It is a measure of the distance between the peaks of a multiplet that arises after segmentation. This is known as the coupling constant. It is denoted by J and its unit is Hertz.
It is a useful tool in the measurement of the effectiveness of spin–spin coupling and in predicting proton NMR in complex structures.
Types of Coupling Constants:
1) Geminal Coupling: This type of coupling occurs exclusively between protons present on the single carbon in the terminal vinyl system. It is between 10-18 and is denoted by 2 But one condition is that the protons present must be non-equivalent.
2) Visine Coupling: Three bonds take part in the coupling. it is denoted by 3J. It depends on the dihedral angle. 180. but the magnitude is 16Hz0, Newman’s projection is used to obtain the dihedral angle and then calculate it.
3) Long distance coupling: This occurs when there are more than three bonds. Usually occurs within 4-5 bonds. But with polyalkynes, it is up to 9 bonds. > it. is denoted by3h. Its range is 4-5.