New Two-Dimensional Ice Models

This paper presents a new approach for enumerating all hydrogen bond arrangements of ice-like systems with periodic boundary conditions. It is founded on a topological procedure for the dimensional reduction and a new variant of the transfer matrix method based on small conditional transfer matrices. We consider a couple of new two-dimensional ice models on very unusual lattices. One of them is the twisted square ice model with crossing H-bonds. The other is the digonal-hexagonal model with double H-bonds. In spite of their uncommonness, these models are quite realistic, because from the standpoint of combinatorics and topology they are equivalent to the layers of usual hexagonal ice Ih under periodic boundary conditions in one of the directions. The exact proton configuration statistics for a number of 2D-expanded unit cells of hexagonal ice Ih and the residual entropy of the new ice models in the large system limit are presented.     

利用拉曼光谱分析冰Ih相的表面薄层的氢键结构

水是生活中一种最基本且最重要的物质,由于它的一些奇特性质和反常物性,得到了广泛的研究,而拉曼光谱是研究水分子结构的一种非常合适的方法,它通过获得分子的振动和转动信息来理解分子结构和分子间的相互作用。在常压下测量了-20～-190 ℃温度范围内冰Ih相的表面薄层的拉曼光谱,实验结果发现随温度降低,冰Ih相的O∶H范德瓦尔斯键向高波数方向移动,而O-H极性共价键向低波数方向移动;且拉曼频移与温度呈线性关系,通过对不同振动模式的斜率进行比较,判断其键长的伸缩变化关系,从而证明了冰Ih相密度随温度的减小而增大,采用氢键理论(结构)给予了解释。同时,发现在-150 ℃时,O-H键反对称伸缩振动模式和O∶H键振动模式的拉曼峰强发生了突变,这表明冰Ih相发生了相变--冰Ⅺ相(冰Ih的质子有序相)。     

Correlated Tunneling in Hydrogen Bonds

We study the quantum nature of the protons participating in hydrogen bonds in several ice structures by analyzing the one particle density matrix. We find that in all cases, including ice Ih, the most common form of ice, and the high pressure phases, ice VIII, VII, and X, the system is ground-state dominated. However, while the dynamics is uncorrelated in the structures with standard asymmetric hydrogen bonds, such as ice Ih and VIII, local correlations among the protons characterize ice VII and, to a lesser extent, ice X in the so-called low barrier hydrogen bond regime. The correlations appear along the path to hydrogen bond symmetrization, when quantum fluctuations delocalize the proton on the two bond sides. The correlations derive from a strong requirement for local charge neutrality that favors concerted motion along the bonds. The resulting behavior deviates substantially from mean field theory, which would predict in ice VII coherent tunneling of the proton between the two bond sides, thereby causing an ionization catastrophe. Due to the correlations, the quantum state of the proton is entangled.     

Anomalous behavior of proton zero point motion in water confined in carbon nanotubes

The momentum distribution of the protons in ice Ih, ice VI, high density amorphous ice, and water in carbon nanotubes has been measured using deep inelastic neutron scattering. We find that at 5 K the kinetic energy of the protons is 35 meV less than that in ice Ih at the same temperature, and the high momentum tail of the distribution, characteristic of the molecular covalent bond, is not present. We observe a phase transition between 230 and 268 K to a phase that does resemble ice Ih. Although there is yet no model for water that explains the low temperature momentum distribution, our data reveal that the protons in the hydrogen bonds are coherently delocalized and that the low temperature phase is a qualitatively new phase of ice.     

Structures of high and low density amorphous ice by neutron diffraction

Neutron diffraction with isotope substitution is used to determine the structures of high (HDA) and low (LDA) density amorphous ice. Both "phases" are fully hydrogen bonded, tetrahedral networks, with local order similarities between LDA and ice Ih, and HDA and liquid water. Moving from HDA, through liquid water and LDA to ice Ih, the second shell radial order increases at the expense of spatial order. This is linked to a fifth first neighbor "interstitial" that restricts the orientations of first shell waters. This "lynch pin" molecule which keeps the HDA structure intact has implications for the nature of the HDA-LDA transition that bear on the current metastable water debate.     

On the importance of zero-point effects in molecular level classical simulations of water

We discuss the fundamental difficulties involved in comparing energetic results obtained via classical simulations of bulk water with the observed values. Emphasis is placed on the difference between quantum and classical dynamics, and correction techniques, which can be used to emulate quantum effects in a classical system, are investigated. We present molecular dynamics simulation results for liquid water using the ‘Thole-type’ all atom polarizable water model, which has previously been shown to give reasonable results for both ice Ih and small water clusters. We employ expressions for the density of states power spectrum in the liquid in either atomic or rigid-body coordinates that are appropriate for rigid molecule simulations. It is demonstrated that the atomic power spectra can be written as a linear combination of the center of mass and rotational power spectra via the use of the ‘coupling matrix’ of linear coefficients. This approach allows us to introduce the concept of ‘fractional degrees of freedom’ (DOF) for nuclei in rigid molecule simulation. Within this framework, it is illustrated that in a rigid water molecule the oxygen and hydrogen atoms have 2.82 and 1.59 DOF, respectively (for the TIP4P geometry). Within our suggested approach, we finally demonstrate that Debye–Waller factors can be obtained from the coupling matrix and show that quantum corrections to the structure can be accounted for by raising the temperature of the system in a classical simulation by approximately 50°, a result consistent with previous suggestions.     

