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Determination of coupling constants:The coupling constant JAX is defined as follows:
JAX = Separation in Hertz of two neighboring lines of the multiplet A
However, the spectra are typically labelled in the d scale. To convert from the d scale to the n scale, the following relation holds:
Basically, you have to convert the separation of two neighboring lines in ppm to the difference in frequency JAX in Hertz.
A similar conversion between the different scales used to label NMR spectra was already covered on page 42!
An additional example: