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At least 1,063 records · Page 59Linked to original sources
PubMed2004

BPA-PC on a Ni111 surface: the interplay between adsorption energy and conformational entropy for different chain-end modifications.

We extend a previous dual scale modeling approach for the behavior of polymers near a metal surface to a variety of end groups. Our approach combines a coarse-grained polymer model with ab initio DFT calculations. Such a procedure was applied to a melt of phenolic-like terminated Bisphenol A-polycarbonate (BPA-PC) interacting with a (111) nickel surface (Delle Site, L.; Abrams, C. F.; Alavi, A.; Kremer, K. Phys. Rev. Lett. 2002, 89, 156103. Abrams, C. F.; Delle Site, L.; Kremer, K. Phys. Rev. E 2003, 67, 021807). This work extends this study to different chain-end modifications of BPA-PC, p-tert-butylphenolic, p-tetramethylpropylphenolic, and p-cumylphenolic. We show how the interplay between adsorption energies and conformational entropy selects different morphologies for the various melts at the interface. Implications of these results for realistic technical materials are finally discussed.

Journal Article↗
PubMed2004

Temperature effects on the catalytic activity of the D38E mutant of 3-oxo-Delta5-steroid isomerase: favorable enthalpies and entropies of activation relative to the nonenzymatic reaction catalyzed by acetate ion.

3-oxo-Delta5-steroid isomerase (ketosteroid isomerase, KSI) catalyzes the isomerization of 5-androstene-3,17-dione (1) to 4-androstene-3,17-dione (3) via a dienolate intermediate (2-). KSI catalyzes this conversion about 13 orders of magnitude faster than the corresponding reaction catalyzed by acetate ion, a difference in activation energy (DeltaG) of approximately 18 kcal/mol. To evaluate whether the decrease in DeltaG by KSI is due to enthalpic or entropic effects, the activation parameters for the isomerization of 1 catalyzed by the D38E mutant of KSI were determined. A linear Arrhenius plot of kcat/KM versus 1/T gives the activation enthalpy (DeltaH = 5.9 kcal/mol) and activation entropy (TDeltaS = -2.6 kcal/mol). Relative to catalysis by acetate, D38E reduces DeltaH by approximately 10 kcal/mol and increases TDeltaS by approximately 5 kcal/mol. The activation parameters for the microscopic rate constants for D38E catalysis were also determined and compared to those for the acetate ion-catalyzed reaction. Enthalpic stabilization of 2- and favorable entropic effects in both chemical transition states by D38E result in an overall energetically more favorable enzymatic reaction relative to that catalyzed by acetate ion.

Catalysis↗
PubMed2004

Chemistry of the t-butoxyl radical: evidence that most hydrogen abstractions from carbon are entropy-controlled.

Absolute rate constants and Arrhenius parameters for hydrogen abstractions (from carbon) by the t-butoxyl radical ((t) BuO.) are reported for several hydrocarbons and tertiary amines in solution. Combined with data already in the literature, an analysis of all the available data reveals that most hydrogen abstractions (from carbon) by (t) BuO. are entropy controlled (i.e., TdeltaS > deltaH, in solution at room temperature). For substrates with C-H bond dissociation energies (BDEs) > 92 kcal/mol, the activation energy for hydrogen abstraction decreases with decreasing BDE in accord with the Evans-Polanyi equation, with alpha approximately 0.3. For substrates with C-H BDEs in the range from 79 to 92 kcal/mol, the activation energy does not vary significantly with C-H BDE. The implications of these results in the context of the use of (t) BuO. as a chemical model for reactive oxygen-centered radicals is discussed.

Journal Article↗
PubMed2006

Ideal mixing in polyelectrolyte complexes and multilayers: entropy driven assembly.

Calorimetry and variable temperature equilibrium doping methods independently show complexation of common polyelectrolytes to be essentially entropy-driven. The complimentary techniques provide enthalpy and free energies of formation or mixing. These findings highlight the importance of polyelectrolyte complexes as ideally mixed polymer blends.

