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The pathways and timescales of vibrational energy flow in nitromethane are investigated in both gas and condensed phases using classical molecular mechanics, with a particular focus on relaxation in liquid water. We monitor the flow of excess energy deposited in vibrational modes of nitromethane into the surrounding solvent. A marked energy flux anisotropy is found when nitromethane is immersed in liquid water, with a preferential flow to those water molecules in contact to the nitro group. The factors that permit such anisotropic energy relaxation are discussed, along with the potential implications on the molecule's non-equilibrium dynamics. In addition, the energy flux analysis allows us to identify the solvent motions responsible for the uptake of solute energy, confirming the crucial role of water librations. Finally, we also show that no anisotropic vibrational energy relaxation occurs when nitromethane is surrounded by argon gas. [ABSTRACT FROM AUTHOR]

Quantitative methods are used for examining the orientational order in the plastic crystal phase of apolar tetrahedral molecules. The results have shown that relaxation is nonhomogeneous in the picosecond range, with a slight dispersion of decay times depending on the initial orientational class.

A theoretical perspective of the different aspects of ultrafast vibrational energy transfer in water and other related systems, which are concerned with the association of the change of population of OH vibrational states is presented. The hydrogen bond plays a very important role in chemical solution processes like proton-transfer reactions and in biochemical processes and systems.

A study of the structural and dynamic properties of electrolyte system ethylene carbonate - Li(super +) both in the liquid and gas forms is discussed. The thinking about C2 equilibrium symmetry of this molecule is reported to be supported by high level ab initio calculations.

The title question is addressed with molecular dynamics simulations for a broad set of molecules: methane (CH4), neopentane (C(CH3)4), carbon tetrafluoride (CF4), carbon tetrachloride (CCl4), silicon tetrachloride (SiCl4), vanadium tetrachloride (VCl4), tin tetrachloride (SnCl4), carbon tetrabromide (CBr4), and tin tetraiodide (SnI4). In all cases the sequence of most populated relative orientations, for increasing distances, is found to be identical: The closest distances correspond to face-to-face followed by a dominant role of edge-to-face, while for larger distances the main configuration is edge-to-edge. The corner-to-face configuration plays an almost negligible role. The range of orientational order is also similar, with remnants of orientational correlation discernible up to the fourth solvation shell. The equivalence does not only hold in the qualitative terms just stated but is also quantitative to a large extent once the center-center distance is properly scaled. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations show that orientational correlations in carbon tetrachloride span a wide range of distances within the phases that are ordinarily described as orientationally disordered. They are long ranged in the plastic crystal phase, reach up to several solvation layers in the liquid phase, and only involve contact neighbors within the gas phase. On the contrary, short range arrangements are rather similar, with the sequence face-to-face, edge-to-face, and edge-to-edge describing the most populated relative orientations for increasing distances. In what concerns rotational relaxation, it is shown that none of the available theories is able to describe the relationship between rotational relaxation and angular velocity relaxation times for the three phases studied. This is at variance with experimental results obtained long ago for carbon tetrafluoride, which were in excellent accord with J-diffusion, but is in line with recent experimental results for deuterated methane in gas-phase mixtures. [ABSTRACT FROM AUTHOR]

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 ≈20 Å, 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 betweeen the ranges of positional and orientational order. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The connection between diffusion and solvent exchanges between first and second solvation shells is studied by means of molecular dynamics simulations and analytic calculations, with detailed illustrations for water exchange for the Li+ and Na+ ions, and for liquid argon. First, two methods are proposed which allow, by means of simulation, to extract the quantitative speed-up in diffusion induced by the exchange events. Second, it is shown by simple kinematic considerations that the instantaneous velocity of the solute conditions to a considerable extent the character of the exchanges. Analytic formulas are derived which quantitatively estimate this effect, and which are of general applicability to molecular diffusion in any thermal fluid. Despite the simplicity of the kinematic considerations, they are shown to well describe many aspects of solvent exchange/diffusion coupling features for nontrivial systems. [ABSTRACT FROM AUTHOR]

It is shown that, for a tightly bound ion-solvation shell complex, the mean square displacement for solvation molecules is characterized by a long lasting transitory. This initial portion is related to the rotational relaxation of the complex and can reach up to several hundred picoseconds for a representative example such as the Mg2+ ion in water. As the diffusion coefficient is usually fitted using much shorter time spans, unnoticed overestimations are possible. It is argued that, instead of computing the aforementioned diffusion coefficient from the mean square displacement, it should be defined taking as a basic guideline the ratio between the rotational relaxation time of the complex and the lifetime within the first solvation shell. [ABSTRACT FROM AUTHOR]

