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.