Andrey G. Kalinichev

Hydrogen bonding in supercritical water: a Monte Carlo simulation

Abstract A detailed analysis of hydrogen bonding in supercritical water is made from Monte Carlo simulations along the 773 K isotherm over a wide range of pressures from 10 to 10000 MPa. It is shown that an energetic definition of H-bonding is much more effective in separating H-bonded and non-bonded molecular pairs than the widely used geometric definition, whereas a combination of both may be yet preferable at high pressures. Compared to H bonds in liquid water, the H bonds at 773 K are, on average, 10% weaker, 5% longer, and more bent. The quantitative characteristics of supercritical H bonds remain almost invariant over the entire pressure range studied.

Molecular modeling of the structure and dynamics of the interlayer and surface species of mixed-metal layered hydroxides: Chloride and water in hydrocalumite (Friedel's salt)

Abstract The dynamical behavior of Cl- and H2O molecules in the interlayer and on the (001) surface of the Ca-aluminate hydrate hydrocalumite (Friedel’s salt) over a range of temperatures from -100 to 300 °C was studied using isothermal-isobaric molecular dynamics computer simulations. This phase is currently the best available model compound for other, typically more disordered, mixed-metal layered hydroxides. The computed crystallographic parameters and density are in good agreement with available X-ray diffraction data and the force field developed for these simulations preserves the structure and density to within less than 2% of their measured values. In contrast to the highly ordered arrangement of the interlayer water molecules interpreted from the X-ray data, the simulations reveal significant dynamic disorder in water orientations. At all simulated temperatures, the interlayer water molecules undergo rapid librations (hindered hopping rotations) around an axis essentially perpendicular to the layers. This results in breaking and reformation of hydrogen bonds with the neighboring Cl- anions and in a time-averaged nearly uniaxial symmetry at Cl-, in good agreement with recent 35Cl NMR measurements. Power spectra of translational, librational, and vibrational motions of interlayer and surface Cl- and H2O were calculated as Fourier transforms of the atomic velocity autocorrelation functions and compared with the corresponding spectra and dynamics for a bulk aqueous solution. The ordered interlayer space has significant effects on the motions. Strong electrostatic attraction between interlayer water molecules and Ca atoms in the principal layer makes the Ca···OH2 bond direction the preferred axis for interlayer water librations. The calculated diffusion coefficient of Cl- as an outer-sphere surface complex is almost three times that of inner-sphere Cl-, but is still about an order of magnitude less than that of Cl- in bulk aqueous solution at the same temperature.

Size and topology of molecular clusters in supercritical water: a molecular dynamics simulation

Abstract A hybrid criterion of hydrogen bonding is applied to the analysis of molecular dynamics trajectories simulated for several near- and supercritical thermodynamic conditions. Even at vapor-like densities supercritical water is shown to contain large molecular clusters, consisting of up to 10 molecules. Relative abundances of topologically different trimers, tetramers, and pentamers are examined, and chain-like clusters are found predominant under supercritical conditions. In contrast to the results of quantum-chemical calculations for isolated water clusters, ring-like clusters are only rarely formed in supercritical water.

Interlayer structure and dynamics of Cl-bearing hydrotalcite: far infrared spectroscopy and molecular dynamics modeling

Abstract Comparison of the observed far-infrared (FIR) spectrum of Cl--containing hydrotalcite, [Mg3Al(OH)8]Cl·3H2O, to a power spectrum calculated using molecular dynamics (MD) computer simulation, provides a greatly increased understanding of the structure and vibrational dynamics in the interlayers of layered double hydroxides. Good agreement between the observed FIR band positions and the simulated power spectrum illustrates the capability of this combination of experimental and computational techniques to effectively probe the structure and dynamics of water in nano-pores and other confined spaces. The simulation model assumes an ordered Mg3Al arrangement in the octahedral sheet and no constraints on the movement of any atoms or on the geometry and symmetry of the simulation supercell. Calculated anisotropic components of the individual atomic power spectra in combination with computed animations of the vibrational modes from normal mode analysis allow for reliable interpretations of the observed spectral bands. For the vibrations related to octahedral cation motions, bands near 145, 180, and 250 cm-1 are due dominantly to Mg vibration in the c direction (perpendicular to the hydroxide layers), Al vibration in the c direction, and Mg and Al vibrations in the a-b plane (parallel to the hydroxide layers), respectively. The low frequency vibrational motions of the interlayer are controlled by a network of hydrogen bonds formed between interlayer water molecules, Cl- ions, and the OH groups of the main hydroxide layers. The bands near 40-70 cm-1 are related to the translational motions of interlayer Cl- and H2O in the a-b plane, and the bands near 120 cm-1 and 210 cm-1 are largely due to translational motions of the interlayer species in the c direction. The three librational modes of interlayer water molecules near 390, 450, and 540 cm-1 correspond to twisting, rocking, and wagging hindered rotations, respectively. The spectral components of the interlayer Cl- motions are remarkably similar to those of bulk aqueous chloride solutions, reflecting the structural and dynamic similarity of the nearest-neighbor Cl- environments in the interlayer and in solution.

