1,n-Radical ions: an abinitio study of racemization and isomerization of the cyclopropane radical cation

Abstract

The surfaces for cis–trans isomerization and racemization of the cyclopropane radical cation (1+) are determined using abinitio calculations. It is found that polarization functions and electron correlation have large effects on the relative energies of 1+. Orbital symmetry arguments predict that conrotatory closure of the 0,0 conformer of 1+ should be preferred. These predictions are supported by the calculations; however, even the conrotatory closure has a significant activation barrier. Conformational changes that accompany single methylene rotation are also discussed. The implication of these results to ion cyclotron resonance and mass spectral experiments is that isomerization of 1+ to the 0,0 conformer can occur at energies far below the barrier required to isomerize 1+ to the propylene radical cation. However, the 0,0 conformer of 1+ and the propylene radical cation should display similar reactivities (e.g. deprotonation). The results also indicate that, contrary to reported esr evidence, 1+ should not exist preferentially in the 90,0 conformation.