Ethane rotations

The ethane rotational barrier and wave function analysis... 

As a simple example, let s consider the case of ethane. Rotation about the carbon-carbon sigma bond results in a number of different conformations for this molecule. There are two extremes for these conformations, as shown in Figure 6.4. The eclipsed conformation has each hydrogen on one carbon as close as possible to one hydrogen on the other carbon. The staggered conformation has the hydrogens on one carbon as far from the hydrogens on the other as possible. Other conformations are intermediate between these two extremes. 

Unlike the C-C single bond in ethane, rotation about the C-C double bond in ethylene is restricted. It can occur only if the it bond first breaks and then re-forms, a process that requires... 

Recall from Section 1.9A that rotation occurs around carbon-carbon a bonds. Thus, the two CH3 groups of ethane rotate, allowing the hydrogens on one to adopt different orientations relative to the hydrogens on the other. These arrangements are called conformations. 

FIGURE 7.2 The two —CH3 groups in ethane rotate easily about the bond that joins them. 

The rotational profile of n-butane can be understood as a superimposition of van der Waals repulsion on the ethane rotational energy profile. The two gauche conformations are raised in energy relative to the anti by an energy increment resulting from the van der Waals repulsion between the two methyl groups of 0.6 kcal/mol. The... 

Several explanations have been given for the physical origin of the ethane rotational barrier. One analysis attributes the barrier to the Pauli-exclusion-prindple repulsion (Section 10.3) between the eclipsing localized C—H bonding electron pairs in eclipsed ethane [R. M. Pitzer, Acc. Chem. Res., 16,207 (1983)], but several workers have rejected this explanation and offered other explanations [R. F. W. Bader et al.,... 

Figure 24-7 Two possible conformations of ethane. Rotation ofoneCHj group about the C — C single bond, as shown by the curved arrows, converts one conformation to the other.

A good illustration is provided by the simple hydrocarbon ethane. Rotation about the C-C bond results in two distinct conformations, one in which all the hydrogens on one carbon line up directly behind those of the second carbon, and one where the hydrogens on one carbon fall direcdy between two hydrogens on the second carbon. The first of these... 

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