Takuma Yagasaki

Ultrafast intermolecular dynamics of liquid water: A theoretical study on two-dimensional infrared spectroscopy

Physical and chemical properties of liquid water are dominated by hydrogen bond structure and dynamics. Recent studies on nonlinear vibrational spectroscopy of intramolecular motion provided new insight into ultrafast hydrogen bond dynamics. However, our understanding of intermolecular dynamics of water is still limited. We theoretically investigated the intermolecular dynamics of liquid water in terms of two-dimensional infrared (2D IR) spectroscopy. The 2D IR spectrum of intermolecular frequency region (<1000 cm(-1)) is calculated by using the equilibrium and nonequilibrium hybrid molecular dynamics method. We find the ultrafast loss of the correlation of the libration motion with the time scale of approximately 110 fs. It is also found that the energy relaxation from the libration motion to the low frequency motion takes place with the time scale of about 180 fs. We analyze the effect of the hindered translation motion on these ultrafast dynamics. It is shown that both the frequency modulation of libration motion and the energy relaxation from the libration to the low frequency motion significantly slow down in the absence of the hindered translation motion. The present result reveals that the anharmonic coupling between the hindered translation and libration motions is essential for the ultrafast relaxation dynamics in liquid water.

Effect of bubble formation on the dissociation of methane hydrate in water: a molecular dynamics study.

We investigate the dissociation of methane hydrate in liquid water using molecular dynamics simulations. As dissociation of the hydrate proceeds, methane molecules are released into the aqueous phase and eventually they form bubbles. It is shown that this bubble formation, which causes change in the methane concentration in the aqueous phase, significantly affects the dissociation kinetics of methane hydrate. A large system size employed in this study makes it possible to analyze the effects of the change in the methane concentration and the formation of bubbles on the dissociation kinetics in detail. It is found that the dissociation rate decreases with time until the bubble formation and then it turns to increase. It is also demonstrated that methane hydrate can exist as a metastable superheated solid if there exists no bubble.

Ultrafast energy relaxation and anisotropy decay of the librational motion in liquid water: A molecular dynamics study

We theoretically investigate intermolecular motions in liquid water in terms of third-order infrared (IR) spectroscopy. We calculate two-dimensional (2D) IR spectra, pump-probe signals, and three-pulse stimulated photon echo signals from the combination of equilibrium and nonequilibrium molecular dynamics simulations. The 2D IR spectra and the three-pulse photon echo peak shift exhibit that the frequency correlation of the librational motion decays with a time scale of 100 fs. The two-color 2D IR spectra and the pump-probe signals reveal that the energy transfer from the librational motion at 700 cm(-1) to the low frequency motion below 300 cm(-1) occurs with a time scale of 60 fs and the subsequent relaxation to the hot ground state takes place on a 500 fs time scale. The time scale of the anisotropy decay of the librational motion is found to be approximately 115 fs. The energy dissipation processes are investigated in detail by using the nonequilibrium molecular dynamics simulation, in which an electric field pulse is applied. We show that the fast energy transfer from the librational motion to the low frequency motion is mainly due to the librational-librational energy transfer. We also show that the fast anisotropy decay mainly arises from the rapid intermolecular energy transfer.

Adsorption Mechanism of Inhibitor and Guest Molecules on the Surface of Gas Hydrates

The adsorption of guest and kinetic inhibitor molecules on the surface of methane hydrate is investigated by using molecular dynamics simulations. We calculate the free energy profile for transferring a solute molecule from bulk water to the hydrate surface for various molecules. Spherical solutes with a diameter of ∼0.5 nm are significantly stabilized at the hydrate surface, whereas smaller and larger solutes exhibit lower adsorption affinity than the solutes of intermediate size. The range of the attractive force is subnanoscale, implying that this force has no effect on the macroscopic mass transfer of guest molecules in crystal growth processes of gas hydrates. We also examine the adsorption mechanism of a kinetic hydrate inhibitor. It is found that a monomer of the kinetic hydrate inhibitor is strongly adsorbed on the hydrate surface. However, the hydrogen bonding between the amide group of the inhibitor and water molecules on the hydrate surface, which was believed to be the driving force for the adsorption, makes no contribution to the adsorption affinity. The preferential adsorption of both the kinetic inhibitor and the spherical molecules to the surface is mainly due to the entropic stabilization arising from the presence of cavities at the hydrate surface. The dependence of surface affinity on the size of adsorbed molecules is also explained by this mechanism.

Roles of the ether oxygen in hydration of tetrahydrofuran studied by IR, NMR, and DFT calculation methods

We studied the concentration dependence of nu(C-H)s in IR and (1)J(C,H) in NMR for binary water-tetrahydrofuran (THF) mixtures and found different trends for the two types of CH(2) groups in the five-membered ring. The changes of the nu(C-O) spectra showed that complexes of THF associated with water are formed, in which the number of water molecules increases with the water concentration. We suggested that hydration proceeds through the formation of 1:1, and 1:2 complexes of [THF:water] up to X(H(2)O) approximately 0.9, where X(H)((2))(O) is the mole fraction of the water in the mixtures. We carried out ab initio MO and DFT calculations to optimize the geometries of a THF dimer as a model of THF molecules in pure liquid, and 1:1 and 1:2 complexes of [THF:water] to simulate observed concentration dependence of nu(C-H)s in IR and (1)J(C,H) in NMR. The changes of the calculated nu(C-H) spectra and (1)J(C,H) values for the optimized complexes are in agreement with those observed with varying X(H)((2))(O), supporting our proposal. From the vibrational and NBO analyses of the optimized complexes, the observed blue shift of nu(C-H)s and the increase of (1)J(C,H) for the CH(2) groups neighboring to the ether oxygen were explained in terms of the changes in the stereoelectronic effect, resulting from HO-H...O< hydrogen bonding. The optimized 1:2-complex contains two weak C-H...OH(2) hydrogen bonds, and blue shift of nu(C-H)s and increase of (1)J(C,H) were demonstrated from the same analyses of the complexes. This result of simulation also supports that the blue shift of nu(C-H)s and increase of (1)J(C,H) observed for both the type of CH(2) groups at 0.6 X(H)((2))(O) < 0.9 are attributed to these interactions. On the basis of all these results, we propose that the formation of the 1:2-complex involving weak C-H...OH(2) hydrogen bonds is responsible dominantly for the hydrophobic hydration of THF.

