Hyperconjugation not steric repulsion leads to the staggered structure of ethane
Pophristic, Vojislava, and Lionel Goodman. "Hyperconjugation Not Steric Repulsion Leads to the Staggered Structure of Ethane." Nature. Nature Publishing Group. Web. 12 Mar. 2012. <http://www.nature.com/nature/journal/v411/n6837/abs/411565a0.html>.
  • In the text book example of ethane, two methyl group will rotate around a central carbon
    • Alternates three times between the unstable eclipsed conformation and most stable staggered conformation
    • Usually the preferred conformation was attributed to steric hindrance
      • As ethane rotates toward the eclipsed conformation the electrons on the different C-H bonds experienced increased repulsion
  • Instead, the author proposes stabilization of the staggered structure through rotation-induced weakening of the central C-C bond and hyperconjugation
  • Test removal of different interaction
    • Hyperconjugation
      • Electron transfer from an occupied to an unoccupied orbital leading to delocalization of charge
    • Steric Factor
      • Exchange
        • Uses Pauli exclusion principle which states that pairs of electrons not to occupy the same spatial region
        • Electrostatic
          • Also known as Coulomb interaction
            • Different than exchange because works over long range
  • Potential curves in terms of torsional angle
    • First curve with all interactions present representing the energy of the real system
    • Second with the energy of the system with exchange repulsion absent
    • Concludes that minima of both curves prefer the staggered configuration regardless of exchange repulsion
  • Tests for hyperconjugation in terms of examining the internal rotation-induced change in nuclear electron attraction via molecular orbitals
    • Potential energy measured in ∆Vne and split between σ and π contributions
    • In ethane only two types of hyperconjugation exist
      • vicinal (between methyl groups)
        • Only π orbitals overlap
        • geminal (within a single methyl group)
  • Natural Bond Orbitals are used to further analyze interactions
    • Doubly Occupied
    • Hyperconjugation is expressed by electron transfer
      • Staggered conformation is lost on removal of all charge transfers
        • Shows that vicinal charge transfer interactions are the ones that keep the molecule in the staggered conformation
          • No inversion of structure occurs on removal of coulombic and exchange repulsions