Free energies and mechanisms of water exchange around Uranyl from first principles molecular dynamics
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Free energies and mechanisms of water exchange around Uranyl from first principles molecular dynamics Raymond Atta-Fynn, Eric J. Bylaska, and Wibe A. de Jong Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352 ABSTRACT From density functional theory (DFT) based ab initio (Car-Parrinello) metadynamics, we compute the activation energies and mechanisms of water exchange between the first and second hydration shells of aqueous Uranyl (UO22+) using the primary hydration number of U as the reaction coordinate. The free energy and activation barrier of the water dissociation reaction [UO2(OH2)5]2+(aq) ĺ [UO2(OH2)4]2+(aq) + H2O are 0.7 kcal and 4.7 kcal/mol respectively. The free energy is in good agreement with previous theoretical (-2.7 to +1.2 kcal/mol) and experimental (0.5 to 2.2 kcal/mol) data. The associative reaction [UO2(OH2)5]2+(aq) + H2O ĺ [UO2(OH2)6]2+(aq) is short-lived with a free energy and activation barrier of +7.9 kcal/mol and +8.9 kca/mol respectively; it is therefore classified as associative-interchange. On the basis of the free energy differences and activation barriers, we predict that the dominant exchange mechanism between [UO2(OH2)5]2+(aq) and bulk water is dissociative. INTRODUCTION Exchange of a solvent molecule between the first and second coordination shells of a metal ion is the simplest measure of the ion’s reactivity in aqueous or non-aqueous solution. The replacement of a solvent molecule from the first coordination shell represents an important step in complex-formation reactions of metal cations and in many reduction/oxidation (redox) processes [1,2]. This is the case for Uranium (U) which exists in multiple oxidation states and form a wide range of species in aqueous solution and aqueous/mineral interface. A fundamental understanding the reactivity of U ions in the environment (particularly aqueous, mineral or aqueous/mineral) is crucial to the long term storage and safe disposal of U-based nuclear waste. This work is focused on the water exchange pathways of the Uranyl dication UO22+ (+6 oxidation state of U) due to its high mobility and solubility is aqueous solution. A variety of experimental probes have indicated that UO22+ is equatorially coordinated by 5 water ligands (see Ref. [3] for discussions of the various experiments). However, high energy x-ray scattering (HEXS) experiments have indicated the coexistence of [UO2(OH2)5]2+ and [UO2(OH2)4]2+ in solution [4,5]. There are three basic pathways for solvent exchange around a metal cation [2]: (i) the associative (A) process where an intermediate of increased coordination number can be detected; (ii) the dissociative (D) process where an intermediate of reduced coordination number can be detected; (iii) the interchange (I) process, where there is no kinetically detectable intermediate. Denoting the metal ion by M, the departing first shell ligand by S, the in-coming ligand by S*, and the spectator ligand(s) by Ln, we express the A, D, and I exchange mechanisms as follows:
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