Free energies and mechanisms of water exchange around Uranyl from first principles molecular dynamics

摘要

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 (UO_2~(2+)) using the primary hydration number of U as the reaction coordinate. The free energy and activation barrier of the water dissociation reaction [UO_2(OH_2)_5]~(2+)(aq) → [UO_2(OH_2)_4]~(2+)(aq) + H_2O 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 [UO_2(OH_2)_5]~(2+)(aq) + H_2O → [UO_2(OH_2)_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 [UO_2(OH_2)_5]~(2+)(aq) and bulk water is dissociative.