What Causes Carbon-Centered Radicals to Pyramidalize? It Depends on the Type of Radical

Carbon-centered
radicals, with a single electron on the carbon
atom, could potentially adopt a planar geometry similar to that of
alkyl carbocations, a tetrahedral geometry similar to that of alkyl
carbanions, or a geometry somewhere in between. Simple alkyl radicals
are known to adopt a slightly pyramidalized geometry due to a combination
of torsional and hyperconjugative interactions. Which of these interactions
is primarily responsible for determining the final adopted geometries
has, up until now, remained unknown. Harnessing the power of natural
bond orbital calculations to perform geometry optimizations with specific
donor–acceptor interactions deleted, it is now demonstrated
that hyperconjugative interactions primarily dictate radical geometries.
For simple alkyl radicals, negative hyperconjugative interactions
are most important, while for bridgehead radicals, positive hyperconjugative
interactions are most important.