text: Magnetic inequivalence may occur with HC-CH fragments that are subdivided into two groups of two in either geminal relationships via a mirror plane along the C-C bond, i.e. HHC-CHH, or in vicinal relationships via a mirror plane bisecting the C-C bond, i.e. in HHC-CHH, or via a rotational axis of symmetry (a C-axis), i.e. HHC-CHH. The coupling constants then differ because of geometry (cis vs. trans) or connectivity (2-bond vs. 3-bond) and the level of complexity will depend on the differences. Conformational dynamics may reduce or even obliterate the difference between cis and trans couplings, if fast compared to the NMR timescale. There may also be additional couplings to other nuclei.
  The ethylene fragment in 2-substituted dioxolanes can thus show a high level of complexity if the substituent is large. Symmetrical norbornanes and similarly rigid compounds (e.g. 7-oxabicyclo[2.2.1]heptane) also show complex signals for the ethylene fragments, made more complicated by additional splitting by the bridgehead protons. Reich gives several additional examples of magnetic inequivalence in acyclic and cyclic systems containing HC-CH fragments.