Oxidation of methanol by FeO2+ in water: DFT calculations in the gas phase and ab initio MD simulations in water solution

We investigate the mechanism of methanol oxidation to formaldehyde by ironoxido ([Fe(IV)O]2+), the alleged active intermediate in the Fenton reaction. The most likely reaction mechanisms are explored with density functional theory (DFT) calculations on microsolvated clusters in the gas phase and, for a selected set of mechanisms, with constrained Car-Parrinello molecular dynamics (CPMD) simulations in water solution. Helmholtz free energy differences are calculated using thermodynamic integration in a simulation box with 31 water molecules at 300 K. The mechanism of the reaction is investigated with an emphasis on whether FeO2+ attacks methanol at a C-H bond or at the O-H bond. We conclude that the most likely mechanism is attack by the oxido oxygen at the C-H bond ("direct CH mechanism"). We calculate an upper bound for the reaction Helmholtz free energy barrier in solution of 50 kJ/mol for the C-H hydrogen transfer, after which transfer of the O-H hydrogen proceeds spontaneously. An alternative mechanism, starting with coordination of methanol directly to Fe ("coordination OH mechanism"), cannot be ruled out, as it involves a reaction Helmholtz free energy barrier in solution of 44 +/- 10 kJ/mol. However, this coordination mechanism has the disadvantage of requiring a prior ligand substitution reaction, to replace a water ligand by methanol. Because of the strong acidity of [FeO(H2O)5]2+, we also investigate the effect of deprotonation of a first-shell water molecule. However, this is found to increase the barriers for all mechanisms.     

Homolytic versus Heterolytic Dissociation of Alkalimetal Halides: The Effect of Microsolvation

Herein we report density functional calculations of homolytic and heterolytic dissociation energies of the diatomic alkalimetal halides MX (M=Li, Na, K, Rb, and Cs and X=F, Cl, Br, I, and At) and their corresponding microsolvated structures MX?(H₂O)_n (n=1 to 4). Our results show that the homolytic dissociation energy of the MX?(H₂O)_n species increases with the number of water molecules involved in the microsolvated salts. On the other hand, the heterolytic dissociation energy follows exactly the opposite trend. As a result, while for the isolated diatomic alkalimetal halides, homolytic dissociation is always favored over heterolytic dissociation, the latter is preferred for CsF and CsCl already for n=2, and for n=4 it is the preferential mode of dissociation for more than half of the species studied.     

Competitive sorption of carbonate and arsenic to hematite: combined ATR-FTIR and batch experiments

The competitive sorption of carbonate and arsenic to hematite was investigated in closed-system batch experiments. The experimental conditions covered a pH range of 3-7, arsenate concentrations of 3-300 μM, and arsenite concentrations of 3-200 μM. Dissolved carbonate concentrations were varied by fixing the CO(2) partial pressure at 0.39 (atmospheric), 10, or 100 hPa. Sorption data were modeled with a one-site three plane model considering carbonate and arsenate surface complexes derived from ATR-FTIR spectroscopy analyses. Macroscopic sorption data revealed that in the pH range 3-7, carbonate was a weak competitor for both arsenite and arsenate. The competitive effect of carbonate increased with increasing CO(2) partial pressure and decreasing arsenic concentrations. For arsenate, sorption was reduced by carbonate only at slightly acidic to neutral pH values, whereas arsenite sorption was decreased across the entire pH range. ATR-FTIR spectra indicated the predominant formation of bidentate binuclear inner-sphere surface complexes for both sorbed arsenate and sorbed carbonate. Surface complexation modeling based on the dominant arsenate and carbonate surface complexes indicated by ATR-FTIR and assuming inner-sphere complexation of arsenite successfully described the macroscopic sorption data. Our results imply that in natural arsenic-contaminated systems where iron oxide minerals are important sorbents, dissolved carbonate may increase aqueous arsenite concentrations, but will affect dissolved arsenate concentrations only at neutral to alkaline pH and at very high CO(2) partial pressures.     

Understanding Solvent Effects in Vibrational Circular Dichroism Spectra: [1,1'-Binaphthalene]-2,2'-diol in Dichloromethane, Acetonitrile, and Dimethyl Sulfoxide Solvents

We present a combined experimental and computational investigation of the vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra of [1,1'-binaphthalene]-2,2'-diol. First, the sensitive dependence of the experimental VA and VCD spectra on the solvent is demonstrated by comparing the experimental spectra measured in CH(2)Cl(2), CD(3)CN, and DMSO-d(6) solvents. Then, by comparing calculations performed for the isolated solute molecule to calculations performed for molecular complexes formed between solute and solvent molecules, we identify three main types of perturbations that affect the shape of the VA and VCD spectra when going from one solvent to another. These sources of perturbations are (1) perturbation of the Boltzmann populations, (2) perturbation of the electronic structure, and (3) perturbation of the normal modes.     

Stability of sputter-deposited gold nanoparticles in imidazolium ionic liquids

The stability of gold nanoparticles synthesised by sputter deposition has been studied in situ in 1-butyl-3-methylimidazolium ionic liquids with bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, hexafluorophosphate and dicyanamide anions with UV-VIS absorption spectroscopy and transmission electron microscopy. Besides the growth of the gold nanoparticles, two other processes were observed after sputtering, namely aggregation and sedimentation of these nanoparticles. To model the absorption spectra of the sputtered gold nanoparticles, generalized multiparticle Mie calculations were performed. These theoretical calculations confirm the increase in absorbance at longer wavelength for larger aggregates and are in agreement with the experimental observations. It was found that the kinetics of aggregation and sedimentation scale with the viscosity of the ionic liquid. Small amounts of water were found to have a large detrimental influence on the stability of the colloidal suspensions of the gold nanoparticles in ionic liquids. From the large discrepancy between the theoretical and the experimentally observed stability of the NPs, it was concluded that structural forces stabilize the gold nanoparticles. This was also borne out by AFM measurements.     

