J. A. Padró

Na+–Na+ and Cl−–Cl− ion pairs in water: Mean force potentials by constrained molecular dynamics

Molecular dynamics simulations of Na++Na+, Na++Cl−, and Cl−+Cl− ions in dilute aqueous solution were carried out using a flexible single point charge (SPC) model for water. The resulting structural and dynamic properties are compared with experimental data and other computer simulation results. The potentials of mean force [W(r)] between the like ions were determined from constrained molecular dynamics simulations. The resulting W(r) for the Na+–Na+ ion pair is in qualitative agreement with other computer simulation findings, whereas the discrepancies are important in the case of the Cl−–Cl− ion pair. Our Cl−–Cl− mean force potential shows a moderate minimum which does not involve the unexpected strong attraction between chloride ions at short distances as predicted in earlier papers. The solvent structure around the ion pairs for separations corresponding to the maxima and minima of the W(r)’s is analyzed.

Dielectric properties and infrared spectra of liquid water: Influence of the dynamic cross correlations

Molecular dynamics simulation is applied to the study of the infrared spectra associated to translational, librational, and vibrational motions as well as to dielectric relaxation of liquid water. Flexible simple point charge (SPC) potentials are used in the simulations. Both dielectric and infrared spectra are calculated and compared with available experimental data. The effects of the dynamic cross correlations are analyzed by comparing the results for the time correlation functions of collective and individual properties.

Structure of liquid ethylene glycol: A molecular dynamics simulation study with different force fields

The structure of liquid ethylene glycol at room temperature is examined by performing molecular dynamics (MD) simulation studies for several different liquid phase force fields. We compare the properties obtained and analyze the differences which arise from the use of these models. A thorough study of molecular conformation and intermolecular structure for the different potential models is carried out given that three of the studied force fields have the same intermolecular parameters and different intramolecular interactions. In addition, the effect of molecular shape on the intermolecular structure is discussed. Due to the important role played by the highly directional forces occurring in hydrogen bonded systems, in their intermolecular structure and in the macroscopic properties of the system, we pay special attention to the analysis of the features of the hydrogen bonding patterns present in the liquid. Revealing an overall agreement with the available structural experimental data, the results obtained show that, for the simulated models, the intermolecular structure is rather similar. The dynamics of the system is studied through the self-diffusion coefficients and, in contrast to the structural properties, the results obtained for the distinct models are quite different.

Dynamics in hydrogen bonded liquids: water and alcohols

Molecular dynamics (MD) has been revealed as a powerful tool to investigate the structure and dynamics of hydrogen bonded liquids. This paper reviews recent works in which MD simulations have been used to study the influence of hydrogen bonding on different dynamic properties of liquid water and alcohols. The analysed properties include intermolecular vibrations, self-diffusion coefficients and reorientational correlation times. Finally, we present a MD study of the translational and reorientational dynamics of supercooled water at pressures up to 400 MPa. The influence of hydrogen bonding on the anomalous behaviour of the dynamic properties of liquid water at high pressures and low temperatures is discussed.

An interpretation of the low-frequency spectrum of liquid water

The controversy concerning the microscopic mechanical interpretation of the main spectral features appearing in the low-frequency spectrum of liquid water (0–400 cm−1) at room temperature is addressed. Information obtained from molecular dynamics simulations indicates the connection of the highest frequency band of the low-frequency spectrum of liquid water at ambient conditions (centered around 170 cm−1) with the existence of the hydrogen-bond network characteristic of liquid water, whereas no relation to hydrogen bonding is found for the lowest frequency mode, located at 60 cm−1.

Mean force potential for the calcium–chloride ion pair in water

The potential of mean force for the Ca2+–Cl− ion pair in water has been calculated from constrained MD simulations assuming a rigid simple point charged (SPC/E) model for water. Unlike for monovalent ion pairs the resulting potential does not show any noticeable minimum at short distances.

Molecular dynamics calculation of the infrared spectra in liquid H2O-D2O mixtures

Abstract The frequency distributions corresponding to the vibrational and librational molecular motions in liquid water and isotopically substituted mixtures are investigated. The infrared spectra is calculated from molecular dynamics simulations assuming a flexible single point charge (SPC) potential. A new set of parameters for the intramolecular potential which allows a reliable reproduction of the peaks of the experimental infrared spectra in the gas and liquid phases is proposed. Pure H 2 O, D 2 O and HDO liquids as well as dilute HDO solutions are analyzed. It has been verified that the OH and OD stretching modes in HDO are very close to those in H 2 O and D 2 O and that the HDO spectra is little influenced by different environments (HDO, H 2 O and D 2 O).

HYDROGEN BONDING INFLUENCE ON THE INTERMOLECULAR VIBRATIONAL SPECTRA OF LIQUID METHANOL

Abstract Intermolecular vibrational spectra of liquid methanol at three thermodynamic states are studied by molecular dynamics simulation. A geometrical criterion is used to determine the number of hydrogen bonds that each molecule has. The power spectra of the oxygen velocity autocorrelation functions for molecules with different number of hydrogen bonds are calculated separately. It is observed that the shoulder around 200 cm−1 is due to molecules participating in two hydrogen bonds whereas the peak at lower frequencies (30–50 cm−1) appears for all molecules independently of their bonding state.

Dynamics and hydrogen bonding in liquid ethanol

A detailed knowledge of the microscopic properties of liquids such as water and alcohols is indispensable for a deep understanding of many chemical and biological processes. It is well known that properties of these liquids are inuenced markedly by the existence of hydrogen bonds among their molecules. Thus, it should be useful to investigate the relationship between hydrogen bonding and molecular behaviour. The direct study of molecular motions in liquids from experiments is rather adiA cult task, and computer simulation methods such as molecular dynamics (MD) (1)are used widely to obtain microscopic information. Though based on eA ec- tive interaction potential models, MD provides detailed pictures of atomic behaviour which are very helpful for the development of new theories and models as well as for the reliable interpretation of the experimental data. Dielectric properties of polar liquids are strongly related to the characteristics of the reorientational mol- ecular motions. The frequency-dependent dielectric per- mittivity of water and simple liquid alcohols in the microwave and up to the far-infrared regions has been measured (2, 3). These experimental data were analysed by ® tting the results to a multiple Debye relaxation model. In the case of liquid ethanol, the best ® t was obtained by considering an analytical expression with three relaxation times. However, as far as we know, there has been no detailed study of the reorientational motions of diA erent molecules in liquid ethanol. Pre- liminary results were reported in an MD study of dif- ferent structural and dynamic properties of this liquid (4). The main objective of this work is to investigate the molecular reorientations in liquid ethanol at diA erent temperatures. Special attention will be paid to the hydrogen bonds and to their inuence on the character- istics of these molecular motions.

Friction kernels for the relative dynamics of ion pairs in water

The dynamic properties of Cl−–Cl−, Cl−–Na+, and Na+–Na+ ion pairs in water are investigated. It is assumed that the interionic separations obey the generalized Langevin equation. The random forces on the ion pairs are calculated from constrained molecular‐dynamics simulations at three interionic distances for each ion pair. The Gaussianity of the random forces is checked. The time‐dependent friction kernels for the relative motions are determined from the random forces by using the fluctuation–dissipation theorem. The dependence of the friction kernels on both the solute species and the intersolute separation is explored. Special attention is paid to the relationship of the friction kernels with the characteristics of the configurations of the solvent molecules in the neighborhood of the ions. The coupling between the intramolecular motions of water and the relative ionic motions is discussed. The association–dissociation processes for the three ion pairs are analyzed within the framework of the Kramers a...