Hyperconjugation not steric repulsion leads to the staggered structure of ethane

V. Pophristic, L. Goodman

Research output: Contribution to journalArticlepeer-review

467 Scopus citations

Abstract

Many molecules can rotate internally around one or more of their bonds so that during a full 360° rotation, they will change between unstable and relatively stable conformations. Ethane is the text-book example of a molecule exhibiting such behaviour: as one of its two methyl (CH3) groups rotates once around the central carbon-carbon bond, the molecule will alternate three times between an unstable eclipsed conformation and the preferred staggered conformation. This structural preference is usually attributed to steric effects; that is, while ethane rotates towards an eclipsed structure, the electrons in C-H bonds on the different C atoms are drawing closer to each other and therefore experience increased repulsion, introducing a rotation barrier that destabilizes the eclipsed structure. Stabilization of the staggered structure through rotation-induced weakening of the central C-C bond and hyperconjugation has been considered to be involved, but evaluation of the contributions of these effects to ethane's internal rotation barrier and conformational preference remains difficult. Here we report a series of ethane structure optimizations, where successive removal of different interactions indicates that ethane's staggered conformation is the result of preferential stabilization through hyperconjugation. Removal of hyperconjugation interactions yields the eclipsed structure as the preferred conformation, whereas repulsive forces, either present or absent, have no influence on the preference for a staggered conformation.

Original languageEnglish (US)
Pages (from-to)565-568
Number of pages4
JournalNature
Volume411
Issue number6837
DOIs
StatePublished - May 31 2001
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • General

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