Anomalous slowing down of the vibrational relaxation of liquid water upon nanoscale confinement

We study the vibrational dynamics of nanodroplets of liquid water with femtosecond two-color midinfrared pump-probe spectroscopy. For the smallest nanodroplet, containing 10-15 water molecules, the lifetime T1 of the O-H stretch vibrations is equal to 0.85+/-0.1 ps, which is more than 3 times as long as in bulk liquid water. We find that the truncation of the hydrogen-bond network of water leads to a dramatic change of the relaxation mechanism.     

Pressure evolution of the high-frequency sound velocity in liquid water

The high-frequency sound velocity v( infinity ) of liquid water has been determined to densities of 1.37 g/cm(3) by inelastic x-ray scattering. In comparison to the hydrodynamic sound velocity v(0), the increase of v( infinity ) with density is substantially less pronounced, indicating that, at high density, the hydrogen-bond network is decreasingly relevant to the physical properties of liquid water. Furthermore, we observe an anomaly in v( infinity ) at densities around 1.12 g/cm(3), contrasting the smooth density evolution of v(0).     

Structural Parameters and Thermodynamics of the Formation of Molecular Water Clusters

The formation of molecular water clusters is simulated using the theoretical density functional theory/ B3LYP/6-311+G(d,p) method. The spatial configurations of 29 clusters with 2 to 28 water molecules are calculated. The dipole moments, the complete complex-formation enthalpy, and the enthalpy of the successive joining of water molecules are determined with the basis-set superposition error taken into account. The features of the geometric structure and the hydrogen-bond strength of water clusters are analyzed on the basis of the obtained theoretical data. The complex-formation enthalpy is revealed to depend periodically on the number of water molecules in a cluster. It is found that clusters with molecules whose number is a multiple of four are energetically most advantageous. When a molecular cluster is built starting with 17 molecules, the cluster structure is changed, resulting in that one end of the complex rolls up into a prismatic configuration.     

The multipole polarizabilities and hyperpolarizabilities of the water molecule in liquid state: an ab initio study

A charge perturbation variant of the finite-field method has been used to calculate dipole and quadrupole moments, dipole polarizability, hyper- and principal components of high-order polarizabilities of the water molecule in gas and in liquid phase conditions. Calculations were performed for the ground-state water molecule at the MP2 and MP4 levels of theory. The gas phase values determined allow our methodology for extracting polarizabilities to be tested and a properly balanced, moderate-sized basis set to be selected; the results obtained are in very good agreement with experiment and the most accurate previous theoretical estimates. A local field approach is introduced to mimic the electrostatic environment experienced by a water molecule in the liquid. Within this approach, sets of fixed charges are used to generate the desired electric fields and field gradients. Three different liquid phase models and the corresponding sets of electrical properties are examined. The values obtained from these models and for gas-phase are compared. The magnitudes of the dipole and the quadrupole moments increase moving from gas to liquid phase, where the latter shows greater sensitivity to the choice of liquid model. For a liquid phase water molecule the first hyperpolarizability (β) and first higher polarizability (A) increase markedly, actually changing sign, the second hyperpolarizability (γ) also increases but much less dramatically, and components of the second high-order polarizability tensor (B) demonstrate a rearrangement of contributions. The values reported for the hyper- and high-order polarizability tensors are the first such theoretical estimates for liquid water.     

基于周期性边界条件的分块量子化学方法对液态水的从头算分子动力学模拟

本文利用分块量子化学方法,实现了在周期性边界条件下应用二阶微扰(MP2)理论对液态水的从头算分子动力学模拟. 通过采用aug-cc-pVDZ基组,MP2理论可以精确地描述水分子之间的相互作用势能面,因而在描述水的各项理化性质方面,MP2有望提供比密度泛函理论更加精确的结果. 本研究计算了多种水的结构及动力学性质,包括径向分布函数,扩散系数,偶极矩,三个临近氧原子的角度分布,氢键结构,都得到了与实验观测一致的结果. 因此,周期性边界量子分块方法可以作为一种研究水的物质结构的可靠理论方法,并且有望促进水科学领域争议性问题的解决. 同时,该方法具有普适性和可扩展性,为有效应用高精度量子化学从头算方法计算其他凝聚态体系提供了理论框架.     

