A Quick Dip into DEPT

In many cases, it would be helpful to have the information about the attached hydrogens that a proton-coupled spectrum provides, but frequently the spectrum becomes too complex, with overlapping multiplets that are difficult to resolve or assign correctly. A compromise technique called off-resonance decoupling can often provide multiplet information while keeping the spectrum relatively simple in appearance. 

In this technique, the frequency of a second radiofrequency transmitter (the decoupler) is set either upheld or downfield from the usual sweep width of a normal proton spectrum (i.e., off resonance). In contrast, the frequency of the decoupler is set to coincide exactly with the range of proton resonances in a trae decoupling experiment. Furthermore, in off-resonance decoupling, the power of the decoupling oscillator is held low to avoid complete decoupling. 

Despite its utility, off-resonance decoupling is now considered an old-fashioned technique. It has been replaced by more modern methods, the most important of which is Distortionless Enhancement by Polarization Transfer, better known as DEPT. The DEPT technique requires an FT-pulsed spectrometer. It is more complicated than off-resonance decoupling, and it requires a computer, but it gives the same information more reliably and more clearly. Chapter 9 will discuss the DEPT method in detail only a brief introduction to the method and the results it provides will be provided here. 

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In the DEPT technique, the sample is irradiated with a complex sequence of pulses in both the and channels. The result of these pulse sequences is that the signals for the carbon atoms in the molecule will exhibit different phases, depending on the number of hydrogens attached to each carbon. Each type of carbon will behave slightly differently, depending on the duration of the complex pulses. These differences can be detected, and spectra produced in each experiment can be plotted. 

CUSS the DEPT method in detail only a brief introduction to the method and the results it provides will be provided here. 

194 Nuclear Magnetic Resonance Spectroscopy Part Two Carbon lS Spectra 

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DEPT-135 and DEPT-90 Spectra. In some cases, the problems list information that can provide valuable information on the types of carbon atoms present in the unknown compound. Review Chapter 4, Section 4.10 (p. 182), A Quick Dip into DEPT) for information on how to determine the presence of CH3, CH2, CH, and C atoms in a carbon spectrum. 

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