Product Operator Analysis of the DEPT Experiment

To understand the pulse sequence, we will try to get an overview of what is happening and then look at some simplified product operator analysis. Consider first the CH case in the DEPT-90 experiment. Ignoring the 180° pulses, the DEPT-90 sequence can be viewed as an INEPT sequence in which the coherence transfer is split up into two steps (Fig. 7.41) the two 90° pulses are no longer simultaneous and between them we have an intermediate state in coherence transfer multiple-quantum coherence (ZQC and DQC). 

The 90°x H pulse puts the H magnetization on the — / axis, and /-coupling evolution for a period of exactly 1/(2J) allows this in-phase magnetization to evolve into antiphase. For simplicity, we assume that the 13C and H are on-resonance so we can ignore chemical shift evolution during the delays. The 13C 90° pulse then converts this to a mixture of ZQ and 

DQ (-2I Sy = ZQy - DQy). Both operators in the product are in the xr-yr plane, so this can be thought of as an intermediate state in coherence transfer. 

we need to make the pulse width of the final H pulse variable, with a rotation angle of ( = 45°, 90°, or 135°). For the CH case, we have already discussed the full coherence transfer, so in the final pulse we have  

Note that the 180 -y pulse on the 13C channel has no effect on Sv. The cosine term is just the product operator we started with, unaffected by the1H pulse, and the sine term is the operator we would get with a full 90° 1H pulse. Note that rotation of the lx magnetization vector by a XH B field on the / axis goes from x to — z to — x to +z as is incremented from 0° to 90° to 180° to 270° in the trigonometric expression. The first term is DQC/ZQC, which will not be observable in the FID—there are no more pulses in the sequence to convert it to observable magnetization. Only the second term represents full coherence transfer to antiphase 13C coherence, which will refocus during the final 1/(27) delay into in-phase 13C coherence  

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