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Molecular dynamics simulations of charged and neutral solutes in dimethyl sulfoxide (DMSO)-chloroform mixtures reveal pronounced nonideality in the solute diffusion with changes of composition of the mixtures. The diffusion coefficient of the anionic solute first decreases, passes through a minimum at DMSO mole fraction of about 0.50, and then increases to reach its value for pure DMSO. The diffusion coefficients of the cationic and neutral solutes are found to decrease with increase in DMSO content of the solvent mixture. The extent of nonideality in the diffusion and orientational relaxation of solvent molecules is found to be somewhat stronger than that in diffusion of the anionic solute in these mixtures. We have also calculated the relaxation of hydrogen bonds formed between DMSO and chloroform molecules. The lifetimes of DMSO-chloroform hydrogen bonds are found to increase monotonically with increase in DMSO concentration. The average number of hydrogen bonds and their average energies are also computed. It is found that an increase in DMSO concentration causes a decrease in the number of DMSO-chloroform hydrogen bonds per DMSO or chloroform molecules but increases the strength of these hydrogen bonds., (Copyright © 2011 Wiley Periodicals, Inc.)

A first principles study of the dynamics of supercritical methanol is carried out by means of ab initio molecular dynamics simulations. In particular, the fluctuation dynamics of hydroxyl stretch frequencies, hydrogen bonds, dangling hydroxyl groups, and orientation of methanol molecules are investigated for three different densities at 523 K. Apart from the dynamical properties, various equilibrium properties of supercritical methanol such as the local density distributions and structural correlations, hydrogen bonding aspects, frequency-structure correlations, and dipole distributions of methanol molecules are also investigated. In addition to the density dependence of various equilibrium and dynamical properties, their dependencies on dispersion interactions are also studied by carrying out additional simulations using a dispersion corrected density functional for all the systems. It is found that the hydrogen bonding between methanol molecules decreases significantly as we move to the supercritical state from the ambient one. The inclusion of dispersion interactions is found to increase the number of hydrogen bonds to some extent. Calculations of the frequency-structure correlation coefficient reveal that a statistical correlation between the hydroxyl stretch frequency and the nearest hydrogen-oxygen distance continues to exist even at supercritical states of methanol, although it is weakened with increase of temperature and decrease of density. In the supercritical state, the frequency time correlation function is found to decay with two time scales: One around or less than 100 fs and the other in the region of 250-700 fs. It is found that, for supercritical methanol, the times scales of vibrational spectral diffusion are determined by an interplay between the dynamics of hydrogen bonds, dangling OD groups, and inertial rotation of methanol molecules and the roles of these various components are found to vary with density of the supercritical solvent. Effects of system size on the calculated structural and dynamical properties are also investigated in the present study. [ABSTRACT FROM AUTHOR]

A theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in aqueous ionic solutions is presented from first principles without employing any empirical potential models. The present calculations are based on ab initio molecular dynamics for trajectory generation and wavelet analysis of the simulated trajectories for time dependent frequency calculations. Results are obtained for two different deuterated aqueous solutions: the first one is a relatively dilute solution of a single Cl- ion and the second one is a concentrated solution of NaCl (∼3M) dissolved in liquid D2O. It is found that the frequencies of OD bonds in the anion hydration shell, i.e., those which are hydrogen bonded to the chloride ion, have a higher stretch frequency than those in the bulk water. Also, on average, the frequencies of hydration shell OD modes are found to increase with increase in the anion-water hydrogen bond distance. On the dynamical side, when the vibrational spectral diffusion is calculated exclusively for the hydration shell water molecules in the first solution, the dynamics reveals three time scales: a short-time relaxation (∼200 fs) corresponding to the dynamics of intact ion-water hydrogen bonds, a slower relaxation (∼3 ps) corresponding to the lifetimes of chloride ion-water hydrogen bonds, and another longer-time constant (∼20 ps) corresponding to the escape dynamics of water from the anion hydration shell. Existence of such three time scales for hydration shell water molecules was also reported earlier for water containing a single iodide ion using classical molecular dynamics [B. Nigro et al., J. Phys. Chem. A 110, 11237 (2006)]. Hence, the present study confirms the basic results of this earlier work using a different methodology. However, when the vibrational spectral diffusion is calculated over all the OD modes, only two time scales of ∼150 fs and ∼2.7 ps are found without the slowest component of ∼20 ps. This is likely because of the very small weight that the hydration shell water molecules carry to the overall spectral diffusion in the solution containing a single ion. For the concentrated solution also, the slowest component of ∼20 ps is not found in the spectral diffusion of all water molecules because a distinct separation between the hydration shell and bulk water in terms of their stretch frequencies does not hold at this high concentration regime. The present first principles results are compared with those of the available experiments and classical simulations. [ABSTRACT FROM AUTHOR]

