Interfacial water structures have been studied intensively by probing the O-H stretch mode of water molecules using sum-frequency generation (SFG) spectroscopy. This surface-specific technique is finding increasingly widespread use, and accordingly, computational approaches to calculate SFG spectra using molecular dynamics (MD) trajectories of interfacial water molecules have been developed and employed to correlate specific spectral signatures with distinct interfacial water structures. Such simulations typically require relatively long (several nanoseconds) MD trajectories to allow reliable calculation of the SFG response functions through the dipole moment-polarizability time correlation function. These long trajectories limit the use of computationally expensive MD techniques such as ab initio MD and centroid MD simulations. Here, we present an efficient algorithm determining the SFG response from the surface-specific velocity-velocity correlation function (ssVVCF). This ssVVCF formalism allows us to calculate SFG spectra using a MD trajectory of only ∼100 ps, resulting in the substantial reduction of the computational costs, by almost an order of magnitude. We demonstrate that the O-H stretch SFG spectra at the water-air interface calculated by using the ssVVCF formalism well reproduce those calculated by using the dipole moment-polarizability time correlation function. Furthermore, we applied this ssVVCF technique for computing the SFG spectra from the ab initio MD trajectories with various density functionals. We report that the SFG responses computed from both ab initio MD simulations and MD simulations with an ab initio based force field model do not show a positive feature in its imaginary component at 3100 cm(-1).

Specialized bacteria trigger ice formation by controlling the molecular structure and energy transfer in interfacial water.

We examine the temperature dependence of the interfacial molecular structure at the water-air interface by combining experimental and simulated sum-frequency generation (SFG) spectroscopy. The experimental SFG spectra of the OH-stretching mode show a decrease in the amplitude at ∼3300 cm(-1) with increasing temperature, while the 3700 cm(-1) 'free OH' SFG feature is insensitive to temperature changes. The simulated spectra are in excellent agreement with experiment. A comparison between interfacial SFG spectra and bulk infrared/Raman spectra reveals that the variation of the SFG signal due to the temperature change is not caused by a temperature-dependent OH bond orientation of the interfacial water molecules, but can be fully accounted for by the temperature dependence of the optical response of water. These results indicate that while the thickness of the interfacial region varies with temperature, the molecular organization of interfacial water at the water-air interface is surprisingly insensitive to temperature changes.

Ab initio molecular dynamics (AIMD) simulations in trimethylamine N-oxide (TMAO)-D2O solution are employed to elucidate the effects of TMAO on the reorientational dynamics of D2O molecules. By decomposing the O-D groups of the D2O molecules into specific subensembles, we reveal that water reorientational dynamics are retarded considerably in the vicinity of the hydrophilic TMAO oxygen (O(TMAO)) atom, due to the O-D···O(TMAO) hydrogen-bond. We find that this reorientational motion is governed by two distinct mechanisms: The O-D group rotates (1) after breaking the O-D···O(TMAO) hydrogen-bond, or (2) together with the TMAO molecule while keeping this hydrogen-bond intact. While the orientational slow-down is prominent in the AIMD simulation, simulations based on force field models exhibit much faster dynamics. The simulated angle-resolved radial distribution functions illustrate that the O-D···O(TMAO) hydrogen-bond has a strong directionality through the sp(3) orbital configuration in the AIMD simulation, and this directionality is not properly accounted for in the force field simulation. These results imply that care must be taken when modeling negatively charged oxygen atoms as single point charges; force field models may not adequately describe the hydration configuration and dynamics.

Evaporation is the process by which water changes from a liquid to a gas or vapor, and is a key step in Earth's water cycle. At the molecular level, evaporation requires breaking at least one very strong intermolecular bond between two water molecules at the interface. Despite the importance of this process the molecular mechanism by which an evaporating water molecule gains sufficient energy to escape from the surface has remained elusive. Here, we show, using molecular dynamics simulations at the water-air interface with polarizable classical force field models, that the high kinetic energy of the evaporated water molecule is enabled by a well-timed making and breaking of hydrogen bonds involving at least three water molecules at the interface, the recoil of which allows one of the molecules to escape. The evaporation of water is thus enabled by concerted, ultrafast hydrogen-bond dynamics of interfacial water, and follows one specific molecular pathway.

