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DEPT

 

 

DEPT is a very useful 13C technique.   The procedure is used to determine the hydrogen substitutions on the carbon atoms responsible for the 13C signals.  The method uses a sequence of 1H and 13C rf pulses and delays to generate 1H-13C double quantum coherence and then transfer the information back to the 13C resonances.  The final results (display) of the 13C spectrum are determined by the last 1H pulse.  If the last 1H pulse is set to 450 , the  resonances of all H bearing  carbons will be positive in phase.  It is important to stress that quaternary carbon atoms do not give DEPT signals.  If that last pulse is set to 900 only methine carbons will show and they will be positive in phase.   The most useful 1H pulse is 1350.  All methyl and methine resonances will be positive in phase and methylene resonances will be negative in phase.

 

Here is the structure of trans- fenretidine with our best guess of 13C assignments.   (Don’t use this as a reference; the assignments may be in error.)

 

 

We have many resonances in the spectrum as we can see in the nOe enhanced 13C spectrum.

 

 

The three lines at 77.00 ppm are from the deuterochloroform used as the solvent.   To help assign all these resonances, it will be very helpful to know which signals correspond to methyl, methylene, methine, quaternary carbon atoms.  The first step would  be to run DEPT-135.

 

 

Notice the three negatively phased signals at 18.9 ppm, 32.8, 39.3.  These are the three methylenes.   There are four positively phased signals at low frequency, 12.6ppm, 13.3, 21.5, 28.7.  By also considering the chemical shifts, these have to be from the methyl groups.  In the high frequency end of the spectrum we have seven move positive signals.  Two are from the aromatic ring, 115.2ppm, 121.4, which we can determine by chemical shifts and also their intensities give a clue.  This leaves five more methine signals that we can assign to the alkene carbons.    Notice that some of the signals that are observable in the nOe enhanced spectrum are not seen in the DEPT-135.  These are from  the quaternary carbon atoms, viz,  33.9ppm, 137.4, 129.5, 138.3, 153.3, 165.0, 130.6, 148.6.

 

At this point we have gleaned all the information possible from the DEPT technique.  In some cases though it may be useful to differentiate methyl signals from methine signals.  Here is the DEPT-90 spectrum, where only the methine signals are observed.

 

 

 

 

DEPT-45 isn’t useful so it is very seldom done.

 

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