Cyclopropanes.
Dihedral angles in cyclopropanes are rigidly fixed by the geometry of the ring system. We therefore find that Jcis (7-10 Hz) is always larger than Jtrans (2-6 Hz), and this can be reliably used for structure assignment. The same relationship holds for the 3-membered ring heterocycles, although the range of observed couplings is wider.
Cyclobutanes.
Cyclobutanes are even flatter than cyclopentanes, so that cis couplings are almost always larger (6-9 Hz) than trans (2-8). However, if structural features which promote strong puckering of the ring such as a trans ring fusion, large or electronegative substituents are present, then trans couplings can become larger than cis, as shown for 1,3-dibromocyclobutane and cyclobutanol below.
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