Exploring the ability of frozen-density embedding to model induced circular dichroism

In this study, we present calculations of the circular dichroism (CD) spectra of complexes between achiral and chiral molecules. Nonzero rotational strengths for transitions of the nonchiral molecule are induced by interactions between the two molecules, which cause electronic and/or structural perturbations of the achiral molecule. We investigate if the chiral molecule (environment) can be represented only in terms of its frozen electron density, which is used to generate an effective embedding potential. The accuracy of these calculations is assessed in comparison to full supermolecular calculations. We can show that electronic effects arising from specific interactions between the two subsystems can reliably be modeled by the frozen-density representation of the chiral molecule. This is demonstrated for complexes of 2-benzoylbenzoic acid with (-)-(R)-amphetamine and for a nonchiral, artificial amino acid receptor system consisting of ferrocenecarboxylic acid bound to a crown ether, for which a complex with l-leucine is studied. Especially in the latter case, where multiple binding sites and interactions between receptor and target molecule exist, the frozen-density results compare very well with the full supermolecular calculation. We also study systems in which a cyclodextrin cavity serves as a chiral host system for a small, achiral molecule. Problems arise in that case because of the importance of excitonic couplings with excitations in the host system. The frozen-density embedding cannot describe such couplings but can only capture the direct effect of the host electron density on the electronic structure of the guest. If couplings play a role, frozen-density embedding can at best only partially describe the induced circular dichroism. To illustrate this problem, we finally construct a case in which excitonic coupling effects are much stronger than direct interactions of the subsystem densities. The frozen density embedding is then completely unsuitable.     

Coupled-perturbed density-matrix functional theory equations. Application to static polarizabilities

Starting from the variational equations for the natural occupation numbers and the recently proposed eigenequations for the natural spin-orbitals, we derive coupled-perturbed density-matrix equations that furnish a linear response of the one-electron reduced density matrix to a static perturbation when the total energy is a functional of the one-electron reduced density matrix. Cases when some occupation numbers achieve exactly 0 or 1 or when the total number of the particles in a system is not preserved are taken into consideration. The scheme is applied to computing static polarizabilities from two simple density-matrix functionals. The behavior of the functionals is erratic and they provide only little or no improvement over the coupled-perturbed Hartree-Fock results.     

Effect of mass transfer on the film drainage between colliding drops

The influence of mass transfer on the drainage behaviour of the thin liquid film between two drops immersed in another liquid colliding at constant approach velocity has been studied experimentally. The liquid-liquid system used is glycerol in silicone oil. The transferred solute is acetone and the volume concentration difference across the interface ranges from 1 to 5%. The film thickness evolution has been measured using a laser interferometry technique. The direction of mass transfer (from the drops towards the film phase and inversely) has been investigated and the results compared to the case with no mass transfer. When the solute transfers from the drops towards the continuous phase, the drainage rate is significantly higher than in the case with no mass transfer. This result is interpreted as a consequence of the mass transfer induced surface mobility in the film region (the so-called Marangoni effect) due to localized surface tension differences. This effect has been demonstrated by the visualization of the flow patterns in the drops and in the film phase (using a particle tracer technique). In this case, the slope of the film height as a function of time seems to be independent of the approach velocity condition imposed on the drop and appears to be controlled by the interfacial tension gradient. In the opposite case, when the solute transfers from the continuous phase towards the drops, the film drainage rate is lowered with respect to the case of no mass transfer, goes to zero or even changes its sign depending on the mass transfer intensity. The results also show that in the range of solute concentration studied, the effect of mass transfer on the film drainage process takes place at large distances compared to the scales at which lubrication theory is valid.     

Reactive adsorption of hydrogen sulfide by promoted sorbents Cu‐ZnO/SiO2: active sites by experiment and simulation

In the Cu_x‐Zn_(1‐x)O/SiO₂ sorbents for ultradeep adsorptive removal of H₂S from gaseous fuel reformates for fuel cells at room temperature, Cu promoter sites significantly increase sulfur uptake capacity of the sorbents. We report characterization of the family of Cu_x‐Zn_(1‐x)O/SiO₂ sorbents for reactive adsorption of H₂S using X‐ray diffraction (XRD), Brunauer‐Emmett‐Teller (BET) surface area analysis, electron spin resonance (ESR), ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy (DRS) and calculations by the density functional theory (DFT). Both the supported ZnO phase and Cu promoter sites in the Cu_x‐Zn_(1‐x)O/SiO₂ sorbents are nano‐dispersed, as shown by XRD. The Cu_x‐Zn_(1‐x)O/SiO₂ sorbents contain Cu promoter as the Cu²⁺ site of octahedral geometry, as found by the complementary ESR and UV–vis DRS. Mechanism of the promoter effect of the Cu²⁺ site in the Cu_x‐Zn_(1‐x)O/SiO₂ sorbents in reaction with H₂S is proposed based on DFT calculations. Copyright © 2012 John Wiley & Sons, Ltd.