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* antibonds, the cis C-H/C-F* interactions are substantial (∼25% of the anti interaction). The established significantly >60° 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° and the smaller cis interactions that maximize at a much larger dihedral angle. This explanation does not invoke repulsive forces to rationalize the 72° 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°. 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.