We have performed a series of molecular dynamics simulations of water-acetone mixtures containing either an ionic solute or a neutral hydrophobic solute to study the extent of nonideality in the dynamics of these solutes with variation of composition of the mixtures. The diffusion coefficients of the charged solutes, both cationic and anionic, are found to change nonmonotonically with the composition of the mixtures showing strong nonideality of their dynamics. Also, the extent of nonideality in the diffusion of these charged solutes is found to be similar to the nonideality that is observed for the diffusion and orientational relaxation of water and acetone molecules in these mixtures which show a somewhat similar changes in the solvation characteristics of charged and dipolar solutes with changes of composition of water-acetone mixtures. The diffusion of the hydrophobic solute, however, shows a monotonic increase with increase of acetone concentration showing its different solvation characteristics as compared to the charged and dipolar solutes. The links between the nonideality in diffusion and solvation structures are further confirmed through calculations of the relevant solute-solvent and solvent-solvent radial distribution functions for both ionic and hydrophobic solutes. We have also calculated various pair dynamical properties such as the relaxation of water-water and acetone-water hydrogen bonds and residence dynamics of water molecules in water and acetone hydration shells. The lifetimes of both water-water and acetone-water hydrogen bonds and also the residence times of water molecules are found to increase steadily with increase in acetone concentration. No maximum or minimum was found in the composition dependence of these pair dynamical quantities. The lifetimes of water-water hydrogen bonds are always found to be longer than that of acetone-water hydrogen bonds in these mixtures. The residence times of water molecules are also found to follow a similar trend. [ABSTRACT FROM AUTHOR]

We have carried out a series of molecular-dynamics simulations of water-methanol mixtures containing either an ionic or a neutral atomic solute to investigate the effects of composition of the mixture on the diffusion of these solutes. Altogether, we have considered 17 different systems of varying composition ranging from pure water to pure methanol. The diffusion coefficients of ionic solutes are found to show nonideal behavior with variation of composition of the solvent mixture. The extent of nonideality of the solute diffusion is found to be similar to the nonideality that is observed for the diffusion and orientational relaxation of water and methanol molecules in these mixtures and is attributed to the enhanced stability of the hydrogen bonds and formation of interspecies complexes in the mixtures. The neutral solute shows characteristics of hydrophobic solvation and its diffusion decreases monotonically with increase of methanol concentration. The present simulation results are compared with those of experiments wherever available. [ABSTRACT FROM AUTHOR]

Detailed molecular-dynamics simulations are carried out to investigate the equilibrium and dynamical properties of water-acetonitrile mixtures of varying composition. Altogether, we have simulated eight different systems of different concentrations of acetonitrile. The inhomogeneous density and anisotropic orientational profiles at interfaces, surface tension, and also the distribution of hydrogen bonds are calculated for both water and acetonitrile molecules. The dynamical aspects of the interfaces are investigated in terms of the anisotropic diffusion and dipole orientational relaxation of interfacial water and acetonitrile molecules. For both structural and dynamical properties, the behaviors of the interfaces are compared with those of the corresponding bulk phases. A comparison between the present theoretical results and experimental findings, wherever available, is also made to verify the usefulness of the molecular models employed in the present study for predicting interfacial properties. [ABSTRACT FROM AUTHOR]

