Flexing analysis of ethane internal rotation energetics

A flexing analysis of the ethane barrier energy in terms of structural (ΔE_struct), steric exchange (ΔE_steric), and hyperconjugative charge-transfer (ΔE_deloc) 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 ΔE_struct and ΔE_deloc have very different stereoelectronic dependencies, but that the ΔE_steric and ΔE_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 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 ΔE_struct 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.