Nuclear magnetic resonance scalar coupling

Di Bari L, Kowalewski J and Bodenhausen G 1990 Magnetization transfer modes in scalar-coupled spin systems investigated by selective 2-dimensional nuclear magnetic resonance exchange experiments J. Chem. Rhys. 93 7698-705... 

The NMR spectra presented in Fig. 20.9 [11] for the same PP samples whose H NMR spectra appear in Fig. 20.8 make the superior microstructural sensitivity of NMR plainly evident. While resonances are spread over an 30 ppm range, aU H resonances observed for PPs are within < 1 ppm of each other. In addition, the absence of homonuclear ( C- C) and the easy removal of hetero-nuclear ( C- H) scalar couplings further simplify the spectra. Both of these advantages result in the kind of microstructural sensitivity seen in the methyl carbon region of the PP spectra note that in atactic PP sample all ten possible pentad stereosequences mmmm, rrrr, mmrm, etc.) are distinctly observed. (Also see in Fig. 20.10 an expansion of the methyl region of the atactic PP spectrum observed at a higher magnetic field, which we will subsequently discuss.)... 

R. Weiss and R. N. Grimes, J. Amer. Chem. Soc., 1978, 100, 1401. Sources of linewidth in boron-11 nuclear magnetic resonance spectra. Scalar relaxation and boron-boron coupling in tetraborane(lO) and pentaborane(9). 

As we shall see, each of these two terms, one for each nucleus, describes a second-rank scalar interaction between the electric field gradient at each nucleus and the nuclear quadrupole moment. De Santis, Lurio, Miller and Freund [44] included two other terms which involve the nuclear spins. One is the direct dipolar coupling of the 14N nuclear magnetic moments, an interaction which we discussed earlier in connection with the magnetic resonance spectrum of D2 its matrix elements were given in equation (8.33). The other is the nuclear spin-rotation interaction, also discussed in connection with H2 and its deuterium isotopes. It is represented by the term... 

Scalar coupling—coupling of nuclear spins that is mediated by chemical bonds commonly utilized to transfer magnetization in triple resonance experiments. 

The nuclear Overhauser effect is used to measure distances between nuclei. It is a relaxation effect that, like a chemical reaction, is incoherent in nature as opposed to the scalar coupling that is coherent and is detected as a frequency splitting of resonances in the spectrum. Relaxation can be best viewed at as the flow of population between different states as it would occur in a chemical reaction where one molecule is transformed, e.g., by a reaction of zero-order into a second molecule. The same is true for the NOE where magnetization of spin A by a zero-order reaction is transferred to mag-... 

On a system equipped with multiple RF channels and receivers several such schemes can be executed in parallel. Let s consider one of the simplest NMR experiments - the two-dimensional COSY experiment. The basic homonuclear COSY pulse sequence consists of an excitation pulse followed by the evolution period, t, a read pulse and an acquisition period, t2 (see Fig. 2a). The same scheme can be executed in parallel on two or more RF channels (Fig. 2b). If the cross-talk between the different nuclear species could be avoided, such an experiment would produce two independent 2D COSY spectra. However, in practise the magnetically active and in particular spin 1/2 nuclei from the same molecule are usually coupled via the scalar spin-spin couplings and in such a simple pulse scheme cross-talk is unavoidable. Therefore we should also observe heteronuclear correlations arising ffran coherence transfer A X and X A, i.e. in total four two-dimensional spectra in a single measurement Indeed, all the correlations can be observed in instances where the gyrranagnetic ratios of the nuclei and their natural abundances are similar. In fact, as a cmisequence of the close proximity of the H-1 and F-19 resonance frequencies at the Earth magnetic field, their two-dimensional correlation spectra can be observed even with a single receiver [29]. 

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