Line broadening interactions sequences

The influence of the line broadening interactions described above (see Sect. 2.1) can be suppressed at least partly by the appHcation of special multiple-pulse sequences [8,13-15]. A typical example is the so-called WAHUHA (Waugh, Huber, Haeberlen) sequence [8] which consists of four "/2-pulses with alternating phases -[-r- nl2)x-2T- nl2) -T-(nl2)y-2r-(nl2).y-]-. This pulse cycle which exhibits a cycle time of t =6ris appHed repeatedly to the resonating spins. Sam-pHng the FID at multiples of leads to a suppression of the influence of the homonuclear magnetic dipole-dipole interaction. It has, however, to be stated... 

The faithful representation of the shape of lines broadened greatly by dipolar and, especially, quadrupolar interactions often requires special experimental techniques. Because the FID lasts for only a very short time, a significant portion may be distorted as the spectrometer recovers from the short, powerful rf pulse. We saw in Section 2.9 that in liquids a 90°, t, 180° pulse sequence essentially recreates the FID in a spin echo, which is removed by 2r from the pulse. As we saw, such a pulse sequence refocuses the dephasing that results from magnetic field inhomogeneity but it does not refocus dephasing from natural relaxation processes such as dipolar interactions. However, a somewhat different pulse sequence can be used to create an echo in a solid—a dipolar echo or a quadrupolar echo—and this method is widely employed in obtaining solid state line shapes (for example, that in Fig. 7.10).The formation of these echoes cannot readily be explained in terms of the vector picture, but we use the formation of a dipolar echo as an example of the use of the product operator formalism in Section 11.6. 

Slichter s spin echo approach is a variation on the spin echo double resonance (SEDOR) experiment originally proposed by Hahn I03. To see how this experiment works, consider the spin echo sequences shown in Fig 53. The first case, Fig 53a, describes Hahn s original echo experiment 103. In a hetero-nuclear case (always observing the rare spin) any heteronuclear dipolar interactions (along with the chemical shift and inhomogeneous line broadening processes) will be refocused as a result of this sequence. The second example. Fig 53b, is Hahn s SEDOR experiment (74). Here, the heteronuclear dipole-dipole interaction is not refocused because of the application of the second tt pulse to... 

In the preceding section, we discussed methods of suppressing dipolar interactions as a means of eliminating spin diffusion in relaxation measurements, but dipolar interactions are also the principal source of line-broadening in solid-state spectra, and therefore suppression of dipolar interactions also offers the possibility of obtaining high resolution spectra of solids in which chemical shift information is discernible. It is this, rather than the suppression of spin diffusion that has been the motivation in the development of these techniques, particularly the multiple pulse sequences. 

---