Journal Article↗
PubMed2001

Entropy-controlled selectivity in the vinylation of a cyclic chiral nitrone. An efficient route to enantiopure polyhydroxylated pyrrolidines.

A short synthesis of 1,4-dideoxy-1,4-imino-L-arabinitol (LAB1) (4) and of 1,4-dideoxy-1,4-imino-D-galactitol (5), two azasugars active as enzymatic inhibitors, is reported. The key reaction is the addition of vinylmagnesium chloride to (3S,4S)-3,4-bis(benzyloxy)-3,4-dihydro-2H-pyrrole 1-oxide (3), a nitrone easily available from L-tartaric acid. Unexpectedly, the reaction affords the corresponding (2S,3S,4S)-1-hydroxy-2-ethenyl-3,4-bis(benzyloxy)pyrrolidine (9) in very good yield and in 93/7 diastereomeric ratio (dr) independently of the reaction temperature, thus representing a unique case of entropy-controlled reaction in a 100 K interval (from +20 degrees C to -80 degrees C). The trans intermediate 9 is converted in two steps (reduction, N-protection) into the common intermediate (2S,3S,4S)-1-(benzyloxycarbonyl)-3,4-bis(benzyloxy)-2-ethenylpyrrolidine (11). Double bond oxidation followed by reductive debenzylation opens a route to the target pyrrolidine azasugars 4 and 5.

Journal Article↗
PubMed2005

Grand potential, helmholtz free energy, and entropy calculation in heterogeneous cylindrical pores by the grand canonical Monte Carlo simulation method.

The adsorption of fluids in porous media is still an open area of research, since no model is able to explain all experimental features. The difficulties rise from the complexity of the real porous materials which present surface heterogeneities, large pore size distributions, and complex networks of interconnected pores. In parallel to experimental efforts trying to produce more ordered porous materials, theoreticians try to introduce more disorder in their models, with the help of molecular simulation for instance. This grand canonical Monte Carlo simulation study concentrates on the adsorption of a simple Lennard-Jones fluid in three porous substrates, to compare the effect of purely geometric heterogeneity (spatial deformation of the external potential) as opposed to purely chemical heterogeneity (amplitude variations of the external potential). This separation is unrealistic, since geometric fluctuations of a real pore diameter along its axis generally induce variations in the amplitude of the external potential created by the pore. However it enables one to compare both effects. In this paper, a thermodynamic integration scheme is applied to a complete set of adsorption/desorption isotherms. The grand potential, free energy, and entropy are calculated, which allows one to discuss the features of the phase diagrams. It is shown that a purely geometric deformation (undulation) of the external potential does not affect the thermodynamic characteristics of the confined fluid. On the other hand, amplitude modulation of the external potential (chemical heterogeneity) strongly distorts the phase diagram. This heterogeneity is actually able to stabilize a "bridgelike" phase which corresponds to an accumulation of molecules in the most attractive region of the pore.

Journal Article↗
PubMed2005

Molecular dynamics and residual entropy in the soft crystal, SmE phase, of 4-butyl-4'-isothiocyano-1,1'-biphenyl.

Molecular dynamics and resulting disorder in the soft crystal, smectic E (SmE) phase, were studied in detail for the title compound, 4-butyl-4'-isothiocyano-1,1'-biphenyl (4TCB), by (1)H NMR spectroscopy and adiabatic calorimetry. The ordered crystal phase of 4TCB was realized for the first time under ambient pressure after long two-step annealing and used as the reference state in the analysis of the experimental results. Four motional modes were identified in the SmE phase through the analysis of the (1)H NMR T(1). The residual entropy was determined as ca. 6 J K(-1) mol(-1). This magnitude implies that most of the disorder in the SmE phase at high temperatures is removed on cooling except for the head-to-tail disorder of the rod-shaped 4TCB molecule. Standard thermodynamic functions are tabulated below 375 K.

Journal Article↗
PubMed2006

Excess entropy in alcohol-water solutions: a simple clustering explanation.

