SIGNAL SHAPE IN13C-NMR:
SPECTRA "EDITING"
We started this section pointing out that 13C spectra were usually recorded "1H-decoupled". Look at methane.
It is evident that spectra become simpler because every 13C signal turns into a "singlet".
Unfortunately, 1H-decoupling make us lose the information as to how many 1H's are bonded to each 13C.
Fortunately, there are techniques that allow us to recover that information.
The theory behind those techniques exceeds the purpose of this course but that does not preclude us from observing their result and learning how to interprete it.
In the next Figure several 13C spectra were recorded from propionylsalycilyc acid.
The "normal" 1H-decoupled spectrum is at the top. One may see the signals from the non-hydrogenated 13C's (1, 2, 9 and 12) displaying their characteristic lower intensity.
The spectrum in the middle is named after the acronym “DEPT135”.
Is it alike the "normal" spectrum?
Yes, all the signals are singlets and consequently it was recorded with 1H-decoupling.
But there are two main differences:
1) Non-hydrogenated 13C's didn't show up.
2) CH and CH3 groups (3, 4, 5, 6 y 15) came out in “antiphase” with respect to the CH2 group (11), the former up (3, 4, 5, 6 and 15) and the latter (11) down.
The spectrum at the bottom is called “DEPT90”.
What do you see on it?
Again the signals are singlets meaning that it was recorded 1H-decoupled.
Non-hydrogenated 1H's didn't show up either.
Only CH groups are detected in "DEPT90".
The experiment "DEPT" allows us to "edit" the 13C signals.
From the "normal" spectrum and
“DEPT135” and “DEPT90” one can figure out what signal belongs to every kind of 13C:
1) From "DEPT90" which 13C's are CH groups (3, 4 , 5 and 6).
2) Comparing "DEPT90" and "DEPT135" one can single out which 13C's are CH2 groups ("antiphase") and which are CH3, in phase with the discerned CH groups.
All signals in the normal spectrum not showing up in the DEPT spectra are from non-hydrogenated 13C's.
This cross-checked 1H-13C information is crucial for the structural determination of a given molecule.
