[HB]

Is it true that compounds containing large number of N atoms give poor signals in 13C NMR due to long relaxation time and what is meant by that?

[Babcock_Hall]

A long relaxation time means that the nucleus is slow to give away the energy given to it by the pulse in an NMR experiment (I am assuming T1, as opposed to T2 relaxation).  One could get around this problem by increasing the delay time between scans.  I have not heard anyone claim that nitrogen atoms increased relaxation times.  A lack of nearby hydrogen atoms could increase the relaxation time.