Natural bond orbital deletion calculations show that whereas the gauche preference arises from vicinal hyperconjugative interaction between anti C-H bonds and C-F* antibonds, the cis C-H/C-F* interactions are substantial (approximately 25% of the anti interaction). The established significantly >60 degrees FCCF dihedral angle for the equilibrium conformer can then be rationalized in terms of the hyperconjugation model alone by taking into account both anti interactions that maximize near 60 degrees and the smaller cis interactions that maximize at a much larger dihedral angle. This explanation does not invoke repulsive forces to rationalize the 72 degrees equilibrium conformer angle. The relative minimum energy for the trans conformer is the consequence of a balance between decreasing hyperconjugative stabilization and decreasing steric destabilization as the FCCF torsional angle approaches 180 degrees . The torsional coordinate is predicted to be strongly contaminated by CCF bending, with the result that approximately half of the trans --> gauche stabilization energy stems from mode coupling.

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.

Relaxation calculations using natural bond orbitals are carried out for the Puali exchange steric repulsion changes associated with ethane internal rotation using sufficiently extended Dunning correlation-consistent and Gaussian orbitals, so that interpretations are not basis set dependent. These calculations show that the total exchange repulsion is strongly dependent on the C–C bond lengthening that accompanies rotation. The effect of this relaxation is that the total exchange repulsion favors the eclipsed conformer by ∼6 kcal/mol, far greater than previous estimates obtained without skeletal relaxation. The source of the skeletal flexing dependence stems from the strong dependence of the orthogonalization-induced-energy-shift of the σCH NBO and carbon core orbitals on RC–C. Strong basis set dependence is found for the pairwise repulsions. Extended basis set calculations attribute an antibarrier energy change to anti/syn repulsions and a barrier-forming one to gauche repulsions. These senses are opposite to the barrier-forming anti/syn CH pairwise interactions found in previous ethane steric considerations.