What is it about?
Structural optimization of low-lying singlet and triplet excited states in benzene, fluorobenzene, phenol, aniline, toluene, and nitrobenzene were carried out quantum chemically.The equilibrium geometries, adiabatic excitation energies, charges, dipole moments, and degree of aromaticity are discussed for the various excited states. Planarity vs. nonplanarity of the ring and the substituents are investigated in connection with delocalized (ring) or localized (substituent) excitation. The calculated results compare well with the available experimental data.
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Why is it important?
The study reveals that the excited singlet and triplet states of benzene and its derivatives have pronounced quinoidal character. At the site of substitution the ring is fairly nonplanar in the singlet, less so in the triplet state. Except in the toluene triplet, the substituent atoms suffer substantial out-of-plane deformation.
Perspectives
Aromaticity is only moderate or lost in the excited states, particularly upon substitution. An interesting feature is the repulsive character of the Triplet state of nitrobenzene. It is caused by an excitation to an orbital which is antibonding for the CN bond. It explains naturally the lack of phosphorescence in this molecule.
E.J. Padma Malar
University of Madras
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This page is a summary of: Structures and properties of excited states of benzene and some monosubstituted benzenes, The Journal of Physical Chemistry, August 1984, American Chemical Society (ACS),
DOI: 10.1021/j150660a027.
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