Ethane hyperconjugation

Still another way to picture the conformational preferences is to visualize the C=C double bond in terms of two equivalent banana bonds (Fig. 3.50). In this picture the preferred conformations are those with C—F in staggered orientation with respect to the three bonds (two banana bonds and one C—H bond) of the vinyl moiety, analogous to the preferred conformations of ethane. However, in using this ethane-like mnemonic one should recall that its essential origin lies in the hyperconjugative interactions of E(SL> rather than the steric and electrostatic interactions of (L). 

Figure 3.55 Leading cjch-och hyperconjugative donor-acceptor interactions in the staggered (left) and eclipsed (right) conformers of ethane.

Numerous small geometrical and energetic differences contribute to the cis-trans net energy difference. However, it is evident that ordinary hyperconjugative donor-acceptor interactions (akin to those in ethane-like molecules) can qualitatively account for the surprising stability of the cis configuration, without invocation of steric attraction or other ad-hoc mechanisms. 

In comparison with previous plots of this section, the no-crco anomeric interaction of Fig. 3.65 can be seen to be a rather typical example of hyperconjugative donor-acceptor interactions. Consequently, there seems to be no valid reason to invoke a special effect for the conformational preferences of sugars, obscuring their essential conformity with a unified donor-acceptor picture of ethane-like rotation barriers. 

The strikingly different characteristics of transition-metal hyperconjugative interactions are particularly apparent in their influence on internal rotation barriers. To illustrate, let us first consider ethane-like Os2H6, whose optimized staggered and eclipsed conformations (displaying conspicuous deviations from those of ethane) are shown in Fig. 4.81. 

As described in Section 3.4.2, hyperconjugative donor-acceptor stabilizations favor conformers in which one of the rotor C—H bonds eclipses an adjacent double bond. (This is equivalent to an ethane-like staggered preference if the double bond is pictured in terms of two bent banana bonds. ) Hence, in the case of a perfectly localized Lewis structure I, the methyl group would be expected to adopt the preferred pseudo-cA conformation la (with in-plane C—H syn to A=C),... 

This structural preference Is usually attributed to steric effects. Here, we report... that ethane s staggered conformation is the result of... hyperconjugation. (From Pophrlstlc and Goodman, 2001)... 

Textbooks... pin It on so-called steric effects... but... Pophristic looked at the other known influence on ethane s twisting—a quantum mechanical effect known as hyperconjugation. The electrons of one methyl group jump over to the other methyl group, says Goodman. 

Pophristic, V. Goodman, L. Hyperconjugation Not Steric Repulsion Leads to the Staggered Structure of Ethane. Nature 2001, 411, 565-568. 

Fig. 7.4 Molecular oribital interaction in ethane, and their interaction diagram emphasising the stabill sation by C—H hyperconjugation.

In the ethane case, however, the AIM analysis helps in understanding the overlap of the bonds and the location of the electrons as derived from the density picture, but it does not tell us anything about the origin of the rotational barrier. For that, we need methods that quantitatively give us energies that can be associated with the effects of donor-acceptor bonding (hyperconjugation) and electron-electron repulsion (Pauli repulsion) as noted above. 

The results of a valence bond treatment of the rotational barrier in ethane lie between the extremes of the NBO and EDA analyses and seem to reconcile this dispute by suggesting that both Pauli repulsion and hyperconjugation are important. This is probably closest to the truth (remember that Pauli repulsion dominates in the higher alkanes) but the VB approach is still imperfect and also is mostly a very powerful expert method [43]. VB methods construct the total wave function from linear combinations of covalent resonance and an array of ionic structures as the covalent structure is typically much lower in energy, the ionic contributions are included by using highly delocalised (and polarisable) so-called Coulson-Fischer orbitals. Needless to say, this is not error free and the brief description of this rather old but valuable approach indicates the expert nature of this type of analysis. 

Hyperconjugation between sp3 hybridized atoms can have important implications for the ground-state conformation of organic compounds. It has, for example, been suggested that the energy difference between the staggered and the eclipsed conformations of ethane is due to both hyperconjugation and repulsion [2-5]. The fact that... 

V. Pophristic, L. Goodman, Hyperconjugation not steric repulsion leads to the staggered structure of ethane. Nature 411, 565-568 (2001)... 

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