The nuclei contributing to an observed signal sense the presence of other NMR-active nuclei in the vicinity; the interaction between them is called coupling. There are several different types of possible coupling, but the one of most interest in simple NMR experiments is J, or scalar, coupling. This arises when two or more nuclei with spin (a fundamental nuclear property) of 1/2 are located within a few bonds of one another (spin-1/2 nuclei that may be found in organic molecules include ¹H, ¹³C, ¹⁹F, ³¹P, ¹⁵N, and ²⁹Si).

To explain the origin of J coupling, take, as an example, methine carbons, ¹³C-¹H. Some of the methine ¹H's in the sample are aligned with the magnetic field (a low-energy state), and some are aligned against the field (a high-energy state). The exact resonant frequency of the 13C depends on which type it is bonded to; ¹³C's next to low-energy ¹H's resonate at a slightly lower frequency than those next to high-energy ¹H's. This gives rise to two subtly shifted peaks in the ¹³C spectrum (blue indicates low E; red, high):

The two peaks in this doublet are typically about 125 Hz apart. Because this heteronuclear coupling constant does not change with magnetic field strength, the extent of the separation varies, from 2.5 ppm at 4.6T (200 MHz for ¹H) to 0.8 ppm at 13.8T (600 MHz for ¹H).

Similarly, it can be shown that, for methylenes (¹³C-¹H₂), three peaks (a triplet) are expected, with the central one being twice the height of the outer ones because red/blue = blue/red), and that methyls (¹³C-¹H₃) will give four peaks (a quartet), in a 1:3:3:1 ratio. Obviously, quaternary carbons, which are not attached to any protons, are not split in this way; they give singlets.

While the presence of J-couplings can clearly be a tremendous aid to spectral interpretation, they can also complicated a spectrum if they lead to complex and/or overlapping splitting patterns. It is not easy to remove ¹H-¹H couplings from a ¹H spectrum, so they are an accepted feature of the final spectrum. If necessary, more sophisticated approaches can be employed to simpify ¹H data.

¹³C-¹H couplings, on the other hand, are relatively easy to remove. One irradiates the set of ¹H's at or near their Larmor frequency. The distribution of energy states is then averaged, so that all of the 1H's appear to be purple in the color scheme of the figures above, and the resonances appear as singlets at the average frequency of the multiplet (between the center peaks if the number of peaks is even, and at the central resonance if the number is odd). Such ¹H decoupling is typically used to acquire spectra of any nuclide other than ¹H; this is designated by ⁿX{¹H}, indicating that protons are being decoupled from nuclide ⁿX. This procedure is represented graphically in the pulse diagram below, which is slightly different from the sequences shown earlier. A second channel has been added to show the activity of the ¹H radiofrequency. This rf field is effectively turned on at the beginning of the experiment, and off at the end.

¹H decoupling has another effect on the observed nuclei (¹³C, for example) as well; this is called the nuclear Overhauser effect (NOE), and can lead to signal enhancements. If you are only interested in simple 1H NMR, click here to skip this section.