Coupling constant is a quantitative measure of spin-spin splitting. It denotes how strongly a nucleus is affected by the spin-state of its neighbour. It is
Types of Coupling Constant
1. Based on the coupling partner:
2. Based on the number of intervening bonds:
highly significant for analysis.
3. Others: Specific couplings of higher order can be extremely useful
in NMR analysis. Ex: meta coupling, allylic coupling, W-coupling.
One Bond (1J) Coupling
Adjacent nuclei prefer to be in opposite spin states in the ground state. For a typical C—H bond, two peaks are observed in the 13C spectrum, due to the two shown transitions.
Some common one-bond coupling constants have been displayed below:
Two Bond (2J) Coupling
Commonly called as geminal coupling, these are usually smaller in magnitude than 1 bond coupling.
Some common 2 bond coupling constants:
Variation in 2J Coupling
Geminal coupling increases in magnitude as the decreases, due to higher electron spin correlation.
Three Bond (3J) Coupling
Commonly called as vicinal coupling, these usually follow the (n+1) rule in simple aliphatic hydrocarbon chains. Nuclear and electronic spin interactions carry the spin information from one hydrogen to its neighbour.
As a result, the best overlap occurs when the C—H bonds are at a dihedral angle of 0°.
Variation in 3J Coupling
The magnitude of vicinal coupling is directly related to the dihedral angle between the C—H bonds in question. The Karplus equation relates dihedral angle to coupling constant between two vicinal hydrogen nuclei.
Cyclopropane/Epoxide Systems: Jcis > Jtrans (unlike in alkenes)
Long Range Coupling
Very rarely, but often significantly, coupling takes place between two atoms separated by four bonds or more (nJ; n≥4). The common systems where this is exhibited are: allylic systems, rigid bicyclic systems and aromatic rings. Since these couplings are observed through a large number of bonds, a highly specific stereochemical arrangement is essential.