Theoretical analysis of the rotational barrier in ethane: cause and consequences

On the basis of energy decomposition analysis, the rotational energy profile of ethane is explained by using two models: rigid rotation with instantaneous geometry relaxations of the eclipsed and staggered conformations and relaxed rotation with continuous geometry relaxations. Both models can be applied to the real system. A distinction between the cause of an initial energy rise and energetic consequences of structural changes accompanying the rotation is made. It is concluded that the increased Pauli repulsion is the main cause for the initial energy rise and geometry changes. However, after the structural changes take place, the Pauli repulsion is not responsible for the higher energy of the eclipsed state. It then originates from energetic consequences of geometry changes, which include decrease in electrostatic and orbital stabilization energies, mainly due to the C-C bond lengthening, and an energy rise due tomethyl groups bending.