text: Another potential complication results from the presence of large one bond J-coupling constants between carbon and hydrogen (typically from 100 to 250 Hz). While potentially informative, these couplings can complicate the spectra and  reduce sensitivity.  For these reasons, C-NMR spectra are usually recorded with proton NMR decoupling. Couplings between carbons can be ignored due to the low natural abundance of C.  Hence in contrast to typical proton NMR spectra, which show multiplets for each proton position, carbon NMR spectra show a single peak for each chemically non-equivalent carbon atom.
  In further contrast to H NMR, the intensities of the signals are often not proportional to the number of equivalent C atoms.  Instead, signal intensity is strongly influenced by (and proportional to) the number of surrounding spins (typically H). Integrations are more quantitative if the delay times are long, i.e. if the delay times greatly exceed relaxation times.
  The most common modes of recording C spectra are proton-noise decoupling (also known as noise-, proton-, or broadband- decoupling), off-resonance decoupling, and gated decoupling.  These modes are meant to address the large J values for C - H (110–320 Hz), C - C - H (5–60 Hz), and C - C - C - H (5–25 Hz) which otherwise make completely proton coupled C spectra difficult to interpret.
  With proton-noise decoupling, in which most spectra are run, a noise decoupler strongly irradiates the sample with a broad (approximately 1000 Hz) range of radio frequencies covering the range (such as 100 MHz for a 23,486 gauss field) at which protons change their nuclear spin.  The rapid changes in proton spin create an effective heteronuclear decoupling, increasing carbon signal strength on account of the nuclear Overhauser effect (NOE) and simplifying the spectrum so that each non-equivalent carbon produces a singlet peak.  The relative intensities are unreliable because some carbons have a larger spin-lattice relaxation time and others have weaker NOE enhancement.
  In gated decoupling, the noise decoupler is gated on early in the free induction delay but gated off for the pulse delay.  This largely prevents NOE enhancement, allowing the strength of individual C peaks to be meaningfully compared by integration, at a cost of half to two-thirds of the overall sensitivity.
  With off-resonance decoupling, the noise decoupler irradiates the sample at 1000–2000 Hz upfield or 2000–3000 Hz downfield of the proton resonance frequency.  This retains couplings between protons immediately adjacent to C atoms but most often removes the others, allowing narrow multiplets to be visualized with one extra peak per bound proton (unless bound methylene protons are non-equivalent, in which case a pair of doublets may be observed).