Z-magnetization transfer

Figure 6.18 Stop-flow 2D WET-TOCSY spectrum of 3,5-dichloro-2-hydroxybiphenyl. Protons (H4.H6) of the chlorinated ring can be identified due to the magnetization transfer between the signals at 7.23 (H4) and 7.41 (H6) ppm. Conditions spectrometer, Bruker DRX 600 probe head, 4mm z-gradient LC probe. 13C decoupling was applied during the WET-pulse train and acquisition. The spectral size was 8k x 512 data points, with 16 scans per increment, and a 64 ms TOCSY mixing time...

Rumboldt Z, Marotti M (2003) Magnetization transfer, HASTE, and FLAIR imaging. Magn Reson Imaging Clin North Am 11 471-492... 

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. 

The remaining two are the longitudinal spin order, which results when the macroscopic z magnetization of one nucleus (e.g., H) is opposite depending on the microscopic z magnetization (a or P) state of the other nucleus (e.g., 13C), and the identity (1) operator, which simply represents the vast majority of spins that cancel each other out and play no role in NMR experiments. Longitudinal spin order can be viewed as an intermediate state in coherence transfer 2 US- 21-S- 2I-S.r = 2SX z. Like z magnetization, it is not affected... 

In Section 7.9, we saw how phase cycling can be used to remove the 13C coherence that comes from the original 13C z magnetization (Sz), so that only the coherence transferred from ll z magnetization (Iz) is observed. This is a subtraction process that requires more than one scan to accomplish. With gradients we can do it in one scan alone ... 

The final read pulse rotates the Hb z magnetization into the x -y plane and the FID is recorded with the frequency Fourier transformation of the FIDs gives in each one a peak at F2 = 2b whose amplitude is oscillating as a function of t at the frequency 2a. Fourier transformation of the t FID gives a crosspeak at F2 = 2b, F = 2a. Because for small molecules, the transferred z magnetization is opposite in sign from the original... 

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