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

Abstract

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.

Structure and dynamics of the hydration shells of the Zn2+ ion from ab initio molecular dynamics and combined ab initio and classical molecular dynamics simulations

“…Therefore, many-body interactions, including polarization, are included in a parameter free way as the simulation proceeds. For highly charged ions, it has been demonstrated that AIMD works reliably [12][13][14]21 and we will show that there is very good agreement between simulated extended x-ray absorption fine structure ͑EXAFS͒ spectra and the results of measurements.…”

“…21͒ and Fe 3+ . 14 However, in contrast to Al 3+ , the structure of this shell is not completely trigonal. As shown in Fig.…”

“…10,11 Modeling charged ions in solution is challenging because the water molecules surrounding the ion are strongly polarized; [12][13][14] we have recently reported water dipole moments increasing by Ϸ1 D relative to bulk water values for the first hydration shells of the Al 3+ and Fe 3+ ions. 14 In this article, we report simulation of the properties of the Zn 2+ ion. This transition metal ion has only the +2 oxidation state and shares the same charge and some of the chemistry of the group 2 elements Ca 2+ and Mg 2+ .…”

Molecular Dynamics Simulation Study of the Early Stages of Nucleation of Iron Oxyhydroxide Nanoparticles in Aqueous Solutions

“…13,17,18 The experimental result for the CN of the second hydration shell is ~12. 7 Our simulated CN of the second shell is ~14, close to those (13.3 -13.6) obtained from quantum mechanical / first principle simulations.…”

“…[9][10][11][12][13][14][15][16][17][18] Efforts have been made to correctly reproduce the experimentally derived structure of the hydration shells around an iron cation using molecular dynamics (MD) simulations 9,10,14,15,19,20 , first principle calculations 11,18 and ab initio molecular dynamics simulations 13,17,18,20 . Density functional theory (DFT) also has been used to study the structure, energetics and electronic properties of iron-water/hydroxyl 4 molecular species in aqueous solutions.…”