Net Magnetization

Each atom that is NMR-active in a sample (each "spin") contributes its magnetism to the bulk or net magnetization of the sample. This spin produces a tiny magnetic field, and we can represent this little magnet with a vector pointing in the direction of the spin axis (Fig. 7.1(a)). If we move the origin (the magnet's south pole) of each vector to a common point, we will have the spins in the "up" or a state precessing at the Larmor frequency forming a 45° angle to the +z axis, tracing out an upward-facing cone. The spins in the "down" or j state form a 135° angle to the z axis and trace out a downward-facing cone as they precess (Fig. 7.1(b)). To trace the path of each spin in the sample would be an impossibly

NMR Spectroscopy Explained: Simplified Theory, Applications and Examples for Organic Chemistry and Structural Biology, by Neil E Jacobsen Copyright © 2007 John Wiley & Sons, Inc.

Spinning nucleus

Bar magnet

Individual magnetic dipole

Spinning nucleus

Bar magnet

Individual magnetic dipole

Ensemble of identical spins

Vector sum

Vector sum

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