Ring-Opening Dynamics of the Cyclopropyl Radical and Cation: the Transition State Nature of the Cyclopropyl Cation

We
provide compelling experimental and theoretical evidence
for
the transition state nature of the cyclopropyl cation. Synchrotron
photoionization spectroscopy employing coincidence techniques together
with a novel simulation based on high-accuracy ab initio calculations
reveal that the cation is unstable via its allowed disrotatory ring-opening
path. The ring strains of the cation and the radical are similar,
but both ring opening paths for the radical are forbidden when the
full electronic symmetries are considered. These findings are discussed
in light of the early predictions by Longuet-Higgins alongside Woodward
and Hoffman; we also propose a simple phase space explanation for
the appearance of the cyclopropyl photoionization spectrum. The results
of this work allow the refinement of the cyclopropane C–H bond
dissociation energy, in addition to the cyclopropyl radical and cation
cyclization energies, via the Active Thermochemical Tables approach.