The three main methods to implement molecular polarization (point dipoles, fluctuating charges, and shell model) are tested against high level ab initio calculations for a molecule (water, carbon tetrachloride) close to a point charge (at the distance of a lithium or magnesium ion). The goal is to check whether an approximation (linear polarization) strictly valid at large intermolecular distances is sufficiently accurate for liquid state molecular dynamics simulations, where strong polarization effects are to be expected at short separations. The monitored observable is the molecular dipole moment as a function of the charge-molecule distance for selected molecular orientations. Analytic formulas are derived for the components of the molecular polarization tensor, facilitating the optimization of the performance for each polarization method as a function of its underlying parameters. Overall, the methods studied provide a remarkably good representation of the induced dipole, with no divergences appearing even at the shortest distances. For water close to a monovalent point charge the point dipole model, implemented with one or three dipoles, accurately reproduces the water dipole moment at all distances. Deficiencies appear as the molecular polarizability and/or charge increase: basically, the ab initio induced moments grow faster at intermediate distances than the linear increase characteristic of the phenomenological polarization methods, suggesting that nonlinear effects (hyperpolarizability) cannot be neglected in these cases. Regarding the capabilities of each method, the point dipole method is the one that performs best overall, with the shell model achieving acceptable results in most instances. The fluctuating charge method shows some noticeable limitations for implementations of comparable complexity (in terms of the number of sites required). © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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 CHCl[sub 3] solute molecule coupled to a classical bath of CHCl[sub 3] 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 for energy loss to the solvent. The bottleneck for relaxation back to the ground state is predicted to be the slow 100–200 ps relaxation of the CH bend and CCl stretch fundamentals. Several aspects of the incoherent anti-Stokes scattering that follows strong infrared excitation of the CH fundamental as observed by Graener, Zu¨rl, and Hoffman [J. Phys. Chem. B 101, 1745 (1997)] are elucidated in the present study. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The molecular dynamics simulations were performed to investigate the key concern related to the ultrafast infrared spectroscopic experiments for OH frequency for HOD molecules in the hydration shell of the [I.super.-] ion solution system. An argument is made to the effect that the long modulation time related to the spectral diffusion of the OH frequency should be interpreted as showing the escape time of an HOD from the first hydration shell of the [I.super.-] ion.

A report is presented on a comprehensive study of structural and dynamic properties of gamma-butyrolactone (GBL) and the extent to which they are affected in the vicinity of a lithium ion, both in gas and liquid phases. The isolated GBL molecule is found to be nonplanar, with a barrier of approximately equal to 9 kJ/mol to ring inversion.

A study is conducted to examine several key aspects of recent ultrafast infrared experiments on liquid water dynamics with the help of molecular dynamics simulations. The findings indicate that the relation between the OH stretch frequency and the length of the hydrogen bond in which the OH is involved is characterized by considerable dispersion.

The solvent exchange rate of aqueous Li (super+) was determined by employing the reactive flux method in combination with very long molecular dynamics simulations based on effective three-body ion-water potentials. The mechanisms for the resulting exchange events were studied in considerable details and analyzed in terms of the five commonly discussed exchange classes, associative (A), dissociative (D), and interchange (I, I (sub a), I (sub d).

Hydration shell exchange can be analyzed using reaction rate perspective. The present system overestimates the rate constant by approximately a factor of 2.

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, including the solute. Relaxation by Coriolis coupling plays a minor but non-negligible role, while the centrifugal coupling contribution to the relaxation is negligible. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Rate constants have been numerically computed for a cubic oscillator subject to a Gaussian friction kernel. The system is studied over a wide range of system–bath coupling, as measured by the static friction coefficient, and the obtained rate constants are compared with theoretical predictions as well as with previous simulations. The results are consistent with variational bounds in the spatial diffusion regime; values lower or equal to the theoretical estimates are obtained throughout. This should resolve the paradox that existed for very high friction, where values higher than the theoretical predictions has been reported and tentatively assigned to a possible nonergodicity of the model. The origin of the discrepancy between the present results and previous simulations, together with the range of validity of several theoretical approaches, are discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the vibrational phase and energy relaxation of cyanide ion in liquid water. Adoption of molecular dynamics simulations and semiclassical perturbation theory; Role of Coulomb and non-Coulomb forces; Relevance of intermolecular terms in the vibration-solvent coupling on frequency shift.

The key aspect of the ultrafast infrared experiments on liquid water dynamics was examined using molecular dynamics (MD). It was found that the relation between the OH stretch frequency and the length of the hydrogen bond, in which the OD was involved, that was assumed to be one-to-one was instead characterized by considerable dispersion.

Dynamic properties and the possible influence of polarization at different temperature are performed for the methylchloromethane family of organic liquids is studied. The result reveals that the inclusion of polarization does not affect structure and slightly slows down the dynamics at all temperatures.