Abstract: The effects of dispersion interactions on the dynamics of hydrogen bonds and vibrational spectral diffusion in liquid methanol are investigated through first principles simulations with a dispersion corrected density functional. Calculations are done at two different temperatures of 300 and 350K and the results are compared with those of an earlier study where no such dispersion corrections were included. It is found that inclusion of dispersion interactions slightly increases the number of molecules held through non-hydrogen-bonded dispersion interactions in the neighborhood which, in turn, makes the dynamics faster. The inclusion of dispersion corrections gives rise to a faster hydrogen bond dynamics compared to the case when no such dispersion corrections are made. Also, the time scale of vibrational spectral diffusion obtained with the dispersion corrected density functional is found to be in better agreement with experiments. [Copyright &y& Elsevier]

We present a first-principles simulation study of vibrational spectral diffusion and hydrogen-bond dynamics in solution of a fluoride ion in deuterated water. The present calculations are based on ab initio molecular dynamics simulation for trajectory generation and wavelet analysis for calculations of frequency fluctuations. The O-D bonds of deuterated water in the anion hydration shell are found to have lower stretching frequency than the bulk water. The dynamical calculations of vibrational spectral diffusion for hydration shell water molecules reveal three time scales: a short-time relaxation (~100 fs) corresponding to the dynamics of intact ion-water hydrogen bonds, a slower relaxation (~7.5 ps) corresponding to the lifetimes of fluoride ion-water hydrogen bonds, and an even longer time scale (~26 ps) associated with the escape dynamics of water from the anion hydration shell. However, the slowest time scale is not observed when the vibrational spectral diffusion is calculated over O-D bonds of all water molecules, including those in the bulk. [ABSTRACT FROM AUTHOR]

Voids in a medium are defined as the regions that are located outside an appropriately defined occupied space associated with molecules. Dynamical properties like diffusion can be related to the structure and distribution of voids present in the medium. This work deals with an analysis of voids and diffusion in liquid ammonia. The analysis of voids is done by the construction of Voronoi polyhedra and Delaunay tessellation. We have performed a series of molecular dynamics simulations of monovalent cations and anions of varying size in liquid ammonia at two different temperatures of 210 and 240 K to investigate the effects of ion size on the diffusion of ions and roles of voids in determining the observed diffusion behavior. It is found that with the increase of ion size, the diffusion coefficients first increase and then pass through a maximum similar to the behavior observed earlier for diffusion in water. The observed results are explained in terms of passage through voids and necks that are present in liquid ammonia. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: A theoretical study of the dynamics of water–acetone mixtures is carried out by using the methods of ab initio molecular dynamics for trajectory generation, time series analysis for frequency calculations and time correlation function approach for calculations of various dynamical quantities. It is found that diffusion coefficients and orientational relaxation times of water and acetone molecules change in a nonlinear manner with variation of composition of the mixture. The lifetimes of acetone–water hydrogen bonds are found to be shorter than that of water–water hydrogen bonds and both are found to increase with decrease of water concentration. The vibrational spectral diffusion of OD stretch modes of deuterated water molecules is also investigated for the equimolar mixture of water and acetone and the results are correlated with the dynamics of water–water and water–acetone hydrogen bonds and also of dangling OD modes. [Copyright &y& Elsevier]

