Conformational preference analysis in C₂H₆ using orbital forces and non-covalent interactions; comparison with...

Dynamic Orbital Forces (DOF) and Non-Covalent Interactions (NCIs) are used to analyze the attractive/repulsive interactions responsible of the conformational preference of ethane and some related compounds. In ethane, it is found that the stabilization of the staggered conformation with respect to the adiabatic eclipsed one arises from both attractive and repulsive interactions in CH₃⋯CH₃. Attractive ones are predominant in a ratio 2 : 1, with an important role of a σ MO. On the contrary, the stabilization of the staggered conformation with respect to the vertical eclipsed one arises almost only from repulsive π interactions. Weak long-range H⋯H repulsions also favour the staggered conformation. From the sum of DOFs, yielding intrinsic bond energies, the rotation barrier can be decomposed into a weakening of the C-C bond (ca. 7 kcal mol^-1), moderated by a strengthening of C-H ones (ca. 4 kcal mol^-1). This evidences the decrease of hyperconjugation in the eclipsed conformation with respect to the staggered one. In the compounds CH₃-SiH₃, SiH₃-SiH₃, CH₃-CF₃ and CF₃-CF₃, the conformational preference is predominantly or exclusively due to repulsive interactions, with respect as well to adiabatic as to vertical eclipsed structures.