Flexing analysis of ethane internal rotation energetics

Lionel Goodman, Hongbing Gu, Vojislava Pophristic

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78 Scopus citations


A flexing analysis of the ethane barrier energy in terms of structural (ΔEstruct), steric exchange (ΔEsteric), and hyperconjugative charge-transfer (ΔEdeloc) energy contributions has been carried out using natural bond orbitals. No evidence is found for the view that the ethane staggered equilibrium geometry or the C-C bond expansion that accompanies rotation results from steric exchange repulsion interactions. The analysis shows that ΔEstruct and ΔEdeloc have very different stereoelectronic dependencies, but that the ΔEsteric and ΔEdeloc dependencies are antagonistic. All of their contributions are strongly affected by the C-C bond expansion, with the result that the barrier mechanism cannot be understood without taking into account their different relaxation dependencies. Neglect of C-C expansion leaves the charge-transfer interactions paramount by subduing the steric and structural contributions. These interactions are found to be an important determinant for the expansion. The strong expansion dependence found for ΔEstruct is largely controlled by weakening of the C-C bond, and to a lesser extent by concomitant strengthening of the C-H bonds. Most of this dependence can be mimicked by C-C expansion in the absence of methyl torsion indicating that C-C bond weakening does not arise from the symmetry change accompanying ethane torsion.

Original languageEnglish (US)
Pages (from-to)4268-4275
Number of pages8
JournalJournal of Chemical Physics
Issue number9
StatePublished - Mar 1 1999
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • General Physics and Astronomy
  • Physical and Theoretical Chemistry


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