Ab initio calculations on the isomerization of alkene radical cations. 2. Ring opening of cycloalkane radical cations

Abstract

Ab initio calculations on the ring opening of the cyclopentane and cyclohexane radical cations do not give an explanation of previous experimental findings that, upon electron ionization, the cyclopentane ion ring opens whereas the cyclohexane ion is stable. The main difference between the two cases is that the cyclohexane ion first has to isomerize to the methylcyclopentane structure before ring opening. The barrier heights for ring opening in the two cases are comparable and significantly lower than the difference between the vertical and adiabatic ionization energies. The results suggest that, for an explanation of the experiments, accurate theoretical studies of the ionization process itself will be needed.