HR: 11:05h
AN: MR12A-04    [Abstracts]
TI: Quinone-Hydroquinone complexes as components of humic acids: Theoretical studies of structure, stability and visible-UV spectra
AU: * Tossell, J A
EM: tossell@umd.edu
AF: Univ. of Maryland, Dept. of Chem. and Biochem., College Park, MD 20742, United States
AB: Quinones and hydroquinones form complexes called quinhydrones which have optical absorption energies lying below those of either of the separate components. It has been suggested that these and similar complexes are components of humic acids which strongly influence their visible-UV spectra and are responsible for the smooth, featureless exponentially rising absorption of humics in the visible and near UV. We have used the methods of molecular quantum mechanics to calculate the structure, energetics and spectra of such complexes. Since the chemical forces holding the quinone and hydroquinone together are of van der Waals and H-bonding type, it is necessarily to use a high level quantum mechanical method which accurately treat such interactions. We rely mainly upon 2nd order Moller-Plesset theory with doubly polarized triple zeta basis sets for the structural and energetic studies and time dependent density functional theory with the B3LYP potential for the spectral studies. Our calculated C-C intermolecular distance for quinhydrone is in excellent agreement with the experimental value. Our energetic calculations indicate that the free energy of formation of quinhydrone is negative in the gas-phase but positive in aqueous solution. We have established that our TD DFT methods give accurate spectra for the gas-phase molecule quinone. The complexes give lower energy absorptions, but these are quite sensitive to geometry and to the charge state of the quinhydrone complex. The transitions within the quinhydrone complex are mainly from the highest energy occupied MO to the lowest energy empty MO, as expected from the donor acceptor model. We are now carrying out studies of the effects of dynamics and structural perturbation on the spectra.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting