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Figure 7.37

(a different RF source that is often limited to proton frequency only). The normal method for calibrating pulses uses pulses from the transmitter, which does not necessarily put out the same power level as the decoupler.

Figure 7.37 shows the three DEPT spectra of cholesterol in CDQ3, run on a Bruker AM-250 (62.9 MHz 13C). The DEPT-90 spectrum was acquired with twice the number of scans of the other two. The DEPT-45 spectrum (bottom) shows all of the "protonated" (nonquaternary) carbon peaks as normal, positive peaks. The three quaternary carbons in cholesterol—C5 in the C=C group, C10 at the A-B ring juncture, and C13 at the C-D ring juncture—are missing from the DEPT spectra. The DEPT-90 spectrum (middle) clearly shows eight methine (CH) carbons: one olefinic (C6 at 103 ppm), one "alcohol" (C3 at 61 ppm), three "crowded aliphatic" (C9, C14, and C17 between 40 and 50 ppm) and three "normal aliphatic" (C8, C20, and C25 between 24 and 34 ppm). From the DEPT-135 spectrum (top), we can count 11 negative peaks, corresponding to the 11 methylene (CH2) groups in cholesterol: C1, 2, 4, 7, 11, 12, 15, 16, 22-24. Those peaks that are missing from the DEPT-90 and positive in the DEPT-135 are CH3 groups: the five most upfield positive peaks in the DEPT-135 can be assigned to C18, 19, 21, 26, and 27. Note that the CDQ3 peak (1:1:1 "triplet" at 77 ppm) is missing from all DEPT spectra.

Figure 7.38 shows the pure subspectra derived from the "raw" DEPT data in Figure 7.37 by linear combination. Although the interpretation of the "raw" DEPT spectra is straightforward, this presentation is especially simple and beautiful to look at. The eight CH carbons, 11 CH2 carbons, and five CH3 carbons can directly be read from the three subspectra. Keep in mind, however, that these are not in fact "spectra": they are the results of adding and subtracting the "raw" DEPT spectra.

When you have a very small amount of compound, a full DEPT analysis is a luxury. All you really need is a 13C spectrum, a DEPT-90, and a DEPT-135 to assign all of the

18 21 22 24 27

ch3 Cholesterol

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