Benzene Excitation Energies

The first study was made on the benzene molecule [79], The S ISi photochemistry of benzene involves a conical intersection, as the fluorescence vanishes if the molecule is excited with an excess of 3000 crn of energy over the excitation energy, indicating that a pathway is opened with efficient nonradiative decay to the ground state. After irradiation, most of the molecules return to benzene. A low yield of benzvalene, which can lead further to fulvene, is, however, also obtained. 

Pariser and Pair adjusted the necessary parameters to the empirical singlet and tr iplet excitation energies in benzene to obtain... 

Kolos, W., J. Chem. Phys. 27, 592, Excitation energies of C6H6. The above method applied to benzene. 

An obvious remedy to this situation is to use potentials that by construction exhibit the correct asymptotic behavior. Indeed, using the LB94 or the HCTH(AC) potentials yields significantly improved Rydberg excitation energies. As an instructive example, we quote the detailed study by Handy and Tozer, 1999, on the benzene molecule. These authors computed a number of singlet and triplet n->n valence and n —> n = 3 Rydberg excitations... 

Handy, N. C., Tozer, D. J., 1999, Excitation Energies of Benzene from Kohn-Sham Theory , J. Comput. Chem., 20, 106. 

The first chemiluminescent paracyclophanes have been described recently 208> the compounds 138 and 139 both contain a phthalhydrazide group as that part of the molecule producing the excitation energy which is transferred to the substituted benzene resp. anthracene moiety. 139 chemiluminesces with about double the amount of 2.3-anthracene dicarboxylic hydrazide on oxidation by oxygen/potassium tert. butoxide... 

In photoassociating solvents (e.g., benzene and its alkyl derivatives) the transfer of electronic excitation energy from solvent to fluorescent solute is characterized37,63 by rate constants kt which are approximately twice those computed from the less approximate form of Eq. (7) ... 

A detailed comparison of several methods for local and charge-transfer excitation energies in benzenes substituted with donor and acceptor groups has been provided by Jamorski et al. 

Many examples of collisions with changes in local permutational symmetry are known. The Cundall technique31,32,190 used to measure triplet yields to photoexcited benzene involves the transfer of triplet-state excitation energy... 

The situation at wavelengths below 2000 A has improved recently. The upper electronic state of benzene excited by absorption of the 1849-A line of mercury would be the 1 lu state.47 No light emission from this state has been reported and no evidence exists relative to crossover from this state to any of the various triplet states of lower energy. 

Table 1. ji- and <7- Distortion Energies (AEn, AE,) of Analogues of Benzene, and 3jtjt Excitation Energies of a. r Bond (AEst)"/ ... 

The first direct time-resolved evidence for energy transfer from an upper excited triplet state in solution at room temperature was published in 1987 [50]. This study made use of the two-color technique to photoexcite the 7, state of benzophenone, 70, in benzene solvent. As the extensive (almost quantitative) triplet depletion was not accompanied by any product formation, it was concluded that the excitation energy was transferred to the triplet manifold of the benzene solvent. The energetics of this donor-acceptor system are certainly conductive to this process. The benzophenone 7, and Tn energies (69 kcal/mol and ca. 120 kcal/ mol, respectively—the second photon in the two-color excitation provides roughly 50 additional kcal/mol to the 71 state) bracket the benzene 7j energy (85 kcal/mol) and therefore benzene acts in the same way toward benzophenone as 1,3-cyclohexadiene acts toward anthracene, i.e., as an exclusive upper triplet energy accepter. 

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