The radical cation of syn-tricyclooctadiene and its rearrangement products

The syn dimer of cyclobutadiene ne (tricyclo[4.2.0.0(2,5)]octa-3,7-diene, T OD) is subjected to ionization under different conditions and the resulting species are probed by optical and ESR spectroscopy. By means of quantum ch emical modelling of the potential energy surfaces and the optical spectra, it is possible to assign the different products that arise spontaneously af ter ionization or after subsequent warming or illumination of the samples, Based on these Findings, we propose a mechanistic scheme which involves a p artitioning of the incipient radical cation of TOD between two electronic s tates. These two states engage in (near) activationless decay to the more s table valence isomers, cyclooctatetraene (COT.+) and a bis-cyclobutenylium radical cation BCB.+. The latter product undergoes further rearrangement, f irst to tetracyclo[4.2.0.0(2,4).0(3,5)]oct-7-ene (TCO.+) and eventually to bicyclo[4.2.0]octa-2,4,7-triene (BOT.+) which can also be generated photoch emically from BCB.+ or TCO.+. The surprising departure of syn-TOD.+ from th e least-motion reaction path leading to BOT.+ can be traced to strong vibro nic interactions (second-order bhn-Teller effects) which prevail in both po ssible ground states of syn-TOD.+. Such effects seem to be more important i n determining the intramolecular reactivity of radical cations than orbital or stale symmetry rules.