We present an ab initio molecular dynamics study of vibrational spectral diffusion and hydrogen bond dynamics in aqueous solution of acetone at room temperature. It is found that the frequencies of OD bonds in the acetone hydration shell have a higher stretch frequency than those in the bulk water. Also, on average, the frequencies of hydration shell OD modes are found to increase with increase in the acetone-water hydrogen bond distance. The vibrational spectral diffusion of the hydration shell water molecules reveals three time scales: A short-time relaxation (~80fs) corresponding to the dynamics of intact acetone-water hydrogen bonds, a slower relaxation (~1.3ps) corresponding to the lifetime of acetone-water hydrogen bonds and another longer time constant (~12ps) corresponding to the escape dynamics of water from the solute hydration shell. The present first principles results are compared with those of available experiments and classical simulations. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Abstract: The structure and dynamics of supercritical aqueous ionic solutions are investigated through ab initio molecular dynamics simulations. Explicit results are obtained for two different densities and, for the higher density, two different concentrations are considered. The structure of the solutions is investigated through ion–water correlations, hydrogen bond and coordination numbers of the ions and water molecules and also through frequency–structure correlations of water molecules in the hydration shells and bulk phases. It is found that the stretch frequencies of water in the anion hydration shell are lower than those in the bulk water contrary to the behavior of the corresponding ambient solutions. On the dynamical side, calculations are made for diffusion, orientational relaxation, dynamics of hydrogen bonds and dangling OD groups. The dynamics of vibrational spectral diffusion is also investigated and its connections to the underlying molecular dynamics of water in hydration shells and bulk phases are established. [Copyright &y& Elsevier]

Abstract: We have performed molecular dynamics simulations of alkali metal (Li+, Na+, K+, Rb+, Cs+) and halide (F−, Cl−, Br−, I−) ions in supercritical water at 673K. The calculations were done for water at three different densities of 1.0, 0.7 and 0.35gcm−3 to investigate the effects of solute size on the diffusion of ions in supercritical water. On increase of ion size, we observe a maximum for diffusion of ions in supercritical water of higher densities (1.0 and 0.7gcm−3). However, no such maximum is found for ion diffusion in the supercritical water of low density (0.35gcm−3) or for diffusion of neutral solutes at all densities. These results are analyzed in terms of passage through voids and necks present in supercritical water. Correlations of the observed diffusion behavior with the sizes of ions and voids present in the systems are discussed. [Copyright &y& Elsevier]

The dynamical properties of acetone-methanol mixtures containing either an ionic or a neutral hydrophobic solute are investigated by means of a series of molecular dynamics simulations. The primary goal has been to study how the solute and solvent dynamical properties change with variation of composition of the mixture ranging from pure acetone to pure methanol. The variations of structure and energetics of the mixture with composition are also calculated. The diffusion coefficients of both ionic and neutral solutes are found to show nonlinear variation with composition of the mixture, although the extent of nonlinearity in the diffusion of the neutral solute is much weaker. Calculations of appropriate solute-solvent distribution functions reveal the extent and nature of selective solvation of these solute species which play a role in determining the nonideal dynamical characteristics of these solutes. The free energies of solvation of the ionic solutes are also calculated and the results are discussed in the context of their dynamical behavior. The hydrogen bond statistics and dynamics of these mixtures are also calculated over their entire composition range. The energies and lifetimes of hydrogen bonds between an acetone and a methanol molecule or between two methanol molecules are found to increase with increase of acetone mole fraction of the mixture. Residence times of methanol molecules in solvation shells of acetone and methanol are also found to follow the same trend as relaxation times. However, these pair dynamical properties show essentially linear dependence on composition, thus behave almost ideally with respect to changes in composition of the mixture. [ABSTRACT FROM AUTHOR]

Abstract: Structural, energetics and dynamical properties of acetone–chloroform mixtures containing an ionic or a neutral solute are investigated using molecular dynamics simulations. The primary goal has been to look at the nonideality in dynamical properties on changes of composition. Our calculations reveal the nature of selective solvation that plays a role in determining the nonideal dynamical characteristics of these solutes. The free energies of solvation of the ionic solutes are also calculated and the results are discussed in the context of their dynamical behavior. The nonideality in the dynamics of solvent molecules is also investigated. We have also calculated a pair dynamical property, namely the hydrogen bond lifetimes of acetone–chloroform pairs in these mixtures. The distribution of hydrogen bonds in these mixtures and their average energies are also calculated. The present simulation results are compared with those of experiments wherever available. [Copyright &y& Elsevier]

The article discusses the hydroxide solvation and structural diffusion in aqueous basic solutions. It says that problems on clearing the microscopic data of structural diffusion methods in hydrogen-bonded media allows for studies pursued by ab initio molecular dynamics (AIMD) method. It mentions that the solvation patterns of hydroxide (OH-) (aq) and hydrated hydronium (H+) ions have key asymmetry that support the dynamical hypercoordination scenario.

