Benjamin P. Hay

Molecular statics calculations of proton binding to goethite surfaces: A new approach to estimation of stability constants for multisite surface complexation models

Abstract A new approach to estimating stability constants for proton binding in multisite surface complexation models is presented. The method is based on molecular statics computation of energies for the formation of proton vacancies and interstitials in ideal periodic slabs representing the (100), (110), (010), (001), and (021) surfaces of goethite. Gas-phase energies of clusters representing the hydrolysis products of ferric iron are calculated using the same potential energy functions used for the surface. These energies are linearly related to the hydrolysis constants for ferric iron in aqueous solution. Stability constants for proton binding at goethite surfaces are estimated by assuming the same log K- ΔE relationship for goethite surface protonation reactions. These stability constants predict a pH of zero charge of 8.9, in adequate agreement with measurements on CO2-free goethite. The estimated stability constants differ significantly from previous estimations based on Pauling bond strength. We find that nearly all the surface oxide ions are reactive; nineteen of the twenty-six surface sites investigated have log Kint between 7.7 and 9.4. This implies a site density between fifteen and sixteen reactive sites/nm for crystals dominated by (110) and (021) crystal faces.

Ewald methods for polarizable surfaces with application to hydroxylation and hydrogen bonding on the (012) and (001) surfaces of α-Fe2O3

We present a clear and rigorous derivation of the Ewald-like method for calculation of the electrostatic energy of the systems infinitely periodic in two dimensions and of finite size in the third dimension (slabs). We have generalized this method originally developed by Rhee et al. (Phys. Rev. B 40 (1989) 36) to account for charge-dipole and dipole-dipole interactions and therefore made it suitable for treatment of polarizable systems. This method has the advantage over exact methods of being significantly faster and therefore appropriate for large-scale molecular dynamics simulations. However, it involves a Taylor expansion which has to be demonstrated to be of sufficient order. The method was extensively benchmarked against the exact methods by Leckner and Parry. We found it necessary to increase the order of the multipole expansion from 4 (as in the original work by Rhee et al.) to 6. In this case the method is adequate for aspect ratios (thickness/shortest side length of the unit cell) ≤ 0.5. Molecular dynamics simulations using the transferable/polarizable model by Rustad et al. were applied to study the surface relaxation of the nonhydroxylated, hydroxylated and solvated surfaces of α-Fe2O3 (hematite). We find that our nonhydroxylated structures and energies are in good agreement with previous LDA calculations on α-alumina by Manassidis et al. (Surf. Sci. 285 (1993) L517). Using the results of molecular dynamics simulations of solvated interfaces, we define end-member hydroxylated-hydrated states for the surfaces which are used in energy minimization calculations. We find that hydration has a small effect on the surface structure, but that hydroxylation has a significant effect. Our calculations, both for gas-phase and solution-phase adsorption, predict a greater amount of hydroxylation for the α-Fe2O3 (012) surface than for the (001) surface. Our simulations also indicate the presence of four-fold coordinated iron ions on the (001) surface.

Molecular statics calculations for iron oxide and oxyhydroxide minerals: Toward a flexible model of the reactive mineral-water interface

Molecular statics calculations are used to model the major FeOOH polymorphs and hematite. The potentials were taken from a previous investigation of Fe(III) in aqueous solutions which involved the extrapolation of the gas-phase Fe(III)-OH2 potential energy surface to the solvated hexaaqua complex. Using this model for the solid phases, lattice parameters for goethite, akaganeite, lepidocrocite, and hematite are generally within 4% of experiment. Internal energies (at 0 K) were computed for each structure; lepidocrocite is energetically the most stable polymorph, followed by akaganeite, with goethite being the least stable. While the model exhibits some variances with experiment, it performs remarkably well, despite the challenging constraint of being consistent with a dissociating molecular dynamics model for water in its gas, aqueous, and solid phases. Because of this consistency, the model allows qualitative theoretical treatment of previously unapproachable problems in mineral-water interface geochemistry. We apply the model to identify surface species on the solvated (110) surface of goethite.

Molecular dynamics simulation of iron(III) and its hydrolysis products in aqueous solution

A simple potential model is described which allows molecular dynamics simulations to be performed for ferric iron ions in dissociating aqueous solutions. The model was parametrized by fitting the polarizable dissociating water model of Halley et al. [J. Chem. Phys. 98, 4110 (1993)] to a single water molecule–ferric iron ion potential energy surface taken from the work of Curtiss et al. [J. Chem. Phys. 86, 2319 (1987)]. The model gives very good results for the structure of the solvated hexaaqua iron(III) complex; the proper coordination number of 6 was obtained when the Fe–O interaction was fit directly to the ab initio calculations without further modification. The model produces adequate results for the first hydrolysis constant, but breaks down for the second hydrolysis constant, which is overestimated by 18 kcal/mol.

