Laura Castañar

Full sensitivity and enhanced resolution in homodecoupled band-selective NMR experiments.

Chemical shifts and coupling constants (J) are fundamentals in the analysis and interpretation of NMR spectra. Multiplicity information and J values can be extracted from the analysis of the fine multiplet structure, and they can be related to structural parameters, such as the number of neighbouring spins, the trace of trough-bond connectivities or dihedral angle constraints. Over recent years, a significant interest has emerged to develop homodecoupled H NMR spectroscopy techniques that offer increased resolution by simplifying the homonuclear splitting pattern, and therefore reducing signal overlapping. The simplest approach for homodecoupling is the use of semiselective shaped pulse decoupling during signal detection, where the receiver and the decoupling are alternatively activated. If the semiselective pulse is applied in a region A of the spectrum, the multiplet structure of J coupled signals resonating in a different region B appear simplified while they are detected. However, this is not a broadband method because protons from a third region C would not be decoupled, and therefore the corresponding coupling splittings will remain in the partially decoupled spectrum. Although the use of sophisticated multiple-region decoupling using different and simultaneous decoupling waveforms could be applied, it is difficult to achieve a perfect decoupling for all resonances and, moreover, without the interference of undesired decoupling sidebands. Alternatively, the internal projection in the chemical shift dimension of J-resolved experiments or the diagonal signals in anti-z-COSY experiments have been also proposed to obtain broadband homodecoupled NMR spectra. They require the collection of more time consuming 2D/3D data and post-processing tasks can be further required. Some years ago, the so-called Zangger–Sterk (ZS) method based on the implementation of the spatially encoded concept along the z-dimension was also proposed. The ZS method has been further refined and several applications have been reported to obtain high-resolved pure-shift multidimensional NMR spectra. The main drawbacks of ZS methods are their low sensitivities because signal only comes from selected z slices and, on the other hand, the need for an FID reconstruction method by means of a time-consuming 2D/3D mode acquisition. Very recently, a new NMR detection scheme has been proposed for the instant and speed-up acquisition of ZS-decoupled spectra in a one-shot single-scan experiment. The instant technique greatly improves the sensitivity per time unit ratio although the attainable sensitivity is still far from a regular H spectrum. Analogous ZS methods incorporating isotopic C editing by using BIRD elements have been also reported to efficiently minimise the effects of strong coupling, but an important penalty in sensitivity remains due to the low natural abundance of C (1.1 %). Based on the instant ZS experiment, a novel NMR spectroscopy method for the fast acquisition of full-sensitive, homodecoupled band-selective (HOBS) NMR spectra is proposed here. It is noteworthy that the spatial encoding gradients applied simultaneously with the selective pulses in the original instant scheme are here omitted, avoiding sensitivity losses due to spatial slice selection. In addition, the HOBS method incorporates a number of advantages, such as: 1) an effective homodecoupling NMR block consisting of a pair of hard/selective 1808 pulses flanked by pulsed field gradients (Figure 1), 2) an excellent spectral quality related to the use of selective gradient echoes, 3) real-time data collection without need of additional reconstruction methods that also allows conventional FID data processing, and 4) an easy implementation in multidimensional experiments. In our hands, the best results in terms of selectivity and optimum

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. Copyright

Implementing homo- and heterodecoupling in region-selective HSQMBC experiments

An NMR method to enhance the sensitivity and resolution in band-selective long-range heteronuclear correlation spectra is proposed. The excellent in-phase nature of the selHSQMBC experiment allows that homonuclear and/or heteronuclear decoupling can be achieved in the detected dimension of a 2D multiple-bond correlation map, obtaining simplified cross-peaks without their characteristic fine J multiplet structure. The experimental result is a resolution improvement while the highest sensitivity is also achieved. Specifically, it is shown that the (1)H-homodecoupled band-selective (HOBS) HSQMBC experiment represents a new way to measure heteronuclear coupling constants from the simplified in-phase doublets generated along the detected dimension.

Simultaneous Multi-Slice Excitation in Spatially Encoded NMR Experiments

Improved sensitivity: A novel strategy to enhance the experimental sensitivity in spatially encoded NMR experiments has been developed. The use of a multiple-frequency modulated pulse applied simultaneously to an encoding gradient can afford a substantial sensitivity gain with respect to single-slice selected experiments.

Enantiodifferentiation through Frequency‐Selective Pure‐Shift 1H Nuclear Magnetic Resonance Spectroscopy

A frequency-selective 1D (1) H nuclear magnetic resonance (NMR) experiment for the fast and sensitive determination of chemical-shift differences between overlapped resonances is proposed. The resulting fully homodecoupled (1) H NMR resonances appear as resolved 1D singlets without their typical J(HH) coupling constant multiplet structures. The high signal dispersion that is achieved is then exploited in enantiodiscrimination studies by using chiral solvating agents.

Pure in-phase heteronuclear correlation NMR experiments.

A general NMR approach to provide pure in-phase (PIP) multiplets in heteronuclear correlation experiments is described. The implementation of a zero-quantum filter efficiently suppresses any unwanted anti-phase contributions that usually distort the multiplet pattern of cross-peaks and can hamper their analysis. The clean pattern obtained in PIP-HSQMBC experiments is suitable for a direct extraction of coupling constants in resolved signals, for a peak-fitting process from a reference signal, and for the application of the IPAP technique in non-resolved multiplets.

Measurement of T1/T2 relaxation times in overlapped regions from homodecoupled 1H singlet signals

The implementation of the HOmodecoupled Band-Selective (HOBS) technique in the conventional Inversion-Recovery and CPMG-based PROJECT experiments is described. The achievement of fully homodecoupled signals allows the distinction of overlapped (1)H resonances with small chemical shift differences. It is shown that the corresponding T1 and T2 relaxation times can be individually measured from the resulting singlet lines using conventional exponential curve-fitting methods.

Suppression of phase and amplitude J(HH) modulations in HSQC experiments

The amplitude and the phase of cross peaks in conventional 2D HSQC experiments are modulated by both proton–proton, J(HH), and proton–carbon, 1J(CH), coupling constants. It is shown by spectral simulation and experimentally that J(HH) interferences are suppressed in a novel perfect‐HSQC pulse scheme that incorporates perfect‐echo INEPT periods. The improved 2D spectra afford pure in‐phase cross peaks with respect to 1J(CH) and J(HH), irrespective of the experiment delay optimization. In addition, peak volumes are not attenuated by the influence of J(HH), rendering practical issues such as phase correction, multiplet analysis, and signal integration more appropriate. Copyright

Pure shift 1H 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

Straightforward measurement of individual 1J(CH) and 2J(HH) in diastereotopic CH2 groups

The C-H(A) cross-peak corresponding to a diastereotopic CHAHB methylene spin system exhibits a characteristic 1:0:1 multiplet pattern along the indirect dimension of a ω1-coupled HSQC spectrum. It is shown here that the use of the initial (13)C Boltzmann polarization instead of the regular INEPT-based (1)H Boltzmann polarization makes visible the central lines of this multiplet pattern. A spin-state-selective method is proposed for the efficient measurement of both (1)J(CHA) and (1)J(CHB) along the indirect dimension of a 2D spectrum as well as to the magnitude and the sign of the geminal (2)J(HAHB) coupling constant from the straightforward analysis of a single four-component E.COSY cross-peak. Additionally, the extraction of (1)J(CH) values for CH and CH3 multiplicities can be also performed from the same spectrum. The success of the method is also illustrated for the determination of residual dipolar (1)D(CH) and (2)D(HH) coupling constants in a small molecule weakly aligned in a PMMA swollen gel.