Cyclobutadiene radical cation. An ab initio study of the Jahn-Teller surface

The Jahn-Teller potential energy surface of the cyclobutadiene radical cation is reinvestigated at the CCSD(T)/TZP//RMP2/6-311G(2d, p) level. It is found that out-of-plane deformations come into play as one moves away from the D4h geometry. The relaxed radical cation has two potential energy minima, a planar rectangle 1.7 kcal/mol below a puckered rhombus, which are interconnected by a transition state lying 1.5 kcal/mol above the latter. However, upon inclusion of zero-point energies, the transition state falls 0.1 kcal/mol below the puckered minimum and the enthalpies at 0 K of the three stationary points all lie within 0.54 kcal/mol. This indicates that the potential energy surface of this cation is strongly anharmonic. The repercussions of these findings with regard to recent experiments are discussed.