Density Functional Theory Analysis of Molybdenum Isotope Fractionation^†

Analytical studies have found an enrichment of the lighter Mo isotopes in oxic marine sediments compared to seawater, with isotope fractionation factors of −1.7 to −2.0 ‰ for Δ^97/95Mo_{sediment-seawater}. These data place constraints on the possible identities of dissolved and adsorbed species because the equilibrium isotope fractionation depends on the energy differences between the isotopomers of the adsorbed species, minor dissolved species, and the dominant solution species, MoO₄^2-. Adsorption likely involves molybdic acid, whose structure is indicated by previous studies to be MoO₃(H₂O)₃. Here we used DFT calculations of vibrational frequencies to determine the isotope fractionation factors versus MoO₄^2-. The results indicate that isotope equilibration of MoO₄^2- with MoO₃(H₂O)₃, yielding Δ^97/95Mo_{molybdic acid-molybdate} = −1.33 ‰, is most likely responsible for the isotope fractionation of Mo between oxic sediments and seawater. The difference between the calculated value of Δ^97/95Mo_{molybdic acid-molybdate} for MoO₃(H₂O)₃ and the value observed in natural sediments and experiments is probably due to effects of solvation and adsorption onto the manganese oxyhydroxide surface.