Solvation of the Ra2+ ion in ammonia and water. A hybrid density functional theory Born–Oppenheimer molecular dynamics study

Abstract

We present a comprehensive theoretical investigation of Ra(II) solvation in water and ammonia micro-solvation environments using hybrid density functional theory Born–Oppenheimer molecular dynamics simulations. While other alkaline earth dications (Mg2+–Ba2+) have been extensively studied, Ra2+ remains poorly characterized due to experimental challenges. Our simulations reveal that Ra2+ exhibits exceptional aqueous solvation dynamics, with a broad first hydration shell (2.7–3.8 Å) showing large temporal coordination number (CN) fluctuations between 9 and 12 and short-lived coordination states. The calculated average Ra–O distance (2.92 Å) and CN (10.9) are in good agreement with EXAFS experimental data. In the ammonia environment, Ra2+ displays a similar but better-defined solvation structure, with a dominant tenfold coordination, longer coordination lifetimes, and rapid NH3 exchange. Comparative analysis across group IIA cations shows systematic trends: increasing cation size (M2+) correlates with longer M2+–L distances and more extended solvation shells, while structural flexibility at 300 K increases dramatically from Mg2+ to Ra2+, with Ba2+ and Ra2+ showing the closest structural analogy, consistent with their similar crystal ionic radii. This work provides fundamental reference data for radium solution chemistry and presents possible implications of the observed solvation differences between water and ammonia for radiochemical separation strategies, as well as the ability of Ra2+ to mimic the coordination properties of Ca2+ in biochemical environments.