Surface energy and surface proton order of ice Ih

Ice Ih is comprised of orientationally disordered water molecules giving rise to positional disorder of the hydrogen atoms in the hydrogen bonded network of the lattice. Here we arrive at a first principles determination of the surface energy of ice Ih and suggest that the surface of ice is significantly more proton ordered than the bulk. We predict that the proton order-disorder transition, which occurs in the bulk at approximately 72 K, will not occur at the surface at any temperature below surface melting. An order parameter which defines the surface energy of ice Ih surfaces is also identified.     

Neutron scattering studies of structural transformations and vibrational spectra of ice after high pressure treatment

Abstract

The transitions of the recovered high-pressure phase ice VIII first to high-density amorphous (hda) and low-density amorphous ices, and finally to cubic Ic, and hexagonal Ih ice were observed at heating using real-time neutron diffraction. Inelastic incoherent neutron scattering measurements on the hdu ice, ice Ih and high-pressure phase ice VI revealed similarity between the amorphous phase and crystalline ice VI and led to the new proposition that hda ice consists of two interpenetrating hydrogen-bounded networks with no hydrogen bonds between “sublattices”.     

NEUTRON SCATTERING AND LATTICE DYNAMICAL STUDIES OF THE HIGH-PRESSURE PHASE ICE (I)

Lattice dynamical calculations of ice VIII have been carried out by using a slightly modified set of force constants obtained recently for ice Ih (Li J C and Ross D K 1993 Nature 365 327). A weak interaction was introduced between the two interpenetrated sublattices in the ice VIII structure. The calculated results for H₂O and D₂O ice VIII are in reasonable agreement with the measured inelastic neutron scattering spectra. The eigenvectors of phonon modes in the range of translational and librational bands have been studied in order to understand the properties of the vibrational modes. It is found that the third peak at 26.7meV in the translation results from weak hydrogen bond interactions, and the first peak (14.7meV) is much higher than it is in ice Ih (～7.1meV), which is partially due to the interactions between the two sublattices.     

Thermodynamics of high-pressure ice polymorphs: ice II

Thermodynamic properties of high-pressure solid phase ice II are studied theoretically. The P-V-T equation of state of ice II is derived and its thermodynamic functions are calculated based on the available experimental data. New results are presented concerning the equilibrium solid-solid phase transitions between ice II and ice Ih, ice II and III, and ice II and V.     

Dispersion interactions and vibrational effects in ice as a function of pressure: a first principles study

We present a first principles theoretical framework that accurately accounts for several properties of ice, over a wide pressure range. In particular, we show that, by using a recently developed nonlocal van der Waals functional and by taking into account hydrogen zero point motion, one can properly describe the zero temperature equation of state, the vibrational spectra, and the dielectric properties of ice at low pressure and of ice VIII, a stable phase between 2 and 60?GPa. While semilocal density functionals yield a transition pressure from ice XI to VIII that is overestimated by almost an order of magnitude, we find good agreement with experiments when dispersion forces are taken into account. Zero point energy contributions do not alter the computed transition pressure, but they affect structural properties, including equilibrium volumes and bulk moduli.     

Structure and electronic properties of Hn@C20 molecule

Density functional theory has been employed to optimize the structure of endohedral doped C₂₀ fullerene. We have also investigated electronic properties. We have found that C₂₀ cage can accommodate up to 8 hydrogen atoms. Some hydrogen atoms get chemisorbed on the inner surface of C₂₀ cage and form C-H bond. Structural deformation is found to increase with increase in H-atoms. From the analysis of electronic properties, we observe that due to endohedral doping of hydrogen atoms inside C₂₀, H-atoms acquire net negative charge by accepting electrons and fullerene molecules acquire positive charge by donating electrons to H-atoms. For endohedral complexes where H₃ triangular molecule formation takes place, the nature of net charge transfer changes, i.e. fractional electronic charge is transferred from H-atoms to fullerene. C₂₀ doped with odd number of H-atoms should be more reactive compared to the even number case. Most of the present results are similar to those of endohedral C₆₀.     

Thermodynamics of ice at high pressures and low temperatures

A new equation of state of ice Ih recently proposed by Feistel and Wagner [J. Phys. Chem. Ref. Data 35 (2006) 1021-1047] is used to study the phenomena related to the equilibrium isentropic compression of an ice-water mixture and dynamic loading of solid ice. New results are presented concerning the properties of the new equation of state, equilibrium solid-liquid phase transitions and Hugoniots of low-temperature (100 K) and temperate (263 K) shock-compressed ice.     

