451 research outputs found
Polarizability anisotropy relaxation in liquid ethanol: A molecular dynamics study
No full textMolecular dynamics simulation is used to study the relaxation of the polarizability anisotropy in liquid ethanol at temperatures of 298 and 348 K. Ethanol molecules are represented by a four-site semi-flexible model in which the internal degree of freedom, corresponding to the torsional motion around the C-O bond, is taken into consideration. The molecular polarizability is calculated using an interaction-site model, based on the modified dipole-induced dipole model developed by Thole. The collective polarizability induced by intermolecular interactions is included using first-order perturbation theory and calculated considering both center-center and site-site models. Results are analyzed in terms of projected variables that allow the decomposition of the total relaxation into orientational and collision-induced components, both of which are influenced by molecular flexibility. We compare our data with the results of low-frequency depolarized Rayleigh light scattering experiments, examining the possibility of separating different relaxation processes from spectroscopic signals in the time and frequency domains. We find that even though the largest contributor to polarizability anisotropy dynamics is orientational relaxation, collision-induced contributions are important, especially at shorter times. Moreover, we show that torsional motion also plays a significant role in the fast decay of the polarizability anisotropy of the system. Although liquid ethanol is strongly associated and its dipolar relaxation is highly collective, we find orientational pair correlation effects on polarizability relaxation to be negligible. A comparison with simulation data previously obtained for methanol suggests that increasing the alcohol chain length decreases the relative importance of OH dynamics in polarizability relaxation. We further show that, as expected for such a structured liquid composed of strongly asymmetric molecules, polarizability and dipole relaxation processes exhibit marked differences. (C) 2002 American Institute of Physics
Solvation of Coumarin 153 in Supercritical Fluoroform
No full textWe present a study of local density augmentation around an attractive solute (i.e., giving rise to more attractive interaction with the solvent than solvent-solvent interactions) in supercritical fluoroform. This work is based on molecular dynamics simulations of coumarin 153 in supercritical fluoroform at densities both above and below the critical density, ranging from dilute gaslike to liquidlike, at a reduced temperature (T/Tc) of 1.03. We focused on studying the structure of the solvation shell and the variation of the solute electronic absorption and emission shifts with density. Quantum calculations at the density functional theory (DFT) level were run on the solute in the ground state, and time-dependent DFT calculations were performed in the solute excited state in order to determine the solute-solvent potential parameters. The results obtained for the Stokes shift are in agreement with the experimental measurements. To evaluate local density augmentation from simulations, we used two different definitions, one based on the solvation number and the other derived from solvatochromic shifts. In the former case, the agreement with experimental results is good, while, in the latter case, better agreement is achieved by perturbatively including the induced-dipole contribution to the solvation energy
Solvation Dynamics in Acetonitrile: A Study Incorporating Solute Electronic Response and Nuclear Relaxation
No full textThe solvent reorganization process after electronic excitation of a polar solute in a polar solvent such as acetonitrile is related mainly to the time evolution of the solute-solvent electrostatic interaction. Modern laser-based techniques have sufficient time resolution to follow this decay in real time, providing information to be confirmed and interpreted by theories and models. We present here a study aimed at the investigation of the different steps involved in the process taking place after a vertical S0 f S1 excitation of a large size chromophore, coumarin 153 (C153), in acetonitrile, from both the solute and the solvent points of view. To do this, we use accurate quantum mechanical calculations for the solute properties within the polarizable continuum model (PCM) and classical molecular dynamics (MD) simulations, both equilibrium and nonequilibrium, for C153 in the presence of the solvent. The geometry of the solute is allowed to change in order to study the role of internal motions in the time-dependent solvation process. The solvent response function has been obtained from the simulation data and compared to experiment, while the comparison between equilibrium and nonequilibrium MD results for the solvation response confirms the validity of the linear response approximation in the C153-acetonitrile system. The MD trajectories have also been used to monitor the structure of the solvation shell and to determine its change in response to the change in the solute partial charges
Molecular Dynamics Simulation Of Solvation Dynamics In Methanol-water Mixtures
No full textThe solvation dynamics following charge-transfer electronic excitation of diatomic solutes immersed in methanol-water mixtures is investigated through molecular dynamics simulations. The solvation response functions associated with an instantaneous reversal of the solute's dipole moment for two different solute sizes in mixtures with methanol mole fractions, xm = 0.2, 0.5, and 0.8, are calculated and compared to the corresponding ones in the pure liquids. The solvation response of the mixtures is separated into methanol and water contributions in order to elucidate the role played by each molecular species on the solvation dynamics. We find significantly different responses for the two solutes and relate them to the fact that the solute with the smaller site diameters is a much better hydrogen (H)-bond acceptor than the larger diameter solute. For the small solute in methanol and in the mixed solvents, we have also calculated H-bond response functions, which measure the rate of solute-solvent H-bond formation after the solute's excitation and find that, at longer times, the solvation and H-bond formation response functions decay at similar rates. The implications of this finding for solvation dynamics of H-bonding solutes in H-bonding solvents are discussed and related to recent experimental results for such systems. © 1996 American Chemical Society.100461825818268Heitele, H., (1993) Angew. Chem., Int. Ed. Engl., 32, p. 359Hynes, J.T., (1993) Ultrafast Dynamics of Chemical Systems, , Simon, J. D., Ed.Kluwer: Dordrecht, Chapter 13Rossky, P.J., Simon, J.D., (1994) Nature (London), 370, p. 263Refs 5-8 contain reviews of various aspects of solvation dynamicsMaroncelli, M., (1993) J. Mol. 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Dynamics of biological water: insights from molecular modeling of light scattering in aqueous trehalose solutions
