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 reveal
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.+.