Application of NMR to structure confirmation in some drug molecules

Application of NMR to structure confirmation in some drug molecules 

It is actually possible with a closer view to see additional splitting of all of the aromatic proton signals and this is due to long-range coupling between the protons meta to each other, i.e. H-1 and H3 and H2 and H4, which can occur in aromatic systems and can be up to 3 Hz. Para coupling can also occur but it is only ca 1 Hz. 

Proton NMR spectrum of salbutamol A more complex example 

The NMR spectrum is perturbed in an AB system so that the ratio of line intensities composing the doublet instead of being 1 1 are given by  

If the differences between the frequencies are substituted in the equation this gives a difference in frequency of Bl and B2 of 22 Hz, i.e. the germinal coupling (coupling of protons on the same carbon) of the two signals gives shifts of 7.5 Hz in one direction and 4.5 Hz in the other direction instead of the usual even splitting. 

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