An X-ray diffraction study on the first and the second hydration shell of the Fe(III) ion in nitrate solutions

Cd(II) adsorption on earth-abundant serpentine in aqueous environment: Role of interfacial ion specificity

Adsorption of heavy metal ions (e.g., Cd(II)) on clay minerals significantly affects their transport and fate in natural and engineered waterbodies. To date, the role of interfacial ion specificity in the adsorption of Cd(II) on earth-abundant serpentine remains elusive. In this work, the adsorption of Cd(II) on serpentine at typical environment conditions (pH 4.5-5.0), particularly under the complex influence of common environmental anions (e.g., NO₃^-, SO₄^2-) and cations (e.g., K⁺, Ca²⁺, Fe³⁺, Al³⁺) was systemically investigated. It was found that the adsorption of Cd(II) on serpentine surface due to the inner-sphere complexation could be negligibly affected by the anion type, yet the cations specifically modulated the Cd(II) adsorption. The presence of mono- and divalent cations moderately enhanced the Cd(II) adsorption by weakening the electrostatic double layer (EDL) repulsion between Cd(II) and Mg-O plane of serpentine, while trivalent cations significantly suppressed the adsorption of Cd(II) due to the competitive adsorption. Based on the spectroscopy analysis, Fe³⁺ and Al³⁺ were found to robustly bind the surface active sites of serpentine, thereby preventing the inner-sphere adsorption of Cd(II). The density functional theory (DFT) calculation indicated that Fe(III) and Al(III) exhibited the larger adsorption energy (E_ad = -146.1 and -516.1 kcal mol^-1, respectively) and stronger electron transfer capacity with serpentine compared to Cd(II) (E_ad = -118.1 kcal mol^-1), thus resulting in the formation of more stable Fe(III)-O and Al(III)-O inner-sphere complexes. This study provides valuable insights into the influence of interfacial ion specificity on the Cd(II) adsorption in terrestrial and aquatic environments.

BuRNN: Buffer Region Neural Network Approach for Polarizable-Embedding Neural Network/Molecular Mechanics Simulations

Hybrid quantum mechanics/molecular mechanics (QM/MM) simulations have advanced the field of computational chemistry tremendously. However, they require the partitioning of a system into two different regions that are treated at different levels of theory, which can cause artifacts at the interface. Furthermore, they are still limited by high computational costs of quantum chemical calculations. In this work, we develop the buffer region neural network (BuRNN), an alternative approach to existing QM/MM schemes, which introduces a buffer region that experiences full electronic polarization by the inner QM region to minimize artifacts. The interactions between the QM and the buffer region are described by deep neural networks (NNs), which leads to the high computational efficiency of this hybrid NN/MM scheme while retaining quantum chemical accuracy. We demonstrate the BuRNN approach by performing NN/MM simulations of the hexa-aqua iron complex.

Polarizable and Non-Polarizable Force Field Representations of Ferric Cation and Validations

The AMOEBA polarizable force field of ferric ion was optimized and applied to study the hydration of ferric ion and its complexation with porphine in the aqueous phase. The nonpolarizable force field was also optimized for comparison. The AMOEBA force field was found to give a more accurate hydration free energy than the nonpolarizable force field with respect to experimental data, and correctly predict the most stable electronic state of hydrated Fe³⁺, which is the sextet state, and of the Fe^(III)-Por complex, which is the quartet state, consistent with the literature that was carried out using the DFT method. The explicit inclusion of charge transfer between Fe³⁺ and ligand was found to be important in order to obtain a precise picture of polarization energy and van der Waals energy, which otherwise deviate from the corresponding energy components derived from ab initio calculations. The successful application of the AMOEBA force field in the characterization of aquo Fe^(III)-Por complexes suggests that its use may be extended to the study of the dynamics of metalloenzyme containing highly charged metal ions in the condensed phase with reliable treatment of the interactions between metal atom and protein.

First principles simulation of the bonding, vibrational, and electronic properties of the hydration shells of the high-spin Fe(3+) 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.

Thermodynamics, kinetics, and mechanism of the stepwise dissociation and formation of Tris(L-lysinehydroxamato)iron(III) in aqueous acid

pK(a) values for the hydroxamic acid, alpha-NH(3)(+), and epsilon-NH(3)(+) groups of L-lysinehydroxamic acid (LyHA, H(3)L(2+)) were found to be 6.87, 8.89, and 10.76, respectively, in aqueous solution (I = 0.1 M, NaClO(4)) at 25 degrees C. O,O coordination to Fe(III) by LyHA is supported by H(+) stoichiometry, UV-vis spectral shifts, and a shift in nu(CO) from 1648 to 1592 cm(-1) upon formation of mono(L-lysinehydroxamato)tetra(aquo)iron(III) (Fe(H(2)L)(H(2)O)(4)(4+)). The stepwise formation of tris(L-lysinehydroxamato)iron(III) from Fe(H(2)O)(6)(3+) and H(3)L(2+) was characterized by spectrophotometric titration, and the values for log beta(1), log beta(2), and log beta(3) are 6.80(9), 12.4(2), and 16.1(2), respectively, at 25 degrees C and I = 2.0 M (NaClO(4)). Stopped-flow spectrophotometry was used to study the proton-driven stepwise ligand dissociation kinetics of tris(L-lysinehydroxamato)iron(III) at 25 degrees C and I = 2.0 M (HClO(4)/NaClO(4)). Defining k(n) and k(-n) as the stepwise ligand dissociation and association rate constants and n as the number of bound LyHA ligands, k(3), k(-3), k(2), k(-2), k(1), and k(-1) are 3.0 x 10(4), 2.4 x 10(1), 3.9 x 10(2), 1.9 x 10(1), 1.4 x 10(-1), and 1.2 x 10(-1) M(-1) s(-1), respectively. These rate and equilibrium constants are compared with corresponding constants for Fe(III) complexes of acetohydroxamic acid (AHA) and N-methylacetohydroxamic acid (NMAHA) in the form of a linear free energy relationship. The role of electrostatics in these complexation reactions to form the highly charged Fe(LyHA)(3)(6+) species is discussed, and an interchange mechanism mediated by charge repulsion is presented. The reduction potential for tris(L-lysinehydroxamato)iron(III) is -214 mV (vs. NHE), and a comparison to other hydroxamic acid complexes of Fe(III) is made through a correlation between E(1/2) and pFe.