Polarisation transfer

The experiment described above is termed selective population transfer (SPT), or more precisely in tbis case with proton spin inversion, selective population inversion, (SPl). It is important to note, however, that the complete inversion of spin populations is not a requirement for the SPT effect to manifest itself. Any unequal perturbation of the lines 

The greatest limitation of the SPI experiment is its lack of generality. Although it achieves the desired polarisation transfer, it is only able to produce this for one resonance in a spectrum at a time. To accomplish this for all one would have to repeatedly step through the spectrum, inverting satellites one-by-one and performing a separate experiment at each step. Clearly, a more efficient approach would be to invert one half of each proton doublet simultaneously for all resonances in a single experiment and this is precisely what the INEPT sequence achieves. 

The INEPT experiment [30] (Insensitive Nuclei Enhanced by Polarisation Transfer) was one of the forerunners of many of the pulse NMR experiments developed over subsequent years and still constitutes a feature of some of the most widely used multidimensional experiments in modem pulse NMR. Its purpose is to enable non-selective polarisation transfer between spins, and its operation may be readily understood with reference to the vector model. Most often it is the proton that is used as the source nucleus and this discussion will relate to XH spin systems throughout, although it should be remembered that any high-y spin-14 nucleus constitutes a suitable source. 

The INEPT sequence (Fig. 4.23a) provides a method for inverting one-half of each XH doublet in a manner that is independent of its chemical shift, requiring the use of non-selective pulses only. The sequence begins with excitation of all protons, which then evolve under the effects of chemical shift and heteronuclear coupling to the X spin. After a period A/2, the proton vectors experience a 180° pulse that serves to refocus chemical shift evolution (and field inhomogeneity) during the second A/2 period. The simultaneous 

To appreciate the sensitivity gains from the application of the INEPT sequence, one should compare the results with those obtained from the usual direct observation of the low-7 species. This invariably means the spectrum obtained in the presence of proton broadband decoupling for which signal enhancement occurs by virtue of the H-X NOE (Chapter 8). Thus, to make a true comparison, we need to consider the signal arising from polarisation transfer versus that from observation with the NOE, which/or an XH pair is given by  

The experiment described above is termed selective population transfer (SPT), or more precisely in this case with proton spin inversion, selective population inversion, (SPI). It is important to note, however, that the complete inversion of spin populations is not a requirement for the SPT effect to manifest itself. Any unequal perturbation of the lines within a multiplet will suffice, so, for example, saturation of one proton line would also have altered the intensities of the carbon resonance. In heteronuclear polarisation (population) transfer experiments, it is the heterospin-coupled satellites of the parent proton resonance that must be subject to the perturbation to induce SPT. The effect is not restricted to heteronuclear systems and can appear in proton spectra when homonuclear-coupled multiplets are subject to unsymmetrical saturation. Fig. 4.20 illustrates the effect of selectively but unevenly saturating a double doublet and shows the resulting intensity distortions in the multiplet structure of its coupled partner, which are most apparent in a difference spectrum. Despite these distortions, the integrated intensity of the proton multiplet is unaffected by the presence of the SPT because of the equal positive and negative contributions (see Fig. 4.19d). Distortions of this sort have particular relevance to the NOE difference experiment described in Chapter 8. 

The asynmietrical peak intensities produced by the SPI effect, —3 and -1-5 for a CH pair, arise from the contribution of the natural X-spin magnetisation, which may be removed by the use of a simple phase cycle. Thus, repeating the 

One problem with the basie INEPT sequence described above is that it precludes the applieation of proton spin decoupling during the acquisition of the X-spin FID. Since this removes the J-splitting it will cause the antiphase 

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The pulse sequence which is used to record CH COSY Involves the H- C polarisation transfer which is the basis of the DEPT sequence and which Increases the sensitivity by a factor of up to four. Consequently, a CH COSY experiment does not require any more sample than a H broadband decoupled C NMR spectrum. The result is a two-dimensional CH correlation, in which the C shift is mapped on to the abscissa and the H shift is mapped on to the ordinate (or vice versa). The C and //shifts of the //and C nuclei which are bonded to one another are read as coordinates of the cross signal as shown in the CH COSY stacked plot (Fig. 2.14b) and the associated contour plots of the a-plnene (Fig. 2.14a and c). To evaluate them, one need only read off the coordinates of the correlation signals. In Fig. 2.14c, for example, the protons with shifts Sh= 1.16 (proton A) and 2.34 (proton B of an AB system) are bonded to the C atom at c = 31.5. Formula 1 shows all of the C//connectivities (C//bonds) of a-pinene which can be read from Fig. 2.14. 

