Flexing analysis of ethane internal rotation energetics

A flexing analysis of the ethane barrier energy in terms of structural (Del ta E-struct), steric exchange (Delta E-steric), and hyperconjugative charge -transfer (Delta E-deloc) energy contributions has been carried out using n atural bond orbitals. No evidence is found for the view that the ethane sta ggered equilibrium geometry or the C-C bond expansion that accompanies rota tion results from steric exchange repulsion interactions. The analysis show s that Delta E-struct and Delta E-deloc have very different stereoelectroni c dependencies, but that the Delta E-steric and Delta E-deloc 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 und erstood without taking into account their different relaxation dependencies . Neglect of C-C expansion leaves the charge-transfer interactions paramoun t by subduing the steric and structural contributions. These interactions a re found to be an important determinant for the expansion. The strong expan sion dependence found for Delta E-struct is largely controlled by weakening of the C-C bond, and to a lesser extent by concomitant strengthening of th e C-H bonds. Most of this dependence can be mimicked by C-C expansion in th e absence of methyl torsion indicating that C-C bond weakening does not ari se from the symmetry change accompanying ethane torsion. (C) 1999 American Institute of Physics. [S0021-9606(99)31209-5].