Subtraction experiments, DEPT

A disadvantage of DEPT is that it is a subtraction experiment and, therefore, much more sensitive to certain problems than are typical one-dimensional techniques. One remedy that is used in many experiments that suffer fi om stability problems is the employment of steady-state, or dummy, scans (parameter 2 in the foregoing list Section 2-4i). Poor signal cancelation is generally the result of difficulties in one or more of the following areas ... 

Of the multitude of ID 13C NMR experiments that can be performed, the two most common experiments are a simple broadband proton-decoupled 13C reference spectrum, and a distortionless enhancement polarization transfer (DEPT) sequence of experiments [29]. The latter, through addition and subtraction of data subsets, allows the presentation of the data as a series of edited experiments containing only methine, methylene and methyl resonances as separate subspectra. Quaternary carbons are excluded in the DEPT experiment and can only be observed in the 13C reference spectrum or by using another editing sequence such as APT [30]. The individual DEPT subspectra for CH, CH2 and CH3 resonances of santonin (4) are presented in Fig. 10.9. 

Steady-state, or dummy, scans are used to allow a sample to come to equilibrium before data collection begins. As in a regular experiment, a number of scans are taken, but data are not collected during what would be the normal acquisition time. Steady-state scans are usually performed before the start of an experiment, but, for certain experiments on older instruments, may be acquired before the start of each incremented time value. This technique is not necessary in typical one-dimensional NMR experiments, but is employed in onedimensional methods that involve spectral subtraction (e.g., DEPT Section 7-2b) and virtually all two-dimensional experiments. 

The J-coupling patterns in spectra can help to elucidate chemical structure, but they also comphcate the spectrum. Spectral-editing experiments allow such interactions to be observed in a controlled way. One such technique, DEPT (ifistortionless enhancement by polarization transfer), edits the spectrum based on scalar coupUng the pulse sequence is depicted in Fig. 15. DEPT is actually a set of three experiments, with tip angles, /, of 45°, 90°, and 135°. In NMR, appropriate addition and subtraction of the resulting subspectra separate the contributions from methyl, methylene, and methine carbons quaternaries are not observed. 

---