Dissecting the hindered rotation of ethane

The existence of a rotational barrier of ca. 3 kcal mol À1 around the CÀC single bond in ethane has been known The steric repulsion still remains the most popular explanation of the hindered rotation of ethane. This effect is often understood as the increase in energy that accompanies the antisymmetrization of a wave function originally formed by strictly localized descriptions of two methyl groups brought up to the final ethane geometry where they overlap. This so-called Pauli repulsion is considered to be more important for the eclipsed conformation, The hyperconjugation [21] The electron delocalization effect can easily be assessed in valence bond (VB) theory calculations Previous energy decomposition analyses relied, in one way or another, on the definition of two methyl fragments. However, in the last years there have been a growing interest in other kinds of energy partitioning schemes, The diatomic terms naturally reflect the attractive or repulsive interactions between the atoms in the molecule. The onecenter terms correspond to the effective energy of each atom in the molecule; its value relative to that of the corresponding free atom accounts for the promotion that occurs upon bond formation. In this sense, it is important to recall that the oneand two-center contributions are static parameters. [26] They measure to which extent the energy of a given atom or atomic pair contributes to the total molecular energy at that geometry and with the wave function used at that point. Thus, the diatomic values cannot be put into direct correspondence with the dissociation energies, as dissociation involves changes in both geometry and wave function. The main advantage of this methodology for the present case is that one can decompose all energetic interactions within the molecule on the basis of a single ab initio calculation, without recurring to an arbitrar