THE C4H8.+ 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. Methyl-, trans-1,2-dimethyl-, trans-1,3-dimethyl-, and all-trans-tetramethyl-CB.+ were calculated at the UMP2/6-31G*//UHF/6-31G* level. These studies revealed that the long-bond trapezoidal structure is favored by the influence of electron-releasing substituents to the extent that it represents the global minimum for trans-1,2-dimethyl-CB.+.