problem persists in the determination of scalar couplings be it between two abundant homonuclear spins or between ¹ H and an abundant heteronuclear spin (viz., ¹⁹ F and ³¹ P). Such problems are encountered in many situations where the determination of homonuclear and heteronuclear couplings is challenging. The several pure shift based one-dimensional and two-dimensional NMR strategies recently developed in our laboratory for the straightforward extraction of homonuclear and heteronuclear interaction parameters in diverse situations are discussed. Initially, the unique application of pure shift technique that paves the way for easy and straightforward extraction of magnitudes of heteronuclear couplings, namely, ⁿ J_HX (where X stands for ¹⁹ F, ³¹ P, etc.), is discussed. Subsequently, several pure shift edited one-dimensional and two-dimensional NMR strategies that are developed for the direct extraction of homonuclear and heteronuclear couplings and for achieving ultra-high-resolved ¹ H spectra with complete eradication of zero frequency peaks and the evolution of unwanted couplings. The enhancement in the sensitivity has also been achieved in the slice-selective pure shift experiments by the rapid acquisition of proton spectrum where the polarization from the adjacent protons is transferred to the selectively excited proton.

Fast and simultaneous determination of 1 H-1 H and 1 H-19 F scalar couplings in complex spin systems: Application of PSYCHE homonuclear broadband decoupling

The present manuscript focuses on fast and simultaneous determination of ¹ H-¹ H and ¹ H-¹⁹ F scalar couplings in fluorinated complex steroid molecules. Incorporation of broadband PSYCHE homonuclear decoupling in the indirect dimension of zero-quantum filtered diagonal experiments (F1-PSYCHE-DIAG) suppresses ¹ H-¹ H scalar couplings; however, it retains ¹ H-¹⁹ F scalar couplings (along F1 dimension) for the ¹⁹ F coupled protons while preserving the pure-shift nature for ¹ H resonances uncoupled to ¹⁹ F. In such cases, along the direct dimensions, ¹ H-¹ H scalar coupling multiplets deconvolute and they appear as duplicated multiplets for the ¹⁹ F coupled protons, which facilitates unambiguous discrimination of ¹⁹ F coupled ¹ H chemical sites from the others. Further, as an added advantage, data acquisition has been accelerated by invoking the known ideas of spectral aliasing in the F1-PSYCHE-DIAG scheme and experiments demand only ~10 min of spectrometer times.

Homonuclear decoupling by projection reconstruction

Similar to J-resolved spectroscopy, also, heteronuclear multiple bond correlation (HMBC), heteronuclear single bond correlation (HSBC), and heteronuclear multiple quantum coherence (HMQC) types of correlation experiments result in homonuclear tilted multiplet patterns. On the example of the high-resolution heteronuclear single bond correlation (HR-HSBC) pulse sequence, it is shown how the tilt angle can be varied within a wide range of positive and negative values. Projection along the tilt angles in all cases results in homonuclear decoupling. Using well-known projection reconstruction techniques, the different tilt angles can be used to reconstruct a homonuclear decoupled two-dimensional correlation spectrum. The concept is proven and further refined by segmental projection reconstruction and the use of a clean in-phase heteronuclear single quantum correlation (CLIP-HSQC) spectrum with an effective zero tilt angle for further filtering. The proof of principle, its application to one-bond coupling measurement, as well as a basic HMBC, and a detailed discussion with comparison to other homodecoupling techniques are given.

A quarter of a century of SERF: The progress of an NMR pulse sequence and its application

SERF, an NMR pulse sequence for selectively measuring a spin coupling constant without interference from other couplings, was published by the current author almost 25 years ago in 1995. Since then, about 35 modifications and extensions of the original have been published by other groups and applied to many chemical problems. This review discusses these modifications and provides pertinent examples. A comparative and critical evaluation of these developments is given in tabular form. The last part focuses on the chemical results.

Extracting unresolved coupling constants from complex multiplets by a real-time J-upscaled SERF experiment

The measurement of small homonuclear coupling constants is often prevented by either their small size and/or overlap with other signal splittings. Here, we present a real-time method to extract such couplings without interference from other splittings, with a resolution that is beyond conventional NMR spectra. In this real-time J-upscaled SERF experiment, homonuclear coupling is removed by slice-selective pure shift NMR, whereas scalar coupling to only one selected signal is reintroduced by selective refocusing. The remaining couplings are enhanced by real-time J-upscaling during interruptions of the FID data acquisition. The resulting spectrum is not only simplified by the restriction of the scalar coupling but also its resolution enhanced. This improved resolution results from a reduction of signal broadening due to magnetic field inhomogeneities from 2 different sources: slice-selective excitation and the spin-echo type J-upscaling element.

