¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)

Distortionless enhancement by polarization transfer (DEPT) is a double resonance technique that distinguishes carbons by the number of attached protons. When broadband proton decoupling simplifies carbon-13 spectra, structural information about attached hydrogens is lost. DEPT recovers this information by using complex pulse sequences with varied pulse delays and flip angles, making it valuable for structure elucidation.

Methyl, methylene, and methine carbons produce distinct signals in DEPT experiments based on the number of attached protons. Quaternary carbons, which have no attached hydrogens, produce no signal. The resulting signals have different phases depending on hydrogen attachment, allowing chemists to identify carbon types and their connectivity patterns.

The DEPT-45 spectrum shows signals from all protonated carbons in a molecule. This spectrum is the first in the DEPT series after the broadband-decoupled carbon-13 spectrum and provides a comprehensive view of all carbons bearing hydrogen atoms, helping distinguish them from quaternary carbons.

The DEPT-90 spectrum shows peaks only from methine carbons (CH groups), isolating a single carbon type. In contrast, the DEPT-135 spectrum displays methyl and methine carbons as positive peaks while methylene carbons appear as negative peaks. Together, these complementary spectra enable complete carbon classification.

Broadband proton decoupling simplifies carbon-13 spectra by removing coupling interactions between carbons and protons, eliminating multiplet splitting patterns. While this produces cleaner spectra showing only chemical shifts of nonequivalent carbons, it removes the structural information encoded in those coupling patterns about hydrogen attachment.

DEPT experiments utilize complex pulse sequences where the pulse delay and flip angle are specifically manipulated to generate signals with different phases based on the number of attached protons. These variable parameters allow the technique to distinguish between methyl, methylene, methine, and quaternary carbons systematically.

DEPT provides a series of complementary spectra that collectively identify all carbon types and their hydrogen attachments. By combining the broadband-decoupled spectrum with DEPT-45, DEPT-90, and DEPT-135 spectra, chemists can determine the complete carbon skeleton and hydrogen connectivity, making DEPT invaluable for determining molecular structures.