Effect of Chemical Shifts and Spin Coupling

The rephasing effect of a 180° pulse not only removes the effects of magnetic field inhomogeneities, it also rephases signals from nuclei that differ in precession frequency because of differing chemical shifts, as illustrated in Fig. 9.1. However, if the two magnetizations differ in frequency because of scalar coupling between them, the situation is more complex. 

FIGURE 9.1 Use of a spin echo pulse sequence to refocus two magnetizations precessing at different frequencies because of difference in chemical shift. 

The third possibility is that the tt pulse affects only S, as illustrated in Fig. 9.2 (bottom). In this case the interchange of S spin states causes the dephasing from the coupling between I and S to be reversed at time 2r, and Fourier transformation would show that the nuclei are decoupled just as though a continuous rf had been applied (Section 5.5). However, there is no echo because the I spins are not affected, and chemical shifts continue to appear in the spectrum. 

It is important to note that in this description of spin-coupled nuclei, we begin to see the limitations of the vector approach to understanding the NMR processes involved. In this case it is fairly simple to graft on the quantum aspect of spin orientation to provide an intuitively satisfying picture of decoupling and of the J modulation of spin echoes. Other cases will not be so clear and will force us to adopt a more powerful approach in Chapter 11. 

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In previous chapters we have described the origin of the chemical shift and of indirect spin coupling, and we have seen a number of illustrations of high resolution NMR spectra. We now need to look more carefully at the way in which the effects of chemical shifts and spin coupling can be added to the basic treatment of spin physics that we studied in Chapter 2. In this chapter we explore the ways in which nuclei interact not only with the applied magnetic field but also with each other. The steady-state quantum mechanical approach of Chapter 2 can easily be expanded by using a Hamiltonian that includes chemical shifts and couplings. 

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