The Dewar Benzene Radical Cation and Its Ring-Opening Reaction

Abstract

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 pi-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 process 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.

Energetic and electron density analysis of hydrogen dissociation of protonated benzene

“…Once the geometry is optimized from 2 without any symmetry constraint, a true minimum, 2 0 , is obtained on this flat portion of the PES with an accompanying ring deformation but with a small energy change (further details are provided in the ESIw). Moreover, Bally et al 47 have reported a degenerate ground state for this radical cation but constrained to a C 2v symmetry in a study of ring opening of the Dewar benzene cation.…”

Two‐ and three‐state conical intersections in the electron capture dissociation of disulfides: The importance of multireference calculations

“…Situations where reaction rates are controlled by the crossing of two or more states are not uncommon in chemistry. Examples of this can be found in processes, which proceed via a charge transfer mechanism 22–24, with a crossing between two states localizing charge in different parts of the molecule. Another well‐known instance is spin‐forbidden reactions, where the spin of reactants and products differs.…”

Electron spin resonance

“…Owing to the orbital symmetry-forbidden nature of the process, the energy of the molecule initially rises steeply with elongation of the transannular C-C bond, but then the system undergoes a crossing to a 2 A 1 surface which leads adiabatically to an excited state. 106 The primary radical cation of cyclooctatetraene has a non-planar geometry. However, a novel version of the structure which is annelated with bicyclo[2.1.1]hexene units in the form of the hexachloroantimonate salt is found to possess a planar structure.…”