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Thus, an "INEPT-45" experiment will give all positive 13C peaks, an "INEPT-90" will give only CH peaks, and an "INEPT-135" will give positive peaks for CH and CH3 and negative peaks for CH2. These three experiments (actually just the last two) give us a complete identification of each peak as CH, CH2, or CH3 while maintaining the 1H-decoupled, singlet character of each peak. We will see how another experiment, called DEPT, achieves exactly the same results with a simpler pulse sequence by varying the width of a pulse rather than a delay time, using 450, 90o, and 1350 pulses.

7.14 DEPT: DISTORTIONLESS ENHANCEMENT BY POLARIZATION TRANSFER

The INEPT method can be used to distinguish CH, CH2, and CH3 carbons in addition to enhancing the signal-to-noise ratio, but a related method called DEPT is superior in many ways. The DEPT pulse sequence is shown in Figure 7.34. We will discuss how it works later; for now, just note that if we forget about the two 180° pulses and the second 1/(27) delay, and we set © = 90°, we have a refocused INEPT that would work well for the CH group. DEPT combines the coherence transfer technique of INEPT (90° on 1H, delay of 1/(27), simultaneous 90° pulses on 1H and 13 C) with the spin-echo technique of APT (1/(27) delay, 180° pulse, 1/(27) delay) to achieve both signal enhancement and spectral editing

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