Rossend Rey

VIBRATIONAL ENERGY RELAXATION OF HOD IN LIQUID D2O

Molecular Dynamics simulation is used to study the vibrational relaxation of the first excited state of the O–H stretch for HOD dissolved in D2O. The technique applied is based on a Landau–Teller type formula, in which the solvent contribution is computed classically, while the quantum nature of the solute enters through the transition moments of the molecular normal modes. The experimental result for the relaxation time (≊8 ps) is accounted for, and the pathway to the ground state is determined. The relaxation proceeds through a sequence of intramolecular transitions initially facilitated by the solute internal anharmonicities. In particular, the anharmonicity allows an initial and rate‐determining transfer to the first overtone of the HOD bend; a corresponding harmonic force field calculation in which this step is precluded yields a relaxation time that is three orders of magnitude larger. The excess energy is removed by the bath modes, which include rotations and translations of all molecules, includin...

Reorientation and Allied Dynamics in Water and Aqueous Solutions

The reorientation of a water molecule is important for a host of phenomena, ranging over--in an only partial listing--the key dynamic hydrogen-bond network restructuring of water itself, aqueous solution chemical reaction mechanisms and rates, ion transport in aqueous solution and membranes, protein folding, and enzymatic activity. This review focuses on water reorientation and related dynamics in pure water, and for aqueous solutes with hydrophobic, hydrophilic, and amphiphilic character, ranging from tetra-methylurea to halide ions and amino acids. Attention is given to the application of theory, simulation, and experiment in the probing of these dynamics, in usefully describing them, and in assessing the description. Special emphasis is placed on a novel sudden, large-amplitude jump mechanism for water reorientation, which contrasts with the commonly assumed Debye rotational diffusion mechanism, characterized by small-amplitude angular motion. Some open questions and directions for further research are also discussed.

Potential of mean force by constrained molecular dynamics: A sodium chloride ion-pair in water

The constrained molecular dynamics simulation method has been used to obtain the mean force and the mean force potential between two particles in solution. The method has been tested for a Lennard-Jones liquid and applied to the study of a Na+-Cl− ion-pair in aqueous solution. A flexible SPC model for water has been assumed. The results have been interpreted in the light of the solvent structure around the ions for separations corresponding to the maxima and minima of the mean force potential. In contrast to earlier studies using rigid water models, the solvent separated ion pair configurations are more stable than the contact ion pair configurations.

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.

Vibrational phase and energy relaxation of CN− in water

Classical molecular dynamics simulations complemented with semiclassical perturbation theory have been applied to the study of the cyanide ion vibrational relaxation in liquid water. The model provides reasonable agreement with known experimental results as well as with ab initio calculations for small clusters. The role of Coulomb and non-Coulomb forces is studied in detail. A dominant role of the former in the vibrational energy (population) relaxation is found, while in contrast, the bandshape—and thus the dephasing—are determined by both forces. Further, and at variance with existing theories, the present model provides the first example in which nonlinear intermolecular terms in the vibration-solvent coupling are critical in the instantaneous frequency shift.

Pathways for H2O bend vibrational relaxation in liquid water.

The mechanism of the H2O bend vibrational relaxation in liquid water has been examined via classical MD simulations and an analysis of work and power contributions. The relaxation is found to be dominated by energy flow to the hindered rotation of the bend excited water molecule. This energy transfer, representing approximately 2/3 of the transferred energy, is due to a 2:1 Fermi resonance for the centrifugal coupling between the water bend and rotation. The remaining energy flow (approximately 1/3) from the excited water bend is dominated by transfer to the excited water molecules first four water neighbors, i.e., the first hydration shell, and is itself dominated by energy flow to the two water molecules hydrogen (H)-bonded to the hydrogens of the central H2O. The energy flow from the produced rotationally excited central molecule is less local in character, with approximately half of its rotational kinetic energy being transferred to water molecules outside of the first hydration shell, whereas the remaining half is preferentially transferred to the two first hydration shell water molecules donating H-bonds to the central water oxygen. The overall energy flow is well described by an approximate kinetic scheme.

