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 90o and DEPT 135o.
| 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
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