Usually for HNMR of aromatic region we get a doublet-doublet, or a tripet with each peak splitted as doublet. Is it possible for some compound to give clean doublet without splitting into doublet-doublet in aromatic region. I'm talking for compounds such as PBIs, NDIs. If yes, I would appreciate a reference literature.
You would only get a doublet of doublets if the ring is para disubstituted. Even then there will be weak long range couplings which will distort the doublets.
Other substitution arrangements will give quite different patterns.
Ortho disubstituted give approximately two quartets (in a high field instrument).
[It is really a AA'BB']
1.4 disubstiuted ring show
A singlet signal if the substiuent the same for example 1.4 dimethylbenzene because all the four aromatic proton are equivalent
Doblet of Doblet if the two groups are different for example para chloro tolouine
Aromatic protons show a coupling of ~ 7Hz if there is another proton in ortho position. Two orthe results in a doublet of doublets often looking like a triplet.
A coupling of ca 2-3 Hz is observe to ta proton in meta and 1 Hz or less to a para proton. All the different substituion patterns on an aromatic ring can lead to a variety of multiplet patters. For example a 1,2,4 substition gives a doublet with small coupling for 3, a doublet of doublets small/large for 5 and a doublet with large coupling for 6.
Dr. Clemens Anklin cited the aromatic protons coupling-values. That infomations you need to contrast with the shimming optimization of your sample. If your sample does not shimming well, a doublet of doublets, you can see like only a doublet or less. If you improve the shimming, you can see better multiplicity for your J-coupling.
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