On the performance of molecular polarization methods. II. Water and carbon tetrachloride close to a cation

Our initial study on the performance of molecular polarization methods close to a positive point charge [M. Masia, M. Probst, and R. Rey, J. Chem. Phys. 121, 7362 (2004)] is extended to the case in which a molecule interacts with a real cation. Two different methods (point dipoles and shell model) are applied to both the ion and the molecule. The results are tested against high-level ab initio calculations for a molecule (water or carbon tetrachloride) close to Li+, Na+, Mg2+, and Ca2+. The monitored observable is in all cases the dimer electric dipole as a function of the ion-molecule distance for selected molecular orientations. The moderate disagreement previously obtained for point charges at intermediate distances, and attributed to the linearity of current polarization methods (as opposed to the nonlinear effects evident in ab initio calculations), is confirmed for real cations as well. More importantly, it is found that at short separations the phenomenological polarization methods studied here substantially overestimate the dipole moment induced if the ion is described quantum chemically as well, in contrast to the dipole moment induced by a point-charge ion, for which they show a better degree of accord with ab initio results. Such behavior can be understood in terms of a decrease of atomic polarizabilities due to the repulsion between electronic charge distributions at contact separations. It is shown that a reparametrization of the Thole method for damping of the electric field, used in conjunction with any polarization scheme, allows to satisfactorily reproduce the dimer dipole at short distances. In contrast with the original approach (developed for intramolecular interactions), the present reparametrization is ion and method dependent, and corresponding parameters are given for each case.

Vibrational relaxation in liquid chloroform following ultrafast excitation of the CH stretch fundamental

Vibrational energy flow in liquid chloroform that follows the ultrafast excitation of the CH stretch fundamental is modeled using semiclassical methods. Relaxation rates are calculated using Landau-Teller theory and a time-dependent method both of which consider a quantum mechanical CHCl3 solute molecule coupled to a classical bath of CHCl3 solvent molecules. Probability flow is examined for several potentials to determine the sensitivity of calculated relaxation rates to the parameters that describe the model potentials. Three stages of relaxation are obtained. Probability is calculated to decay initially to a single acceptor state, a combination state of the solute molecule with two quanta of excitation in the CH bend and one in the CCl stretch, in 13–23 ps depending on the potential model employed. This is followed by rapid and complex intramolecular energy flow into the remaining vibrational degrees of freedom. During this second stage the lowest frequency Cl–C–Cl bend is found to serve as a conduit f...

Ultrafast energy transfer from the intramolecular bending vibration to librations in liquid water

A theoretical study of the water bend-to-libration energy transfer in liquid H(2)O has been performed by means of nonequilibrium classical molecular dynamics computer simulations. Attention has been focused on the time scale and mechanism of the decay of the fundamental H(2)O bend vibration and the related issue of the decay of water librational (hindered rotational) excitations, including the important role of that for the excited molecule itself. The time scales found are 270 fs for the decay of the average energy of an H(2)O molecule excited to the nu = 1 state of the bending oscillator and less than 100 fs for excess rotational (librational) kinetic energy, both consistent with recent ultrafast infrared experimental results. The energy flow to the excited molecule rotation and through the first several solvent shells around the excited water molecule is discussed in some detail.

Quantitative characterization of orientational order in liquid carbon tetrachloride

A simple geometrical construct is proposed for a clear-cut classification of the relative orientation between two tetrahedral molecules in terms of six orientational classes. When applied to sort out configurations from condensed phase simulations, it leads to a quantitative characterization of orientational order: A definite percentage for each class is obtained as a function of the distance between molecular centers. The basic picture that emerges, for liquid carbon tetrachloride, is that the dominant configuration for each distance is such that the number of chlorines in between both carbons diminishes with increasing separation, with a configuration here termed edge-to-face being the dominant one at contact. Regarding the range of orientational order, remnants are still noticeable at approximately 20 A, i.e., up to the fourth solvation shell. Beyond this distance the distributions are hardly distinguishable from the analytical predictions for random orientation. The analysis of the small fluctuations at such long distances shows that there are no significant differences between the ranges of positional and orientational order.