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Theoretical study of the excited singlet and triplet states of alloxan
Author(s) -
Kakkar Rita,
Sarma Bhupendra K.
Publication year - 2001
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.1190
Subject(s) - tautomer , excited state , chemistry , singlet fission , excitation , singlet state , molecule , ground state , atomic physics , computational chemistry , stereochemistry , physics , quantum mechanics , organic chemistry
The possibility of excited‐state protomeric shifts in the biologically important molecule, alloxan, is investigated. We have focused on the S 1 and T 1 excited states of alloxan and its hydroxy tautomers. Modifications brought in by excitation on the relative stabilities, activation barriers, and optimized geometries, computed at the MNDO, AM1, and PM3 levels of approximation, have been discussed for both excited electronic states. The absorption and fluorescence spectra for the three tautomers are also discussed. Results show significant changes in the geometries on excitation, although the changes are similar for the singlet and triplet excited states. Though the relative stability orders do not change, the 2‐hydroxy tautomer is stabilized, while the 4‐hydroxy tautomer gets destabilized on excitation. The excited states are ( n ,π*) states, involving the promotion of a nonbonding oxygen lone pair from the COCOCO moiety, which explains why the oxygens of this group become less basic and the 4‐hydroxy tautomer gets destabilized on excitation. However, the activation barriers do not reduce significantly on excitation, and this precludes the possibility of ground‐ or excited‐state proton transfer in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001