Hindered Rotation III. The Qualitative Nature of the Interactions HIndering Internal Rotation

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Abstract
Experimental evidence shows that an important contribution to the potential barrier hindering internal rotation, is an interaction stabilizing ``opposed'' configurations. A qualitative picture of this interaction which is considered as arising in the bond connecting the two halves of a molecule, is presented, based on the symmetry properties of the molecule and the ideas associated with overlapping of wave functions. This interaction seems to be of chief importance in hydrocarbons where dipole and steric effects are small, such as ethane, propylene, toluene, cyclohexane, dimethylacetylene. Even in compounds where the dipole effect is great, such as tetrachloroethane and dichloroethylene, this interaction plays the most important role.