Stuart Bogatko

Near-Quantitative Agreement of Model-Free DFT-MD Predictions with XAFS Observations of the Hydration Structure of Highly Charged Transition-Metal Ions.

First-principles dynamics simulations (DFT, PBE96, and PBE0) and electron scattering calculations (FEFF9) provide near-quantitative agreement with new and existing XAFS measurements for a series of transition-metal ions interacting with their hydration shells via complex mechanisms (high spin, covalency, charge transfer, etc.). This analysis does not require either the development of empirical interparticle interaction potentials or structural models of hydration. However, it provides consistent parameter-free analysis and improved agreement with the higher-R scattering region (first- and second-shell structure, symmetry, dynamic disorder, and multiple scattering) for this comprehensive series of ions. DFT+GGA MD methods provide a high level of agreement. However, improvements are observed when exact exchange is included. Higher accuracy in the pseudopotential description of the atomic potential, including core polarization and reducing core radii, was necessary for very detailed agreement. The first-principles nature of this approach supports its application to more complex systems.

First Principles Simulation of the Bonding, Vibrational, and Electronic Properties of the Hydration Shells of the High-Spin Fe3+ Ion in Aqueous Solutions

Results of parameter-free first principles simulations of a spin up 3d(5) Fe(3+) ion hydrated in an aqueous solution (64 waters, 30 ps, 300 K) are reported. The first hydration shell associated with the first maximum of the radial distribution function, g(FeO)(r), at d(Fe-O(I)) = 2.11-2.15 A, contains 6 waters with average d(OH) = 0.99 A, in good agreement with observations. A second shell with average coordination number 13.3 can be identified with average shell radius of d(Fe-O(II)) = 4.21-4.32 A. The waters in this hydration shell are coordinated to the first shell via a trigonal H-bond network with d(O(I)-O(II)) = 2.7-2.9 A, also in agreement with experimental measurements. The first shell tilt angle average is 33.4 degrees as compared to the reported value of 41 degrees . Wannier-Boys orbitals (WBO) show an interaction between the unoccupied 3d orbitals of the Fe(3+) valence (spin up, 3d(5)) and the occupied spin down lone pair orbitals of first shell waters. The effect of the spin ordering of the Fe(3+) ion on the WBO is not observed beyond the first shell. From this local bond analysis and consistent with other observations, the electronic structure of waters in the second shell is similar to that of a bulk water even in this strongly interacting system. H-bond decomposition shows significant bulk-like structure within the second shell for Fe(3+). The vibrational density of states shows a first shell red shift of 230 cm(-1) for the v(1),2v(2),v(3) overtone, in reasonable agreement with experimental estimates for trivalent cations (300 cm(-1)). No exchanges between first and second shell were observed. Waters in the second shell exchanged with bulk waters via dissociative and associative mechanisms. Results are compared with an AIMD study of Al(3+) and 64 waters. For Fe(3+) the average first shell tilt angle is larger and the tilt angle distribution wider. H-bond decomposition shows that second shell to second shell H-bonding is enhanced in Fe(3+) suggesting an earlier onset of bulk-like water structure.

Cooperativity in Al3+ Hydrolysis Reactions from Density Functional Theory Calculations

Aqua/hydroxo mononuclear Al(3+) species in aqueous solution are investigated using density functional theory (DFT B3LYP/6-311++G(d,p)) and the polarized continuum model (PCM). Optimized gas-phase geometries have been obtained for the species Al(OH)(n)(H(2)O)(m)((3-n)+) in which n = 0, 1, 2, 3, or 4 while (n + m) = 4, 5, or 6. For Al(OH)(2)(H(2)O)(4)(1+) the cis and trans geometries were considered. The structures were analyzed in terms of water and hydroxide M-O and O-H distances, which are shown to be strongly modulated by water hydrolysis. The atomic charges were computed and the electronic structure of these complexes is discussed. The conversion from one aqua-/hydroxo- species to another follows independent hydrolysis and dehydration reactions for which the aqueous Gibbs free energies have been estimated by means of constructing thermodynamic cycles. Results clearly demonstrate that the dehydration reaction is increasingly favorable as hydrolysis proceeds. Similarly, as the complex coordination number decreases the hydrolysis reaction proceeds increasingly more favorably. The aqueous Gibbs free energy of each species, relative to Al(H(2)O)(6)(3+), has been determined by combining the appropriate Gibbs free energies of the hydrolysis and dehydration reactions demonstrating that the additive effect is quite complex showing a gradual transition from preferring the 6-coordination to 5-coordination to 4-coordination as a function of ligand hydrolysis, in agreement with published experimental and theoretical work. We have also computed the equilibrium constants of each of the above reactions and, using [H(+)] as a parameter, estimated the mole fraction of each species as a function of pH. This offers a clear demonstration that the qualitative hydrolysis behavior, e.g., cooperativity, of aqueous Al(3)+ species is obtained at the B3LYP/IEF-PCM level of theory.