Room temperature ionic liquids (RTILs) have been shown to exhibit spatial heterogeneity or structural heterogeneity in the sense that they form hydrophobic and ionic domains. Yet studies of the relationship between this structural heterogeneity and the ∼picosecond motion of the molecular constituents remain limited. In order to obtain insight into the time scales relevant to this structural heterogeneity, we perform molecular dynamics simulations of a series of RTILs. To investigate the relationship between the structures, i.e., the presence of hydrophobic and ionic domains, and the dynamics, we gradually increase the size of the hydrophobic part of the cation from ethylammonium nitrate (EAN), via propylammonium nitrate (PAN), to butylammonium nitrate (BAN). The two ends of the organic cation, namely, the charged Nhead–H group and the hydrophobic Ctail–H group, exhibit rotational dynamics on different time scales, evidencing dynamical heterogeneity. The dynamics of the Nhead–H group is slower because of the strong coulombic interaction with the nitrate counter-ionic anions, while the dynamics of the Ctail–H group is faster because of the weaker van der Waals interaction with the surrounding atoms. In particular, the rotation of the Nhead–H group slows down with increasing cationic chain length, while the rotation of the Ctail–H group shows little dependence on the cationic chain length, manifesting that the dynamical heterogeneity is enhanced with a longer cationic chain. The slowdown of the Nhead–H group with increasing cationic chain length is associated with a lower number of nitrate anions near the Nhead–H group, which presumably results in the increase of the energy barrier for the rotation. The sensitivity of the Nhead–H rotation to the number of surrounding nitrate anions, in conjunction with the varying number of nitrate anions, gives rise to a broad distribution of Nhead–H reorientation times. Our results suggest that the asymmetry of the cations and the larger excluded volume for longer cationic chain are important for both the structural heterogeneity and the dynamical heterogeneities. The observed dynamical heterogeneities may affect the rates of chemical reactions depending on where the reactants are solvated in ionic liquids and provide an additional guideline for the design of RTILs as solvents.

We propose a new force field for trimethylamine N-oxide (TMAO), which is designed to reproduce the long-lived and highly directional hydrogen bond between the TMAO oxygen (OTMAO) atom and surrounding water molecules. Based on the data obtained by ab initio molecular dynamics simulations, we introduce three dummy sites around OTMAO to mimic the OTMAO lone pairs and we migrate the negative charge on the OTMAO to the dummy sites. The force field model developed here improves both structural and dynamical properties of aqueous TMAO solutions. Moreover, it reproduces the experimentally observed dependence of viscosity upon increasing TMAO concentration quantitatively. The simple procedure of the force field construction makes it easy to implement in molecular dynamics simulation packages and makes it compatible with the existing biomolecular force fields. This paves the path for further investigation of protein-TMAO interaction in aqueous solutions.

Structural disordeling induced by hydrostatic pressure has been observed in a wide vmiety of materials. Stannic iodide, Sni 4 , is a molecular crystal which is known to exhibit pressure-induced amorphization and metallization. Previous works including x-ray diffraction, Raman and Mossbauer measurements led to a structural model of the amorphous phase that consisted of randomly oriented chains of Sn4 tetrahedral molecules. The plimary purpose of this study is to clarify the structural disorde1ing process upon the crystal-mnorphous transformation and to search for a new crystal structure beyond the amorphous state at high pressures.We have car1ied out a high-pressure, synchrotron powder xray diffraction measurement. The present study has revealed that the initial cubic structure of Sni 4 transfonns to a new crystalline phase at 7 GPa before the amorphization begins. This second phase gradually looses its crystalline chm·acter exhibited in the diffraction pattem with increasing pressure. Diffraction pattems chm·ac-teristic of the amorphous state were observed between 20 GPa and 55 GPa. At about 60 GPa, a phase transition to the third crystalline phase takes place discontinuously. A diffraction pattern of the third phase consists of very low background and only diffraction peaks assigned to the fcc structure. This fact leads to a model of the third phase in which iodine ions are located at the ideal fcc-sites and tin ions randomly occupy, for instance, the tetral1edral sites in the fcc lattice. A very small value of its lattice constant indicates that this crystal structure results from the dissociation of Sni 4 molecules. MS18.01.02 HIGH PRESSURE X-RAY STUDY OF AlH3AND AJD3: ANOMALOUS ISOTOPE EFFECT. Stanislav P. Besedin and Andrew P. Jephcoat. Depm·tment ofEm·th Sciences, University of Oxford, Parks Road, Oxford, OXl 3PR, UKThe metal hyd1ides are of interest owing to a number of phenomena in which the quantum chm·acter of a light hydrogen atom in a metallic matlix is exhibited. We report synchrotron powder x-ray diffraction measurements at room temperature for the predominantly ionic (pm·tly covalent) hydrogen-rich metal hydlides AJH3 and AJD3 in a diamond-anvil cell to maximum pressures near 53 GPa. Aluminium hydlide has a rhombohedral unit cell (R-3c space group) with a=4.4493 A and c=ll.8037 A in hexagonal axes. It can be regm·ded as "a three-dimensional polymer" all atoms of which are linked by Al-H-Al bonds. The structure can also be desCiibed in terms of a distorted hcp matlix of hydrogen atoms. in which one third of the octahedral interstices are occupied by aluminium atoms. The pressure-volume data obtained show an unexpected difference in the compressibility between isotopes, with AJD3 more compressible at all pressures. No structural phase transition hom the rhombohedral aluminium lattice is observed in either isotope, which could be associated with the previous prediction of an insulator-to-metal transition at 46 GPa. On the basis of our data for ALH3, we expect that the pressure for which the atomic hy...