There are thre pairs of heterotopic groups and two prochiral centres as illustrated below.
The signal at 5.3ppm is due to Ha and Hb, which
though diastereotopic have overlapping 1H NMR resonances. Both
Ha and Hb couple to Hc: Ha
giving rise to the lines at 1341 and 1323Hz, i.e. an 18Hz coupling constant
typical of hydrogens trans to one another; and Hb giving rise
to the lines at 1323 and 1312Hz, i.e. an 11Hz coupling constant typical
of hydrogens cis to one another. The coupling constant between Ha
and Hb is too small to detect (typically <1Hz).
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