A continuous mixture of two different dimers in liquid water.

It is hitherto thought that liquid water is composed of tetrahedrally coordinated molecules with an asymmetric interaction of the central molecule with neighboring molecules. Kühne et al., Nat. Commun., 2013, 4, 1450 suggested that this asymmetry, energetic rather than geometric, is the cornerstone to reconcile the homogeneous and inhomogeneous viewpoints of liquid water. In order to investigate the geometric origin of that asymmetry, we have scrutinized Molecular Dynamics (MD) simulations of water through a careful analysis of the five-dimensional probability distribution function of Euler angles in which the relative positions and orientations of water molecules are obtained. We demonstrate that, beyond the ubiquitous tetrahedral structure with well-defined molecular dimers, there is a series of possible molecular orientations that define the structure. These orientations are generated by rotating the neighboring molecule around the O-H axis that is involved in the hydrogen bond scheme. Two of the possible orientations have a higher probability, giving rise to two kinds of dimers: one close to the lowest energy of a water dimer in vacuum with an almost perpendicular alignment of the dipole moment, and another one with a parallel orientation of the dipole moment which is less tightly bound. These two different dimers have an effect on the orientation of further water dipole moments up to a distance of ≈6 Å. Liquid water can therefore be described as a continuous mixture of two kinds of dimers where the hydrogen bonds have the same geometry but the interaction energies are different due to a different mutual orientation of the dipoles of the participating water molecules.

Specific ion effects in aqueous eletrolyte solutions confined within graphene sheets at the nanometric scale.

The underlying mechanisms of specific ion effects on structure and dynamics of aqueous solutions have been long debated. On the other hand, the role of polarization at hydrophobic interfaces when aqueous electrolytes are present is of great importance, as it has been observed at the air-vapor interface. In this work, we have explored influence of ionic species on microscopical properties of aqueous sodium halide solutions constrained inside a double layer graphene channel, as a model for a realistic hydrophobic interface. Our systems have been simulated by molecular dynamics techniques, explicitly including polarization in water molecules and ions. Water and ionic density profiles showed the tendency of ionic species to occupy the whole space available, in good agreement with spectroscopic experimental data. The exception to this general behavior was fluoride, which preferred to stay away from interfaces. Two main regions were defined: interfaces and the central part of the slab, the bulklike region. Ionic hydration numbers at interfaces were lower than those at the bulklike area by about one to two units. We have also analyzed water-ion orientations and polarization distributions and obtained a marked dependence on ionic concentration. Residence time of anions suffered important fluctuations and tended to be largest at interfaces. Large variations of the static permittivity between interfacial and bulklike regions were observed. Ionic diffusion was found to be between 10(-5) and 10(-6) cm(2) s(-1) and showed to be mainly dependent on the concentration, whereas the type of anion considered and the polarizability had significantly less relevance. Conductivities were found to be dependent on ionic concentrations and the polarizabilities of anions, as well as on the spatial direction considered.

Effects of concentration on structure, dielectric, and dynamic properties of aqueous NaCl solutions using a polarizable model.

The study of NaCl solutions in water at finite concentration, explicitly including polarization in water molecules and ions, has been carried out by molecular dynamics simulations. A comparison of the RPOL polarizable model with the rigid SPC/E potential for water has been included. Structure obtained with the two models does not show significant differences, although some deviations in the NaNa radial distribution functions at all concentrations are observed. Dielectric properties such as total and molecular dipole moment correlation functions revealed decay times of the order of 10 ps, roughly independent of concentration. The analysis of electric conductivity by means of current-current correlation functions also included the calculation of cross terms corresponding to dipole moment-current correlations, which proved to be non-neglectable at short times and especially relevant at high concentrations (m=4 mol kg(-1)). Frequency dependent dielectric constants and conductivities have been computed and the role of cross correlations has been analyzed. In all cases both concentration and cross correlations have significant influence in the results.

Molecular dynamics simulations of supercritical water confined within a carbon-slit pore.