• Two-dimensional infrared spectra of aqueous solutions are calculated from ab initio simulations. • Dynamics of vibrational spectral diffusion is obtained from the ellipticity of 2D-IR spectral plots. • Vibrational spectral diffusion of local OD modes are obtained for aqueous solutions of NaBr and urea. • Dynamics of spectral diffusion slows down in presence of NaBr but it gets only mildly affected in presence of urea. • 2D-IR and vibrational spectral diffusion are also calculated for water at two different temperatures. Vibrational spectral diffusion of OH/OD modes in liquid water can be influenced by the presence of ions, dipolar solutes or by the change of temperature. In the present work, we have calculated the two-dimensional infrared (2D-IR) spectrum of aqueous NaBr and urea solutions at room temperature and also liquid water at two different temperatures to study the effects of these solutes and varying temperature on the dynamics of vibrational spectral diffusion of OD stretch modes in aqueous media. Time-dependent evolution of the ellipticity of 2D-IR spectra is calculated to obtain timescales of the dynamics of vibrational spectral diffusion. [ABSTRACT FROM AUTHOR]

We present a theoretical study of the structure and dynamics of water-carbon tetrachloride liquid-liquid interface by means of ab initio molecular dynamics simulations. We have studied the density profiles, orientational profiles, hydrogen bond distributions, vibrational power spectra, diffusion, orientational relaxation, hydrogen bond dynamics and vibrational spectral diffusion of bulk and interfacial molecules. We have also provided an analysis of vacancies present in the interfacial system using Voronoi polyhedra method. The hydrogen bonding interaction is found to be weakened at the interface compared to that in the bulk phase of water. Weakly hydrogen bonded and non-hydrogen bonded water molecules at the interface give rise to peaks at different positions of the vibrational power spectrum. Diffusion and orientational relaxation of water molecules are also found to be faster at the interface, which can be correlated with the vacancies present in the system. The dynamics of vibrational spectral diffusion is studied by means of frequency-time correlations calculated through a time-series analysis using the wavelet method and the results of spectral diffusion are correlated with the dynamics of hydrogen bond fluctuations and that of non-hydrogen bonded hydroxyl modes in the bulk and interfacial regions. [ABSTRACT FROM AUTHOR]

Abstract: A theoretical study of vibrational spectral diffusion in aqueous NaBr solutions is presented by means of ab initio molecular dynamics simulations and time series analysis. The OD stretch frequencies of deuterated water hydrogen bonded to the bromide ions are found to be higher than those in the bulk which implies a somewhat weaker Br–water hydrogen bonds than those between water molecules. The dependence of OD stretch frequencies of hydration shell water on the length and angle of the associated ion–water hydrogen bonds is also investigated. The dynamics of frequency fluctuations of hydration shell and all water molecules are studied through calculations of hole dynamics and frequency time correlations. The roles of the dynamics of ion–water and water–water hydrogen bonds, escape dynamics of water from ion hydration shells and also that of other non-hydrogen-bonded dynamical modes in influencing the dynamics of vibrational spectral diffusion are discussed. [Copyright &y& Elsevier]

Abstract: We present a first principles theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in liquid methanol at room temperature. The dynamics of spectral diffusion of OD modes of deuterated methanol reveals two times scales: a short time scale of about 120fs and a longer time scale of about 3.2ps. A damped oscillation is also found at around 120–180fs. Calculations of power spectrum of relative velocities and hydrogen bond correlation functions reveal that the short time dynamics originates from intermolecular motion of hydrogen bonded methanol pairs while the long time relaxation corresponds to the breaking dynamics of hydrogen bonds. The quantitative details of the time constants are found to depend on the frequency of tagged OD bonds. [Copyright &y& Elsevier]