A past paper question on solid state NMR I was looking at asks why, in $^{13}$C CP MAS NMR, carbons directly bonded to nitrogen are "occasionally" split, and why they're usually absent in the dipolar dephased spectra.

I assume this is probably due to quadrupolar interactions, since $^{14}$N has spin 1; the size of the quadrupolar interaction depends on the electric field gradient at the quadrupolar nucleus, and hence the symmetry of the environment, so the "occasionally" presumably occurs when the nitrogen is in a particularly asymmetric electronic environment and so has a very large quadrupolar coupling? I'm not sure why we don't see this splitting all the time. Presumably the absence of these carbons from dipolarly dephased spectra is due to a short T$_2$ time - just like CH and CH$_2$ carbons have a short T$_2$ time due to dipolar coupling, I assume the quadrupolar interaction reduces T$_2$ time. I'd be very grateful for help understanding this.



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