We report the results of a series of molecular dynamics simulations of water inside a carbon-slit pore at supercritical conditions. A range of densities corresponding from liquid (0.66gcm;{-3}) to gas environments (0.08gcm;{-3}) at the supercritical temperature of 673K were considered. Our findings are compared with previous studies of liquid water confined in graphene nanochannels at ambient and high temperatures, and indicate that the microscopic structure of water evolves from hydrogen bond networks characteristic of hot dense liquids to looser arrangements where the dominant units are water monomers and dimers. Water permittivity was found to be very small at low densities, with a tendency to grow with density and to reach typical values of unconfined supercritical water at 0.66gcm;{-3}) . In supercritical conditions, the residence time of water at interfaces is roughly similar to that of water in the central regions of the slabs, if the size of the considered region is taken into account. That time span is long enough to compute dynamical properties such as diffusion or spectral densities. Water diffusion in supercritical states is much faster at low densities, and it is produced in such a way that, at interfaces, translational diffusion is mainly produced along planes parallel to the carbon walls. Spectral frequency shifts depend on several factors, being temperature and density effects the most relevant. However, we can observe corrections due to confinement, important both at the graphene interface and in the central region of the water slab.

Liquid water confined in carbon nanochannels at high temperatures.

Structure, hydrogen bonding, electrostatics, dielectric, and dynamical properties of liquid water confined in flat graphene nanochannels are investigated by molecular dynamics simulations. A wide range of temperatures (between 20 and 360 degrees C) have been considered. Molecular structure suffers substantial changes when the system is heated, with a significant loss of structure and hydrogen bonding. In such case, the interface between adsorbed and bulk-like water has a marked tendency to disappear, and the two preferential orientations of water nearby the graphite layers at room temperature are essentially merging above the boiling point. The general trend for the static dielectric constant is its reduction at high temperature states, as compared to ambient conditions. Similarly, residence times of water molecules in adsorbed and bulk-like regions are significantly influenced by temperature, as well. Finally, we observed relevant changes in water diffusion and spectroscopy along the range of temperatures analyzed.

Molecular dynamics simulation of liquid water confined inside graphite channels: dielectric and dynamical properties.

Electric and dielectric properties and microscopic dynamics of liquid water confined between graphite slabs are analyzed by means of molecular dynamics simulations for several graphite-graphite separations at ambient conditions. The electric potential across the interface shows oscillations due to water layering, and the overall potential drop is about -0.28 V. The total dielectric constant is larger than the corresponding value for the bulklike internal region of the system. This is mainly due to the preferential orientations of water nearest the graphite walls. Estimation of the capacitance of the system is reported, indicating large variations for the different adsorption layers. The main trend observed concerning water diffusion is 2-fold: on one hand, the overall diffusion of water is markedly smaller for the closest graphite-graphite separations, and on the other hand, water molecules diffuse in interfaces slightly slower than those in the bulklike internal areas. Molecular reorientational times are generally larger than those corresponding to those of unconstrained bulk water. The analysis of spectral densities revealed significant spectral shifts, compared to the bands in unconstrained water, in different frequency regions, and associated to confinement effects. These findings are important because of the scarce information available from experimental, theoretical, and computer simulation research into the dielectric and dynamical properties of confined water.

1H nuclear spin relaxation of liquid water from molecular dynamics simulations.

We have investigated the nuclear spin relaxation properties of (1)H in liquid water with the help of molecular dynamics simulations. We have computed the (1)H nuclear spin relaxation times T1 and T2 and determined the contribution of the different interactions to the relaxation at different temperatures and for different classical water models (SPC/E, TIP3P, TIP4P, and TIP4P/2005). Among the water models considered, the TIP4P/2005 model exhibits the best agreement with the experiment. The same analysis was performed with Car-Parrinello ab initio molecular dynamics simulations of bulk water at T = 330 K, which provided results close to the experimental values at room temperature. To complete the study, we have successfully accounted for the temperature-dependence of T1 and T2 in terms of a simplified model, which considers the reorientation in finite angle jumps and the diffusive translation of water molecules.

Neuropsychological alterations in patients with computed tomography-detected basal ganglia calcification.

OBJECTIVE: To investigate the cognitive and mental status of patients with basal ganglia calcification on a computed tomographic scan. DESIGN: Eighteen ambulatory patients with basal ganglia calcification and without other radiological findings who were identified from the computed tomography records of a general hospital in a 2-year period and 16 control subjects who were matched for age, education, sex, and premorbid IQ estimation consented to participate. All subjects underwent a neurological evaluation, a comprehensive neuropsychological battery, and tests with psychiatric rating scales. RESULTS: Significant differences for the control group were found in tests that evaluated motor speed and executive, visuospatial, and some memory functions. Four patients (22%) met criteria for organic mood disorder (minor depression, three patients; bipolar depression, one patient) according to the Diagnostic and Statistical Manual of Mental Disorders, Revised Third Edition, whereas six other patients (33%) met diagnostic criteria for obsessive-compulsive disorder. CONCLUSIONS: These results indicate that patients with basal ganglia calcifications frequently have a subcortical pattern of neuropsychological dysfunction and behavioral changes that are known to be associated with alterations of the frontal-limbic-basal ganglia circuits. The pattern of neuropsychological impairment is consistent with basal ganglia damage. However, poor performance in other neuropsychological tests suggest additional involvement of other connected networks.

