NMR 13 - Carbon coupling

Carbon-13 can couple to attached protons. Carbon-13 is very rare to find in a molecule. The chance of two neighboring carbons being carbon-13 is even rarer. From this you should be able to tell:

1. C-H coupling is negliable in hydrogen NMR - since the chance of any measured proton being attached to carbon-13 is so low.
2. C-C coupling is negliable in carbon NMR - since the chance of a neighbor being carbon-13 is so low.
3. C-H coupling is viewable in carbon NMR - since only carbon-13 is measured, 100% of the measured carbon is coupled with attached protons.

Since it is a one-bond interaction, the J values are quite large, typically 100 - 250 Hz. Since protons have a spin of ½, the carbons are split by the n+1 rule.

There is also C-C-H coupling. This is usually too small to notice. For example, ²J = 0 - 2 Hz for aliphatics.

At undergrad level you will nearly always be shown proton decoupled spectra. This removes the coupling, showing each carbon environment as a single peak:

Proton decoupling is done by using a separate tunable RF transmitter, which saturates the hydrogens without saturating the carbons. The saturated hydrogens undergo such rapid relaxations and excitation that the carbon only detects one average spin state. 

The reason decoupling is useful is that carbon multiplets can easily overlap - the J values are often then larger then the difference between carbon environments.