The diastereotopic protons H(a) and H(b) give different signals, each split into a doublet by H(c). The downfield peaks of the doublets happen to coincide. [The spectrum shows no splitting due to coupling between H(a) and H(b). With the trial run at high gain, however, the spectrum shows this coupling: each doublet is split into a quartet.]
The four-line pattern of c is due to successive splittings by H(a) and H(b). [ If J(ac) and J(bc) were equal- as they would have to be if, for example, H(a) and H(b) were equivalent - then the middle peaks of c would merge to give the familiar 1:2:1 triplet.]
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