Pulse Widths, Spins, and Magnetization Vectors

FIGURE 9.3 A simple pulse sequence, (a) A pulse sequence for gated decoupling (b) a pulse sequence for inverse gated decoupling. 

We will not describe these complex pulse sequences further their description and analysis are beyond the scope of this discussion. Our purpose in describing a few simple pulse sequences in this section is to give you an idea of how a pulse sequence is constructed and how its design may affect the results of an NMR experiment. From this point forward, we shall simply describe the results of experiments that utilize some complex sequences and show how the results may be applied to the solution of a molecular stmcture problem. If you want more detailed information about pulse sequences for the experiments described in the following sections, consult one of the works listed in the references at the end of this chapter. 

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Nuclear Magnetic Resonance Spectroscopy Part Five Advanced NMR Techniques 

Since the small microscopic moments (vectors) from each nucleus add together, what our instrument sees is the net or bulk magnetization vector for the whole sample. We will refer to this bulk magnetization vector in the discussions that foUow. 

In a Fourier transform NMR instrument, the ladiofnequency is transmitted into the sample in a pulse of very short duration— typically on the order of 1 to 10 microseconds (//sec) during this time, the radiofrequency transmitter is suddenly turned on and, after about 10 //sec, suddenly turned off again. The 

What happens to the magnetization vector following a 90° pulse At the end of the pulse, the Bq field is still present, and the nuclei continue to process about it. If we focus for the moment on the 

FIGURE 10.6 The effect of a 90° pulse (M is the bulk magnetization vector for the sample). 

In the laboratory frame, the Y component corresponds to a magnetization vector rotating in the XY plane. The magnetization vector rotates in the XY plane because the individual nuclear magnetization vectors are processing about Z (the principal field axis). Before the pulse, individual nuclei have random precessional motions and are not in phase. The pulse causes phase coherence to develop, so that all of the vectors process in phase (see Fig. 10.7). Because all of the individual vectors process about the Z axis, M, the resultant of all of these vectors, also rotates in the XY plane. 

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In order to rotate the magnetization vectors of all nuclear spins within the range of Larmor frequencies to be observed, the pulse must not only be adjusted for 90", so that yB1 tp = 71/2 (eq. 2.2)), but must also be very strong, so that y B, 2 7i A (eq. (2.3)). These requirements give the relation between pulse width and spectral width ... 

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