Cyclopentane, conformational energy barrier

Any one of the five atoms of the ring can be either above or below the plane defined by the other four in the envelope conformation. The energy barriers separating them are very low, and in cyclopentane or in proline all of the envelope conformations are freely interconvertible through intermediate skew forms.32 Furanose sugar rings are very flexible but the presence of the bulky substituents reduces the number of possible conformations.33-363 See Chapter 5 for further discussion. 

Pseudorotation The progression of one conformer of a five-membered ring to another conformer. In the case of cyclopentane there is no planar intermediate all conformers have at least one carbon atom out of the plane of the other carbon atoms. The maximum pucker can, in this case, rotate with almost no potential energy barrier between conformers. Each of the multitude of possible conformers can be described in terms of the maximum pucker and the pseudorotation phase angle, that is, where the conformer lies on a pseudorotation cycle (with an arbitrarily chosen origin). 

Distortion from planarity to relieve torsional strain also applies to cyclopentane (Figure 3.19). Experimental evidence indicates that the molecule exhibits ten different envelope conformations in which one carbon atom at a time is above or below the plane defined by the other four, as well as ten twist or half-chair conformations in which three carbon atoms at a time define a plane, with the fourth carbon atom above this plane and the fifth below it. The molecule is continually changing its conformation, however, so that each carbon atom is 0.458 A above the molecular plane one-fifth of the time, with the barrier to planarity of 5.16 kcal/mol. In the absence of isotopic labeling, the effect of this shifting permutation is indistinguishable from a rotation of the molecule about an axis through its center. For that reason, the process is called pseudorotation. The energy barrier for pseudorotation is so small that the process is described as essentially barrierless. ... 

The symmetric puckered conformations of cyclopentane are the Cs symmetric envelope (E) (10) with four carbon atoms in a plane and the C2 s)mrmetric twist (T) (11) with three carbon atoms in a plane [88]. Unlike cyclohexane, these conformations are of almost equal energy and are separated by barriers of about RT or less [89]. There are ten envelope conformations, each with one of the five carbon atoms out of the plane in one of the two directions, and ten corresponding twist conformations. The individual conformations freely exchange which atom or atoms are out of the plane, a process termed pseudorotation, and the whole sequence of conformations is called the pseudorotational itinerary (O Fig. 9). 

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