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CDC13

a 13c spectrum 1 III

b

c

d

e

240 220 200 180 160 140 120 100 80 60 40 20 0 ppm

Figure 7.36

240 220 200 180 160 140 120 100 80 60 40 20 0 ppm

Figure 7.36

The pure subspectra are diagramed for 4-hydroxy-3-methyl-2-butanone in Figure 7.36, with simplified linear combinations. We can directly read off the assignments for peaks a-e from the pure subspectra: a is Cq (missing in all DEPT spectra), c is CH, b is CH2, and d and e are CH3. Varian sets up the full DEPT experiment by defining the parameter mult as 0.5, 1.0, or 1.5 and using the parameter pp for the proton 90o pulse. The actual final !H pulse used is then mult*pp, which will be a 45o, 90o, or 135o pulse. To run all three spectra in succession as a single experiment, an array is created with mult = 0.5, 1.0, 1.0, 1.5. This will acquire four FIDs with final 1H pulses of 45o, 90o, 90o, and 135o. Bruker uses three different pulse programs called "dept45", "dept90," and "dept135" in which pi is the transmitter 90o 1H pulse and p0 is the decoupler 1H 90o pulse. These three can be set up in consecutive experiment numbers and run in queued fashion using the "multizg" command.

In the DEPT-90 experiment (Fig. 7.35), if the pulse is slightly too long (e.g., © = 100o), the DEPT-90 spectrum will contain a little bit of the DEPT-135: there will be weak negative peaks for the CH2 carbons and weak positive peaks for the CH3 carbons. If the pulse is slightly too short (e.g., © = 80o), the DEPT-90 spectrum will contain a little bit of the DEPT-45: there will be weak positive peaks for both the CH2 carbons and the CH3 carbons. Thus, the DEPT-90 experiment, using a concentrated sample of a simple molecule like menthol, is the best way to calibrate the decoupler 90o 1H pulse. Simply calibrating the 1H 90o pulse by observing the 1H spectrum may give a different result. In older instruments, the 13C pulses come from the transmitter (a "broadband" RF source that can be set to any frequency up to the proton frequency) and the 1H pulses come from the decoupler

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