DEPT Distortionless enhancement by polarisation transfer, differentiation between CH, CH2 and CH by positive CH, CH3) or negative CH signal amplitudes, using improved sensitivity of polarisation transfer... 

One-dimensional111 and 13C NMR experiments usually provide sufficient information for the assignment and identification of additives. Multidimensional NMR techniques and other multipulse techniques (e.g. distortionless enhancement of polarisation transfer, DEPT) can be used, mainly to analyse complicated structures [186]. 

Scheme 1.14 Possible mechanisms for the transfer of polarisation from parahydrogen onto cyclooctene via [Rh(COD)(dppb)] (a) and (b) are the possible intermediate dihydride species responsible for the polarisation transfer at cyclooctene.

In contrast to applications in structural biology where X/Y correlations are nowadays normally executed as H detected, three-dimensional experiments because of sensitivity reasons,14 many studies on inorganic or organometallic compounds are still performed as two-dimensional experiments with direct detection of one heteronucleus and under -decoupling. As compared to these two categories, one-dimensional polarisation transfer methods such as (semi) selective X/Y-INEPT or INDOR-type techniques, which had in the past been shown to be particularly useful for the characterisation of substrates with only one or two heteronuclei,11 have recently received less attention.15 NOE-based correlations, which are frequently employed for the structure elucidation of bio-molecules, remain rare, and apart from an earlier report of a 13C/6Li HOESY experiment,16 have not been further investigated. 

The 13C chemical shifts were assigned in more detail for monosulfidic and polysulfidic crosslinks occurring in the accelerated sulfur vulcanisation of NR [18]. The NR was cured with a pure thiuram formulation (TMTD alone) in order to predominantly prepare monosulfidic bridges in the network. The distortionless enhancement by polarisation transfer (DEPT) experiments, in which the carbons with different level of protonation can be distinguished [22-24], were performed for the NR cured with extended levels of sulfur. Based on the DEPT results and previously reported model compound results [20], the chemical shifts of the resonances occurring in the spectra were assigned. 

This 2D correlation method uses a multiple-pulse sequence to suppress resonance offsets and promote polarisation transfer driven only by the /-coupling while simultaneously avoiding the recoupling of dipolar interactions. A much... 

Even with the line-narrowing techniques described earlier, NMR experiments on solids with dilute spin- /2 nuclei are still relatively unattractive on two principal counts. One is the lack of sensitivity due to their low net polarisation and the other is the relatively long spin-lattice relaxation time that is often encountered. In solids where both abundant (I) and dilute (S) nuclei coexist, polarisation transfer techniques can usually be used to overcome both these problems. There are many schemes to effect such a transfer but the most common technique is to create and then spin-lock transverse I-magnetisation. This experiment is best understood using ideas from spin thermodynamics. The magnetisation is given by Curie s Law (Eq. 2.21) and the temperature in... 

Contact between the I and S spins to allow polarisation transfer,... 

In this equation Tis determines the rate of polarisation transfer and hence the build up of signal, while Tip is the relaxation time of the spin-locked I-magnetisation in the rotating frame determining the time scale of the decay of the reservoir of magnetisation. The rate of transfer of the magnetisation is found to depend on the second moments (M2) of the I-S and I-I spin systems as... 

Spectrometers now often come equipped with at least two channels (designated X and Y) and often three lower frequency channels. This allows polarisation transfer experiments between X and Y nuclei. Examples of X—>Y magnetisation transfer are rapidly increasing and some examples are given in Table 3.9. 

H shift in the other via one-bond CH coupling J h. The pulse sequence which is used to record it also involves the H C polarisation transfer which is the basis of the DEPT sequence and which increases the sensitivity by a factor of up to four. Consequently, a... 

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