I searched the related questions, some people answered that the nitro group will make this carbon missing in the NMR. But I would like to know the principle how does the nitro group affect?
It is very likely that the C attached to the nitro group has a (very) long relaxation time (as do all C atoms without a H attached).
If you repeat the experiment with a longer wait between pulses you will give the C time to relax (but it will of course increase the experimental time).
This C will also not benefit from NOE as only C with H attached will show this effect (caused by the decoupling).
There may also be some coupling of this C to the quadrupolar 14N which due to the small electric field gradient typical in nitro nitrogens and consequently may not completely decouple itself.
David has given very precise answer. When ever we come across quaternary carbon atoms we have to be careful to observe the signals. Often the signals are very sharp but are very very weak. Give more pulse delay or use shiftless relaxation reagent [such as Cr(acac)3] to see all the signals.
Best wishes
Balakrishna
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