Speciation of the curium(III) ion in aqueous solution: a combined study by quantum chemistry and molecular dynamics simulation

The structures of aquo complexes of the curium(III) ion have been systematically studied using quantum chemical and molecular dynamics (MD) methods. The first hydration shell of the Cm3+ ion has been calculated using density functional theory (DFT), with and without inclusion of the conductor-like polarizable continuum medium (CPCM) model of solvation. The calculated results indicate that the primary hydration number of Cm3+ is nine, with a Cm-O bond distance of 2.47-2.48 A. The calculated bond distances and the hydration number are in excellent agreement with available experimental data. The inclusion of a complete second hydration shell of Cm3+ has been investigated using both DFT and MD methods. The presence of the second hydration shell has significant effects on the primary coordination sphere, suggesting that the explicit inclusion of second-shell effects is important for understanding the nature of the first shell. The calculated results indicate that 21 water molecules can be coordinated in the second hydration shell of the Cm3+ ion. MD simulations within the hydrated-ion model suggest that the second-shell water molecules exchange with the bulk solvent with a lifetime of 161 ps.     

Fe(II) hydrolysis in aqueous solution: A DFT study

Species arising from Fe(II) hydrolysis in aqueous solution have been investigated using density-functional methods (DFT). The different tautomers and multiplicities of each species have been calculated. The solvation energy has been estimated using the UAHF–PCM method. The hydrolysis free energies have been estimated and compared with the available experimental data. The different hydrolysis species have distinct geometries and electronic structures. The estimated ionization potential of the hydrolyzed species is linearly dependent to the number of hydroxyls present in the complex. The estimated Fe(II)/Fe(III) oxidation potential is in good agreement with previously published results about 0.29 V larger than the experimental value. The results highlight the importance of the chemical speciation in describing electron transfer processes at a molecular level. The PBE/TZVP/UAHF–PCM method has been found to describe correctly the hydrolysis free energies of Fe(II) with an average error about 5 kcal mol⁻¹ from the experimental values.     

New example of a non-heme mononuclear iron(IV) oxo complex. Spectroscopic data and oxidation activity

The green complex S=1 [(TPEN)FeO]2+ [TPEN=N,N,N',N'-tetrakis(2-pyridylmethyl)ethane-1,2-diamine] has been obtained by treating the [(TPEN)Fe]2+ precursor with meta-chloroperoxybenzoic acid (m-CPBA). This high-valent complex belongs to the emerging family of synthetic models of Fe(IV)=O intermediates invoked during the catalytic cycle of biological systems. This complex exhibits spectroscopic characteristics that are similar to those of other models reported recently with a similar amine/pyridine environment. Thanks to its relative stability, vibrational data in solution have been obtained by Fourier transform infrared. A comparison of the Fe=O and Fe=(18)O wavenumbers reveals that the Fe-oxo vibration is not a pure one. The ability of the green complex to oxidize small organic molecules has been studied. Mixtures of oxygenated products derived from two- or four-electron oxidations are obtained. The reactivity of this [FeO]2+ complex is then not straightforward, and different mechanisms may be involved.     

Hydration of hydrogentungstate anions at different ph conditions: a car-parrinello molecular dynamics study

Standard density functional theory calculations with a continuous model of solvation as well as Car-Parrinello molecular dynamics simulations with explicit solvent molecules are carried out to analyze the effect of the pH of the solution on the coordination sphere of the W (VI) ion. Both methodologies agree in predicting an expansion of the coordination sphere of the W (VI) ion upon a decrease in the pH. Continuous solvation models, however, are unable to predict as stable some structural isomers of a hydrated hydrogentungstate anion and tungstic acid.     

