H2C-CH2 fragments Magnetic inequivalence

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ch2 region (cdcl3, 300 mhz); starred peaks impurities


magnetic inequivalence may occur h2c-ch2 fragments subdivided 2 groups of 2 in either geminal relationships via mirror plane along c-c bond, i.e. haha′c-chbhb′, or in vicinal relationships via mirror plane bisecting c-c bond, i.e. in hahbc-cha′hb′, or via rotational axis of symmetry (a c2-axis), i.e. hahbc-chb′ha′. coupling constants differ because of geometry (cis vs. trans) or connectivity (2-bond vs. 3-bond) , level of complexity depend on differences. conformational dynamics may reduce or obliterate difference between cis , trans couplings, if fast compared nmr timescale. there may additional couplings other nuclei.


the ethylene fragment in 2-substituted dioxolanes can show high level of complexity if substituent large. symmetrical norbornanes , rigid compounds (e.g. 7-oxabicyclo[2.2.1]heptane) show complex signals ethylene fragments, made more complicated additional splitting bridgehead protons. reich gives several additional examples of magnetic inequivalence in acyclic , cyclic systems containing h2c-ch2 fragments.








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