Theoretical Study on the Hydration Structure of Divalent Radium Ion Using Fragment Molecular Orbital–Molecular Dynamics (FMO–MD) Simulation

Using fragment molecular orbital–molecular dynamics (FMO–MD) simulation at the FMO3-HF/6-31G(d,p) level, the hydration of a Ra2+ ion was theoretically investigated. The first peaks of the radial distribution function (RDF) for Ra–O and Ra–H lengths were predicted to be 2.85 and 3.45 Å with broad envelopes in the ranges of 2.5–3.5 and 2.8–4.3 Å, respectively. The broad peaks shows that the first hydration shell of Ra2+ is much more flexible than those in the other hydrated divalent alkaline earth metal ions, i.e., Ra2+ is a structure-breaking ion. The hydration number of Ra2+ was predicted to be 8.1. From the angular distribution function (ADF), it was clarified that the octa hydrated Ra2+ ion has a flexible square antiprism structure at room temperature.

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