The Dewar Benzene Radical Cation and Its Ring-Opening Reaction

The radical cation of Dewar benzene, 1•+, has been generated and observed by optical
spectroscopy in cryogenic matrices. 1•+ distinguishes itself by a charge resonance band at 600 nm, very
similar in shape and position to that observed for the related radical cation of norbornadiene. This coincidence
indicates that in ground-state 1•+ the odd electron is also located in a π-MO. The energy of the charge
resonance transition, which is very sensitive to the dihedral angle between the four-membered rings in 1•+,
is predicted consistently too low by TD-DFT and CASPT2. Probably this angle is too large in the B3LYP
and CASSCF geometries. As 1•+ can be observed at 77 K, it must be separated by a barrier of at least
7−8 kcal/mol from its very exothermic decay to the radical cation of benzene, 2•+. An analysis shows that
the ring-opening of 1•+ is a multistep proccess involving two avoided crossings between potential surfaces
of different symmetry and electronic nature. Owing to the orbital symmetry-forbidden nature of the process,
the energy of 1•+ starts by increasing steeply on stretching the central C−C bond, but then the system
undergoes a crossing to a 2A1 surface which leads adiabatically to an excited state of 2•+. Therefore, another
avoided crossing must be transited before the molecule can decay on the ground-state surface of 2•+. The
rearrangement of 1•+ to 2•+ is an example of a “pseudodiabatic” thermal reaction that transits between
potential surfaces representing very different electronic structures.