coupling constant
/ KUP-ling /
When a signal splits into a doublet or triplet, the little peaks within it are separated by a definite gap. That gap, measured in hertz (Hz), is the coupling constant, written J. It is not just a curiosity — its size encodes the geometry of how two hydrogens are arranged in space, making it a quiet but precise reporter of three-dimensional structure.
Two hydrogens that split each other share the same coupling constant — that is, the spacing in one's multiplet exactly equals the spacing in the other's, which is how you confirm that two signals are genuinely coupled neighbors rather than a coincidence. The value of J depends on the geometric relationship between the coupled hydrogens, especially the angle of the bonds between them and how many bonds separate them. A few patterns are diagnostic: hydrogens on a double bond that are trans (on opposite sides) couple with a large J, around 12–18 Hz, while cis hydrogens (same side) couple with a smaller J, around 6–12 Hz. So J itself can distinguish cis from trans geometry.
This makes the coupling constant a tool for stereochemistry, not just connectivity. In a ring or across a double bond, comparing J values tells a chemist the spatial arrangement of substituents — axial versus equatorial, cis versus trans, E versus Z. Where chemical shift and integration give the flat 'what and how many,' the coupling constant adds the three-dimensional 'arranged how,' which is often the last piece needed to settle a structure.
In a disubstituted alkene, a measured J of about 16 Hz between the two vinyl hydrogens tells you they are trans (E), while a J of about 8 Hz would tell you they are cis (Z) — the coupling constant settles the double-bond geometry.
J is the spacing within a multiplet; its size reports the geometry between coupled hydrogens.
The coupling constant J is measured in hertz and stays the same regardless of magnet strength, unlike chemical shift in ppm — which is why mutually coupled peaks keep identical spacing on any instrument, a useful way to check which signals are partners.