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The number of peaks in the low-resolution spectrum depends on the number of different environments that the hydrogen atoms have in the molecule.


One environment.

Benzene, C6H6 has only one sort of hydrogen atom, so that the NMR spectrum shows a single peak (the TMS peak is omitted):

 benzeneNMR.gif (2880 bytes)


Two environments:

Ethanal CH3CHO has two sorts of hydrogen atom, those on the methyl group and that on the aldehyde group. It therefore has two peaks in its spectrum (the TMS peak is omitted):

 ethanalNMR.gif (2902 bytes)


Three environments:

Ethanol CH3CH2OH has methyl hydrogen, methylene hydrogen, and hydroxyl hydrogen. It therefore has three peaks in its spectrum (the TMS peak is omitted). However, this spectrum is the high-resolution spectrum, which shows spin-spin coupling; this is dealt with below . It can be neglected as far as A level examinations are concerned, but it is an important topic and merits some consideration.

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Spin-spin coupling.

A given hydrogen nucleus can exist in one of two spin states; it is the transition between these states that gives rise to the NMR spectral line. Other adjacent hydrogen atoms in the same environment, for example all the methyl hydrogen atoms in ethanol, do not affect the absorbtion. However this is no longer true when the hydrogen atoms are non-equivalent. In ethanol, for example, the hydrogen atoms on the methyl group interact with those on the methylene group – their magnetic fields couple. The effect of coupling on the spectrum is that the lines are split into multiplets. Most coupling occurs between hydrogen atoms on adjacent carbon atoms, so in the ethanol spectrum there is splitting of the lines due to the methyl and methylene hydrogen atoms, but not that of the hydroxyl hydrogen – it is too far away.

The spectra are commonly integrated, that is the area under each peak (or set of peaks in a multiplet) is measured. This gives information about the number of hydrogen atoms that have given rise to each absorbtion.

Chemistry contents    NMR introduction    Basis of NMR    The chemical shift   

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