The bisketene radical cation and its formation by oxidative ring-opening of cyclobutenedione.

Parent cyclobutenedione 1 was photolyzed and ionized in an Ar matrix at 10K. The bisketene 2 that results in both cases (in the form of its radical cation after ionization) was characterized by its IR spectrum and by high-level quantum chemical calculations. Experiment and theory show that the neutral bisketene has only a single conformation where the two ketene moieties are nearly perpendicular, whereas the radical cation is present in two stable planar conformations. The mechanism of the ring-opening reaction, both in the neutral and in the radical cation, is discussed on the basis of calculations. In the latter case it is a nonsynchronous process that involves an avoided crossing of states.