Relative strain energies of cycloalkanes

SECTION 3.4. CARBOCYCLIC RINGS OTHER THAN SIX-MEMBERED 

The energy difference between puckered and planar conformations in cyclobutanes appears to be small. The geometries of many-substituted cyclobutanes in the crystal have been determined to be planar by X-ray diffraction, trans-1,3-Cyclobutanedicarboxylic acid, when crystallized by itself, has a planar four-membered ring, but, when cocrystallized with its disodium salt, has a puckered four-membered ring.  

Cyclopentane is nonplanar, with the two most easily described geometries being the envelope (C symmetry) and the half-chair (C2 symmetry)  

In the envelope conformation, one carbon atom is displaced from the plane of the 

As ring size increases, there are progressively more conformations that need to be considered. For cycloheptane, four conformations have been calculated to be particularly stable. Of these four, the twist-chair is indicated to be the most stable by NMR investigation of dimethyl derivatives, in accordance with the calculations.  

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