We show that the anomalous negative excess entropy of mixing characteristic of aqueous lower alcohols containing hydrophobic groups is quantitatively consistent with a model exploiting only the experimentally observed molecular-scale segregation of the components across the entire concentration range. The simple model presented here, which uses plausible interatomic distances as its only free parameters, obviates the need to invoke "iceberg" or other water restructuring concepts which, though frequently postulated in explaining the hydrophobic interaction, are unsupported by recent experiments.

Journal Article↗
PubMed2003

Approximate entropy of symptoms of mood: an effective technique to quantify regularity of mood.

OBJECTIVES: Several psychiatric conditions are associated with lability of affect. In this study, we investigated regularity of mood using APEN (Approximate Entropy) on daily subjective ratings using a Visual Analog Scale with 11 items pertaining to mood. METHODS: APEN was applied to the data in a double-blind placebo controlled investigation on the effects of fluoxetine (n=19), pemoline (n=18) and placebo (n=20) in normal controls. These subjects rated their subjective feelings daily at the end of each day. We analysed 56 data point time series (each value was obtained on each day) after the three experimental conditions. RESULTS: While the mean or the SD of all the 56 points was not significantly different among the three conditions, APEN was highly and significantly lower for pemoline compared with fluoxetine and placebo. There was no significant correlation between average APEN and mean or SD (standard deviation). The one symptom that explained most of this difference among the groups after drug administration was the feeling of 'happiness'. CONCLUSIONS: This result indicates that there was a more consistent subjective sense of happiness during the 8-week period of pemoline administration compared with the other two drugs. Though this study was not designed to address the efficacy of mood stabilizing drugs, such daily subjective ratings may be useful in future studies that evaluate the stability of mood. APEN has been used in several different fields of research with small data sets and this study extends its possible use to evaluate changes in mood in certain populations such as patients with bipolar disorders.

Adult↗
PubMed2001

The relationship between fragility, configurational entropy and the potential energy landscape of glass-forming liquids.

Glass is a microscopically disordered, solid form of matter that results when a fluid is cooled or compressed in such a manner that it does not crystallize. Almost all types of materials are capable of glass formation, including polymers, metal alloys and molten salts. Given such diversity, general principles by which different glass-forming materials can be systematically classified are invaluable. One such principle is the classification of glass-formers according to their fragility. Fragility measures the rapidity with which a liquid's properties (such as viscosity) change as the glassy state is approached. Although the relationship between the fragility, configurational entropy and features of the energy landscape (the complicated dependence of energy on configuration) of a glass-former have been analysed previously, a detailed understanding of the origins of fragility is lacking. Here I use simulations to analyse the relationship between fragility and quantitative measures of the energy landscape for a model liquid whose fragility depends on its bulk density. The results reveal that fragility depends on changes in the vibrational properties of individual energy minima in addition to their total number and spread in energy. A thermodynamic expression for fragility is derived, which is in quantitative agreement with kinetic fragilities obtained from the liquid's diffusivity.

Journal Article↗
PubMed2002

Bond-controlled configurational entropy reduction in chemical vitrification.

Glass formation is usually viewed in terms of physical vitrification: a liquid in a metastable state is cooled or compressed so as to avoid crystallization. However, glasses may also be formed by chemical vitrification, a process involving progressive polymerization of the constituent molecules via the formation of irreversible chemical bonds. The formation of most of the materials used in engineering plastics and the hardening of natural and synthetic resins are based on chemical vitrification. Despite the differences in the molecular processes involved in chemical and physical vitrification, surprising similarities are observed in the slowing down of the dynamics and in the thermodynamical properties of the resulting glasses. Explaining such similarities would improve general understanding of the glass transition and may disclose its universal nature. Here we report dielectric and photon-correlation measurements that reveal the origin of the similarity in the dynamical behaviour of physical and chemical glass formers. We find that the evolution of their configurational restrictions proceeds in a similar manner. In particular, we make a connection between the reduction in configurational entropy and the number of chemical bonds, a quantity that can be controlled in experiments.

Journal Article↗
PubMed2005

Entropy-controlled supramolecular photochirogenesis: enantiodifferentiating Z-E photoisomerization of cyclooctene included and sensitized by permethylated 6-O-benzoyl-beta-cyclodextrin.