Combining pure shift and J-edited spectroscopies: A strategy for extracting chemical shifts and scalar couplings from highly crowded proton spectra of oligomeric saccharides

We report the application of pure shift and J-edited nuclear magnetic resonance spectroscopies to the structural analysis of a protected maltotrioside synthetic intermediate whose crowded ¹ H spectrum displays highly crowded regions. The analytical strategy is based on the implementation of J-edited and TOCSY experiments whose resolution is optimized by the use of broadband homonuclear decoupling and selective refocusing techniques, to assign and measure chemical shifts and homonuclear scalar couplings with high accuracy. The resulting data show a high level of complementarity, providing a detailed insight into each subunit of this oligomeric saccharide, even for proton sites whose nuclear magnetic resonance signals strongly overlap. This approach allowed for fully assigning proton chemical shifts and extracting 80% of the ³ J_HH couplings that are in excellent agreement with those expected for D-gluco-pyranosyl units in ⁴ C₁ conformations.

Selective measurement of 1 H-1 H scalar couplings from crowded chemical shift regions: Combined pure shift and spin-echo modulation approach

J_HH scalar couplings carry rich structural information and their measurements are fundamental in the ¹ H NMR based elucidation of small and medium molecules, which, however, are hampered in the presence of large J-coupling network. Further, enhanced spectral resolution is often essential for precise determination of a specific set of ¹ H-¹ H J-couplings among the complex J-multiplets. In the light of the recent advancements in homodecoupling pure shift strategies, here, we report absorption mode, band-selective refocused pure shift spin-echo method, which helps in determining ¹ H-¹ H J-couplings from crowded spectral regions. The importance of the present band-selective refocused pure shift spin-echo experiment is exemplified for 2 steroid molecules, estradiol and testosterone.

Combining Fourier phase encoding and broadband inversion toward J-edited spectra

Nuclear magnetic resonance (NMR) spectra are often utilized for gathering accurate information relevant to molecular structures and composition assignments. In this study, we develop a homonuclear encoding approach based on imparting a discrete phase modulation of the targeted cross peaks, and combine it with a pure shift experiments (PSYCHE) based J-modulated scheme, providing simple 2D J-edited spectra for accurate measurement of scalar coupling networks. Chemical shifts and J coupling constants of protons coupled to the specific protons are demonstrated along the F₂ and F₁ dimensions, respectively. Polychromatic pulses by Fourier phase encoding were performed to simultaneously detect several coupling networks. Proton-proton scalar couplings are chosen by a polychromatic pulse and a PSYCHE element. Axis peaks and unwanted couplings are complete eradicated by incorporating a selective COSY block as a preparation period. The theoretical principles and the signal processing procedure are laid out, and experimental observations are rationalized on the basis of theoretical analyses.

Current developments in homonuclear and heteronuclear J-resolved NMR experiments

Two-dimensional J-resolved (Jres) NMR experiments offer a simple, user-friendly spectral representation where the information of coupling constants and chemical shifts are separated into two orthogonal frequency axis. Since its initial proposal 40 years ago, Jres has been the focus of considerable interest both in improving the basic pulse sequence and in its successful application to a wide range of studies. Here, the latest developments in the design of novel Jres pulse schemes are reviewed, mainly focusing on obtaining pure absorption lineshapes, minimizing strong coupling artifacts, and also optimizing sensitivity and experimental measurements. A discussion of several Jres versions for the accurate measurement of a different number of homonuclear (J_HH ) and heteronuclear (J_CH ) coupling constants is presented, accompanied by some illustrative examples.

How to face the low intrinsic sensitivity of 2D heteronuclear NMR with fast repetition techniques: go faster to go higher!

Nuclear magnetic resonance (NMR) is one of the most widely used analytical techniques in numerous domains where molecules are objects of investigation. However, major limitations of multidimensional NMR experiments come from their low sensitivity and from the long times needed for their acquisition. In order to overcome such limitations, fast repetition NMR techniques allowed for the reduction of 2D experimental time and for the conversion of the gained time into a higher number of scans leading to a better sensitivity. Thus, fast repetition 2D heteronuclear NMR techniques have allowed new advances in NMR, especially to access infomation on low abundant nuclei, to enhance the detection of low concentrated compounds and to probe weak interactions like hydrogen bonds at natural abundance. Copyright © 2017 John Wiley & Sons, Ltd.

