Requested by no1uno High-resolution NMR Spectra under Inhomogeneous fields via Intermolecular Double-Quantum Coherences Lin,Yanquin;Chen,Zhiwei;Cai,Congbo;Chen,ZhongSpec. Chim. Acta. A: Mol. & Biol. Spect.2008, Vol.70(5), pp.1025-1028DOI: 10.1016/j.saa.2007.10.022
AbstractHigh-resolution NMR spectroscopy is a powerful tool for analyzing molecular structures and compositions. Line-widths of conventional liquid NMR signals are directly proportional to the overall magnetic field inhomogeneity the sample experiences. In many circumstances, spatial and temporal homogeneity of the magnetic field is degraded. In this paper, a modified pulse sequence based on intermolecular double-quantum coherences (iDQCs) was proposed to obtain 1D high-resolution NMR spectra under inhomogeneous fields using 2D acquisition. Analytical expressions were derived from the intermolecular multiple-quantum coherence (iMQC) treatments. Both experimental and simulated spectra provide high-resolution 1D projection spectra similar to conventional 1D high-resolution spectra. Moreover, the apparent J coupling constants are threefold magnified, which allows a more accurate measurement of small J coupling constants.
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Intermolecular Double-Quantum coherence NMR Spectroscopy in Moderate Inhomogeneous FieldsZhang,Wen;Cai,Congbo;Cai,Shuhui;Chen,Xi;Chen,ZhongSpect. Chim. Acta A: Mol & Biol. Spect.2009, Vol.74(5), pp.1138-1144DOI: 10.1016/j.saa.2009.09.023
AbstractIntermolecular multiple-quantum coherences (iMQCs) can be utilized to retrieve high-resolution NMR spectra in inhomogeneous magnetic fields. The application of selective pulses in pulse sequences can greatly simplify 2D iMQC spectra. However, so far high-resolution iMQC methods are mainly used in relatively small field inhomogeneities. In this paper, we took the IDEAL-II sequence as an example to study their applicability in moderate inhomogeneous magnetic fields. The experimental and simulation results show that high-resolution NMR spectra can be obtained in moderate inhomogeneous fields if the excitation range of selective pulse is properly set. Once the field inhomogeneity reaches a certain degree, the appearance of undesirable intermolecular cross-peaks due to the distant dipolar field produced by solute spins is inevitable. The spectral quality may vary with sample even in the same moderate inhomogeneous fields, depending on the chemical shift distributions and the J coupling networks of the components under study. The conclusions drawn in this paper are generally applicable to all high-resolution iMQC methods utilizing selective RF pulses.
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Magnetic Field Gradients in High Resolution NMRHurd,RalphEncyclopedia Spectroscopy & Spectrometry2010 pp.1407-1414
DOI: 10.1016/B978-0-12-374413-5.00195-0
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http://ifile.it/2hcmug8/0127641009.rarAbstractThe use of magnetic field gradients has become important in many areas of high-resolution NMR spectroscopy. This article describes how they can be generated, and outlines their use. The main applications include coherence and coherence pathway selection, multiple-quantum coherence selection and spoiling of magnetization. In addition, they can be used for measurement of diffusion coefficients, for diffusion-based water peak suppression and for spatial encoding
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Signal Selection in High-Resolution NMR by Pulsed Field Gradients: I. Geometrical AnalysisMitschang,LorenzJ. Mag. Res.1999, Vol.137(1), pp.1-9DOI: 10.1006/jmre.1998.1636
AbstractA geometrical description for the selection of coherence transfer pathways in high resolution NMR by the application of pulsed field gradients along three orthogonal directions in space is presented. The response of the spin system is one point of the three-dimensional Fourier transform of the sample volume affected by a sequence of field gradients. The property that a pathway is retained (or suppressed) when a sequence of field gradients is applied is expressed by the property of vectors, representing the pathway and the sequence, respectively, to be orthogonal (or not orthogonal). Ignoring imperfections of RF pulses, and with the exception of sensitivity enhanced experiments and experiments where the relevant coherence order is zero while field gradients are applied, it is shown that at most only half of the relevant pathways, as compared to a phase cycled experiment, are retained when field gradients are used for signal selection.
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Signal Selection in High-Resolution NMR by Pulsed Field Gradients: II. The Design of Gradient Pulse SequencesThomas,David;Mitschang,Lorenz;Simon,Bernd;Oschkinat,HartmutJ. Mag. Res.1999, Vol.137(1), pp.10-24DOI: 10.1006/jmre.1998.1637
AbstractWe describe a new and powerful computer program called TRIPLE GRADIENT which calculates optimized pulsed field gradient sequences for specific coherence pathway selection or rejection. Sequences can be computed for gradient coils acting along one, two, or three perpendicular axes. The program is based on the computational minimization of a penalty function formed from the summed amplitudes of the unwanted signals. The underlying mathematical analysis makes use of a vectorial representation of the way in which a gradient sequence suppresses different signals. It is argued that experiments using well-calculated gradient sequences are quicker and generally perform better than those using extensive phase cycling, especially when suppressing extremely strong solvent signals, and it is shown that in many cases gradient experiments of optimal signal-to-noise ratio can be performed. These claims are illustrated by spectra obtained from an HQQC experiment.
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The Virtual NMR Spectrometer: a Computer Program for Efficient Simulation of NMR Experiments Involving Pulsed Field Gradients.Nicholas,P;Fushman,D;Ruchinsky,V;Cowburn,DJ. Mag. Res.2000, Vol.145(2), pp.262-275DOI: 10.1006/jmre.2000.2108
AbstractThis paper presents a software program, the Virtual NMR Spectrometer, for computer simulation of multichannel, multidimensional NMR experiments on user-defined spin systems. The program is capable of reproducing most features of the modern NMR experiment, including homo- and heteronuclear pulse sequences, phase cycling, pulsed field gradients, and shaped pulses. Two different approaches are implemented to simulate the effect of pulsed field gradients on coherence selection, an explicit calculation of all coherence transfer pathways, and an effective approximate method using integration over multiple positions in the sample. The applications of the Virtual NMR Spectrometer are illustrated using homonuclear COSY and DQF COSY experiments with gradient selection, heteronuclear HSQC, and TROSY. The program uses an intuitive graphical user interface, which resembles the appearance and operation of a real spectrometer. A translator is used to allow the user to design pulse sequences with the same programming language used in the actual experiment on a real spectrometer. The Virtual NMR Spectrometer is designed as a useful tool for developing new NMR experiments and for tuning and adjusting the experimental setup for existing ones prior to running costly NMR experiments, in order to reduce the setup time on a real spectrometer. It will also be a useful aid for learning the general principles of magnetic resonance and contemporary innovations in NMR pulse sequence design.