Thermodynamics and structure of molecular clusters in supercritical water

Abstract The extent of hydrogen-bonded cluster formation in near- and supercritical water has been studied by application of a hybrid hydrogen-bonding criterion to the analysis of Monte Carlo computer simulation results. Up to 10% of water molecules were found to constitute H-bonded clusters even in dilute supercritical water vapor ( T ∗ =1.04 , ρ ∗ =0.06 ), and the maximum size of such molecular complexes formed may be as large as seven molecules per cluster under these conditions. Relative abundance and geometric and energetic characteristics of topologically different trimers, tetramers, and pentamers were also examined. Open chain- and tree-like clusters are preferentially formed in supercritical water, while cyclic ring-like structures occur only rarely. Partitioning of molecules between clusters of the same size, but topologically different structure is found to be virtually independent of temperature and density.

Molecular dynamics of supercritical water: A computer simulation of vibrational spectra with the flexible BJH potential

Abstract Molecular dynamics (MD) computer simulations have been performed for a system of 200 water molecules interacting by means of the Bopp-Jancso-Heinzinger (BJH) intermolecular interaction potential under supercritical conditions (630 T 3 ) and pressures (0.25 P 3 and 0.9718 g/cm 3 , respectively. The spectra of intramolecular vibrations are calculated as Fourier transforms of the velocity autocorrelation functions of hydrogen atoms. The frequencies of both symmetric and asymmetric stretching vibrations increase with temperature and decrease with density (pressure), while the frequency of the Hue5f8Oue5f8H bending vibrations remains almost constant over the wide range of thermodynamic conditions studied. These findings are in good agreement with available IR and Raman spectroscopic measurements and allow us to expect the BJH potential to be able to predict changes in the vibrational behavior of water molecules in response to changes of thermodynamic parameters covering the entire range of temperatures, densities, and compositions characteristic of hydrothermal systems.

Molecular dynamics modelling of hydrated mineral interlayers and surfaces: structure and dynamics

Abstract This paper reviews the results of recent molecular dynamics (MD) modelling studies of the interaction of water and solute species with mineral surfaces and their behaviour in mineral interlayers. Emphasis is on results for single and double hydroxide phases. Computational results are presented for water and anions in the interlayers of the Ca2Al, Mg2Al, and LiAl2 layered double hydroxides and on the surfaces of the Ca2Al phase. Detailed results for water on the (001) surface of brucite (Mg(OH)2) are presented and compared to published results for other phases. In all these cases, hydrogen bonding and the development of a hydrogen-bond network involving the H2O molecules and the solid substrate play very significant roles. The MD methods are especially effective for investigating the structure and dynamics of mineral-fluid interfaces and mineral interlayers, because they can be applied to systems containing hundreds to thousands of atoms and for extended durations of the order of nanoseconds.

Dissociation of carbonic acid: Gas phase energetics and mechanism from ab initio metadynamics simulations

A comprehensive metadynamics study of the energetics, stability, conformational changes, and mechanism of dissociation of gas phase carbonic acid, H2CO3, yields significant new insight into these reactions. The equilibrium geometries, vibrational frequencies, and conformer energies calculated using the density functional theory are in good agreement with the previous theoretical predictions. At 315 K, the cis-cis conformer has a very short life time and transforms easily to the cis-trans conformer through a change in the O=C-O-H dihedral angle. The energy difference between the trans-trans and cis-trans conformers is very small (approximately 1 kcal/mol), but the trans-trans conformer is resistant to dissociation to carbon dioxide and water. The cis-trans conformer has a relatively short path for one of its hydroxyl groups to accept the proton from the other end of the molecule, resulting in a lower activation barrier for dissociation. Comparison of the free and potential energies of dissociation shows that the entropic contribution to the dissociation energy is less than 10%. The potential energy barrier for dissociation of H2CO3 to CO2 and H2O from the metadynamics calculations is 5-6 kcal/mol lower than in previous 0 K studies, possibly due to a combination of a finite temperature and more efficient sampling of the energy landscape in the metadynamics calculations. Gas phase carbonic acid dissociation is triggered by the dehydroxylation of one of the hydroxyl groups, which reorients as it approaches the proton on the other end of the molecule, thus facilitating a favorable H-O-H angle for the formation of a product H2O molecule. The major atomic reorganization of the other part of the molecule is a gradual straightening of the O=C=O bond. The metadynamics results provide a basis for future simulation of the more challenging carbonic acid-water system.

Computer Simulations of Aqueous Fluids at High Temperatures and Pressures

Water is a unique substance in many respects. It is the only chemical compound that occurs in all three physical states (solid, liquid, and vapor) under the thermodynamic conditions unique to the Earth’s surface. It has played a principal role in major natural processes during the long geological and biological history of the planet. Its oustanding properties as a solvent and its general abundance almost everywhere on our planet’s suface have made it also an integral part of many technological processes since the very beginning of human civilization.

Monte Carlo Simulations of Water under Supercritical Conditions. I. Thermodynamic and Structural

Abstract The thermodynamic and structural properties of water along two supercritical isotherms at 673 and 773 K in the pressure range from 0.3 to 30 kbar have been studied by the NPT-ensemble Monte Carlo method using a TIP4P intermolecular pair potential. Simulated values of the configurational enthalpy, molar volume, isobaric heat capacity, isothermal compressibility, and thermal expansion coefficient are found to be in a rather good agreement with experimental data, although the critical point of the TIP4P water model is supposed to lie about 50 degrees lower than observed experimentally. The analysis of simulated atom-atom radial distribution functions as well as the dimerization energy distributions clearly shows that hydrogen bonding persists under the conditions studied, despite the fact that the water structure may be considered as argon-like in terms of oxygen-oxygen distribution functions (i.e. close to that of a simple liquid)