Nuclear magnetic resonance decoupling (NMR decoupling for short) is a special method used in nuclear magnetic resonance (NMR) spectroscopy where a sample to be analyzed is irradiated at a certain frequency or frequency range to eliminate or partially the effect of coupling between certain nuclei.
Since the percentage of 13C is so low in natural isotopic abundance samples, the 13C coupling effects on other carbons and on 1H are usually negligible, and for all practical purposes splitting of 1H signals due to coupling with natural isotopic abundance carbon does not show up in 1H NMR spectra.
In real life, however, the 13C coupling effect does show up on non-13C decoupled spectra of other magnetic nuclei, causing satellite signals.
Similarly for all practical purposes, 13C signal splitting due to coupling with nearby natural isotopic abundance carbons is negligible in 13C NMR spectra.
The coupling constant, indicating a small frequency difference between split signal peaks, would be smaller than in an undecoupled spectrum.
[1] Looking at a compound's off-resonance proton-decoupled 13C spectrum can show how many hydrogens are bonded to the carbon atoms to further help elucidate the chemical structure.
Here the selected "narrow" 1H frequency band of the (soft) decoupling RF pulse covers only a certain part of all 1H signals present in the spectrum.
In other words this specific decoupling method is useful for signal assignments which is a crucial step for further analyses e.g. with the aim of solving a molecular structure.
This is exploited e.g. with chemical exchange saturation transfer (CEST) contrast agents in in vivo magnetic resonance spectroscopy.