Ab initio SCF and CI study of the cyclopropane/trimethylene radical cations

Abstract The ab initio UHF energy surface for the ring opening process leading from the cyclopropane cation to the trimethylene radical cation has been calculated. The trimethylene radical cation is found to correspond to a saddle point on the energy hypersurface, having C s geometry with one of the methylene groups twisted 90° and a π electronic state. Further optimization of this species at the MP2/6-31G* level converges to a geometry of C 2v symmetry. Hyperfine coupling constants for the cyclopropane and trimethylene radical cations at their UHF and MP2/6-31G* optimized geometries have been calculated using accurate CI wavefunctions, and the calculated data are discussed together with published ESR data. For the cyclopropane cation, no evidence for a ground state of symmetry lower than C 2v has been found.