同核二维中总相关谱检测方案(核磁共振)

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Application Note 8 Spin Locking： Total CorrelationSpectroscopy (TOCSY) In an NMR experiment， magnetisations perpendicular to thestatic magnetic field B0 will rotate about the BO field at itscharacteristic Larmor frequency， often referred to as chemicalshift precession. In addition to the chemical shift precessionthe magnetisations will also evolve under the influence of themutual coupling between the spins， the scalar or J-coupling.The chemical shift evolution accounts for the position of aparticular resonance in the spectrum， while the J-couplingis the source of the peak splitting patterns. When theJ-couplings are small in comparison to the difference betweenthe resonance frequencies of the coupled spins， the evolutionis dominated by the chemical shift. This may be true becausethe BO field is high or because the pair of spins are separatedin the molecule by several bonds producing a small coupling.In some experiments it is desirable to suppress the chemicalshift evolution and allow the spin system to evolve under theJ-coupling only. This is often referred to as isotropic mixing. The suppression of the chemical shift evolution can beachieved by applying a strong RF pulse along a chosendirection keeping the magnetisations locked in alignmentwith the field. This is referred to as spin locking and the RFfield as the spin lock field. There are a number of methodsexploiting composite pulses designed to provide a spin lockwhere imperfections in the RF-pulse can be continuallyrefocused to provide a clean spin lock. The two most commonare the MLEV-17 [Bax， A and Davis， DG， Journal of MagneticResonance 65， 355-360，1985] and DIPSI [Shaka， AJ， Lee， CJ，and Pines， A， Journal of Magnetic Resonance 77(2)， 274-293，19881. TOCSY Total correlation spectroscopy TOCSY， is a homonuclear 2Dexperiment similar to COSY (see application note 7) in whichthe J-coupling between two hydrogen nuclei manifests asa cross peak in the spectrum. Unlike COSY， however， thedetection of the coupled spins is not limited to nearestneighbours. The TOCSY experiment exploits the isotropicmixing condition during spin locking to produce cross peaksbetween all hydrogen nuclei that form part of an unbrokenchain of coupled spins. Consider a chain of four hydrogens， labelled A， B， C andD， where hydrogen A is coupled to B which is coupled to Cwhich， in turn， is coupled to D. In a COSY experiment we would expect to see a cross peak between hydrogen nucleiin the following pairs of positions， (A， B)， (B，C) and (C，D).In contrast， the TOCSY spectrum of these hydrogens wouldshow cross peaks between all pairs of nuclei (see Figure 1). COSY D BA TOCSY D. .C ● .B A Figure 1： Comparison of 2D COSY and 2D TOCSY spectra for ahypothetical molecule in which hydrogen A is coupled to hydrogen B，which is coupled to C， which in turn， is coupled to D. Lines are drawnto connect the peaks below the diagonal， illustrating the through-bond connectivity shown by each spectrum； heavier lines indicatecouplings shown in both spectra， lighter lines indicate connectivityshown in the TOCSY spectrum but not the COSY. To further illustrate this concept， we consider the moleculetrans-2-hexenoic acid shown in figure 2(top). The 1D 1HNMR spectrum (figure 2 bottom) of this molecule shows sixresonances， five which have been labelled A to E correspondingto hydrogens attached to the carbon backbone of the moleculeand a sixth which corresponds to the -OH of the carboxylicacid group. The resonances A to E all show peak splittingcorresponding to the manner in which they are coupled to theneighbouring hydrogens. The COSY spectrum (figure 3) revealsthe coupling between A，B and B，C and C，D and D，E. It alsoshows the coupling between C and E， which might be expectedbecause of the double bond between D and E resulting in astronger coupling between C and E than D and B for instance. The TOCSY spectrum (figure 4) shows the coupling between allpairs of hydrogen nuclei confirming the fact that resonances Ato E comprise a single unbroken chain of coupled spins. If wecompare figure 3 to the TOCSY spectrum of ethyl crotonatein figure 5 we can see that the ethyl crotonate comprises twoseparate chains of spins， the ethyl group， resonances A andC and the crotonyl group， resonances B， D and E. The lackof cross peaks between the two sets of spins confirms thepresence of a nucle which effectively breaks the J-couplingchain. In this case the oxygen from the ester coupling. Figure 2： The structure (top) and 1D 1H spectrum (bottom) oftrans-2-hexenoic acid. Hydrogen positions on the carbon backboneare labelled A to E to identify the appropriate resonance in thespectrum. The unlabelled singlet ay 12.5 ppm corresponds to the-OH group of the carboxylic acid. Figure 4：. Spin lock was implemented using MLEV-17 Figure 5： TOCSY spectrum visit nmr.oxinst.com/x-pulse for more information or email magres@oxinst.com ( This publication is the copyright of Oxford Instruments plc and provides outline informa t ion o n ly， whi c h ( un l e ss ag reed by the com p a ny inwriting) may not be used， applied or reproduced for any purpose or form part of any order o r cont r act o r reg a rde d a s the represe n tation relating to the products or services concerned. Oxford Instruments’ policy is one o f co nti nued impr ov em en t. T h e com p any re s e r v es t h e r i ghtto alter， without notice the specification， design or conditions of supply of any p rod u c t or se r vic e. Ox f or d I ns tr u ment s ac k n o wle d ges alltrademarks and registrations.O Oxford Instruments plc， 2019. All rights r e s erve d . Pa rt no ：OII A / 1 6 6/0919 ) 总相关谱TOCSY是一个类似于COSY（见应用7 60MHz同核二维核磁共振）的同核二维实验，其中两个氢原子核之间的J-耦合显示为谱图中的交叉峰。