Tünde Megyes

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.

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.

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.

Benchmarking polarizable molecular dynamics simulations of aqueous sodium hydroxide by diffraction measurements

Results from molecular dynamics simulations of aqueous hydroxide of varying concentrations have been compared with experimental structural data. First, the polarizable POL3 model was verified against neutron scattering using a reverse Monte Carlo fitting procedure. It was found to be competitive with other simple water models and well suited for combining with hydroxide ions. Second, a set of four polarizable models of OH- were developed by fitting against accurate ab initio calculations for small hydroxide-water clusters. All of these models were found to provide similar results that robustly agree with structural data from X-ray scattering. The present force field thus represents a significant improvement over previously tested nonpolarizable potentials. Although it cannot in principle capture proton hopping and can only approximately describe the charge delocalization within the immediate solvent shell around OH-, it provides structural data that are almost entirely consistent with data obtained from scattering experiments.

X-ray diffraction study of lithium halides in methanol

Abstract The structure of solutions of lithium halide (LiCl and LiI) in methanol has been studied by X-ray diffraction. The parameters for ion-solvent interaction are in good agreement with those found in earlier studies both in methanol and aqueous solutions. In low concentrated solutions six solvate molecules surround the Li + ion. The solvation shell of Cl − ion is composed of six methanol molecules and that of I − of eight methanol molecules. The coordination numbers decrease with increase in concentration. The concentration dependence for chloride and iodide ion shows a significant difference. On the basis of the stoichiometry and parameters obtained, different models are suggested to explain the structure of the solutions.

Ab Initio Quantum Chemical and Molecular Dynamics Simulation Study of Lithium Iodide in Acetonitrile

Abstract Ab initio calculations for clusters containing Li+ and up to six acetonitrile molecules have been performed by using the B3LYP, MP2(FC) and MP2(Full) methods with the 6-311+G** basis set. The three computational methods give similar results for highly symmetric structures. In the case of MP2 calculation the inclusion of core electrons does not effect the values of the intermolecular distances and binding energy in the Li+-acetonitrile complex. The average ion–ligand distance increases with increasing number of acetonitrile molecules in the cluster. It has been shown that the energy required to incorporate an additional acetonitrile molecule in the Li-acetonitrile clusters containing 5 and 6 acetonitrile molecules is almost equal to the acetonitrile–acetonitrile interaction energy in the most stable acetonitrile dimer and therefore it is very difficult to predict whether the coordination sphere of Li+ contains four, five or six acetonitrile molecules. A molecular dynamics simulation has also been performed on LiI-acetonitrile system. The coordination number of the Li+ and I− have been found to be 5.97 and 14, respectively. The first solvation shell of Li+ has an octahedral geometry; the coordination sphere of the I− ion does not show any special arrangement.