It is the measurement of the distance between the peaks of a multiplet which arises after splitting. It is known as the coupling constant. It denoted by the J and its unit is Hertz.
It is a useful tool in the measurement of the effectiveness of spin-spin coupling and predicting the protons NMR in complex structures.
Types of coupling constant:
1) Geminal coupling: this type of coupling occurs between protons present on single carbon specifically in the terminal vinyl system. It ranges from 10-18 and denoted by 2 But one condition is applied that the proton present should be non-equivalent.
2) Vicinal coupling: three bonds participate in coupling. It is denoted by 3J. It depends on the dihedral angle. Magnitude is 16Hz at 1800. Newman’s projection is used to get a dihedral angle and then calculate it.
3) long-range coupling: it occurs when there are more than three bonds. Normally occurred within 4-5 bonds. But with polyalkynes, it is up to 9 bonds. It is denoted by >3J. Its range is 4-5.