Radicals - Introduction
The Structure of Carbon-Centered Radicals
Which three-dimensional structure is displayed by carbon-centered radicals? Are these rigid pyramids? Do they form flexible pyramids whose configuration may easily be inverted or do they display a planar structure?
Reactions with enantiomerically pure starting products in which the asymmetric carbon is intermediately converted into a carbon-centered radical usually yield racemates.
Therefore, carbon-centered radicals do not possess a rigid pyramidal structure, since, in that case, an enantiomerically pure product would be formed.
In a flexible pyramid, the energy barrier of configurational inversion must be rather low in order for a complete racemization to occur. Otherwise, the respective result would not be a complete racemization, but an enantiomeric excess instead.
Accordingly, it is assumed that the carbon-centered radicals will gather to form a planar-like structure. However, ab initio molecular orbital calculations conclude that the carbon-centered radicals will instead form a relatively flat, pyramidal structure with rapid configurational inversions.