Hydrogen-bond dynamics of interior and surface molecules in a water nanocluster: temperature and size effects

Molecular dynamics simulations are employed to investigate the effects of temperature and size on the hydrogen-bond dynamics of interior molecules and surface molecules in a water nanocluster. The flexible three-centred (F3C) water model is invoked in the simulations. To inspect the dynamics of the interior hydrogen bonds and the surface hydrogen bonds, a spherical water nanocluster is modelled and then divided into interior molecules and surface molecules according to the density profile of the water nanocluster. It is observed that at higher temperatures the average number of hydrogen bonds decreases and yields faster hydrogen-bond relaxation for both interior molecules and surface molecules of the water nanocluster. Furthermore, the surface molecules have a lower average number of hydrogen bonds than the interior molecules. The lifetime of the surface hydrogen bonds is slightly longer than that of the interior hydrogen bonds, whereas the hydrogen-bond structural relaxation time of the surface molecules is more obviously slower than that of the interior molecules. Regarding the size effect, a larger water nanocluster is seen to have a larger average number of hydrogen bonds and a longer hydrogen-bond structural relaxation time.     

Dipolar response and hydrogen-bond kinetics in liquid water in square-wave time-varying electric fields

Non-equilibrium molecular dynamics simulations of liquid water have been performed at 298 K in the presence of external time-varying electric fields, approximating a square wave, of varying peak intensity (0.005–0.1 V/Å) in the microwave to far-infrared frequency range (20–500 GHz). Significant non-thermal field effects were noted in terms of dipolar response and acceleration of hydrogen-bond kinetics. The coupling between the total dipole moment and the external field has been investigated and autocorrelation functions (ACFs) of both the total dipole moment and the average of the individual molecular dipole moment along the laboratory axis of the applied fields exhibited coupling, with the former showing a stronger coupling and the latter showing coupling to lower magnitude fields. The maximum alignment achieved has been computed as a function of field intensities and frequencies: the lower frequencies show a greater maximum alignment as the system had more time within each field cycle to respond. The normalised probability distribution and the hydrogen-bond ACFs have been computed: the ACF showed a clear effect over shortening the hydrogen-bond relaxation time. The field effects over the molecules’ transitions from four to five hydrogen bonds have been computed. There was an enhancement of fewer molecules undergoing transitions and a dampening for a larger proportion of molecules, depending on the external fields’ periods.     

Phase diagram of water based on a lattice model

This model is a variant on Baker's lattice model for liquids; it takes certain features of water into account by means of a hydrogen-bond parameter in the Hamiltonian. Solutions for the complete phase diagram are obtained by means of the cluster variation method. The model displays three phases chosen to correspond with the vapor, open ice and liquid state. The isotherms show a density maximum. We discuss the shortcomings of the model and what to do about them.     

Microwave spectra and structure of an isoxazole-water complex

The rotational spectrum of a hydrogen-bonded isoxazole-water complex has been measured between 6–18 GHz with a pulsed nozzle Fourier transform microwave spectrometer. In addition to isoxazole-H₂O, the complexes with HDO and D₂O as well as isoxazole-¹⁵ N-H₂O have been investigated in order to determine the structure of the complex. Rotational constants, quartic centrifugal distortion constants and quadrupole coupling constants, where applicable, have been fitted to the measured transition frequencies of the isotopomers. Structural data, which have been deduced from the planar moments of inertia and the quadrupole coupling constants of the isotopomers, have established conclusively that water binds to nitrogen in the ring plane of isoxazole. Ab initio calculations have revealed that complexes with a hydrogen-bond to nitrogen or to oxygen are both stable. The complex with water attached to nitrogen has been found to be more strongly bound than that with water attached to oxygen. Small splittings of the rotational transitions of the two complexes with H₂O have been interpreted as being the result of an internal rotation of water with respect to isoxazole.     

