Quaternary carbons have long relaxation times. They can often not yet be seen when all other signals are present. Make sure the carbon spectrum was acquired with a sufficiently long relaxation delay. Carbons next to nitrogen can be broadened to such an extent that they are difficult to recognize.  If the chemical shift in the HMBC makes sense with the proposed structure then you need to optimize the conditions for the 13C spectrum to be able to see it. If it does not make sense it could be an artefact.

I agree with Prof Clemens Anklin's answer.  You need to make sure about the relaxation time.

Thanks Prof Clemens ! highly appreciated for your great explanation !

Sometime dynamic  process in molecules are responsible for the absence of carbon resonances when hydrogen spectra have no special peculiarities

hindered rotation, pyramidal nitrogene  invertion

to detect that will you change temperature

Dear Rachal,

have you tried to process the 13C spectrum with other windowfunctions (like lb 10 or even lb 50)...it might be - if you are lucky - that you find additional broad signals...

I hope it helps

Alfred

Agree with Professor Clemens and others that the slow relaxation of quaternary carbons is the most probable cause. The following experiment might be useful in identifying quaternary carbon resonances: http://onlinelibrary.wiley.com/doi/10.1002/mrc.882/pdf

Also - I did some work a long time ago looking at long-range polarization transfer to quaternary carbons.

http://onlinelibrary.wiley.com/doi/10.1002/mrc.1260300203/full

yes, I am agree with others for the long relaxation time of the carbon atom specially quaternary carbon. to solve this problem:

1- elongate Tau (adjust time between ON-OFF) of the experiment or

3-  left sample over night to ensure that all carbons are relaxed including quaternary carbons

3- change deuterated solvent (used viscose one like DMSO or Pyridine

4- you have used high strength of NMR device (500MHz or more)

Prototropic tautomerism when using DMSO - D6 espesially not dried from water can be the reason of broadening and complete dissapearense

of some signals in 13C NMR spectra. See our open acsess paper New cases of prototropic tautomerism in substituted piridines

Recently we have faced with the absence of 13C NMR signals in heterocyclyc compound solution in DMSO -D6 due to interesting case of prototropic tautomerism. We failed to detect the absent signals by common temperature increase to 70 C when using our usual APT (jmode) pulse program. May be Dept programs also would give the same result. Not the first time pulsprogram udeft appears to be nice solution for the problem in question. Signals wanted were detected even at room temperature and became approapriately narrowed at 70C.

I encountered the same in one my compound spectra.

First, confirm the different C environments. Similar environment C shows at one place.

Second, try to take more number of scans. Sometimes peaks too close and small to high intense are buried.

Finally, try to do it in another solvent.

The situations happen to me and I confirmed by doing all the three steps.

Good Luck!!!