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

The “gauche effect” designates the tendency of many 1,2 polar group substituted molecules, such as 1,2-difluoroethane (C₂H₄F₂) and 1,2-difluoroethylene (C ₂H₂F₂), to assume a gauche conformation as their stable conformer, rather than the minimum repulsion anti or trans conformation. In this thesis we focus on understanding the machinery that controls the potential surface of the benchmark molecule, 1,2-difluoroethane (DFE).; The questions addressed here are: why is the FCCF dihedral angle in difluoroethane 71–72° rather than the 60° angle preferred by a hyperconjugation model in terms of anti C-H/C-F* interactions? What is the origin of the metastable trans conformer? What is the effect of relaxation on the potential surface? We address these questions by examining the hyperconjugation interactions through systematic deletion calculations using Natural Bond Orbital (NBO) analysis.; Analysis of the gauche stabilization energy at the B3LYP/6-311+G(d,p) level of theory shows that skeletal relaxation accounts for ∼1/3 of the 0.8 kcal/mol trans⇒gauche stabilization energy. Decomposition of the relaxation energy into individual bond length and bond angle relaxations reveals that CCF angle opening, increasing separation of the two C-F bonds in the gauche conformer, has the major effect.; The deletion calculations show that while the dominant hyperconjugative interaction in the gauche conformer arises from anti C-H bonds to vicinal C-F* antibonds as is widely accepted in the literature, the cis C₁-H/C₂-F* interactions are substantial (∼25% of the anti interaction). Although the dominance of the anti C₁-H donor—C2-F* acceptor interaction is preserved, understanding of the gauche-trans potential surface requires account to be taken of the cis interaction.; The well established substantially >60° FCCF dihedral angle for the equilibrium conformer can be rationalized in terms of the hyperconjugation model alone by taking into account both dominant anti C ₁-H/C2-F* interactions which maximize near 60° and the smaller cis interactions which maximize at a much larger dihedral angle. As the dihedral angle increases, there is progressive increase of orbital overlap between the main lobe of the cis C-H bond and the main lobe of the C-F* antibond, accounting for the dominance of the cis interaction at larger angles. This explanation does not invoke repulsive forces to rationalize the 71° equilibrium conformer angle.; The existence in DFE of a metastable trans conformer is linked to lower steric exchange repulsion at the trans geometry. The metastable trans conformer in this view arises from competition between both decreasing hyperconjugative stabilization and decreasing steric exchange repulsion as the FCCF dihedral angle increases.