In-vitro antidiabetic activity,theoretical investigation by DFT and molecular docking of mono oxo vanadyl (IV) azomethine macromolecule

Prospective antidiabetic applicant [VO(IV)HBDB] was synthesized qualitatively and rapidly, spending HBDB of the mode N, N′-bis (2-hydroxy benzylidene)-1,2-diaminobenzene/Salophen a well familiar Schiff base with myriad property. Interesting and valuable details of physical and electronic properties, band gap energy, electronic configuration, and NLO possessions of the complex were reported. The complex consent with extraordinary NLO property accords with first-order hyperpolarizability. The impressiveness of the monooxo Vanadyl complex is elevated further by biological applications. The antibacterial evaluation accounts that [VO(IV)HBDB] shows greater activity than HBDB. DPPH scavenging report demonstrates that [VO(IV)HBDB] exhibits greater antioxidant potential than standard assay. Subsequently, in-vitro antidiabetic activity with α-glucosidase enzyme was also excellent. Molecular docking with α-glucosidase protein of PDB id: 3 WY1 illustrates greater binding energy of about −12.3 kcal/mol defined for [VO(IV)HBDB] than HBDB roughly −7.6 kcal/mol with more amino acid binding residues. The above-disclosed findings of HBDB and its complex firmly sustenance the biochemists and pharmacological field.     

Kinetic investigation of sulfur(IV) oxidation by peroxo compounds R-OOH in aqueous solution

Summary Normal and rapid-scan stopped-flow spectrophotometry in the range of 260–300 nm was used to study the kinetics of sulfur(IV) oxidation by peroxo compounds R-OOH (such as hydrogen peroxide, R=H; peroxonitrous acid, R=NO; peroxoacetic acid, R=Ac; peroxomonosulfuric acid, R=SO ₃ ^– ) in the pH range 2–6 in buffered aqueous solution at an ionic strength of 0.5 M (NaClO₄) or 1.0 M (R=NO; Na₂SO₄). The kinetics follow a three-term rate law, rate=(k_H[H]+k_HX[HX]+k_p)[HSO ₃ ^– ][ROOH] ([H] = proton activity; HX = buffer acid = chloroacetic acid, formic acid, acetic acid, H₂PO ₄ ^– ). Ionic strength effects (I=0.05–0.5 M) and anion effects (Cl^–, ClO ₄ ^– , SO ₄ ^2– ) were not observed. In addition to proton-catalysis (k_H[H]) and general acid catalysis (k_HX[HX]), the rate constant k_p characterizes, most probably, a water induced reaction channel with k_p=k_HOH[H₂O]. It is found that k_Hf(R) with k_H(mean)=2.1·10⁷ M^–2 s^–1 at 298 K. The rate constant k_HX ranges from 0.85·10⁶ M^–2 s^–1 (HX=ClCH₂–COOH; R=NO; 293 K) to 0.47·10⁴ M^–2 s^–1 (HX=H₂PO ₄ ^– ; R=H; 298 K) and the rate constant k_p covers the range 0.2·M^–1 s^–1 (R=H) to 4.0·10⁴ M^–1 s^–1 (R=NO). LFE relationships can be established for both k_HX, correlating with the pK_a of HX, and k_p, correlating with the pK_a of the peroxo compounds R-OOH. These relationships imply interesting aspects concerning the mechanism of sulfur(IV) oxidation and the possible role of peroxonitrous acid in atmospheric chemistry. A UV-spectrum of the unstable peroxo acid ON-OOH is presented.     

A combined QM/MM molecular dynamics simulations study of nitrate anion (NO3-) in aqueous solution

The structural and dynamical properties of NO3- in dilute aqueous solution have been investigated by means of two combined quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations, namely HF/MM and B3LYP/MM, in which the ion and its surrounding water molecules were treated at HF and B3LYP levels of accuracy, respectively, using the DZV+ basis set. On the basis of both HF and B3LYP methods, a well-defined first hydration shell of NO3- is obtainable, but the shell is quite flexible and the hydrogen-bond interactions between NO3- and water are rather weak. With respect to the detailed analysis of the geometrical arrangement and vibrations of NO3-, the experimentally observed solvent-induced symmetry breaking of the ion is well reflected. In addition, the dynamical information, i.e., the bond distortions and shifts in the corresponding bending and stretching frequencies as well as the mean residence time of water molecules surrounding the NO3- ion, clearly indicates the "structure-breaking" ability of this ion in aqueous solution. From a methodical point of view it seems that both the HF and B3LYP methods are not too different in describing this hydrated ion by means of a QM/MM simulation. However, the detailed analysis of the dynamics properties indicates a better suitability of the HF method compared to the B3LYP-DFT approach.     

