Szabolcs Bálint

The structure of aqueous sodium hydroxide solutions: A combined solution x-ray diffraction and simulation study

To determine the structure of aqueous sodium hydroxide solutions, results obtained from x-ray diffraction and computer simulation (molecular dynamics and Car-Parrinello) have been compared. The capabilities and limitations of the methods in describing the solution structure are discussed. For the solutions studied, diffraction methods were found to perform very well in describing the hydration spheres of the sodium ion and yield structural information on the anions hydration structure. Classical molecular dynamics simulations were not able to correctly describe the bulk structure of these solutions. However, Car-Parrinello simulation proved to be a suitable tool in the detailed interpretation of the hydration sphere of ions and bulk structure of solutions. The results of Car-Parrinello simulations were compared with the findings of diffraction experiments.

Association of frustrated phosphine–borane pairs in toluene: molecular dynamics simulations

Explicit solvent molecular dynamics simulations of the ((t)Bu)(3)P/B(C(6)F(5))(3) pair in toluene allowed the estimation of the degree of intermolecular association and the population of encounter complex states in solution phase.

Solution structure of NaNO3 in Water: Diffraction and molecular dynamics simulation study

The structure of a series of aqueous sodium nitrate solutions (1.9-7.6 M) was studied using a combination of experimental and theoretical methods. The results obtained from diffraction (X-ray, neutron) and molecular dynamics simulation have been compared and the capabilities and limitations of the methods in describing solution structure are discussed. For the solutions studied, diffraction methods were found to perform very well in description of hydration spheres of the sodium ion but do not yield detailed structural information on the anions hydration structure. Molecular dynamics simulations proved to be a suitable tool in the detailed interpretation of the hydration sphere of ions, ion pair formation, and bulk structure of solutions.

Hydrogen bonded network properties in liquid formamide.

Molecular dynamics simulations have been performed for liquid formamide using two different types of potential model (OPLS, Cordeiro). The structural results obtained from simulation were compared to experimental (x-ray and neutron diffraction measurements) outcomes. A generally good agreement for both models examined has been found, but in the hydrogen bonded region (2.9 A) the Cordeiro model shows a slightly better fit. Besides the evaluation of partial radial distribution functions, orientational correlation functions and energy distribution functions, describing the hydrogen bonded structure, have been calculated based on the statistical analysis of configurations, resulting into a new insight in the clustering properties and topology of hydrogen bonded network. It has been shown that in liquid formamide exists a continuous hydrogen bonded network and from the analysis of the distribution of small rings revealed the ring size distribution in liquid formamide. Our study resulted that the ring size distribution of the hydrogen bonded liquid formamide shows a broad distribution with a maximum around 11. It has been found that the topology in formamide is significantly different than in water.

Hydrogen bond network topology in liquid water and methanol: a graph theory approach

Networks are increasingly recognized as important building blocks of various systems in nature and society. Water is known to possess an extended hydrogen bond network, in which the individual bonds are broken in the sub-picosecond range and still the network structure remains intact. We investigated and compared the topological properties of liquid water and methanol at various temperatures using concepts derived within the framework of graph and network theory (neighbour number and cycle size distribution, the distribution of local cyclic and local bonding coefficients, Laplacian spectra of the network, inverse participation ratio distribution of the eigenvalues and average localization distribution of a node) and compared them to small world and Erdős-Rényi random networks. Various characteristic properties (e.g. the local cyclic and bonding coefficients) of the network in liquid water could be reproduced by small world and/or Erdős-Rényi networks, but the ring size distribution of water is unique and none of the studied graph models could describe it. Using the inverse participation ratio of the Laplacian eigenvectors we characterized the network inhomogeneities found in water and showed that similar phenomena can be observed in Erdős-Rényi and small world graphs. We demonstrated that the topological properties of the hydrogen bond network found in liquid water systematically change with the temperature and that increasing temperature leads to a broader ring size distribution. We applied the studied topological indices to the network of water molecules with four hydrogen bonds, and showed that at low temperature (250 K) these molecules form a percolated or nearly-percolated network, while at ambient or high temperatures only small clusters of four-hydrogen bonded water molecules exist.

Structure of liquid nitromethane: Comparison of simulation and diffraction studies

Simulation (molecular dynamics and Car-Parrinello [Phys. Rev. Lett. 55, 2471 (1985)]) and diffraction (x-ray and neutron) studies on nitromethane are compared aiming at the determination of the liquid structure. Beyond that, the capabilities of the methods to describe liquid structure are discussed. For the studied liquid, the diffraction methods are performing very well in the determination of intramolecular structure, but they do not give detailed structural information on the intermolecular structure. The good agreement between the diffraction experiments and the results of molecular dynamics simulations justifies the use of simulations for the more detailed description of the liquid structure using partial radial distribution functions and orientational correlation functions. Liquid nitromethane is described as a molecular liquid without strong intermolecular interactions such as hydrogen bonding, but with detectable orientational correlations resulting in preferential antiparallel order of the neighboring molecules.