A simultaneous multi-slice selective J-resolved experiment for fully resolved scalar coupling information

Proton-proton scalar coupling plays an important role in molecular structure elucidation. Many methods have been proposed for revealing scalar coupling networks involving chosen protons. However, determining all J_HH values within a fully coupled network remains as a tedious process. Here, we propose a method termed as simultaneous multi-slice selective J-resolved spectroscopy (SMS-SEJRES) for simultaneously measuring J_HH values out of all coupling networks in a sample within one experiment. In this work, gradient-encoded selective refocusing, PSYCHE decoupling and echo planar spectroscopic imaging (EPSI) detection module are adopted, resulting in different selective J-edited spectra extracted from different spatial positions. The proposed pulse sequence can facilitate the analysis of molecular structures. Therefore, it will interest scientists who would like to efficiently address the structural analysis of molecules.

Accurate measurement of JHH in overlapped signals by a TOCSY-edited SERF Experiment

Selective refocusing (GSERF or the recent PSYCHEDELIC) experiments were originally designed to determine all proton-proton coupling constants (J_HH ) for a selected proton resonance. They work for isolated signals on which selective excitation can be successfully applied but, as it happens in other selective experiments, fail for overlapped signals. To circumvent this limitation, a doubly-selective TOCSY-GSERF scheme is presented for the measurement of J_HH in protons resonating in crowded regions. This new experiment takes advantage of the editing features of an initial TOCSY transfer to uncover hidden resonances that become accessible to perform the subsequent frequency-selective refocusing. Copyright © 2016 John Wiley & Sons, Ltd.

Pure shift edited ultra high resolution NMR spectrum with complete eradication of axial peaks and unwanted couplings

Poor chemical shift dispersion and pairwise interaction among the entire coupled network of spins results in complex one dimensional ¹H NMR spectra, severely hampering the analysis and the accurate determination of ⁿJ_HH. Available two dimensional selective refocusing based techniques suffer from the evolution of undesirable couplings and intense axial peaks, creating ambiguity in the analysis and the extraction of coupling values. In this work, we report a novel two dimensional experiment for the complete elimination of axial peaks and unwanted couplings, while retaining only the couplings of the selected proton to its partners, with a blend of ultra-high resolution achieved by real time broad band homonuclear decoupling.

Clean G-SERF an NMR experiment for the complete eradication of axial peaks and undesired couplings from the complex spectrum

The Clean-G-SERF spectrum is free from axial peaks and undesired couplings and is very useful in the measurement of proton–proton couplings.

Measuring JHH values with a selective constant-time 2D NMR protocol

Proton-proton scalar couplings play important roles in molecule structure elucidation. However, measurements of J_HH values in complex coupled spin systems remain challenging. In this study, we develop a selective constant-time (SECT) 2D NMR protocol with which scalar coupling networks involving chosen protons can be revealed, and corresponding J_HH values can be measured through doublets along the F1 dimension. All J_HH values within a network of n fully coupled protons can be separately determined with (n-1) SECT experiments. Additionally, the proposed pulse sequence possesses satisfactory sensitivity and handy implementation. Therefore, it will interest scientists who intend to address structural analyzes of molecules with overcrowded spectra, and may greatly facilitate the applications of scalar-coupling constants in molecule structure studies.

Determination of unresolved heteronuclear scalar coupling constants by J(up)-HSQMBC

Long-range heteronuclear scalar coupling constants provide important structural information, which is necessary for obtaining stereospecific assignment or dihedral angle information. The measurement of small proton-carbon splittings is particularly difficult due to the low natural abundance of carbon-13 and the presence of homonuclear couplings of similar size. Here we present a real-time J-upscaled HSQMBC, which allows the measurement of heteronuclear coupling constants even if they are hidden in the signal linewidth of a regular spectrum.

Achieving high resolution and optimizing sensitivity in spatial frequency encoding NMR spectroscopy: from theory to practice

A detailed analysis of NMR spectra based on spatial frequency encoding is presented.