Reductions of oxo species by aquatitanium(II) as catalyzed by titanium(IV)

Titanium(II) solutions, prepared by dissolving titanium wire in triflic acid + HF, contain equimolar quantities of Ti(IV). These preparations react readily with oxidizing species such as VO(2)(+), substituted benzoquinones, and the biguanide complex of Mn(IV). Reactions with excess Ti(II) yield Ti(III). Reductions of 1,4-benzoquinones and the Mn(IV) complex are catalyzed by added Ti(IV) but stoichiometry is unaffected. Rate laws for the Ti(IV)-catalyzed reactions are consistent with formation of a Ti(II)-Ti(IV) complex (K(formation) 4 x 10(2) M(-1)), which, in some cases, also partakes in contributing deprotonated and halogen-catalyzed paths.     

Kinetics and mechanism of the oxidation of iron(II) ion by chlorine dioxide in aqueous solution

The mechanism by which an excess of iron(II) ion reacts with aqueous chlorine dioxide to produce iron(III) ion and chloride ion has been determined. The reaction proceeds via the formation of chlorite ion, which in turn reacts with additional iron(II) to produce the observed products. The first step of the process, the reduction of chlorine dioxide to chlorite ion, is fast compared to the subsequent reduction of chlorite by iron(II). The overall stoichiometry is The rate is independent of pH over the range from 3.5 to 7.5, but the reaction is assisted by the presence of acetate ion. Thus the rate law is given by At an ionic strength of 2.0 M and at 25°C, k_u = (3.9 ± 0.1) × 10³ L mol^?1 s^?1 and k_c = (6 ± 1) × 10⁴ L mol^?1 s^?1. The formation constant for the acetatoiron(II) complex, K_f, at an ionic strength of 2.0 M and 25°C was found to be (4.8 ± 0.8) × 10^?2 L mol^?1. The activation parameters for the reaction were determined and compared to those for iron(II) ion reacting directly with chlorite ion. At 0.1 M ionic strength, the activation parameters for the two reactions were found to be identical within experimental error. The values of ΔH? and ΔS? are 64 ± 3 kJ mol^?1 and + 40 ± 10 J K^?1 mol^?1 respectively. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 554–565, 2004     

Mechanism of oxidation of benzaldehyde by polypyridyl oxo complexes of Ru(IV)

The oxidation of benzaldehyde and several of its derivatives to their carboxylic acids by cis-[Ru(IV)(bpy)2(py)(O)]2+ (Ru(IV)=O2+; bpy is 2,2'-bipyridine, py is pyridine), cis-[Ru(III)(bpy)2(py)(OH)]2+ (Ru(III)-OH2+), and [Ru(IV)(tpy)(bpy)(O)]2+ (tpy is 2,2':6',2'-terpyridine) in acetonitrile and water has been investigated using a variety of techniques. Several lines of evidence support a one-electron hydrogen-atom transfer (HAT) mechanism for the redox step in the oxidation of benzaldehyde. They include (i) moderate k(C-H)/k(C-D) kinetic isotope effects of 8.1 +/- 0.3 in CH3CN, 9.4 +/- 0.4 in H2O, and 7.2 +/- 0.8 in D2O; (ii) a low k(H2O/D2O) kinetic isotope effect of 1.2 +/- 0.1; (iii) a decrease in rate constant by a factor of only approximately 5 in CH3CN and approximately 8 in H2O for the oxidation of benzaldehyde by cis-[Ru(III)(bpy)2(py)(OH)]2+ compared to cis-[Ru(IV)(bpy)2(py)(O)]2+; (iv) the appearance of cis-[Ru(III)(bpy)2(py)(OH)]2+ rather than cis-[Ru(II)(bpy)2(py)(OH2)]2+ as the initial product; and (v) the small rho value of -0.65 +/- 0.03 in a Hammett plot of log k vs sigma in the oxidation of a series of aldehydes. A mechanism is proposed for the process occurring in the absence of O2 involving (i) preassociation of the reactants, (ii) H-atom transfer to Ru(IV)=O2+ to give Ru(III)-OH2+ and PhCO, (iii) capture of PhCO by Ru(III)-OH2+ to give Ru(II)-OC(O)Ph+ and H+, and (iv) solvolysis to give cis-[Ru(II)(bpy)2(py)(NCCH3)]2+ or the aqua complex and the carboxylic acid as products.     

