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

Abstract

Abstract To investigate whether polyoxometallate α-PW 12 O 40 3− Keggin anions (noted PW 3− ) repulse each other in water, we calculated the changes in free energy ΔG( 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 H 3 O + , UO 2 2+ or Eu 3+ counterions. The results obtained with different methodological variants (water models, PW 3− charge models, sampling procedures) support our previous finding that PW 3− 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 PW 3− to S 3− spherical analogues and to PW 3+ cations (with all atomic charges of PW 3− 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 A, corresponding to an ion separated pair PW 3− …Eu(MeOH) 9 3+ …PW 3− . These findings are important for understanding processes like condensation and assembling of POMs and macro-ions in water or at aqueous interfaces.