Spontaneous primary intraventricular hemorrhage: clinical data, etiology and outcome.

The clinical features, etiology, and neurological outcome in patients with primary intraventricular hemorrhage (PIVH) have rarely been reported. We retrospectively reviewed the clinical data, complementary examinations, outcome, computed tomography (CT) blood amount, and ventricle size of 13 patients (mean age 60 years, five men). We defined PIVH as hemorrhage detected by CT in the ventricular system only. The major symptoms included headache (n = 13), decreased level of consciousness (n = 9), and nausea/vomiting (n = 7). The cause was unknown in five patients; and was associated with arterial hypertension in five, vascular malformations in two, and tumor in one, although arteriography was performed in only five patients. Outcomes were death in three, asymptomatic in six, mild disability in three, and moderate disability in one. Prognosis was not related to clinical or CT data. Clinical features can suggest the diagnosis of PIVH, but cerebral CT is required for confirmation.

Nuclear chromatin texture to differentiate follicular and papillary carcinoma of the thyroid.

This study presents the results of an automatic classification of six purely papillary, five purely follicular and five mixed papillary-follicular carcinomas of the thyroid. All cases were correctly classified with only two chromatin textural parameters obtained in the Olga's scan software package developed in our laboratory. Chromatin texture lines were generated by a unidirectional grey level gradient, obtained as a natural logarithmic function of the integrated optical density of each nucleus; this increased the difference between clear papillary and granular follicular nuclei. The accuracy of the method allowed a follicular carcinoma to be correctly classified as a mixed follicular-papillary. Finally a columnar-cell carcinoma of the thyroid was detected by means of its atypical DNA histogram.

Density and temperature effects on the orientational and dielectric properties of supercritical water.

The influence of temperature and density changes on the reorientational motions and dielectric properties of supercritical water is investigated by molecular dynamics simulations. A rigid simple point charged potential has been used to model water-water interactions. This model has revealed to be very satisfactory to reproduce thermodynamic, diffusive, and static dielectric properties of supercritical water. Reorientation times of water molecules have been computed along specific molecular directions and compared with available experimental data. The imaginary part of the frequency dependent dielectric constant, spectral densities of states, and residence times of water molecules in their first hydration shells are also evaluated.

[Cerebral venous thrombosis. Study of 17 cases]. / Trombosis venosa cerebral. Estudio de 17 casos.

BACKGROUND: We undertook this study to determine the clinical, neuroimaging findings, etiologies and final outcome of 17 patients with cerebral venous thrombosis diagnosed in a single center. PATIENTS AND METHODS: In this retrospective study we analyzed the clinical and neuroimaging findings of patients with cerebral venous thrombosis collected at our hospital from 1980 to 1997. The diagnosis of cerebral venous thrombosis was made by angiography and/or magnetic resonance imaging. Final outcome was assessed with the modified Rankin scale and patients were included in two groups. Differences between groups were tested using uni and multivariate analysis. RESULTS: Seventeen patients (10 women) with a mean age of 41.9 years were analyzed. The most frequent clinical pattern was focal cerebral signs (70.5%) followed by symptoms/signs of increased intracranial pressure (12%) and diffuse encephalopathy (12%). The diagnosis of cerebral venous thrombosis was made by conventional angiography in 12 cases (70.5%) and by magnetic resonance imaging in 5 (29.5%). The most frequent site of venous occlusion was superior sagittal sinus (47%) followed by lateral sinus (35%). Etiologies were hematologic disease (29%), neoplasms (23.5%), oral contraceptives (12%), infection (12%) and unknown in 18%. The majority of the patients (59%) had minor neurological sequelae during follow-up. A decreased level of consciousness and neoplasm were associated with a worse functional outcome. CONCLUSIONS: In our series the most frequent clinical pattern was focal cerebral signs and the main etiology was hematologic disease. A relatively good prognosis was observed in those patients. A decreased level of consciousness and presence of neoplasm were the factors associated with a bad prognosis in these cases.

[Thyroid ophthalmopathy: clinical and tomographic study]. / Oftalmopatía tiroidea: estudio clínico y tomográfico.

