Search results for "SIM"
showing 10 items of 10139 documents
Static and Dynamic Properties of the Nonframework Cations in Na-Mordenites Zeolite
2004
This work consists of exploring the effect of Si/Al ratio and water content on both the static and dynamic properties of the extra-framework cations in a typical zeolite system Na+-Mordenites. Atomistic simulations based on interatomic potentials and minimisation techniques have been used to determine the location of the cations as a function of the Si/Al ratio and the hydration level. Our calculations showed two different cation behaviours depending on the type of channels that they occupy, the positions of the cations in the main channels being substantially perturbed upon the sorption of water molecules whereas those of the cations located in the small side channels being only slightly s…
Does glycosyl transfer involve an oxacarbenium intermediate? Computational simulation of the lifetime of the methoxymethyl cation in water
2011
2D free-energy surfaces for transfer of the methoxymethyl cation between two water molecules are constructed from molecular dynamics (MD) simulations in which these atoms are treated quantum-mechanically within a box of 1030 classical solvent water molecules at 300 K. This provides a simple model for glycosyl transfer in water. The AM1/TIP3P surfaces with 2D-spline corrections at either MPWB1K/6-31+G(d,p) or MP2/6-31+G(d,p) contain a shallow free-energy well corresponding to an oxacarbenium ion intermediate in a DN*AN mechanism. MD analysis at three temperatures leads to a classical estimate of the lifetime of the methoxymethyl cation in water; when quantum corrections for vibrational zero-…
Erratum: “Computing absolute free energies of disordered structures by molecular simulation” [J. Chem. Phys. 131, 231102 (2009)]
2011
Solute-induced Water Structure: Computer Simulation on a Model System
1988
Abstract Two series of Monte Carlo simulations have been carried out on a system consisting of 125 water molecules, one of which is kept fixed to simulate a water molecule whose mobility is restricted by a solute. The results are checked against similar simulations without restrictions, used as a control, and they show how the blocked molecule helps increase both the structural order and the connectivity of the hydrogen bond network. Cooperativity originating from proton polarizability of H-bonds and/or from many-body terms of interaction potentials cannot be involved since we use a rigid water model and ab initio pair potentials. The present findings are interpreted as indicative of a moti…
Study of percolation and clustering in supercritical water-CO2 mixtures
2008
The microscopic structure of supercritical water-CO(2) mixture is investigated by neutron diffraction experiments exploiting the isotopic HD substitution. The investigated water reach mixtures are in the liquidlike region of the phase diagram, according to the behavior of the radial distribution functions, yet a reduction of the average number of hydrogen bonds, compared to equivalent states of pure water, is found. As a consequence, the average dimension of water clusters is reduced and the system stays below the percolation threshold. These results, along with the shift of the main peaks of the site-site radial distribution functions, suggest that the excess volume in these supercritical …
Experimental evidence for fractional time evolution in glass forming materials
2002
The infinitesimal generator of time evolution in the standard equation for exponential (Debye) relaxation is replaced with the infinitesimal generator of composite fractional translations. Composite fractional translations are defined as a combination of translation and the fractional time evolution introduced in [Physica A, 221 (1995) 89]. The fractional differential equation for composite fractional relaxation is solved. The resulting dynamical susceptibility is used to fit broad band dielectric spectroscopy data of glycerol. The composite fractional susceptibility function can exhibit an asymmetric relaxation peak and an excess wing at high frequencies in the imaginary part. Nevertheless…
Assessment of a single-acquisition imaging sequence for oxygen-sensitive (3)He-MRI.
2001
MRI of the lungs using hyperpolarized helium-3 (3He) allows the determination of intrapulmonary oxygen partial pressures (pO2). The need to separate competing processes of signal loss has hitherto required two different imaging series during two different breathing maneuvers. In this work, a new imaging strategy to measure pO2 by a single series of consecutive scans is presented. The feasibility of the method is demonstrated in three healthy human volunteers. Maps and histograms of intrapulmonary pO2 are calculated. Changes in the oxygen concentration of the inhaled gas mixture are well reproduced in the histograms. Monte Carlo (MC) simulations of the temporal evolution of 3He hyperpolariza…
Self-assembly and rheology of dipolar colloids in simple shear studied using multi-particle collision dynamics.
2017
Magnetic nanoparticles in a colloidal solution self-assemble in various aligned structures, which has a profound influence on the flow behavior. However, the precise role of the microstructure in the development of the rheological response has not been reliably quantified. We investigate the self-assembly of dipolar colloids in simple shear using hybrid molecular dynamics and multi-particle collision dynamics simulations with explicit coarse-grained hydrodynamics, conduct simulated rheometric studies and apply micromechanical models to produce master curves, showing evidence of the universality of the structural behavior governed by the competition between the bonding (dipolar) and erosive …
Growing and Shaping Metal–Organic Framework Single Crystals at the Millimeter Scale
2020
Controlling and understanding the mechanisms that harness crystallization processes is of utmost importance in contemporary materials science and, in particular, in the realm of reticular solids where it still remains a great challenge. In this work, we show that environments mimicking microgravity conditions can harness the size and shape of functional biogenic crystals such as peptide-based metal–organic frameworks (MOFs). In particular, we demonstrate formation of the largest single crystals with controlled nonequilibrium shapes of peptide-based MOFs reported to date (e.g., those featuring curved crystal habits), as opposed to the typical polyhedral microcrystals obtained under bul…