With reference to J. Am. Chem. Soc. 1968, 90 (19), 5266–5267, they state:
The most favorable geometry of aliphatic free radicals remains an unsolved problem, since existing data are consistent with a geometry which is either pyramidal and rapidly inverting, or planar but easily deformed.2-4 One approach to the problem is generation of the radical at a bridgehead of a bridged bicyclic system. [...] From their aldehyde decarbonylation study, however, Applequist and Kaplan concluded that 1 -adamantyl and 1 -bicyclo[2.2.2]octyl were appreciably more stable than t-butyl. 3
3. J. Am. Chem. Soc. 1965, 87 (10), 2194–2200
which is later concluded as:
[...] The extent of geometrical destabilization is considerable, and we conclude that alkyl radicals tend toward planar geometry. The lesser extent of destabilization, relative to that for the carbonium ions, might mean that radicals seek a flattened pyramidal state. [...] This issue can evidently not be settled without spectroscopic information about alkyl radicals.
Has there been any further studies on the geometry of free radicals at the bridgehead carbon?