Fine Structure in an NMR Spectrum
As mentioned previously, it is most common for the lines in an NMR spectrum to be split into several components. This is referred to as the fine structure of the spectrum, and it leads to the NMR spectrum of ethanol (C2H5OH) having the following appearance:
The appearance of such a spectrum makes it hard to accurately measure the intensity of an absorption (which corresponds to the total area under all the components which make up a line). Thus most NMR spectra include bars by each absorption line, the height of which indicates the relative intensity of that absorption.
The splitting of the lines into components occurs because each magnetic nucleus can contribute to the local field experienced by adjacent nuclei. However, since a magnetic nucleus can only take up discrete orientations (distinguished by the value of mI), the interaction splits resonance lines into discrete component lines rather than acting to broaden the resonance out over a range of frequencies.
The strength of the scalar coupling interaction (the splitting observed in the spectrum) is measured by the scalar coupling constant, J, which has units of hertz. It is independent of the strength of the applied field. Since the interaction is between the spins of nearby nuclei, this interaction is commonly known as spin-spin coupling.
If the resonance of a nucleus is split by a certain amount by a second nucleus, then the resonance of the second nucleus is split by the same amount due to the presence of the first nucleus.
The following diagram represents the NMR spectrum generated by two spin-½ nuclei, A and X, which resonate at different frequencies and have a scalar coupling constant of J:
The points A and X mark the positions of the resonance of each nucleus if there were no coupling between them.
The effect of the scalar coupling is to split each line into two components, J hertz apart. The components are centred on the resonance frequency in the absence of coupling. Note that the splitting is greatly exaggerated in the above diagram – the components of a resonance line are usually much more closely spaced than above.