Configurations and conformations in acyclic, unsaturated hydrocarbons. A 13C NMR study

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Abstract
13C NMR (CMR) spectra of a number of di‐ and trisubstituted ethylenes have been measured. Very consistent values are found for the differential shieldings of allylic carbons in a number of linear, (Z)‐ and (E)‐disubstituted ethylenes. The discrepancies between the several structural elements are explained tentatively by assuming different rotational preferences in the (Z)‐isomers, the (E)‐isomers, or both. The results obtained for some branched olefins are qualitatively in agreement with conclusions reached in previous 1H NMR (PMR) investigations, with the possible exception of 2,5‐diMe‐3‐hexene.Differential shieldings in trisubstituted ethylenes are larger than in disubstituted ethylenes, probably as a consequence of steric interaction between geminal substituents.It is confirmed, that API's recent relabelling of the isomers of 3‐Me‐2‐hexene was justified. Furthermore, the (E)‐configuration is assigned to the high‐boiling isomer of 3,4,4‐triMe‐2‐pentene. On the other hand, the high‐boiling isomer of 3‐Et‐4‐Me‐2‐pentene is shown to be the (Z)‐isomer. For the isomeric pair of geraniol and nerol, the (E)‐configuration is assigned to the former and the (Z)‐configuration to the latter, in agreement with our recent PMR investigations.