Rotational barriers origin

Historically, rotational barriers have played an important role in structural chemistry because the relationship between electronic and molecular structures is basic to barrier height and shape. The quantum mechanical nature of hindered rotation in molecules was recognized in the early years of quantum theory by Nielson, whose work provided the stepping stone for K. S. Pitzer s famous 1936-7 papers on internal rotation in ethane. E. B. Wilson, early on, realized that elucidation of rotational barrier origins would lead to an in-depth understanding of the electronic-molecular structure relationship. He and his students published many papers on this topic, ... 

Wiberg, K.B. (2000). Origin of the amide rotational barrier. In The Amide Linkage. Structural Significance in Chemistry, Biochemistry and Materials Science, Greenberg, A., Breneman, C.M. and Liebman, J.F. (eds), p. 33. John Wiley Sons, Inc., New York... 

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. 

Peter R. Schreiner Teaching the Right Reasons Lessons from the Mistaken Origin of the Rotational Barrier in Ethane, Angew. Chem. 114(19), 3729-3731 (2002), Angew. Chem. Int. Ed. 41(19), 3579-3581 (2002)... 

Figure 12. Relevant ONNO components of primary Lewis-type VB structures needed for a VB rationalization of the origin of the rotation barrier for the D2h isomer of N2O4.

The staggered conformation observed for / is interesting when compared with RuCl2(CO)(C2H4)(PMe2Ph)2 [mutually cw-chlorines, mutually fra/ts-phosphines] in which the ethylene eclipses the phosphines. The origin of the ethylene rotational barrier in complexes of this type has been discussed by Hoffmann and others and has been shown to be due primarily to repulsions between the filled ethylene n orbital and the filled metal d, orbital. Obviously this type of interaction is much less important for V -S02. 

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