Conformational preference analysis in C2H6 using Orbital Forces and Non-Covalent Interactions; comparison with related systems

Dynamic Orbital Forces (DOF) and Non-Covalent Interactions (NCIs) allow an analysis of the attractive/repulsive interactions whose variations lead to the staggered preference of ethane and some related compounds. In ethane, it is found that CH3···CH3 attractive interactions are predominant in the staggered preference with respect to adiabatic eclipsed conformation, with an important  contribution. On the contrary, vertical eclipsed ethane is destabilized almost only by repulsive  interactions. Weak long-range H···H repulsions favor the staggered conformation. From the sum of DOFs, the energy barrier can be decomposed into C-H and C-C bond energies. It is found due to the weakening of the C-C bond (ca. 7 kcal/mol), moderated by a strengthening of C-H ones (ca. 4 kcal/mol) arising from the decrease of hyperconjugation with respect to the staggered conformation. 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.