DEPT (Distortionless Enhancement of Polarization Transfer)

DEPT (Distortionless Enhancement of Polarization Transfer): The DEPT technique has proven superior to others in providing information on attached protons reliably, efficiently and with high selectivity. It is a proton-carbon polarization transfer method, so DEPT spectra are actually more sensitive than normal acquisitions. A set of spectra with pulse delays adjusted for π/2 (DEPT-90) and 3π/4 (DEPT-135) are taken. 

The DEPT-90 spectrum shows only CH carbons, the DEPT-135 shows positive CH₃ and CH, and negative CH₂ signals. It is important to understand that the appearance of positive and negative signals can be reversed by phasing, so it is necessary to have some way of determining whether the spectrum has been phased for CH₂ positive or negative. Quaternary carbons are invisible

  

  Figure The normal ¹³C NMR spectrum and a typical set of DEPT spectra of an alkyne. Note the absence of the quaternary alkyne carbons in the DEPT spectra, and the presence of small peaks for the CH₂ and CH₃ signals in the DEPT-90 spectrum, which, in principle, should have only CH signals.

  "Leakage" can occur in DEPT-90 spectra because ¹J_C-H varies as a function of environment, and the technique assumes that all ¹J_C-H are identical. This can result in small peaks for CH₂ and CH₃ signals, which should have zero intensity. For similar reasons the C-H of terminal acetylenes (C≡C-H) will show anomalous intensities in DEPT spectra (either nulled or very small in DEPT-90, or present in DEPT-135) because the C-H coupling is much larger (around 250 Hz) than the normal value of 125 Hz for which the DEPT experiment is usually parameterized. Of course, leakage can also result from an incorrectly calibrated pulse width for the spectrometer.