Computational studies on the ring openings of cyclopropyl radical and cyclopropyl cation

Three computational methods – B3LYP, CASPT2//CASSCF, and CCSD(T) – and three basis sets – 6-311G(2d), cc-pVTZ, and cc-pVQZ – are compared for the ring opening of cyclopropyl radical to allyl radical. The same comparison is made for the ring opening of cyclopropyl cation, except that MP2 is used instead of CASPT2//CASSCF. Good agreement is found among the calculations, and with most but not all experimental data. The ring opening of the radical is found to be topologically disrotatory and nominally forbidden, with a barrier of 90 kJ/mol. However, cyclopropyl cation is found to be a transition state with consequently no barrier to ring opening.