Net z-magnetization

An assemblage of equivalent protons precessing in random phase around the z axis (i.e., in the direction of the stationary magnetic field B0) has a net macroscopic magnetization M0 along the z axis, but none in the xy plane (Figure 3.4). 

In NMR, there is a very important distinction between the z-axis component of the net magnetization ( z magnetization ) and the component of the net magnetization that lies in the x-y plane ( coherence ). z-Magnetization, which is the result of unequal populations in... 

In the two examples above on the right, observable magnetization (antiphase coherence) is transferred by the 90° 1H pulse from Hb to Ha (top) and from Ha to Hb (bottom). This is a key process in all advanced NMR experiments that depend on / couplings. The role of the two operators is reversed as the operator in the x-y plane (the observable net magnetization) rotates to the z axis and the operator on the z axis (the multiplier that represents microscopic z magnetization) rotates to the x-y plane. After the rotations, we reverse the order of the two operators because we always write the observable operator first in the product. 

Frequently in this book we will want to depict nuclear spin orientations like those shown in Figure 2.7. More often than not, we will focus our attention on the net nuclear magnetic moment (M) rather than on the individual nuclear spins. Because M will sometimes precess around B0 (i.e., the z axis), we need a more convenient way than the dashed ellipses used so far to depict M as it precesses and changes orientation. Henceforth we will use another convention to represent this precessional motion of M, the rotating frame of reference, which is designed to show the effects of B on M. 

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