High energy vibrational excitations of nitromethane in liquid water.

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.

Ultrafast Rotational and Translational Energy Relaxation in Neat Liquids.

The excess energy flow pathways during rotational and translational relaxation induced by rotational or translational excitation of a single molecule of and within each of four different neat liquids (H2O, MeOH, CCl4, and CH4) are studied using classical molecular dynamics simulations and energy flux analysis. For all four liquids, the relaxation processes for both types of excitation are ultrafast, but the energy flow is significantly faster for the polar, hydrogen-bonded (H-bonded) liquids H2O and MeOH. Whereas the majority of the initial excess energy is transferred into hindered rotations (librations) for rotational excitation in the H-bonded liquids, an almost equal efficiency for transfer to translational and rotational motions is observed in the nonpolar, non-H-bonded liquids CCl4 and CH4. For translational excitation, transfer to translational motions dominates for all liquids. In general, the energy flows are quite local; i.e., more than 70% of the energy flows directly to the first solvent shell molecules, reaching almost 100% for CCl4 and CH4. Finally, the determined validity of linear response theory for these nonequilibrium relaxation processes is quite solvent-dependent, with the deviation from linear response most marked for rotational excitation and for the nonpolar liquids.

Solvation Dynamics in Water. 4. On the Initial Regime of Solvation Relaxation.

It is shown, by means of numerical and analytic work, that initial molecular momenta play little significant role in the initial fast solvation relaxation that follows electronic excitation of, and charge creation for, a standard model system of a solute in water. Instead, the nonequilibrium dynamics are predominantly described by noninertial "steering" by the torques directly generated by the newly created charge distribution. It is this process that largely overcomes inertia and drives the relaxation dynamics on a time scale of a few tens of femtoseconds in the key initial regime of the dynamics. These results are discussed in the context of commonly employed descriptions such as inertial, Gaussian, and underdamped dynamical behavior.

Self-thermophoresis at the nanoscale using light induced solvation dynamics.

Downsizing microswimmers to the nanoscale, and using light as an externally controlled fuel, are two important goals within the field of active matter. Here we demonstrate using all-atom molecular dynamics simulations that solvation relaxation, the solvent dynamics induced after visible light electronic excitation of a fluorophore, can be used to propel nanoparticles immersed in polar solvents. We show that fullerenes functionalized with fluorophore molecules in liquid water exhibit substantial enhanced mobility under external excitation, with a propulsion speed proportional to the power dissipated into the system. We show that the propulsion mechanism is quantitatively consistent with a molecular scale instance of self-thermophoresis. Strategies to direct the motion of functionalized fullerenes in a given direction using confined environments are also discussed.

Solvation Dynamics in Liquid Water. III. Energy Fluxes and Structural Changes.

In previous installments it has been shown how a detailed analysis of energy fluxes induced by electronic excitation of a solute can provide a quantitative understanding of the dominant molecular energy flow channels characterizing solvation-and in particular, hydration- relaxation dynamics. Here this work and power approach is complemented with a detailed characterization of the changes induced by such energy fluxes. We first examine the water solvent's spatial and orientational distributions and the assorted energy fluxes in the various hydration shells of the solute to provide a molecular picture of the relaxation. The latter analysis is also used to address the issue of a possible "inverse snowball" effect, an ansatz concerning the time scales of the different hydration shells to reach equilibrium. We then establish a link between the instantaneous torque, exerted on the water solvent neighbors' principal rotational axes immediately after excitation and the final energy transferred into those librational motions, which are the dominant short-time energy receptor.

Solvation Dynamics in Water: 2. Energy Fluxes on Excited- and Ground-State Surfaces.

This series' first installment introduced an approach to solvation dynamics focused on expressing the emission frequency shift (following electronic excitation of, and resulting charge change or redistribution in, a solute) in terms of energy fluxes, a work and power perspective. This approach, which had been previously exploited for rotational and vibrational excitation-induced energy flow, has the novel advantage of providing a quantitative view and understanding of the molecular-level mechanisms involved in the solvation dynamics via tracing of the energy flow induced by the electronic excitation's charge change or redistribution in the solute. This new methodology, which was illustrated for the case in which only the excited electronic state surface contributes to the frequency shift (ionization of a monatomic solute in water), is here extended to the general case in which both the excited and ground electronic states may contribute. Simple monatomic solute model variations allow a discussion of the (sometimes surprising) issues involved in assessing each surface's contribution. The calculation of properly defined energy fluxes/work allows a more complete understanding of the solvation dynamics even when the real work for one of the surfaces does not directly contribute to the frequency shift, an aspect further emphasizing the utility of an energy flux approach.

Solvation Dynamics in Liquid Water. 1. Ultrafast Energy Fluxes.

