I noticed that in the DEPT experiment that I did, some peaks show up in all three DEPTs (45, 90 and 135). If I'm not mistaken, DEPT-90 is supposed to show only CH peaks while in DEPT-135, CH peaks should appear in the positive region. However, as shown in the attached image, some peaks are negative in DEPT-135 but positive in DEPT-90. Can anyone explain? How should I interpret this?
From Wikipedia:
The DEPT experiment differentiates between CH, CH2 and CH3 groups by variation of the selection angle parameter (the tip angle of the final 1H pulse): 135° angle gives all CH and CH3 in a phase opposite to CH2; 90° angle gives only CH groups, the others being suppressed; 45° angle gives all carbons with attached protons (regardless of number) in phase.
So, DEPT-45 contains all your Carbons, DEPT-90 contains CH-only.
Now in DEPT-135:
- Peaks that are common with DEPT-90 CHs (regardless of their phase).
- Peaks with the same phase as the CHs are CH3s
- Peaks with the opposite phase are CH2s
If you are wondering why CH peaks are negative in DEPT-135, this is because of the automatic phase correction algorithm. If this is bothering you, you can simply invert the phase of this spectrum.
Hi Ahmed. Thanks for the response. As attached, the other peaks in the DEPT-135 "seems" to be in the correct phase, only those at lower ppm don't look right. Is this normal?
Hi,
If your sample is pure, the response of the different experiments seem to be good. In dept 90 the intensity of the peaks allow you to differentiate between CH and other peaks (CH2 and CH3). If your P90 is not well calibrate you have residual peak (CH2 and CH3) in your spectrum. Try again your experiments but calibrate first the P90.
- I can give you the theoretical interpretation. 45 ,90 and 135 are the variable flip angles of a last pulse applying on 1H in DEPT sequence, which can convert antiphase magnetization to observable magnetization. So, the intensity of the signal depends on the magnitude of the flip angle and the number of proton attached to a carbon. Formula is I= n sinθ cos^(n-1)θ.If you put n=1 for 90 , intensity will be a positive number, otherwise it's 0, so only for CH you will get spectrum in positive side, In case of 45 you will get positive signal for n=1,2,3, so all in phase peak you'll get, but for 135 Intensity will be positive only when n=1&3, it will be negetive when n=2, Peaks with the opposite phase are CH2s.You can check it by computing the numbers.
Hi Marc Gaysinski and Ipsita Chakraborty, thanks a lot for your help.
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