Do Keggin anions repulse each other in solution? The effect of solvent, counterions and ion representation investigated by free energy (PMF) simulations

作者：Chaumont Alain; Wipff Georges^*

来源：Comptes Rendus Chimie, 2012, 15(2-3): 107-117.

DOI：10.1016/j.crci.2011.07.001

• POM
• Aggregation
• Assembling
• Molecular dynamics
• Solvation
• Counterions

To investigate whether polyoxometallate alpha-PW12O403- Keggin anions (noted PW3-) repulse each other in water, we calculated the changes in free energy Delta G(d) as a function of the P ... P distance d (potential of mean force "PMF" calculations). As the anions approach each other, the free energy profiles are found to be quite flat, with a tiny minimum at ca. 11 A, showing that the anions can form "contact ion pairs" in the presence of either H3O+, UO22+ or Eu3+ counterions. The results obtained with different methodological variants (water models, PW3- charge models, sampling procedures) support our previous finding that PW3- ions can form dimers or oligomers in water (A. Chaumont and G. Wipff (2008) [13]). The importance of stabilizing bridging water molecules and solute granularity is demonstrated by comparing PW3- to S3- spherical analogues and to PW3+ cations (with all atomic charges of PW3- inverted). With these analogues, a somewhat repulsive behavior (ca +2 to 3 kcal/mol) is observed at short distances. The role of water is further demonstrated by comparing PMFs in water and in methanol solution where there is no contact ion pair, but a free energy minimum at ca. 17 angstrom, corresponding to an ion separated pair PW3- ... Eu(MeOH)(9)(3+) ... PW3-. These findings are important for understanding processes like condensation and assembling of POMs and macro-ions in water or at aqueous interfaces.

• 出版日期2012-3