Ion Association in AlCl3 Aqueous Solutions from Constrained First-Principles Molecular Dynamics

The Car-Parrinello-based molecular dynamics (CPMD) method was used to investigate the ion-pairing behavior between Cl(-) and Al(3+) ions in an aqueous AlCl(3) solution containing 63 water molecules. A series of constrained simulations was carried out at 300 K for up to 16 ps each, with the internuclear separation (r(Al-Cl)) between the Al(3+) ion and one of the Cl(-) ions held constant. The calculated potential of mean force (PMF) of the Al(3+)-Cl(-) ion pair shows a global minimum at r(Al-Cl) = 2.3 Å corresponding to a contact ion pair (CIP). Two local minima assigned to solvent-separated ion pairs (SSIPs) are identified at r(Al-Cl) = 4.4 and 6.0 Å. The positions of the free energy minima coincide with the hydration-shell intervals of the Al(3+) cation, suggesting that the Cl(-) ion is inclined to reside in regions with low concentrations of water molecules, that is, between the first and second hydration shells of Al(3+) and between the second shell and the bulk. A detailed analysis of the solvent structure around the Al(3+) and Cl(-) ions as a function of r(Al-Cl) is presented. The results are compared to structural data from X-ray measurements and unconstrained CPMD simulations of single Al(3+) and Cl(-) ions and AlCl(3) solutions. The dipole moments of the water molecules in the first and second hydration shells of Al(3+) and in the bulk region and those of Cl(-) ions were calculated as a function of r(Al-Cl). Major changes in the electronic structure of the system were found to result from the removal of Cl(-) from the first hydration shell of the Al(3+) cation. Finally, two unconstrained CPMD simulations of aqueous AlCl(3) solutions corresponding to CIP and SSIP configurations were performed (17 ps, 300 K). Only minor structural changes were observed in these systems, confirming their stability.

Density functional theory interpretation of the 1H photo-chemically induced dynamic nuclear polarization enhancements characterizing photoreduced polyazaaromatic Ru(II) coordination complexes.

The unprotonated and protonated monoreduced forms of the polyazaaromatic Ru(II) coordination complexes [Ru(tap)(3)](2+) and [Ru(tap)(2)(phen)](2+) (tap = 1,4,5,8-tetraazaphenanthrene ; phen = 1,10-phenanthroline), that is, [Ru(tap)(3)](*+), [Ru(tap)(2)(phen)](*+), [Ru(tap)(2)(tap-H)](*2+), and [Ru(tap)(tap-H)(phen)](*2+), were studied by Density Functional Theory (DFT). The electron spin density of these radical cations, the isotropic Fermi-contact, and the anisotropic dipolar contributions to the hyperfine coupling constants of the H nuclei were calculated in vacuo and using a continuum model for water solvation. For [Ru(tap)(2)(phen)](*+), as well as for its protonated form, the DFT results show that the unpaired electron is not localized on the phen ligand. For both [Ru(tap)(3)](*+) and [Ru(tap)(2)(phen)](*+), they reveal high electron spin density in the vicinity of tap H-2 and tap H-7 (the H atoms in the ortho position of the tap non-chelating N atoms). These results are in full agreement with recent steady-state (1)H photo-Chemically Induced Dynamic Nuclear Polarization (photo-CIDNP) measurements. The DFT calculations performed for the protonated species also predict major (1)H photo-CIDNP enhancements at these positions. Interestingly, they indicate significantly different polarization for tap H-9,10, suggesting that the occurrence of a photoinduced electron transfer with protonation of the reduced species might be detected by high-precision photo-CIDNP experiments.

Improved DFT-Based Interpretation of ESI-MS of Aqueous Metal Cations

AbstractWe present results showing that our recently developed density functional theory (DFT)-based speciation model of the aqueous Al3+ system has the potential to improve the interpretations of ESI-MS studies of aqueous metal cation hydrolytic speciation. The main advantages of our method are that (1) it allows for the calculation of the relative abundance of a given species which may be directly assigned to the signal intensity in a mass spectrum; (2) in cases where species with identical m⁄z ratios may coexist, the assignment can be unambiguously assigned based on their theoretical relative abundances. As a demonstration of its application, we study four pairs of monomer and dimer aqueous Al3+ species, each with identical m/z ratio. For some of these pairs our method predicts that the dominant species changes from the monomer to the dimer species under varying pH conditions. Figureᅟ

Rydberg electron capture by neutral Al hydrolysis products.

We predict that electron attachment may be used with ESI-MS techniques to observe neutral Al metal aqua-oxo-hydroxo species and the complex polymerization and precipitation reactions in which they participate. Neutral aqueous metal species have, so far, been invisible to ESI-MS techniques.

On the coupling of solvent characteristics to the electronic structure of solute molecules

We present the results of a theoretical investigation focusing on the solvent structure surrounding the -1, 0 and +1 charged species of F, Cl, Br and I halogen atoms and F2, Cl2, Br2 and I2 di-halogen molecules in a methanol solvent and its influence on the electronic structure of the solute molecules. Our results show a large stabilizing effect arising from the solute-solvent interactions. Well-formed first solvation shells are observed for all species, the structure of which is strongly influenced by the charge of the solute species. Detailed analysis reveals that coordination number, CN, solvent orientation, θ, and solute-solvent distance, d, are important structural characteristics which are coupled to changes in the electronic structure of the solute. We propose that the fundamental chemistry of any solute species is generally regulated by these solvent degrees of freedom.