The importance of second shell effects in the simulation of hydrated Sr²⁺hydroxide complexes

Abstract

Density functional theory at the meta-GGA level is employed to study the microsolvation of Sr(2+) hydroxides, in order to establish likely candidate species for the interaction of nuclear fission-generated strontium with corroded Magnox fuel cladding in high pH spent nuclear fuel storage ponds. A combination of the COSMO continuum solvation model and one or two shells of explicit water molecules is employed. Inclusion of only a single explicit solvation shell is unsatisfactory; open regions are present in the strontium coordination shell which would not exist in real aqueous complexes, and many optimised structures possess unavoidable energetic instabilities. Incorporation of a second shell of explicit waters, however, yields energetically minimal structures without open regions in the first strontium coordination shell. The most stable systems with one, two or three hydroxide ions are all 6-coordinated with a distorted trigonal antiprismatic geometry, whereas systems with four OH(-) ions have a most stable coordination number of five. Transformation, via a proton transfer mechanism, from one coordination mode to another (e.g. from a system with two hydroxides bound directly to the strontium to one in which a hydroxide ion migrates into the second coordination shell) is found to be energetically facile. It is concluded that the most likely strontium-hydroxide complexes to be found in high pH aqueous solutions are mono- and dihydroxides, and that these coexist.

Ab initiomolecular dynamics studies of hydroxide coordination of alkaline earth metals and uranyl

“…To fully investigate the interaction between these ions and a mineral surface it is essential to have a detailed understanding of the microsolvation of the ions in both the absence and presence of hydroxide. While the former has been well documented in literature there is severely limited research on the later with literature focusing on gas phase [13][14][15] investigations, while the dynamics of hydrated hydroxide complexes have received limited attention. [16][17][18][19] This contribution aims to develop understanding of ion interactions with hydroxide in aqueous environments.…”

“…Beyond the broad goal of characterising the nature of hydrated hydroxide complexes in bulk environments, our motivation here was to further investigate the recent finding of a low barrier to proton transfer between the first and second solvation shells of strontium dihydroxide complexes, along with the apparent similar stabilities of complexes in which hydroxide resides in the first and second solvation shells. 13 To minimise bias with respect to initial conditions, we trebled the number of trajectories calculated for the alkaline earth aquo complexes discussed in Section 3.1.1. For each ion, we considered five DFT-optimised starting structures in which either zero, one or two hydroxide species were present in the first solvation shell, giving a total of 15 starting structures.…”

“…Here we consider how the coordination of hydrated hydroxide complexes of alkaline earth metals compares to those of aquo complexes. We have previously investigated this for strontium systems containing a partially and fully occupied second solvation shell, 13,14 finding a decrease in total coordination number as hydroxide coordination increases, but these simulations did not allow for migration of the hydroxide species into the bulk.…”

Development of Nonbonded Models for Metal Cations Using the Electronic Continuum Correction

“…However, the exact number of water molecules coordinating Ca 2+ is not known in the literature, and theoretical simulations suggest a CN number in the range of 6–8 . Changes in the CN are detected also for Sr 2+ , for which an average CN of 7.3 has been calculated: although not in agreement with the CN of 8 suggested by Marcus, this value is within the range of 6–8 defined by several computational results provided by the literature …”

“…[54,55] Changes in the CN are detected also for Sr 2+ , for which an average CN of 7.3 has been calculated: although not in agreement with the CN of 8 suggested by Marcus, this value is within the range of 6-8 defined by several computational results provided by the literature. [56][57][58][59] Highly charged ions. The difficulty in the modeling of highly charged metal ions (both tri-and tetravalents) by means of the standard 12-6 Lennard-Jones model has been already experienced.…”