The C4H8.bul.+ potential energy surface. 1. The cyclobutane radical cation

The potential energy surface of the cyclobutane radical cation (CB .+ ) has been explored at the QCISD-(T)/6-31G * //UMP2/6-31G * level of theory. Thereby it was found that the first-order Jahn-Teller rhombic and rectangular structures are more stable than the long-bond trapezium structure reported to be the global minimum on the CB .+ surface in previous semiempirical and ab initio SCF calculations. In agreement with ESR experiments, a rhombic structure very flexible to ring puckering was found to be the most stable one