Computer Science
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005gecoa..69.2497t&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 69, Issue 10, p. 2497-2503.
Computer Science
Scientific paper
Erickson and Helz (2000) established that molybdate, MoO4-2, is quickly and completely transformed to thiomolybdate, MoS4-2, by reaction with sulfide. They monitored the equilibria and kinetics of this process by measuring the UV-visible spectra of solutions containing the different oxythiomolybdate species. There is interest in the analogous reactions for the ReO4- species but little experimental data. We have therefore calculated quantum-chemically the equilibrium constants in solution for the sulfidation reactions of both MoO4-2 and ReO4-, as well as recalculating their UV-visible charge-transfer spectra. Calculations using configuration interaction singles and time-dependent hybrid Hartree-Fock density functional methods give good absolute values and trends in UV-vis energies for both series. For MoO4-2 the calculated equilibrium constants for the various sulfidation steps match reasonably well against the experimental values (within two log K units). For the ReO4- sulfidation reaction the first two steps are considerably less favorable than for MoO4-2, suggesting that the “geochemical switch” of Erickson and Helz, a rapid transformation of oxyanion to thioanion highly dependent on sulfide concentration, will be less effective in the Re case. However, both our calculations and experiment indicate that ReO4- and ReS4- are both easier to reduce than their Mo analogs, so that reduction of these Re(VII) species will be the preferred mechanism for their removal from seawater. A previous suggestion that the neutral species MoO4H2 is actually the hydrated octahedral Mo(OH)6 species is found to be incorrect, but the MoO3(OH2)3 species, a highly distorted six-coordinate complex, is almost competitive in energy with MoO4H2 plus two H2O.
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