Solvation dynamics in liquid water is addressed via nonequilibrium energy-transfer pathways activated after a neutral atomic solute acquires a unit charge, either positive or negative. It is shown that the well-known nonequilibrium frequency shift relaxation function can be expressed in a novel fashion in terms of energy fluxes, providing a clear-cut and quantitative account of the processes involved. Roughly half of the initial excess energy is transferred into hindered rotations of first hydration shell water molecules, i.e., librational motions, specifically those rotations around the lowest moment of inertia principal axis. After integration over all water solvent molecules, rotations account for roughly 80% of the energy transferred, while translations have a secondary role; transfer to intramolecular water stretch and bend vibrations is negligible. This picture is similar to that for relaxation of a single vibrationally or rotationally excited water molecule in neat liquid water, although solvation relaxation is more nonlocal. In addition, we find a remarkable independence of the main relaxation channels on the newly created charge's sign. Although the methodology is applied here to the simplest solute case, the approach is rather general, and it should be at least equally useful in more realistic and complex scenarios.

Ultrafast librational relaxation of H2O in liquid water.

The ultrafast librational (hindered rotational) relaxation of a rotationally excited H2O molecule in pure liquid water is investigated by means of classical nonequilibrium molecular dynamics simulations and a power and work analysis. This analysis allows the mechanism of the energy transfer from the excited H2O to its water neighbors, which occurs on a sub-100 fs time scale, to be followed in molecular detail, i.e., to determine which water molecules receive the energy and in which degrees of freedom. It is found that the dominant energy flow is to the four hydrogen-bonded water partners in the first hydration shell, dominated by those partners' rotational motion, in a fairly symmetric fashion over the hydration shell. The minority component of the energy transfer, to these neighboring waters' translational motion, exhibits an asymmetry in energy reception between hydrogen-bond-donating and -accepting water molecules. The variation of the energy flow characteristics with rotational axis, initial rotational energy excitation magnitude, method of excitation, and temperature is discussed. Finally, the relation of the nonequilibrium results to equilibrium time correlations is investigated.

Tracking energy transfer from excited to accepting modes: application to water bend vibrational relaxation.

We extend, via a reformulation in terms of Poisson brackets, the method developed previously (Rey et al., J. Phys. Chem. A, 2009, 113, 8949) allowing analysis of the pathways of an excited molecule's ultrafast vibrational relaxation in terms of intramolecular and intermolecular contributions. In particular we show how to ascertain, through the computation of power and work, which portion of an initial excess molecular energy (e.g. vibrational) is transferred to various degrees of freedom (e.g. rotational, translational) of the excited molecule itself and its neighbors. The particular case of bend excess energy relaxation in pure water is treated in detail, completing the picture reported in the work cited above. It is shown explicitly, within a classical description, that almost all of the initial water bend excitation energy is transferred-either indirectly, via Fermi resonance centrifugal coupling to the bend-excited water's rotation, or directly via intermolecular coupling- to local water librations, only involving molecules in the first two hydration shells of the vibrationally excited water molecule. Finally, it is pointed out that the Poisson bracket formulation can also be applied to elucidate the microscopic character of solvation and rotational dynamics, and should prove useful in developing a quantum treatment for energy flow in condensed phases.

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.

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 molecule's 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.

Is there a common orientational order for the liquid phase of tetrahedral molecules?

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.

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.

Thermodynamic state dependence of orientational order and rotational relaxation in carbon tetrachloride.

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.

Orientational order and rotational relaxation in the plastic crystal phase of tetrahedral molecules.

A methodology recently introduced to describe orientational order in liquid carbon tetrachloride is extended to the plastic crystal phase of XY4 molecules. The notion that liquid and plastic crystal phases are germane regarding orientational order is confirmed for short intermolecular distances but is seen to fail beyond, as long range orientational correlations are found for the simulated solid phase. It is argued that, if real, such a phenomenon may not to be accessible with direct (diffraction) methods due to the high molecular symmetry. This behavior is linked to the existence of preferential orientation with respect to the fcc crystalline network defined by the centers of mass. It is found that the dominant class accounts, at most, for one-third of all configurations, with a feeble dependence on temperature. Finally, the issue of rotational relaxation is also addressed, with an excellent agreement with experimental measures. It is shown that relaxation is nonhomogeneous in the picosecond range, with a slight dispersion of decay times depending on the initial orientational class. The results reported mainly correspond to neopentane over a wide temperature range, although results for carbon tetrachloride are included, as well.

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.

On the ultrafast infrared spectroscopy of anion hydration shell hydrogen bond dynamics.

Molecular Dynamics simulations are used to examine the title issue for the I-/HOD/D2O solution system in connection with recent ultrafast infrared spectroscopic experiments. It is argued that the long "modulation time" associated with the spectral diffusion of the OH frequency, extracted in these experiments, should be interpreted as reflecting the escape time of an HOD from the first hydration shell of the I- ion, i.e., the residence time of an HOD in this solvation shell. Shorter time features related to the oscillation of the OH ...I- hydrogen bond and the breaking and making of this bond are also discussed.

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.

Diffusion coefficient of ionic solvation shell molecules.

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 Mg(2+) 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.

On the coupling between molecular diffusion and solvation shell exchange.

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.