A General Method for Extracting Individual Coupling Constants from Crowded (1)H NMR Spectra

Couplings between protons, whether scalar or dipolar, provide a wealth of structural information. Unfortunately, the high number of ¹H‐¹H couplings gives rise to complex multiplets and severe overlap in crowded spectra, greatly complicating their measurement. Many different methods exist for disentangling couplings, but none approaches optimum resolution. Here, we present a general new 2D J‐resolved method, PSYCHEDELIC, in which all homonuclear couplings are suppressed in F₂, and only the couplings to chosen spins appear, as simple doublets, in F₁. This approaches the theoretical limit for resolving ¹H‐¹H couplings, with close to natural linewidths and with only chemical shifts in F₂. With the same high sensitivity and spectral purity as the parent PSYCHE pure shift experiment, PSYCHEDELIC offers a robust method for chemists seeking to exploit couplings for structural, conformational, or stereochemical analyses.

Faster and cleaner real-time pure shift NMR experiments

Real-time pure shift experiments provide highly resolved proton NMR spectra which do not require any special processing. Although being more sensitive than their pseudo 2D counterparts, their signal intensities per unit time are still far below regular NMR spectra. In addition, scalar coupling evolution during the individual data chunks produces decoupling sidebands. Here we show that faster and cleaner real-time pure shift spectra can be obtained through the implementation of two parameter alterations. Variation of the FID chunk lengths between individual transients significantly suppresses decoupling sidebands for any kind of real-time pure shift spectra and thus allows for example the analysis of minor components in compound mixtures. Shifting the excitation frequency between individual scans of real-time slice-selective pure shift spectra increases their sensitivity obtainable in unit time by allowing faster repetitions of acquisitions.

Magnetic field dependence of spatial frequency encoding NMR as probed on an oligosaccharide

The magnetic field dependence of spatial frequency encoding NMR techniques is addressed through a detailed analysis of (1)H NMR spectra acquired under spatial frequency encoding on an oligomeric saccharide sample. In particular, the influence of the strength of the static magnetic field on spectral and spatial resolutions that are key features of this method is investigated. For this purpose, we report the acquisition of correlation experiments implementing broadband homodecoupling or J-edited spin evolutions, and we discuss the resolution enhancements that are provided by these techniques at two different magnetic fields. We show that performing these experiments at higher field improves the performance of high resolution NMR techniques based on a spatial frequency encoding. The significant resolution enhancements observed on the correlation spectra acquired at very high field make them valuable analytical tools that are suitable for the assignment of (1)H chemical shifts and scalar couplings in molecules with highly crowded spectrum such as carbohydrates.

Broadband 1H homodecoupled NMR experiments: recent developments, methods and applications

In recent years, a great interest in the development of new broadband 1H homonuclear decoupled techniques providing simplified JHH multiplet patterns has emerged again in the field of small molecule NMR. The resulting highly resolved 1H NMR spectra display resonances as collapsed singlets, therefore minimizing signal overlap and expediting spectral analysis. This review aims at presenting the most recent advances in pure shift NMR spectroscopy, with a particular emphasis to the Zangger-Sterk experiment. A detailed discussion about the most relevant practical aspects in terms of pulse sequence design, selectivity, sensitivity, spectral resolution and performance is provided. Finally, the implementation of the different reported strategies into traditional 1D and 2D NMR experiments is described while several practical applications are also reviewed.

Pure shift NMR

Although scalar-coupling provides important structural information, the resulting signal splittings significantly reduce the resolution of NMR spectra. Limited resolution is a particular problem in proton NMR experiments, resulting in part from the limited proton chemical shift range (∼10 ppm) but even more from the splittings due to scalar coupling to nearby protons. "Pure shift" NMR spectroscopy (also known as broadband homonuclear decoupling) has been developed for disentangling overlapped proton NMR spectra. The resulting spectra are considerably simplified as they consist of single lines, reminiscent of proton-decoupled C-13 spectra at natural abundance, with no multiplet structure. The different approaches to obtaining pure shift spectra are reviewed here and several applications presented. Pure shift spectra are especially useful for highly overlapped proton spectra, as found for example in reaction mixtures, natural products and biomacromolecules.

Pushing the limits of signal resolution to make coupling measurement easier

Novel band selective decoupled pure shift selective refocusing experiments allowed simplification of the measurement of all δ¹H, and J_HH couplings with an ultrahigh spectral resolution in peptides.

Enantio-differentiation of molecules with diverse functionalities using a single probe

Chiral discrimination of molecules with diverse functionalities using a single CSA.