Imre Bakó

Car-Parrinello molecular dynamics simulation of the hydrated calcium ion

First principles molecular dynamics has been used to investigate the structural, vibrational, and energetic properties of [Ca(H2O)n]2+ clusters with n=1–9, and the hydration shell of a calcium ion in a periodically repeated box with 54 water molecules. We find that, while stable highly symmetric Ca–water clusters can be formed with up to eight water molecules, the n=9 cluster dissociates into the last stable [Ca(H2O8]2+ complex. In solution the first hydration shell around the Ca2+ ion contains six water molecules in an octahedral arrangement. The electronic structure of nearest neighbor hydration shell water molecules has been examined with a localized orbital analysis. The average dipole moments of hydration water molecules was found to be increased by about 0.4 Debye relative to that of pure water.

Reactivity Models of Hydrogen Activation by Frustrated Lewis Pairs: Synergistic Electron Transfers or Polarization by Electric Field?

Two alternative qualitative reactivity models have recently been proposed to interpret the facile heterolytic cleavage of H2 by frustrated Lewis pairs (FLPs). Both models assume that the reaction takes place via reactive intermediates with preorganized acid/base partners; however, they differ in the mode of action of the active centers. In the electron transfer (ET) model, the hydrogen activation is associated with synergistic electron donation processes with the simultaneous involvement of active centers and the bridging hydrogen, showing similarity to transition-metal-based and other H2-activating systems. In contrast, the electric field (EF) model suggests that the heterolytic bond cleavage occurs as a result of polarization by the strong EF present in the cavity of the reactive intermediates. To assess the applicability of the two conceptually different mechanistic views, we examined the structural and electronic rearrangements as well as the EFs along the H2 splitting pathways for a representative set of reactions. The analysis reveals that electron donations developing already in the initial phase are general characteristics of all studied reactions, and the related ET model provides qualitative interpretation for the main features of the reaction pathways. On the other hand, several arguments have emerged that cast doubt on the relevance of EF effects as a conceptual basis in FLP-mediated hydrogen activation.

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.

LKP model of the inhibition mechanism of thiourea compounds

Abstract Interaction of corrosion inhibitors with metal surfaces in acidic solutions was investigated using a nonlinear model encompassing the Langmuir adsorption isotherm. A regression equation, w...

Water–methanol mixtures: topology of hydrogen bonded network

Molecular dynamics simulation has been performed to study the structure of water-methanol mixtures. Besides the evaluation of partial radial distribution functions describing the hydrogen-bonded structure of the mixtures with different composition, the statistical analysis of configurations was introduced resulting in a new insight in the clustering properties and topology of hydrogen-bonded network. The results have shown that mixtures of methanol and water exhibit extended structures in solution. At low methanol concentration the water molecules form a percolated network, the methanol molecules are incorporated as monomers or short chains and together form a percolated system. In methanol-rich mixtures short water chains and longer methanol chains build up the hydrogen-bonded clusters in the system. On the basis of the statistical analysis of configurations obtained from molecular dynamics simulation it has been found that more methanol molecules are connected to non-cyclic entities, while more water molecules form rings that might have been predicted on the basis of the stoichiometry of the mixtures. This finding can be explained by the presence of microscopic configurational inhomogeneity in water-methanol mixtures.

Polynomial model of the inhibition mechanism of thiourea derivatives

Interaction of corrosion inhibitors with metal surfaces in acidic solutions was modelled by using a polynomial model based on the Langmuir adsorption isotherm. The curve-fitting procedure yielded a regression equation with a multiple correlation coefficient equal to 0.982.

Corrosion inhibition by 1-hydroxy-ethane-1,1-diphosphonic acid : an electrochemical impedance spectroscopy study

Impedance spectra of carbon steel in the presence of 1-hydroxy-ethane-1,1-diphosphonic acid (HEDP) has been investigated in neutral solutions. The aim of the measurements was to study the effect of HEDP on the carbon steel corrosion as a function of inhibitor concentration. An optimal inhibitor concentration was found. A two-time constant equivalent circuit model including a constant phase element was consistent with experimental data in the frequency range studied. The system parameters, obtained by a nonlinear fit procedure are discussed in terms of the model used. Inhibitor efficiencies derived from impedance data are compared with those obtained from polarization and weight loss measurements. Results of these investigations show that a low concentrations, HEDP inhibits carbon steel corrosion by a precipitation mechanism forming insoluble iron complexes and repairing the porous oxide layer formed n the metal surface. Increasing HEDP concentration beyond the optimal value decreases its inhibition efficiency due to the dissolution of the oxide layer.

Ethylene glycol dimers in the liquid phase: A study by x-ray and neutron diffraction

Molecular conformations and intermolecular H bonding in liquid ethylene glycol have been studied by neutron diffraction with hydrogen/deuterium isotopic substitution, and by x-ray diffraction measurements at room temperature. For comparison, conformations of molecules and their dimers in the gas phase have also been calculated, based on the density-functional theory. Energies, geometries, and vibrational frequencies of dimers were analyzed. Diffraction data analyzed by the Monte Carlo method resulted in a molecular structure in agreement with the findings from gas-phase electron diffraction experiments and density-functional calculations. Conformers of ethylene glycol molecules in the liquid phase were identified as a gauche OCCO arrangement, stabilized by intramolecular hydrogen bonding, which is in agreement with the results of gas-phase density-functional calculations. Data analysis resulted in an average of three hydrogen-bonded nearest neighbors of molecules in liquid ethylene glycol. In addition, ex...

Molecular dynamics investigation of the inter- and intramolecular motions in liquid methanol and methanol-water mixtures

Molecular dynamics computer simulations were performed at room temperature and selected density on pure liquid methanol and water, and on methanol-water mixtures with methanol mole fractions of 0·1...

Investigation of the uniqueness of the reverse Monte Carlo method: Studies on liquid water

Reverse Monte Carlo (RMC) simulation of liquid water has been performed on the basis of experimental partial pair correlation functions. The resulted configurations were analyzed in various aspects; the hydrogen bond angle distribution, three body correlation and orientational correlation were calculated. The question of the uniqueness of the RMC method was also examined. In order to do this, two conventional computer simulations of liquid water with different potential models were performed, and the resulted pair correlation function sets were fitted by RMC simulations. The resulted configurations were then compared to the original configurations to study how the RMC method can reproduce the original structure. We showed that the configurations produced by the RMC method are not uniquely related to the pair correlation functions even if the interactions in the original system were pairwise additive. Therefore the difference between the original simulated and the RMC configurations can be a measure of the uncertainty of the RMC results on real water. We found that RMC produces less ordered structure than the original one from various aspects. However, the orientational correlations were reproduced rather successfully. The RMC method exaggerates the amount of the close packed patches in the structure, although these patches certainly exist in liquid water.