Is the hydrophobic effect unique to water? The relation between solvation properties and network structure in water and modified water models

The properties of water as a solvent are related to the structure of its liquid phase which in turn depends on the intermolecular potential. In order to explore this relationship we investigate the properties of liquids formed by a number of modified water models. Changing the molecular geometry changes short-range molecular correlations and the network of hydrogen bonds. The solubility and anomalously low entropy of non-polar solutes is only slightly reduced. Reducing the hydrogen-bond strength increases the solubility and removes the low entropy of solution of uncharged spheres. We conclude that the hydrophobic effect depends on the existence of hydrogen bonds and other strong intermolecular interactions but not on the presence of a three-dimensional network.     

Spin state of ortho-parawater molecules in liquid

The state of the nuclear spin moments of two protons in the water molecule is considered when the latter is subjected to surrounding liquid and, for this reason, is an open quantum system. The form of the density operator of the mixed state for spin moments is postulated. Spin degrees of freedom of the molecule appear strongly protected against external influences and are not susceptible to external magnetic fields due to the zero nuclear magnetic moment.     

Direct observation of hydrogen-bond exchange within a single water dimer

The dynamics of water dimers was investigated at the single-molecule level by using a scanning tunneling microscope. The two molecules in a water dimer, bound on a Cu(110) surface at 6 K, were observed to exchange their roles as hydrogen-bond donor and acceptor via hydrogen-bond rearrangement. The interchange rate is approximately 60 times higher for (H2O)2 than for (D2O)2, suggesting that quantum tunneling is involved in the process. The interchange rate is enhanced upon excitation of the intermolecular mode that correlates with the reaction coordinate.     

A molecular level study of liquid water with a flexible water model

The structural properties of water at different temperatures and pressures have been investigated by using a flexible water model and the inherent structure mechanism. The presence of 60$^\circ$ peak in the O--O--O angle distribution function and the behaviours of the hydrogen bonds in the first shell indicate that some water molecules in the second shell move toward the central molecules through the bending (not breaking) of hydrogen bonds and even become first-shell molecules of the central molecule on the basis of the O--O cutoff distance but not first-shell molecules by means of the hydrogen-bond criterion. The inherent-structure analysis of the O--O radial distribution functions at different pressures shows that the first peak is almost independent of the pressure; the position of second peak moves from 0.45 to 0.32nm as the pressure increases from $1\times 10^5$Pa to $1\times 10^9$Pa. This particularly evident pressure effect, i.e. the constant nearest-neighbours and the transformation of outer-neighbours on the basis of O--O distance, together with the results at different temperatures, gives a positive evidence for the two-state outer-neighbour mixture model: liquid water is a mixture of Ice-Ih-type-bonding and Ice-II-type bonding structures.     

A comparison of effective and polarizable intermolecular potentials in simulations: liquid water as a test case

A comparison of polarizable and effective intermolecular potentials has been carried out by employing simulated properties of liquid water at different temperatures. The effective potentials were obtained by adding a fixed fraction (~80%) of the induced dipole moments of the polarizable potential to the permanent dipole moment of the water molecule. The fraction was fitted to reproduce one structural (the height of the first peak of the oxygen-oxygen radial distribution function) and one dynamic (the self-diffusion coefficient) liquid property predicted by the polarizable potential. The two properties were well reproduced simultaneously by the effective potential at 273 K and 303 K, but less accurately at 373 K. The effective dipole moments were 2.79, 2.75, and 2.68 D at the three respective temperatures. In order to examine the effective potentials further, other liquid properties have been considered, and we found that the molecular rotational relaxation times and the hydrogen bonding properties are reproduced well by the effective potentials, whereas the velocity autocorrelation function, the pressure, the dielectric constant, and the Debye relaxation time are reproduced less accurately.     

