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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:

d = DnPr · 106 / n0 ppm

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:

A quartet appears in the NMR spectrum of acetaldehyde CH3-CHO at d = 9.26 ppm (Spectrometer frequency n0 = 300 MHz *).

Spectrum
  1. Which protons of the acetaldehyde give rise to this quartet?
  2. What is the coupling constant J in this case?

Compare your answer to the solution on page 79!
* Spectrum courtesy of Pacific Lutheran University