Internal rotation in small and larger molecules

The forces which determine the shape of the rotational isomeric potential are predominantly non-bonding interactions, similar to those described above, but now between atoms attached to neighbouring bonds. 

These interactions are very short range and fall off as a high power of the distance. The interactions for the eclipsed and staggered structures of ethane 

The arrows indicate the non-bonding interactions presented in a saw-horse representation. Only the strongest interactions have been shown however, there are longer, and hence weaker, interactions between all the other hydrogen atoms, as well as the strong bonding interactions which retain the tetrahedral symmetry of the methyl groups. 

t state is the lowest energy state and is taken as the reference for aU other states. The conformations created by rotation about either (p or (p are t, =g, t =g, t = t, +.The intramolecular forces defining these conformations are the same, making these conformations energetically degenerate. Rotation about both 2 and cp produces the conformations which are also energeti- 

Extending the chain length makes it difficult to visualise the conformational potential energy space as the dimensionaUty should increase with the number of bonds in the molecule. However, if it is assumed that the important intramolecular 

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