8.23:
¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)
When carbon-13 spectra are simplified by broadband proton decoupling, structural information regarding the attached hydrogens is lost.
Distortionless enhancement by polarization transfer, or DEPT, is a double resonance technique that distinguishes carbons by the number of attached protons.
The DEPT experiment utilizes a complex pulse sequence, where the pulse delay and flip angle are varied.
Now, methyl, methine, and methylene carbons yield distinct signals, whereas quaternary carbons produce no signal.
Observe that the broadband-decoupled carbon-13 spectrum for isopentyl acetate represents the chemical shifts of all nonequivalent carbons.
While signals from all protonated carbons appear in the DEPT-45 spectrum, the DEPT-90 spectrum shows peaks only from the methine carbons.
Finally, in the DEPT-135 spectrum, methyl and methine carbons appear as positive peaks, while methylene carbons appear as negative peaks.
Taken together, the DEPT spectra provide a valuable tool for structure elucidation.
8.23:
¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)
When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on the number of protons attached to the carbon. As a result, methyl, methylene, and methine carbons produce separate signals, whereas quaternary carbons produce no signal.
The DEPT experiment provides a series of spectra. First, the broadband decoupled carbon-13 spectrum is obtained, which shows the chemical shifts of all nonequivalent carbons. Next in the series is the DEPT-45 spectrum, which shows signals from all the protonated carbons. The DEPT-90 spectrum shows peaks only from –CH groups. Finally, in the DEPT-135 spectrum, methyl and methine carbons appear as positive peaks, while methylene carbons appear as negative peaks. Taken together, the DEPT spectra are very useful for structure elucidation.