Quantum Chemical Studies of the Hydration of Sr²⁺ in Vacuum and Aqueous Solution

Abstract

The geometric structures of gas-phase Sr(2+) hydrates are calculated quantum chemically by using hybrid (B3LYP) and meta-GGA (TPSS) density functional theory, and a range of thermodynamic data (including sequential bond enthalpies, entropies and free energies for the reactions Sr(2+)(H(2)O)(n-1)+H(2)O→Sr(2+)(H(2)O)(n)) are shown to be in excellent agreement with experiment. When the number of coordinating water molecules exceeds six, such that water begins to occupy the second solvation shell, it is found that detailed analysis based on both geometrical and conformational entropy is required in order to confidently identify the experimentally observed structures. The significant increase in coordination number observed experimentally between the gas- and aqueous-phase species is successfully reproduced, as is the first solvation shell geometry. Inaccurate second shell geometries imply that larger model systems may be required to achieve agreement with experiment. Candidate species for on-going computational studies of the interaction of hydrated Sr(2+) with brucite surfaces have been identified.

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

“…These results are in a good agreement with the previously published theoretical data in which the preferred coordination was seven 13,21 and are at the lower end of the experimental range. We found 6-and 7-coordinated aqua complexes to be energetically very similar (see ESI).…”

“…13 The present calculations were therefore carried out with version 6.5 of the Turbomole code 34 using resolution-of-the-identity density functional theory (RI-DFT). 13 The present calculations were therefore carried out with version 6.5 of the Turbomole code 34 using resolution-of-the-identity density functional theory (RI-DFT).…”

Ab initio molecular dynamics studies of hydroxide coordination of alkaline earth metals and uranyl

“…37 A ND study by Neilson et al 72 of Sr(ClO 4 ) 2 reported a much higher CN of 15 with Sr-O first shell distance of 2.65 Å, however this data was later re-examined in conjunction with an Anomalous X-ray Diffraction (AXD) study to find a lower CN of 9. 71 Various computational methods have been used to evaluate the first shell solvation structure of Sr 2+ including DFT, 73 Quantum Mechanical/Molecular Mechanics (QMMM), 74 QMSTAT 58 as well as MD and AIMD. 41,64,75 A Sr CN of around 8 40,56,64,[76][77][78][79] is typically identified within a range of CNs between 6.7 41 and 9.8 47 and Sr-O distances of 2.58 40 to 2.69.…”

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 …”