5-HMR-14 A2X2 and A2B2 Patterns
In A2X2 and A2B2 patterns the two A nuclei and the two X (B) nuclei are magnetically equivalent: they have the same chemical shift by symmetry, and each A proton is coupled equally to the two X (or B) protons. True A2X2 patterns are quite rare. Both the A and X protons are identical triplets.
An example of an A2X2 patterns is shown below. Note that the methyne protons signals are a little broader than the CH2, presumably there is a small long-range coupling to the MeO protons.
More complicated patterns are seen when the chemical shift difference approaches or is smaller than the JAB coupling. However, both A2B2 and AA'BB' always give centrosymmetric patters (A2 part mirror image of the B2 part).
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