“…In the radical cation of hexamethyl Dewar benzene, 1-Me 6 •+ , we find the same states, this time separated more clearly in the photoelectron spectrum 20 except that in this case Bieri et al posited that the ordering of states is inverted, i.e., 2 A 1 below 2 B 2 , at the geometry of neutral 1-Me 6 . After some claims to the effect that the 2 B 2 and the 2 A 1 states of 1-Me 6 •+ may coexist (or exist as separate entities), , several independent experiments have, however, converged on demonstrating that the ground state of 1-Me 6 •+ at its equilibrium geometry is 2 B 2 21,25-27 In fact, it has been possible to observe 1-Me 6 •+ by optical and ESR spectroscopy in low-temperature matrices, and by different forms of transient spectroscopy on the nanosecond time scale. ,, We therefore decided to investigate and see if it would be possible to characterize the parent compound, 1 •+ , by subjecting 1 to ionization in solid matrices.…”
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.
“…In the radical cation of hexamethyl Dewar benzene, 1-Me 6 •+ , we find the same states, this time separated more clearly in the photoelectron spectrum 20 except that in this case Bieri et al posited that the ordering of states is inverted, i.e., 2 A 1 below 2 B 2 , at the geometry of neutral 1-Me 6 . After some claims to the effect that the 2 B 2 and the 2 A 1 states of 1-Me 6 •+ may coexist (or exist as separate entities), , several independent experiments have, however, converged on demonstrating that the ground state of 1-Me 6 •+ at its equilibrium geometry is 2 B 2 21,25-27 In fact, it has been possible to observe 1-Me 6 •+ by optical and ESR spectroscopy in low-temperature matrices, and by different forms of transient spectroscopy on the nanosecond time scale. ,, We therefore decided to investigate and see if it would be possible to characterize the parent compound, 1 •+ , by subjecting 1 to ionization in solid matrices.…”
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.
“…A first unambiguous though indirect proof was delivered by Roth et al by means of the chemically induced dynamic nuclear polarization (CIDNP) technique. Direct spectroscopic characterization of 1 • + was finally achieved by electron spin resonance (ESR) spectroscopy, indicating the 2 B 2 state as its ground state. − However, the electronic absorption spectrum has never been measured for hexamethyl(Dewar benzene) radical cation.…”
Section: Introductionmentioning
confidence: 99%
“…Direct spectroscopic characterization of 1 • + was finally achieved by electron spin resonance (ESR) spectroscopy, indicating the 2 B 2 state as its ground state. [5][6][7][8] However, the electronic absorption spectrum has never been measured for hexamethyl(Dewar benzene) radical cation.…”
The electronic absorption spectrum of the hexamethyl(Dewar benzene) radical cation (1 • + ) is presented for the first time. Partial stabilization of 1 • + in organic glasses at cryogenic temperatures is explained in terms of matrix rigidity, which can inhibit the isomerization process. Annealing of the matrix leads to fast isomerization of 1 • + to hexamethylbenzene radical cation (2 • + ) and, at high concentrations of 1, formation of the hexamethylbenzene dimer radical cation can be observed.
“…The radical cation-mediated rearrangement of 1 to 2 has been found to occur in a chain reaction referred to as “quantum amplified isomerization” (QAI). − In polar liquid media, electron transfer to an acceptor ( A ) in an electronic-excited electron-deficient state initiates isomerization of 1 to 2 via a chain mechanism (Scheme ) leading to quantum yields greater than 100 . Since the discovery of this process in 1974, various experimental efforts have been undertaken to identify and characterize the primary radical cation of 1 . − It was originally proposed that two distinct radical cation states of similar energy exist, namely, a 2 B 2 (π) ground state and a less stable 2 A 1 (σ) state. − Unfortunately, further studies have been unable to verify a 2 A 1 state and have only provided definitive evidence of the 2 B 2 ground state. − However, computational methods indicate that in theory two minima should exist. ,, 1 Electron-Transfer-Induced Isomerization of Hexamethyl Dewar Benzene 1 to Hexamethyl Benzene 2 2 “Quantum Amplification” Chain Mechanism for Electron-Transfer-Induced Isomerization of Hexamethyl Dewar Benzene 1 to Hexamethyl Benzene 3 in the Presence of an Acceptor (A) in Polar Solvents …”
The ring-opening reactions of the radical cations of hexamethyl Dewar benzene (1) and Dewar benzene have been studied using density functional theory (DFT) and complete active-space self-consistent field (CASSCF) calculations. Compound 1 is known to undergo photoinitiated ring opening by a radical cation chain mechanism, termed "quantum amplified isomerization" (QAI), which is due to the high quantum yield. Why QAI is efficient for 1 but not other reactions is explained computationally. Two radical cation minima of 1 and transition states located near avoided crossings are identified. The state crossings are characterized by conical intersections corresponding to degeneracy between doublet surfaces. Ring opening occurs by formation of the radical cation followed by a decrease in the flap dihedral angle. A rate-limiting Cs transition state leads to a second stable radical cation with an elongated transannular C-C bond and an increased flap dihedral. This structure proceeds through a conrotatory-like pathway of Cs symmetry to give the benzene radical cation. The role of electron transfer was investigated by evaluating oxidation of various systems using adiabatic ionization energies and electron affinities calculated from neutral and cation geometries. Electron-transfer theory was applied to 1 to investigate the limiting effects of back-electron transfer as it is related to the unusual stability of the two radical cations. Expected changes in optical properties between reactants and products of Dewar benzene compounds and other systems known to undergo QAI were characterized by computing frequency-dependent indices of refraction from isotropic polarizabilities. In particular, the reaction of 1 shows greater contrast in index of refraction than that of the Dewar benzene parent system.
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