Quasi-Chemical Theory with Cluster Sampling from Ab Initio Molecular Dynamics: Fluoride (F-) Anion Hydration.

Accurate predictions of the hydration free energy for anions typically has been more challenging than that for cations. Hydrogen bond donation to the anion in hydrated clusters such as F(H2O) n - can lead to delicate structures. Consequently, the energy landscape contains many local minima, even for small clusters, and these minima present a challenge for computational optimization. Utilization of cluster experimental results for the free energies of gas-phase clusters shows that even though anharmonic effects are interesting they need not be of troublesome magnitudes for careful applications of quasi-chemical theory to ion hydration. Energy-optimized cluster structures for anions can leave the central ion highly exposed, and application of implicit solvation models to these structures can incur more serious errors than those for metal cations. Utilizing cluster structures sampled from ab initio molecular dynamics simulations substantially fixes those issues.

Comparison of single-ion molecular dynamics in common solvents.

Laying a basis for molecularly specific theory for the mobilities of ions in solutions of practical interest, we report a broad survey of velocity autocorrelation functions (VACFs) of Li+ and PF6- ions in water, ethylene carbonate, propylene carbonate, and acetonitrile solutions. We extract the memory function, γ(t), which characterizes the random forces governing the mobilities of ions. We provide comparisons controlling for the effects of electrolyte concentration and ion-pairing, van der Waals attractive interactions, and solvent molecular characteristics. For the heavier ion (PF6-), velocity relaxations are all similar: negative tail relaxations for the VACF and a clear second relaxation for γt, observed previously also for other molecular ions and with n-pentanol as the solvent. For the light Li+ ion, short time-scale oscillatory behavior masks simple, longer time-scale relaxation of γt. But the corresponding analysis of the solventberg Li+H2O4 does conform to the standard picture set by all the PF6- results.

Ion selectivity from local configurations of ligands in solutions and ion channels.

Probabilities of numbers of ligands proximal to an ion lead to simple, general formulae for the free energy of ion selectivity between different media. That free energy does not depend on the definition of an inner shell for ligand-counting, but other quantities of mechanistic interest do. If analysis is restricted to a specific coordination number, then two distinct probabilities are required to obtain the free energy in addition. The normalizations of those distributions produce partition function formulae for the free energy. Quasi-chemical theory introduces concepts of chemical equilibrium, then seeks the probability that is simplest to estimate, that of the most probable coordination number. Quasi-chemical theory establishes the utility of distributions of ligand-number, and sharpens our understanding of quasi-chemical calculations based on electronic structure methods. This development identifies contributions with clear physical interpretations, and shows that evaluation of those contributions can establish a mechanistic understanding of the selectivity in ion channels.

Neutron scattering studies of the hydration structure of Li+.

New results derived from the experimental method of neutron diffraction and isotopic substitution (NDIS) are presented for the hydration structure of the lithium cation (Li(+)) in aqueous solutions of lithium chloride in heavy water (D2O) at concentrations of 6, 3, and 1 m and at 1.5 m lithium sulfate. By introducing new and more-accurate data reduction procedures than in our earlier studies (I. Howell and G. W. Neilson, J. Phys: Condens. Matter, 1996, 8, 4455-4463), we find, in the first hydration shell of Li(+), ∼4.3(2) water molecules at 6 m, 4.9(3) at 3 m, 4.8(3) at 1 m in the LiCl solutions, and 5.0(3) water molecules in the case of Li2SO4 solution. The general form of the first hydration shell is similar in all four solutions, with the correlations for Li-O and Li-D sited at 1.96 (0.02) Å and 2.58 (0.02) Å, respectively. The results resemble those presented in 1996, in terms of ion-water distances and local coordination, but the hydration number is significantly lower for the case at 1 m than the 6.5 (1.0) given at that time. Thus, experimental and theoretical results now agree that lithium is hydrated by a small number of water molecules (4-5) in the nearest coordination shell.