BACKGROUND: To evaluate exophthalmos by Hertel exophthalmometer (HE) compared to computed tomography (CT), and extraocular muscle width on CT and its relation with the clinical course in patients with thyroidal ophthalmopathy. METHODS: On reviewed 118 orbits from 59 patients (50 women, mean age 40.6 y., range 12-84) who suffered a thyroidal ophthalmopathy (Graves' disease, n = 57; Hashimoto's thyroiditis, n = 2). Muscle width was evaluated using coronal and mid-axial scans in all cases. Medial rectus (MR) width was analysed by Hallin and Feldon technique. RESULTS: Exophthalmos by HE was 22.5 +/- 2.5 and 23.2 +/- 3.2 mm (right-RE/left eye-LE), and CT 20.8 +/- 3.2 and 20.9 +/- 2.9 mm (p < 0.00001, r = 0.72 and 0.65, respectively). MR width on mid-axial scan was 4.1 +/- 1.6 and 4.2 +/- 1.5 mm (RE/LE). On coronal scans, MR was the muscle more often enlarged followed by superior, inferior and lateral rectus. CONCLUSIONS: Exophthalmos measured by HE was greater than by CT-measured one, but with a good correlation. Muscle were not equally affected, being MR the most frequently enlarged. MR-width was not related to duration and severity of disease.

Characterization of the Methane-Graphene Hydrophobic Interaction in Aqueous Solution from Ab Initio Simulations.

In this article, the interaction between a methane molecule and a graphene plane in liquid water has been characterized employing DFT-based free energy Molecular Dynamics calculations. This system represents a good model to understand the generic interaction between a small hydrophobic solute (methane molecule) and an extense hydrophobic surface (graphene plane). The structural and dynamical properties of graphene and methane hydration water are analyzed and found to be closely related to the main features of the potential of mean force. The results could be used in coarse-grained models to take into account the effect of the hydrophobic interaction in realistic systems relevant to experiment.

Insights into the determination of molecular structure from diffraction data using a Bayesian algorithm.

The determination of the molecular ordering in a liquid is still a controversial subject. There is no general consensus either on the methods to obtain reliable liquid structures or on the way to analyze them. Regardless of the method, it is very important to have a realistic molecular structure available that allows simulations to faithfully reproduce the sample features, and that minimizes the computing time in structure refinements. However, attention is not always paid to this point and molecular models coming from general force-fields are frequently used to undertake many of the analyses. We propose in this work to use a Bayesian scheme to fit the experimental data and produce reliable molecular models that can be used as the starting point of any simulation or refinement. The algorithm behind the proposed method is based on a Markov chain Monte Carlo procedure, as many other refinement programs such as reverse Monte Carlo or empirical potential structure refinement.

Dynamics of water nanodroplets and aqueous protons in non-ionic reverse micelles.

We present a study of the microscopic dynamics of water trapped in reverse non-ionic micelles by means of a series of molecular dynamics simulations. The analysis of the effects of micellar confinement on spectroscopical properties of an excess proton has also been considered. Our micelles were microemulsions made with the neutral surfactant diethylene glycol monodecyl ether [CH(3)(CH(2))(11)(OC(2)H(4))(2)OH]. Simulation experiments including the proton species were performed using a multistate empirical valence bond Hamiltonian model. Diffusion of water in the micelle is markedly slower than that in the bulk liquid, in the same fashion as happens with reorientational dynamics. Spectral densities of hydrogens revealed a blue-shift of the OH-stretching vibration together with a split of the main band into two components. Absorption lineshapes of the solvated proton in the vicinity of the internal surface of the micelle indicate the coexistence of Eigen-like and Zundel-like structures and a tendency to red-shifting (compared to the aqueous unconstrained excess proton case) of the two relevant spectral bands (around 2000 and 2500 wavenumbers) mainly due to the slower dynamics of proton vibrations in water near interfaces.

Molecular simulation of liquid water confined inside graphite channels: thermodynamics and structural properties.

We carried out molecular dynamics simulations to describe the properties of water inside a narrow graphite channel. Two stable phases were found: a low-density one made of water clusters adsorbed on the graphite sheets and a liquid one that fills the entire channel, forming several layers around a bulk-like region. We analyzed the interfacial structure, orientational order, water residence times in several regions, and hydrogen bonding of this last water phase, calculating also a quantity of electrochemical interest, the probability of electron tunneling through interfacial water. The results are in good qualitative agreement with the available experimental data.