Kinetics of the homogeneous oxidation of hydrogen by platinum(II) and platinum(IV) chloro complexes in aqueous solution

The kinetics of the homogeneous oxidation of hydrogen in the Pt(II)–Pt(IV)–Cl^––H₂O system has been studied for the first time in conditions permitting to avoid the formation of Pt-black. It is shown that platinum (II) [Pt(II)Cl_i(H₂O)_4-i, where i=1, 2, 2], is active in the reaction, whereas the PtCl ₄ ^2– complex and platinum(IV) do not react with hydrogen.

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, Pt-, H₂ Pt^II–Pt^IV–Cl^––H₂O. (II) (Pt^IICl_i(H₂O)_4-i, i=1, 2, 3); PtCl ₄ ^2– (IV) .

     

Mg/Ca partitioning between aqueous solution and aragonite mineral: a molecular dynamics study

We have calculated the concentrations of Mg in the bulk and surfaces of aragonite CaCO(3) in equilibrium with aqueous solution, based on molecular dynamics simulations and grand-canonical statistical mechanics. Mg is incorporated in the surfaces, in particular in the (001) terraces, rather than in the bulk of aragonite particles. However, the total Mg content in the bulk and surface of aragonite particles was found to be too small to account for the measured Mg/Ca ratios in corals. We therefore argue that most Mg in corals is either highly metastable in the aragonite lattice, or is located outside the aragonite phase of the coral skeleton, and we discuss the implications of this finding for Mg/Ca paleothermometry.     

A molecular dynamics study of sodium chenodeoxycholate in an aqueous solution

Hydration structure and dynamics of sodium chenodeoxycholate (CDC) in water are studied by a long-time molecular dynamics calculation. Strong hydration shell around the hydrophobic region of this large solute and strong hydrogen bonds of water with both hydroxyl and carboxyl oxygen atoms have been identified. The rotation of CDC around its longitudinal axis is found to be particularly active in comparison with that around other axes of the molecule. The diffusion coefficient of CDC calculated from the slope of the mean-square displacement, 0.95 × 10⁻⁹ m²/s, is only 1/6 of that for water in the solution, 5.4 × 10⁻⁹ m²/s.     

Oxo iron(IV) as an oxidative active intermediate of p-chlorophenol in the Fenton reaction: a DFT study

Debate continues over which active species plays the role of oxidative agent during the Fenton reaction-the HO˙ radical or oxo iron [Fe(IV)O](2+). In this context, the present study investigates the oxidation of p-chlorophenol by [Fe(IV)O(H(2)O)(5)](2+) using DFT calculations, within gas-phase and micro-solvated models, in order to explore the possible role of oxo iron as a reactant. The results show that the chlorine atom substitution of p-chlorophenol by oxo iron is a highly stabilising step (ΔH = -83 kcal mol(-1)) with a free energy barrier of 5.8 kcal mol(-1) in the micro-solvated model. This illustrates the high oxidising power of the [Fe(IV)O(H(2)O)(5)](2+) complex. On the other hand, the breaking of the Fe-O bond, leading to the formation of hydroquinone, is observed to be the rate-determining step of the reaction. The rather large free energy barrier corresponding to this bond cleavage amounts to 10.2 and 9.3 kcal mol(-1) in the gas-phase and micro-solvated models, respectively. Elsewhere, the lifetime of the HO˙ radical has previously been shown to be extremely small. These facts, combined with observations of oxo iron under certain experimental conditions, suggest that oxo iron is a highly plausible oxidative species of the reaction. In addition, a trigonal bipyramidal iron complex, coordinated either by hydroxyl groups and/or by water molecules, has been found in all described mechanisms. This structure appears to be a stable intermediate; and to our knowledge, it has not been characterised by previous studies.     

Manganese(II) catalysed oxidation of glycerol by cerium(IV) in aqueous sulphuric acid medium: a kinetic and mechanistic study

The manganese(II) catalysed oxidation of glycerol by cerium(IV) in aqueous sulphuric acid has been studied spectrophotometrically at 25 °C and I = 1.60 mol dm⁻³. Stoichiometry analysis shows that one mole of glycerol reacts with two moles of cerium(IV) to give cerium(III) and glycolic aldehyde. The reaction is first order in both cerium(IV) and manganese(II), and the order with respect to glycerol concentration varies from first to zero order as the glycerol concentration increases. Increase in sulphuric acid concentration, added sulphate and bisulphate all decrease the rate. Added cerium(III) retards the rate of reaction, whereas glycolic aldehyde had no effect. The active species of oxidant and catalyst are Ce(SO₄)₂ and [Mn(H₂O)₄]²⁺. A mechanism is proposed, and the reaction constants and activation parameters have been determined.     

