Rotation barrier ethane-like molecules

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

The free rotor model is not adequate for most molecules as there is in general some potential barrier to internal rotation. As an example, consider ethyl fluoride. Ethyl fluoride, just like ethane, has a predominately threefold potential barrier as shown in Figure 7-3. The potential barrier can be expressed as the following function. 

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