Coupling is a scientific term used to describe the interaction between protons. Coupling constant J is a specific way of expressing how these protons behave when this interaction occurs. When the behavior of these protons is mapped out on something called an NMR spectrum, the diagram reveals a series of peaks and valleys representing the strength of the protons' magnetic fields. The distance between these peaks is known as the coupling constant J.
Gather the necessary data about the molecule. You will require access to measurements indicating the height of the molecule's NMR peaks in parts per million. This information can only be gathered with a nuclear magnetic resonance spectroscope. You will also need to know the strength of the NMR spectroscope in megahertz.
Convert each peak's parts per million into hertz. To do this, multiply each peak's parts per million by the strength of the NMR spectroscope. For example, if you had a peak that was measured at 3.632 ppm on a spectroscope operating at 500 mHz, you would multiply 3.632 by 500 to get 1816 hertz. This should be done individually for each of the peaks.
Find the difference in hertz between two of the peaks. To do this, subtract the hertz of one peak from the hertz of the other. The difference is the coupling constant J.