Structure of the hydrated platinum(II) ion and the cis-diammineplatinum(II) complex in acidic aqueous solution: an EXAFS study

Careful analysis of Pt L3-edge extended X-ray absorption fine structure (EXAFS) spectra shows that the hydrated platinum(II) ion in acidic (HClO 4) aqueous solution binds four water molecules with the Pt-O bond distance 2.01(2) A and one (or two) in the axial position at 2.39(2) A. The weak axial water coordination is in accordance with the unexpectedly small activation volume previously reported for water exchange in an interchange mechanism with associative character. The hydrated cis-diammineplatinum(II) complex has a similar coordination environment with two ammine and two aqua ligands strongly bound with Pt-O/N bond distances of 2.01(2) A and, in addition, one (or two) axial water molecule at 2.37(2) A. This result provides a new basis for theoretical computational studies aiming to connect the function of the anticancer drug cis-platin to its ligand exchange reactions, where usually four-coordinated square planar platinum(II) species are considered as the reactant and product. (195)Pt NMR spectroscopy has been used to characterize the Pt(II) complexes.     

Determination by high-frequency and -field EPR of zero-field splitting in iron(IV) oxo complexes: implications for intermediates in nonheme iron enzymes

[Fe(IV)O](2+) species have been implicated as the active form of many nonheme iron enzymes. The electronic structures of iron(IV) oxo complexes are thus of great interest. High-frequency and -field electron paramagnetic resonance is employed to determine accurately the spin Hamiltonian parameters of two stable complexes that contain the FeO unit: [FeO(TMC)(CH 3CN)](CF 3SO 3) 2, where TMC = tetramethylcyclam and [FeO(N4py)](CF 3SO 3) 2, where N4Py = bis(2-pyridylmethyl)bis(2-pyridyl)methylamine. Both complexes exhibit zero-field splittings that are positive, almost perfectly axial, and of very large magnitude: D = +26.95(5) and +22.05(5) cm (-1), respectively. These definitive experimental values can serve as the basis for further computational studies to unravel the electronic structures of such complexes.     

A molecular dynamics study of alkaline earth metal-chloride complexation in aqueous solution

The relative stability of alkaline earth metals (M2+ = Mg2+, Ca2+, Sr2+, and Ba2+) and their chloride complexes in aqueous solution is examined through molecular dynamics simulations using a flexible SPC water model with an internally consistent set of metal ion force field parameters. For each metal-chloride ion pair in aqueous solution, the free energy profile was calculated via potential of mean force simulations. The simulations provide detailed thermodynamic information regarding the relative stability of the different types of metal-chloride pairs. The free energy profiles indicate that the preference for contact ion pair formation increases with ionic radius and is closely related to the metal hydration free energies. The water residence times within the first hydration shells are in agreement with residence times reported in other computational studies. Calculated association constants suggest an increase in metal-chloride complexation with increasing cation radii that is inconsistent with experimentally observed trends. Possible explanations for this discrepancy are discussed.     

Kinetics of the oxidation of octacyanomolybdate(IV) by periodate in aqueous acid

Summary The kinetics of oxidation of [Mo(CN)₈]^4– by IO ₄ ^– in aqueous acid is described by the equation: d[{Mo(CN)₈}^3–]/ dt=2k₃[{Mo(CN)₈}^4–][IO ₄ ^– ][H⁺]. Unlike IO ₄ ^– oxidations of [Fe(CN)₆]^4– and [W(CN)₈]^4–, no [H⁺] independent term exists in the [Mo(CN)₈]^4– reaction, which indicates that, in neutral and alkaline solutions, oxidation of [Mo(CN)₈]^4– is thermodynamically unfavourable. An inner-sphere mechanism, consistent with the rate law, is proposed. This conclusion is based, in the absence of direct evidence, on the observed behaviour of IO ₄ ^– as an inner-sphere oxidant.