Coupling Constant
Definition and meaning of Coupling Constant in chemistry.
Coupling constant (J) is the magnitude, measured in hertz, of the spin–spin interaction between two nuclei observed as multiplet splitting in an NMR spectrum.
In more detail
J arises from indirect, through-bond spin-spin coupling transmitted by bonding electrons, and it splits NMR signals into recognizable patterns (doublets, triplets, quartets) that follow the n+1 rule for equivalent neighboring nuclei. Unlike chemical shift, J is expressed in hertz and does not change with the spectrometer's magnetic field strength, so peak splittings stay the same distance apart in Hz on any instrument. Its size depends on the number of bonds separating the coupled nuclei and, for vicinal (3-bond) coupling, on the dihedral angle between them (the Karplus relationship), making J values a powerful tool for deducing connectivity and conformation.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Symbol | J |
| Unit | hertz (Hz) |
| Typical vicinal (3J) range | 0-18 Hz, depending on dihedral angle |
In the 1H NMR spectrum of ethanol, the CH3 protons appear as a triplet because they couple with the two adjacent CH2 protons, with a typical vicinal coupling constant of about 7 Hz.
Frequently asked questions
Does the coupling constant change with spectrometer field strength?
No. J is measured in hertz and stays the same across spectrometers of different field strength, whereas chemical shift differences (in Hz) scale with field strength even though they are constant in ppm.
What is the Karplus relationship?
The Karplus equation relates the size of a vicinal (3-bond) coupling constant to the dihedral angle between the two coupled protons, allowing chemists to infer molecular conformation from J values.