No full textExtended depolarized light scattering (EDLS) measurements have been recently employed to investigate the dynamics of water solvating biological molecules, giving evidence of the presence of two different dynamical regimes among water molecules. An interpretation of EDLS has been proposed that provides an independent estimate of the retardation factor of slowdown with respect to fast water molecules and of the number of solvent molecules affected by this slowing down. Nevertheless this measure is an inherently complex one, due to the collective nature of the physical property probed. In the present work a molecular dynamics (MD) approach has been used to more deeply understand experimental results. Time correlation functions of the collective polarizability anisotropy have been calculated for the prototype disaccharide trehalose in aqueous solutions as a function of concentration. The unique capability of MD to disentangle the contributions to the dynamics arising from solute, solvent, and cross terms between the two allowed us to check the reliability of an interpretation that assumes a spectral separation of water and sugar dynamics, as well as to highlight the very presence of two distinct relaxation processes in water. The two processes have been attributed to the dynamics of bulk and hydration water, respectively. A retardation factor of ~5 and concentration dependent hydration numbers have been observed, in good agreement with experimental results [Paolantoni, M.; et al. J. Phys. Chem. B 2009, 113, 7874-7878]
Hydration and aggregation in mono- and disaccharide aqueous solutions by gigahertz-to-terahertz light scattering and molecular dynamics simulations
No full textThe relaxation properties of hydration water around fructose, glucose, sucrose, and trehalose molecules have been studied by means of extended frequency range depolarized light scattering and molecular dynamics simulations. Evidence is given of hydration dynamics retarded by a factor ξ = 5-6 for all the analyzed solutes. A dynamical hydration shell is defined based on the solute-induced slowing down of water mobility at picosecond time scales. The number of dynamically perturbed water molecules N(h) and its concentration dependence have been determined in glucose and trehalose aqueous solutions up to high solute weight fractions (ca. 45%). For highly dilute solutions, about 3.3 water molecules per sugar hydroxyl group are found to be part of the hydration shell of mono- and disaccharide. For increasing concentrations, a noticeable solute-dependent reduction of hydration number occurs, which has been attributed, in addition to simple statistical shells overlapping, to aggregation of solute molecules. A scaling law based on the number of hydroxyl groups collapses the N(h) concentration dependence of glucose and trehalose into a single master plot, suggesting hydration and aggregation properties independent of the size of the sugar. As a whole, the present results point to the concentration of hydroxyl groups as the parameter guiding both sugar-water and sugar-sugar interactions, without appreciable difference between mono- and disaccharides
Xp11.2 Translocation Renal Cell Carcinoma With Very Aggressive Course In Five Adult Patients: Editorial Comment
No full text[No abstract available]323365366Argani, P., Antonescu, C.R., Couturier, J., Fournet, J.C., Sciot, R., Debiec-Rychter, M., Hutchinson, B., Ladanyi, M., PRCC-TFE3 renal carcinomas: Morphologic, immunohistochemical, ultrastructural, and molecular analysis of an entity associated with the t(X;1)(p11.2;q21) (2002) Am J Surg Pathol., 26, pp. 1553-1566Salles, P.G., Soto Jr., M., Kidney carcinoma associated with Xp11.2 translocation/TFE3 (ASPL-TFE3) gene fusion (2005) Int Braz J Urol, 31, pp. 251-25
Shear failure in rock using different constant normal load
No full textSEILDEL – HABERFIELD’s [1] work was based on the analysis of the regular triangular asperities
and assumed that the asperities were rigid. They used normal stiffness shear stress (CNS) to measure
the joint dilatation rate and extended the LADANYI – ARCHAMBAULT’s [2] approach. The aim of
this research is to investigate the dependence of the constant normal load (CNL) on the rate of the
dilatation. Three points were chosen on the bilinear failure envelope: the first in the first linear part,
the second in transition stress, and the third in the second part of the bilinear failure envelope. 12
regular triangular cementmortar specimens were used to carry out this research. These cementmortar
specimens were investigated by TISA – KOVARI [3] before so all the material and shearing constants
were well-known. The other purpose of this research was to observe the influence of the normal stress
on the dilatation–displacement curves
Use of the deep penetration test in sensitive clays/ Utilisation de l'essai de penetration en profondeur dans les argiles sensibles
No full textThe deep penetration test was subjected to trials in both the field and laboratory as a possible method of determining the undrained shear strength. It is shown that in sensitive clays the bearing capacity factor N(c) is considerably less than 9. The trials showed that the deep penetration test is a promising method of measuring undrained strength as it is not affected by disturbance and yields a continuous record of undrained strengths.Les auteurs ont mis en oeuvre l'essai de p\ue9n\ue9tration profonde, tant en place qu'au laboratoire, comme nouvelle m\ue9thode de d\ue9termination de la r\ue9sistance au cisaillement des sols non \ue9goutt\ue9s. Ils montrent que pour les argiles sensibles la force portante N(c) est consid\ue9rablement inf\ue9rieure \ue0 9. Les travaux ont montr\ue9 que l'essai de p\ue9n\ue9tration profonde constitue une m\ue9thode fructueuse de mesure des r\ue9sistances des sols non \ue9goutt\ue9s, qu'il n'est pas influenc\ue9 par les bouleversements et qu'il donne une courbe continue des r\ue9sistances en sol non \ue9goutt\ue9.Peer reviewed: NoNRC publication: Ye
Etude théorique et expérimentale du problème de l'expansion dans un sol pulvérulent d'une cavité presentant une symétrie sphérique ou cylindrique
No full textDoctorat en sciences appliquée
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