Gauche effect in 1,2-difluoroethane. Hyperconjugation, bent bonds, steric repulsion

Natural bond orbital deletion calculations show that whereas the gauche preference arises from vicinal hyperconjugative interaction between anti C-H bonds and C-F* aritibonds, the cis C-H/C-F* interactions are substantial (similar to25% of the anti interaction). The established significantly >60degrees 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 60degrees and the smaller cis interactions that maximize at a much larger dihedral angle. This explanation does not invoke repulsive forces to rationalize the 72degrees 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 180degrees. 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.