Thermodynamics of simple models of associating fluids: primitive models of ammonia,methanol, ethanol and water

Thermodynamic P-V-T properties of primitive models that descend directly from realistic Hamiltonians and reproduce the structure of real fluids have been studied both by means of theory and computer simulations. Analytic expressions for the Helmholtz free energy of four typical associating fluids, ammonia, methanol, ethanol and water, have been derived using the thermodynamic perturbation theory. Whereas for the models which allow only single bonding of each site the first-order theory is sufficient, for models in which some sites may form simultaneously up to two bonds the theory has to be extended to the second order. Comparison with simulation data shows that the theory is very accurate and has therefore also been used to determine vapour–liquid equilibria. We have found fundamental differences in the behaviour of different models; these differences are linked to the properties of the hydrogen-bond network that are discussed in detail.     

Effects of ion atmosphere on hydrogen-bond dynamics in aqueous electrolyte solutions

We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations to investigate the effects of ion atmosphere on the dynamics of water-water hydrogen bonds at room temperature. The average number of hydrogen bonds per water molecule decreases with increase of ion concentration. The dynamics of hydrogen-bond breaking is found to accelerate somewhat and that of hydrogen-bond structural relaxation, which occurs at a longer time scale, is found to slow down with increasing ion concentration for both NaCl and KCl solutions.     

原子核多重碎裂的静态理论Ⅱ.间歇类型和临界行为

用温度、密度和动量相关的平均场下的状态方程得到有限核¹⁹⁷Au在温度T=6MeV时的5个不同的相,即热液相、汽相、超热液相、超冷汽相和力学不稳定相,并对其在坐标和动量空间中进行了模拟,采用并合模型对形成的核碎片进行了分类. 定标阶乘矩和条件矩分析表明,间歇类型只来自于力学不稳定相的贡献,临界行为主要来自于力学不稳定相的贡献,超热液相和热液相也对临界行为有微弱贡献.     

Infrared Study of Solid Cycloheptanol

Abstract

Infrared absorption spectra of cycloheptanol-OH have been measured in the 50–3700 cm^?1 frequency range for crystal II', II, I and liquid phases. Order-disorder behaviour concerning orientational, conformational and hydrogen-bond properties is discussed.     

Novel approach to study liquid crystal phase transitions using Legendre moments

A novel approach is proposed to investigate the phase transitions of cholesteric liquid crystals using the Legendre moments. The textures of cholesteric liquid crystals (cholesteryl butyrate, cholesteryl n-valerate, cholesteryl decanoate, and cholesteryl myristate) are captured as a function of temperature using high-resolution camera attached to the arthroscopic mode of polarizing optical microscope with hot stage. A recurrence formula is used to compute the Legendre moments of the liquid crystal textures based on the Legendre polynomial using MATLAB software. The abrupt change in the values of Legendre moments as a function of temperature gives the phase transitions of liquid crystals. The investigated transition temperatures of cholesteric liquid crystals are compared with other techniques.     

Low-frequency spin dynamics in the CeMIn5 materials

We measure the spin lattice relaxation of the planar In(1) nuclei in the CeMIn5 materials, extract quantitative information about the low energy spin dynamics of the lattice of Ce moments in both CeRhIn5 and CeCoIn5, and identify a crossover in the normal state. Above a temperature T(*) the Ce lattice exhibits "Kondo gas" behavior characterized by local fluctuations of independently screened moments; below T(*) both systems exhibit a "Kondo liquid" regime in which interactions between the local moments contribute to the spin dynamics. Both the antiferromagnetic and superconducting ground states in these systems emerge from the Kondo liquid regime. Our analysis provides strong evidence for quantum criticality in CeCoIn5.