Anomalous pressure-induced transition(s) in ice XI

The effects of pressure on the structure of ice XI-an ordered form of the phase of ice Ih, which is known to amorphize under pressure-are investigated theoretically using density-functional theory. We find that pressure induces a mechanical instability, which is initiated by the softening of an acoustic phonon occurring at an incommensurate wavelength, followed by the collapse of the entire acoustic band and by the violation of the Born stability criteria. It is argued that phonon collapse may be a quite general feature of pressure-induced amorphization. The implications of our findings for the amorphization of ice Ih are also discussed.     

Theoretical study of HOCl adsorption on ice surface

Adsorption of HOCl on ice surface was studied using the ab initio molecular orbtial theory. We applied Hartree–Fock (HF) self-consistent field and the second-order Møller–Plesset (MP2) level of theory to cluster models of the (0001) surface ice Ih to optimize adsorption structures and binding energies. In all stable binding configurations, HOCl acts as a proton donor in a hydrogen bond. The presence of neighboring water molecules can strengthen the interaction of HOCl with ice. In the HOCl·(H₂O)₄ system, interaction hydrogen bond length is about 1.85 Å, and binding energies are −10.063−11.149 kcal mol⁻¹. We also calculated the vibrational frequencies of HOCl affected by the ice surface.     

Theoretical investigation of structures,compositions, and phase transitions of neon hydrates based on ices Ih and II

Thermodynamic conditions of existence in the p-T plane and the composition of neon hydrates based on ices Ih and II are determined. The occupancy of neon in cages (channels) of ices Ih and II at temperatures below 0°C is calculated. It is shown that the occupancy of neon in hydrate based on ices cages decreases with growing temperature. Lines of monovariant equilibria between gas phase (neon)-neon hydrate based on ice Ih-liquid water (or ice II) and neon-gas phase (neon)-hydrate based on ice II-liquidwater (or ice II) are found. The line of divariant equilibria between neon hydrate based on ice Ih-neon hydrate based on ice II has been also calculated. The possibility of ice stabilization due to inclusion of neon into ice cages (channels) is shown.     

缩酮席夫碱类阴离子识别的量子化学研究

缩酮席夫碱是良好的阴离子识别剂,利用量子化学密度泛函理论研究了3种缩酮席夫碱阴离子受体与阴离子客体相结合的空间结构、电荷分布、结合能等方面的变化,计算结果表明,阴离子受体分子的阴离子结合位点位于亚氨基-NH-部,受体分子和阴离子间通过氢键相互作用,阴离子有一部分负电荷转移到受体分子中,且转移的电荷量为R3-3 R3-2 R3-1,在考察的阴离子中,F~-与受体分子的结合能最大,而在3种受体分子中,R3-3对阴离子结合能最大.计算模拟结果,与实验情况相吻合.     

Hydrogen-bond topology and the ice VII/VIII and ice Ih/XI proton-ordering phase transitions

The existence of an ice Ih/XI proton-ordering transition to a low-temperature ferroelectric phase has sparked considerable debate in the literature. Electronic density functional theory calculations, extended using graph invariants, confirm that a transition to a low-temperature ferroelectric phase should occur. The predicted transition at 98 K is in qualitative agreement with the observed transition at 72 K, and the low-temperature phase is the ferroelectric phase determined in diffraction experiments. The theoretical methods used to predict the phase transition are validated by comparing their prediction to the well-characterized ice VII/VIII proton-ordering transition.     

Phase diagram of water under an applied electric field

Simulations are used to investigate for the first time the anisotropy of the dielectric response and the effects of an applied electric field E(ex) on the phase diagram of water. In the presence of electric fields ice II disappears from the phase diagram. When E(ex) is applied in the direction perpendicular to the ac crystallographic plane the melting temperatures of ices III and V increase whereas that of ice Ih is hardly affected. Ice III also disappears as a stable phase when E(ex) is applied in the direction perpendicular to the ab plane. E(ex) increases by a small amount the critical temperature and reduces slightly the temperature of the maximum density of liquid water. The presence E(ex) modifies all phase transitions of water but its effect on solid-solid and solid-fluid transitions seems to be more important and different depending on the direction of E(ex).     

A polarizable, four-point-charge model of water with size variation

The proposed model is classical, polarizable and can change its size responding to the impact of the surroundings. We introduce a qualitative theory which simultaneously changes polarization, repulsion and size. Larger electric fields in condensed phases pull the molecules closer and induce larger polarization than in gas phase. However, this causes exponential increase in repulsion as a result of overlapping electron clouds. To decrease the impact of this repulsion the electron cloud contracts slightly, which decrease its polarization. Since the size of the molecule is determined by the extension of its electron cloud, there will be a subtle equilibrium determined by the actual strength of the polarization force, the repulsion and the size of the molecule.Using the principles above, we developed a model and carried out numerical calculations. The properties of the dimer practically reproduce the experimental values. The estimates of the energy, the density and the structure of liquid water and hexagonal ice are also excellent.