Nuclear magnetization vector

The concept of relaxation time was introduced to the vocabulary of NMR in 1946 by Bloch in his famous equations of motion for nuclear magnetization vector M [1] ... 

When these nuclei are placed in a strong external magnetic field Bq (OZ orientation) an interaction will occurs between each individual nuclear magnetic vector and this field. 

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. 

In a normal pulsed NMR experiment, the 90° excitation pulse is followed immediately by a data acquisition phase in which the FID is recorded and the data are stored in the computer. In experiments that use complex pulse sequences, such as DEPT, a preparation phase is included before data acquisition. During the preparation phase, the nuclear magnetization vectors are allowed to process, and information may be exchanged between magnetic nuclei. In other words, a given nucleus may become encoded with information about the spin state of another nucleus which may be nearby. 

To identify which protons couple to each other, the coupling interaction is allowed to take place during ti. During the same period, the individual nuclear magnetization vectors spread as a result of spin-coupling interactions. These interactions modify the signal that is observed during t2. Unfortunately, the mechanism of the interaction of spins in a COSY experiment is too complex to be described completely in a simple manner. A pictorial description must suffice. 

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