Abstract
Ab initio molecular dynamics (AIMD) simulations of the Mg2+, Ca2+, Sr2+ and UO22+ ions in either a pure aqueous environment or an environment containing two hydroxide ions have been carried out at the density functional level of theory, employing the generalised gradient approximation via the PBE exchange-correlation functional. Calculated mean M-O bond lengths in the first solvation shell of the aquo systems compared very well to existing experimental and computational literature, with bond lengths well within values measured previously and coordination numbers in line with previously calculated values. When applied to systems containing additional hydroxide ions, the methodology revealed increased bond lengths in all systems. Proton transfer events (PTEs) were recorded and were found to be most prevalent in the strontium hydroxide systems, likely due to the low charge density of the ion and the consequent lack of hydroxide coordination. For all alkaline earths, intrashell PTEs which occurred outside of the first solvation shell were most prevalent. Only three PTEs were identified in the entire simulation data of the uranium dihydroxide system, indicating the clear impact of the increased charge density of the hexavalent uranium ion on the strength of metal-oxygen bonds in aqueous solution. Broadly, systems containing more charge dense ions were found to exhibit fewer PTEs than those containing ions of lower charge density.
Highlights
The coordination environment of alkaline earth metals in aqueous environments has been the subject of both experimental and theoretical investigations for decades.[1,2,3,4,5,6,7] Both magnesium and calcium are of significant biological importance[8,9] and are key components in natural groundwater,[10] whereas strontium is often associated with nuclear waste
Five 20 ps trajectories were simulated using randomly selected Density Functional Theory (DFT) optimised initial geometries generated from a 20 ps NPT_I 64 water molecule Ab initio molecular dynamics (AIMD) run, each ion was added to the centre of the box and the snapshot optimised before being used as the starting structure
In order to assess the quality of the model, initial AIMD simulations focussed on the hydration of Mg2+, Ca2+ and Sr2+
Summary
The coordination environment of alkaline earth metals in aqueous environments has been the subject of both experimental and theoretical investigations for decades.[1,2,3,4,5,6,7] Both magnesium and calcium are of significant biological importance[8,9] and are key components in natural groundwater,[10] whereas strontium is often associated with nuclear waste. Various computational methods have been used to evaluate the first shell solvation structure of Sr2+ including DFT,[73] Quantum Mechanical/Molecular Mechanics (QMMM),[74] QMSTAT58 as well as MD and AIMD.[41,64,75] A Sr CN of around 840,56,64,76–79 is typically identified within a range of CNs between 6.741 and 9.847 and Sr–O distances of 2.5840 to 2.69.58 A recent paper by D’Angelo et al.[79] combined experimental and computational techniques to investigate the coordination shell of Sr2+ using X-ray absorption near-edge spectroscopy (XANES) of [Sr(H2O)8](OH)[2], MD and CPMD. The impact on the solvation environment of alkaline earth metals due to the presence of hydroxide is investigated here for the first time using AIMD methods, with particular attention paid to identification and characterisation of proton transfer events, along with their timescales and frequency
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