The C₈H₈ Radical Cations of Cyclooctatetraene, Semibullvalene, and Their Common Bisallylic Rearrangement Product: Electronic Structure and Potential Energy Surfaces

The recently discovered access paths to the radical cation of bicyclo[3.3.0]octa-2,6-diene-4,8-diyl (BOD^·+) are explored by electronic absorption (EA) spectroscopy whereby previous ESR results are confirmed. The electronic and molecular structure of BOD^·+ and of its photoprecursor, the radical cation of cyclooctatetraene (COT^·+), are discussed on the basis of their EA spectra and ab initio calculations. The ground and excited state potential surfaces common to the title cations are explored, and it is shown that the COP^·+ → BOD^·+ photorearrangement proceeds mainly by virtue of a pronounced Jahn-Teller distortion of the second excited state of COT^·+ (²E). This distortion competes effectively with internal conversion to the first excited state, leads to an inversion of the ground state symmetry, and covers a substantial part of the reaction path leading to the bisallylic cation.