DEPT (Distortionless Enhancement by Polarization Transfer)

Allow to distinguish signals of quaternary carbon atoms, CH, CH 2 and CH 3 groups.

Experiment DEPT (Distortionless Enhancement by Polarization Transfer) allows to distinguish between signals of quaternary, methine, methylene and methyl carbon atoms in the range of 13 C.

Depending on the multiplicity of the signals may be either positive or negative intensity. The most commonly used kind of experiment is DEPT DEPT135, in which the signals of the methylene carbon atoms - have negative intensity, methine and methyl - positive, and the signals of carbon atoms not directly associated with the protons do not appear. The spectrum exhibited only signals DEPT90 methine carbons and the spectrum DEPT45 quaternary carbons are not shown, and other signals - positive.

Experiment DEPT, usually used in conjunction with experiment 13 C with complete suppression of the spin-spin coupling (13 C { 1 H} experiment). From the analysis of the spectra of 13 C with suppression, DEPT135 and DEPT90 possible to draw conclusions about the multiplicity of all carbon atoms in the molecule.

As a result of polarization transfer to protons on the carbon atoms in the DEPT experiment of the sensitivity of the method is approximately four times greater than that of the standard experimental 13 C to complete suppression, and the experiment APT (Attached Proton Test).

DEPT experiment is less critical to the accuracy of selecting the value of direct constants C-H used to calculate a delay in the pulse sequence than experiment INEPT. This feature is useful in analyzing the structure containing carbon atoms of different natures, CH constants which respectively vary widely.

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.

DEPT (Distortionless Enhancement by Polarization Transfer) technique

Proton – coupled ¹³C-NMR spectra are often difficult to interpret due to large coupling constants and overlaping of signals. For this reason, ¹³C-NMR spectra are taken with proton–decoupled mode in which C/H ratios is lost.

To provide this information while retaining signal strength, DEPT (Distortionless Enhancement by Polarization Transfer) is developed.

In DEPT experiments, methyl, methylene, and methine protons can be distinguishable. There are several variations on the experiment.

sub-spectrum	technique
CH	DEPT 90o
CH2	DEPT 45o - DEPT 135o
CH3	DEPT 45o + DEPT 135o - 0.707DEPT 90o
C	comparing the DEPT with the BB decoupled spectrum

The types of carbon observed with various of DEPTs.

1.	DEPT 45o	signals of all protonated carbons
2.	DEPT 90o	signal of CH groups
3.	DEPT 135o	negative signal of CH2, positive signal of CH and CH3, and no signal of C with no attached H

Nomally, only two DEPT experiments are sufficient, DEPT 90^o and DEPT 135^o.

we can distinguish C, CH, CH2 and CH3 because;          - there is no signal of C with no attached H         - CH2 shows negative signal whereas CH and CH3 show positive signal          - CH carbons absorb at lower field and lower signal intensity than CH3carbons

Example 1: DEPT spectrum of isobutyl acetate

Interpretation :

d (ppm)	type of signal in DEPT	represent
22 (b)	positive	2 CH3
24 (c)	positive	CH3
24 (a)	positive	CH
37 (d)	negative	CH2
62 (e)	negative	CH2
170 (f)	not present	C of  >C=O

Example 2: DEPT spectrum of caryophyllene oxide