Pure shift FESTA: An ultra-high resolution NMR tool for the analysis of complex fluorine-containing spin systems

NMR measurements of molecules containing sparse fluorine atoms are becoming increasingly common due to their prevalence in medicinal chemistry. However, the presence of both homonuclear and heteronuclear scalar couplings severely complicates their analysis by NMR. In complex systems, FESTA, a heteronuclear spectral editing method, allows simplified 1 H NMR spectra to be obtained containing only 1 H signals from the same spin system as a chosen 19 F. Despite spectral simplification, signal overlap due to the presence of scalar couplings is often a problem in FESTA spectra. Here, we report a new experiment that combines FESTA and pure shift methods to provide fully decoupled ultra-high resolution FESTA spectra showing a single signal for each 1 H chemical environment. The utility of the method is demonstrated for the analysis of two complex fluorine-containing mixtures of pharmaceutical and biochemical interest.

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.

Simultaneous Broadband Suppression of Homonuclear and Heteronuclear Couplings in 1 H NMR Spectroscopy

The ¹ H NMR analysis of species containing NMR-active heteronuclei can be difficult due to signal overlap caused by the combined effects of homonuclear and heteronuclear scalar (J) couplings. Here, a general pure shift method is presented for obtaining ultra-high resolution ¹ H NMR spectra where spectral overlap is drastically reduced by suppressing both homonuclear and heteronuclear J-couplings, giving one single signal per ¹ H chemical environment. Its usefulness is demonstrated in the analysis of fluorine- and phosphorus-containing compounds of pharmaceutical and biochemical interest.

Kövér K. Ultrahigh-Resolution Homo- and Heterodecoupled 1H and TOCSY NMR Experiments

The original homonuclear decoupled (pure shift) experiments provide ultrahigh-resolution ¹H spectra of compounds containing NMR-active heteronuclei of low natural isotopic abundance (e.g., ¹³C or ¹⁵N). In contrast, molecules containing highly abundant heteronuclei (like ³¹P or ¹⁹F) give doublets or a multiple of doublets in their homonuclear decoupled spectra, depending on the number of heteronuclear coupling partners and the magnitude of the respective coupling constants. In these cases, the complex and frequently overlapping signals may hamper the unambiguous assignment of resonances. Here, we present new heteronuclear decoupled (HD) PSYCHE ¹H and TOCSY experiments, which result in simplified spectra with significantly increased resolution, allowing the reliable assessment of individual resonances. The utility of the experiments has been demonstrated on a challenging stereoisomeric mixture of a platinum-phosphine complex, where ultrahigh resolution of the obtained HD PSYCHE spectra made the structure elucidation of the chiral products feasible. HD PSYCHE methods can be potentially applied to other important ³¹P- or ¹⁹F-containing compounds in medicinal chemistry and metabolomics.

Selective Nuclear Magnetic Resonance Experiments for Sign-Sensitive Determination of Heteronuclear Couplings: Expanding the Analysis of Crude Reaction Mixtures

State-of-the-art nuclear magnetic resonance (NMR) selective experiments are capable of directly analyzing crude reaction mixtures. A new experiment named HD-HAPPY-FESTA yields ultrahigh-resolution total correlation subspectra, which are suitable for sign-sensitive determination of heteronuclear couplings, as demonstrated here by measuring the sign and magnitude for proton-fluorine couplings (J_HF) from major and minor isomer products of a two-step reaction without any purification. Proton-fluorine couplings ranging from 51.5 to -2.6 Hz could be measured using HD-HAPPY-FESTA, with the smallest measured magnitude of 0.8 Hz. Experimental J_HF values were used to identify the two fluoroketone intermediates and the four fluoroalcohol products. Results were rationalized and compared with the density functional theory (DFT) calculations. Experimental data were further compared with the couplings reported in the literature, where pure samples were analyzed.

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.

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.

Pure shift 1 H NMR: what is next?

Currently, pure shift nuclear magnetic resonance (NMR) is an area of high interest. The aim of this contribution is to describe briefly how this technique has evolved, where it is now and what could be the next challenges in the amazing adventure of the development and application of pure shift NMR experiments. Copyright © 2016 John Wiley & Sons, Ltd.

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.

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.

Precise measurement of long-range heteronuclear coupling constants by a novel broadband proton-proton-decoupled CPMG-HSQMBC method

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 ⁿJ(¹H,³¹P), ⁿJ(¹H,⁷⁷Se), and ⁿJ(¹H,¹³C) values, respectively.

Ultra high-resolution HSQC: application to the efficient and accurate measurement of heteronuclear coupling constants

A rapid acquisition strategy in terms of enhanced resolution per time unit for the simple and efficient determination of multiple coupling constants is proposed. The use of ¹³C spectral aliasing combined with pure shift NMR techniques allows accurate measurements of the magnitude and the sign from ultra high resolved 2D cross-peaks.

Sensitivity enhancement in slice-selective NMR experiments through polarization sharing

Sensitivity enhanced spatially encoded NMR experiments.