NMR 10 - Coupling constants

The distance between peaks in a simple multiplet is called the coupling constant, J. It is a measure of how strongly a nucleus is affected by spin states of it's neighbor - the stronger the coupling the stronger the constant.

J is always expressed in hertz, irrespective of the operating frequency. Chemical shift is expressed in parts per million of the operating frequency. In a 60Mhz machine, each ppm corresponds to 60hz. In a 300Mhz machine, each corresponds to 300hz, and so on. In other words, as the operating frequency gets higher, the distance in ppm between coupled peaks will get smaller. Here is an example of nitropropane at two different operating frequencies:

The magnified peaks in the bottom spectrum are called "expansions" and are added for convenience  Many textbook examples will use low-resolution spectra to avoid having to draw expansions.

J is written with a superscript indicating the number of bonds between the coupled nuclei. So for a typical H-C-C-H coupling we would write ³J.