In contrast to the photosensitization with a non-methylated analogue, supramolecular photochirogenesis with a novel permethylated mono(6-O-benzoyl)-beta-cyclodextrin exhibited a critical dependence of the product's enantiomeric excess upon temperature, and possessed a large differential entropy of activation (-11 J K(-1) mol(-1)) for which the flexible host skeleton is likely to be responsible.

Journal Article↗
PubMed2004

On the use of the quasi-Gaussian entropy theory in the study of simulated dilute solutions.

In a recent paper [M. D'Alessandro, M. D'Abramo, G. Brancato, A. Di Nola, and A. Amadei, J. Phys. Chem. B 106, 11843 (2002)] we showed how to combine molecular dynamics simulations with the quasi-Gaussian entropy theory, in order to model the statistical mechanics and thermodynamics of ionic (water) solutions. In this paper we extend the method to treat nonspherical solutes, describe more thoroughly its theoretical basis and apply it to a set of more complex solute molecules in water (i.e., water, methane, ethane, methanol, and ethanol). Results show that this approach can really provide an excellent theoretical description of solute-solvent systems over a wide range of temperatures.

Journal Article↗
PubMed2004

Free energy, energy, and entropy of swelling in Cs-, Na-, and Sr-montmorillonite clays.

A Monte Carlo method for grand canonical and grand isoshear ensemble simulations has been used to characterize the free energy, energy, and entropy of clay mineral swelling. The Monte Carlo approach was found to be more efficient at simulating water content fluctuations in the highly constrained clay environment than a previously developed molecular dynamics method. Swelling thermodynamics calculated for Cs-, Na-, and Sr-montmorillonite clays indicate a strong dependence of swelling on the interlayer ion identity, in agreement with various experimental measurements. The Sr clay swells most readily, and both the Na and Sr clays prefer expanded states (two-layer hydrate or greater) when in contact with bulk water. In contrast, swelling is inhibited in the Cs clay. Differences in swelling behavior are traced directly to the tendency of the different ions to hydrate. The swelling free energies are decomposed into their energetic and entropic components, revealing an overall energetic driving force for the swelling phenomena. Entropic effects provide a smaller, mediating role in the swelling processes. The results provide a unique molecular perspective on experimentally well-characterized swelling phenomena.

Journal Article↗
PubMed1996

Entropy estimation of symbol sequences.

We discuss algorithms for estimating the Shannon entropy h of finite symbol sequences with long range correlations. In particular, we consider algorithms which estimate h from the code lengths produced by some compression algorithm. Our interest is in describing their convergence with sequence length, assuming no limits for the space and time complexities of the compression algorithms. A scaling law is proposed for extrapolation from finite sample lengths. This is applied to sequences of dynamical systems in non-trivial chaotic regimes, a 1-D cellular automaton, and to written English texts. (c)1996 American Institute of Physics.

Journal Article↗
PubMed1997

On a general concept of multifractality: Multifractal spectra for dimensions, entropies, and Lyapunov exponents. Multifractal rigidity.

We introduce the mathematical concept of multifractality and describe various multifractal spectra for dynamical systems, including spectra for dimensions and spectra for entropies. We support the study by providing some physical motivation and describing several nontrivial examples. Among them are subshifts of finite type and one-dimensional Markov maps. An essential part of the article is devoted to the concept of multifractal rigidity. In particular, we use the multifractal spectra to obtain a "physical" classification of dynamical systems. For a class of Markov maps, we show that, if the multifractal spectra for dimensions of two maps coincide, then the maps are differentiably equivalent. (c) 1997 American Institute of Physics.

Journal Article↗
PubMed2005

Surface entropy of rare-gas clusters.

Abundances of ArN+ and XeN+ clusters produced in a supersonic expansion source are inverted to find relative dissociation energies. The values around the shell and subshell closings at N=55, 71, and 147 differ from theoretical values derived from ground-state energies of Lennard-Jones clusters. A significant part of the difference can be accounted for by the conformational entropies of surface atoms and vacancies.

Journal Article↗