Pd-decorated m-BiVO4/BiOBr ternary composite with dual heterojunction for enhanced photocatalytic activity

We introduce a unique material ensemble to boost the photocatalytic activity of m-BiVO4 by creating dual heterojunction of bismuth oxybromide nanosheets and Pd nanodomains. The m-BiVO4/BiOBr/Pd ternary composite demonstrates substantially higher photocatalytic activity compared to pure m-BiVO4. We demonstrate for the first time the use of such visible light photocatalyst in highly efficient degradation of polychlorinated biphenyls.

Speciation and structure of tin(II) in hyper-alkaline aqueous solution

The identity of the predominating tin(ii)-hydroxide complex formed in hyper-alkaline aqueous solutions (0.2 ≤CNaOH≤ 12 mol dm(-3)) is determined by potentiometric titrations, Raman, Mössbauer and XANES spectroscopy, supplemented by quantum chemical calculations. Thermodynamic studies using a H2/Pt electrode up to free hydroxide concentrations of 1 mol dm(-3) showed the presence of a single monomeric complex with a tin(II) : hydroxide ratio of 1 : 3. This observation together with Raman and Mössbauer spectroscopic measurements supplemented by quantum mechanical calculations proved that the predominating complex is [Sn(OH)3](-), and that the presence of the other possible complex, [SnO(OH)](-), could not be proven with either experiments or simulations. The structure of the trihydroxidostannate(II) complex, [Sn(OH)3](-), was determined by EXAFS and was found to be independent of the applied hydroxide and tin(II) concentrations. The mean Sn-O bond distance is short, 2.078 Å, and in very good agreement with the only structure reported in the solid state. It is also shown that at pH values above 13 the speciation of the predominant trihydroxidostannate(II) complex is not affected by the presence of high concentrations of chloride ions.

Complete Structural Characterization of Metallacyclic Complexes in Solution-Phase Using Simultaneously X-ray Diffraction and Molecular Dynamics Simulation

Wide-angle X-ray diffraction and molecular dynamics simulation has been used to perform complete structural characterization of nitromethane solution of a 16-membered gold(I) ring. The joint application of these two methods was an adequate tool to describe not only the structure of the complex but also the solvation properties of the complex in nitromethane and the effect of the solvation on the bulk structure. It has been found that a relatively diffuse slightly distorted solvation shell is formed around the complex, following the shape of the molecule. Nitromethane molecules in the solvation sphere are distributed randomly; no special orientation can be detected. The interaction energy of the complex with nitromethane molecules is attractive. In bulk, besides the antiparallel orientation of the nitromethane molecules, T-shape orientation and long-range order in antidipole orientation can also be detected.

Speciation and the structure of lead(II) in hyper-alkaline aqueous solution.

The identity of the predominating lead(ii) species in hyper-alkaline aqueous solution has been determined by Raman spectroscopy, and ab initio quantum chemical calculations and its structure has been determined by EXAFS. The observed and calculated Raman spectra for the [Pb(OH)3](-) complex are in agreement while they are different for two-coordinated complexes and complexes containing Pb[double bond, length as m-dash]O double bonds. Predicted bond lengths are also consistent with the presence of [Pb(OH)3](-) and exclude the formation of Pb[double bond, length as m-dash]O double bond(s). These observations together with experimentally established analogies between lead(ii) and tin(ii) in hyper-alkaline aqueous solutions suggest that the last stepwise hydroxido complex of lead(ii) is [Pb(OH)3](-). The Pb-O bond distance in the [Pb(OH)3](-) complex as determined is remarkably short, 2.216 Å, and has low symmetry as no multiple back-scattering is observed. The [Pb(OH)3](-) complex has most likely trigonal pyramidal geometry as all reported three-coordinated lead(ii) complexes in the solid state. From single crystal X-ray data, the bond lengths for O-coordinated lead(ii) complexes with low coordination numbers are spread over an unusually wide interval, 2.216-2.464 Å for N = 3. The Pb-O bond distance is at the short side and within the range of three coordinated complexes, as also observed for the trihydroxidostannate(ii) complex indicating that the hydroxide ion forms short bonds with d(10)s(2) metal ions with occupied anti-bonding orbitals.