Contributions of Internal Rotation to the Thermodynamic Properties

Some molecules undergo an internal motion in which one part of the molecule rotates about a bond connecting it with the rest of the molecule. Some examples are 

In ethane, for example, the two halves of the molecule rotate around the bond so that the molecule passes through eclipsed and staggered conformations and all the intermediate orientations. In methanol, the hydroxyl group swings around the methyl group. 

As the parts of the molecule pass by one another, the potential energy associated with their interaction changes. Repulsive interactions that occur if parts of the molecule get too close will cause the potential energy to increase, while attractive interactions will reduce the potential energy. 

We are interested in understanding how this relative motion of two parts of the molecule contributes to its thermodynamic properties. It turns out that the thermodynamic treatment of this internal rotation depends upon the relative 

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