Fluctuations and Relaxation Dynamics of Liquid Water Revealed by Linear and Nonlinear Spectroscopy

Many efforts have been devoted to elucidating the intra- and intermolecular dynamics of liquid water because of their important roles in many fields of science and engineering. Nonlinear spectroscopy is a powerful tool to investigate the dynamics. Because nonlinear response functions are described by more than one time variable, it is possible to analyze static and dynamic mode couplings. Here we review the intra- and intermolecular dynamics of liquid water revealed by recent linear and nonlinear spectroscopic experiments and computer simulations. In particular, we discuss the population relaxation, anisotropy decay, and spectral diffusion of the intra- and intermolecular motions of water and their temperature dependence, which play important roles in ultrafast dynamics and relaxations in water.

Effects of Nonadditive Interactions on Ion Solvation at the Water/Vapor Interface: A Molecular Dynamics Study

The solvation of halide ions at the water/vapor interface is investigated by using molecular dynamics simulations with nonpolarizable molecular mechanical (MM), polarizable MM, and quantum mechanical (QM)/MM methods. The free energy profile of the ion solvation is decomposed into the energy and the entropic contributions along the ion displacement from inside to the surface of water. It is found that the surface affinity of the ion, relative to the bulk value, is determined by a subtle balance between the energetic destabilization and the entropic stabilization with the ion displacement. The amount of energetic destabilization is found to be reduced when nonadditive interactions are included, as in the polarizable MM and QM/MM models. The structure of water around the ion at the interface is also largely modified when the higher order effects are considered. For example, the induced dipole effect enhances the solvation structure around the ion at the interface significantly and thus reduces the amount of entropic stabilization at the interface, relative to in the bulk. It is found that this induced dipole effect causes the slowing in the ion-water hydrogen bond dynamics at the interface. On the other hand, the higher order induced multipole effects in the QM/MM method suppress both the excessive enhancement of the solvation structure and the slowing of the ion-water hydrogen bond dynamics at the interface. The present study demonstrates that not only the induced dipole moment but also the higher order induced multipole moments, which are neglected in standard empirical models, are essential for the correct description of the ion solvation at the water/vapor interface.

Dissociation of methane hydrate in aqueous NaCl solutions.

Molecular dynamics simulations of the dissociation of methane hydrate in aqueous NaCl solutions are performed. It is shown that the dissociation of the hydrate is accelerated by the formation of methane bubbles both in NaCl solutions and in pure water. We find two significant effects on the kinetics of the hydrate dissociation by NaCl. One is slowing down in an early stage before bubble formation, and another is swift bubble formation that enhances the dissociation. These effects arise from the low solubility of methane in NaCl solution, which gives rise to a nonuniform spatial distribution of solvated methane in the aqueous phase. We also demonstrate that bubbles form near the hydrate interface in dense NaCl solutions and that the hydrate dissociation proceeds inhomogeneously due to the bubbles.

A novel method for analyzing energy relaxation in condensed phases using nonequilibrium molecular dynamics simulations: Application to the energy relaxation of intermolecular motions in liquid water

We present a novel method to investigate energy relaxation processes in condensed phases using nonequilibrium molecular dynamics simulations. This method can reveal details of the time evolution of energy relaxation like two-color third-order IR spectroscopy. Nonetheless, the computational cost of this method is significantly lower than that of third-order response functions. We apply this method to the energy relaxation of intermolecular motions in liquid water. We show that the intermolecular energy relaxation in water is characterized by four energy transfer processes. The structural changes of the liquid associated with the energy relaxation are also analyzed by the nonequilibrium molecular dynamics technique.

Energy relaxation of intermolecular motions in supercooled water and ice: A molecular dynamics study

We investigate the energy relaxation of intermolecular motions in liquid water at temperatures ranging from 220 K to 300 K and in ice at 220 K using molecular dynamics simulations. We employ the recently developed frequency resolved transient kinetic energy analysis, which provides detailed information on energy relaxation in condensed phases like two-color pump-probe spectroscopy. It is shown that the energy cascading in liquid water is characterized by four processes. The temperature dependences of the earlier three processes, the rotational-rotational, rotational-translational, and translational-translational energy transfers, are explained in terms of the density of states of the intermolecular motions. The last process is the slow energy transfer arising from the transitions between potential energy basins caused by the excitation of the low frequency translational motion. This process is absent in ice because the hydrogen bond network rearrangement, which accompanies the interbasin transitions in liquid water, cannot take place in the solid phase. We find that the last process in supercooled water is well approximated by a stretched exponential function. The stretching parameter, β, decreases from 1 to 0.72 with decreasing temperature. This result indicates that the dynamics of liquid water becomes heterogeneous at lower temperatures.