Signal multiplicity multiplets

Spectra of greater complexity may occur for systems where the coupling constant is of similar magnitude to the chemical shift difference between the coupled nuclei. Such a case is referred to as an AmB system, where nucleus A has the smaller and nucleus B the larger chemical shift. 

An AB system (Fig. 1.6) consists, for example, of an A doublet and a B doublet with the common coupling constant JAB, where the external signal of both doublets is attenuated and the internal signal is enhanced. This is referred to as an AB effect, a roofing symmetric to the centre of the AB system 2 roofing is frequently observed in proton NMR spectra, even in practically first order spectra (Fig. 1.2, ethyl quartet and triplet). The chemical shifts vA and vB are displaced from the centres of the two doublets, approaching the frequencies of the more intense inner signals. 

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The splitting (signal multiplicity) of H resonances often reveals the spatial proximity of the protons involved. Thus it is possible to identify structural units such as those which often occur in organic molecules simply from the appearance of multiplet systems and by using the + 1 rule. 

Signals with different multiplicities Multiplet pattern Determination of Relaxation time... 

Multiplicity is brought about due to the splitting of the signal of one set of equivalent nuclei by the magnetic fields of adjacent sets of nuclei i.e., spin-spin interactions. The distance between the peaks of a regular multiplet is termed as the coupling constant, designated as J, and measured in Hz. 

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