A molecular dynamics study of solvated orthosilicic acid and orthosilicate anion using parameterized potentials

Ab initio calculations have been used extensively in the last decade to model silicate mineral surfaces. A major problem with both the use and validation of ab initio calculations is that including more than a few solvent molecules requires enormous amounts of computer time. In this work, we propose a means of alleviating this problem by introducing a parameterized force field for Siue5f8Oue5f8H systems. The parameterized model is constructed by appending an Siue5f8O interaction to a previously published molecular dynamics model for water. By fitting the parameters of the Siue5f8O interaction to the structure and vibrational spectrum of H4Si04, and retaining the Oue5f8H interactions in the water model, we obtain a representation of the H4SiO4 which quite accurately reproduces the gas-phase deprotonation energy of H4SiO4 computed from quantum mechanical calculations. Molecular dynamics calculations are then used to model the free energy change for the deprotonation of H4SiO4 in aqueous solution. While the predicted value of the free energy change (16 kcal/mol) is 2.5 kcal/mol larger than the accepted experimental value of 13.5 kcal/ mol, the difference between the gas-phase deprotonation energy computed from quantum mechanics and the solution phase deprotonation free energy measured experimentally is predicted to within 1%.

Conformational analysis of crown ethers—part 2. 15-crown-5

Abstract An extensive conformational search of 15-crown-5 was performed with the MM3 force field yielding 3623 conformers ranging in steric energy from 21.40 to 56.24 kcal mol −1 . Free energies and conformer population are reported for the gas phase. A continuum dielectric model is used to estimate the conformer populations in a variety of solvents. The three conformers observed in crystal structures containing 15-crown-5 molecules that do not form inner sphere complexes with metal ions are predicted to be among the most populated conformations in condensed phases. Comparison of the structural features of the crystallographically observed conformers with those of the energy minimized MM3 structures demonstrates that the MM3 force field well reproduces that experimentally observed structural features. Predicted conformer populations are consistent with 13 C NMR and vibrational spectra.

Conformational analysis of crown ethers. Part 3. Alkali and alkaline earth cation complexes of 15-crown-5

Abstract An extensive conformational search of 15-crown-5 complexes with the alkali and alkaline earth cations was performed with an extended MM3 force field. A detailed comparison of the search results with crystallographic data establishes that the model accurately predicts the structural features of these compounds. An improved method is reported for scanning a macrocycle conformer cavity to determine the preferred metal ion size. We demonstrate that the variations in ligand strain for different conformers as a function of metal size rationalize the crystallographically observed variations in 15-crown-5 conformation as a function of metal ion size and charge.

An extended MM3(96) force field for complexes of the group 1A and 2A cations with ligands bearing conjugated ether donor groups

Structural requirements for strain-free metal ion complexation by a conjugated ether group are investigated through the use of both electronic structure calculations and molecular mechanics calculations. Hartree-Fock calculations on simple model complexes, [M(anisole)]n+ and [M(1,2-dimethoxybenzene)]n+, reveal a preference for trigonal planar oxygen geometry when conjugated ether oxygens are coordinated to alkali and alkaline earth cations. Potential energy surfaces from the Hartree-Fock calculations are used, in part, to develop MM3(96) force field parameters for calculation on complexes of ligands bearing conjugated ether donor atoms with the alkali (Li to Cs) and alkaline earth (Mg to Ba) cations. The extended MM3(96) model reproduces the experimental crystal structures of 42 complexes of multidentate ether ligands with the alkali and alkaline earth cations. A novel feature of the MM3(96) program is reported which facilitates the treatment of metal-centered bond angles in complexes where there are greater than six donor atoms.

Why the addition of neutral oxygen donor groups promotes selectivity for larger metal ions

Abstract The origin of selectivity enhancement for large metal ions that occurs on the addition of neutral oxygen donors to existing ligands in such a way as to form additional five-membered chelate rings is analyzed in terms of inductive and steric effects. Molecular mechanics calculations are used to examine the degree of strain that develops in five-membered, aliphatic chelate rings of ethers and amines as a function of the size and charge of the metal ion. Although five-membered chelate rings that contain saturated neutral oxygen donors are found to exhibit an inherent steric preference for large metal ions, experimental evidence suggests that for the majority of cases where enhanced selectivity for larger metal ions has been observed after the addition of neutral oxygen donors, the selectivity enhancement is largely the result of steric and inductive changes to other donor groups in the ligand, e.g. amines, rather than the result of increasing the denticity of the ligand.

An extended molecular mechanics (MM3(96)) force field for benzocrown ethers, calixarenes, and spherands

Abstract The X-ray crystal structures of 60 molecules containing aliphatic and conjugated ether functional groups were calculated with an extended version of the molecular mechanics MM3(96) force field. The structures fit well with overall deviations of 0.014 A in bond length, 1.1 ° in bond angle, and 3.2 ° in torsion angle.