Diffusion in periodic two-dimensional channels formed by overlapping circles: comparison of analytical and numerical results

We study two-dimensional diffusion in a channel formed by periodic overlapping circles. Periodic variation of the channel width leads to the slowdown of diffusion along the channel axis. There are several approximate approaches, which allow one to analyze the slowdown. We use these approaches to derive five expressions for the effective diffusion coefficient of a point Brownian particle in the channel. To check the accuracy of the expressions we compare their predictions with the effective diffusion coefficient obtained from Brownian dynamics simulations.     

Diphosphanes derived from phobane and phosphatrioxa-adamantane: similarities, differences and anomalies

The homodiphosphanes CgP-PCg (1) and PhobP-PPhob (2) and the heterodiphosphanes CgP-PPhob (3), CgP-PPh(2) (4a), CgP-P(o-Tol)(2) (4b), CgP-PCy(2) (4c), CgP-P(t)Bu(2) (4d), PhobP-PPh(2) (5a), PhobP-P(o-Tol)(2) (5b), PhobP-PCy(2) (5c), PhobP-P(t)Bu(2) (5d) where CgP = 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-9-yl and PhobP = 9-phosphabicyclo[3.3.1]nonan-9-yl have been prepared from CgP(BH(3))Li or PhobP(BH(3))Li and the appropriate halophosphine. The formation of 1 is remarkably diastereoselective, with the major isomer (97% of the product) assigned to rac-1. Restricted rotation about the P-P bond of the bulky meso-1 is detected by variable temperature (31)P NMR spectroscopy. Diphosphane 3 reacts with BH(3) to give a mixture of CgP(BH(3))-PPhob and CgP-PPhob(BH(3)) which was unexpected in view of the predicted much greater electron-richness of the PhobP site. Each of the diphosphanes was treated with dimethylacetylene dicarboxylate (DMAD) in order to determine their propensity for diphosphination. The homodiphosphanes 1 and 2 did not react with DMAD. The CgP-containing heterodiphosphanes 4a-d all added to DMAD to generate the corresponding cis alkenes CgPCH(CO(2)Me)=CH(CO(2)Me)PR(2) (6a-d) which have been used in situ to form chelate complexes of the type [MCl(2)(diphos)] (7a-d) where M = Pd or Pt. The PhobP-containing heterodiphosphanes 3 and 5a-d react anomalously with DMAD and do not give the products of diphosphination. The X-ray crystal structures of the diphosphanes 2, 3, 4a, and 5a, the monoxide and dioxide of diphosphane 1, and the platinum chelate complex 7c have been determined and their structures are discussed.     

Solution of quantum Langevin equation: approximations, theoretical and numerical aspects

Based on a coherent state representation of noise operator and an ensemble averaging procedure using Wigner canonical thermal distribution for harmonic oscillators, a generalized quantum Langevin equation has been recently developed [Phys. Rev. E 65, 021109 (2002); 66, 051106 (2002)] to derive the equations of motion for probability distribution functions in c-number phase-space. We extend the treatment to explore several systematic approximation schemes for the solutions of the Langevin equation for nonlinear potentials for a wide range of noise correlation, strength and temperature down to the vacuum limit. The method is exemplified by an analytic application to harmonic oscillator for arbitrary memory kernel and with the help of a numerical calculation of barrier crossing, in a cubic potential to demonstrate the quantum Kramers' turnover and the quantum Arrhenius plot.     

Human and environmental risk assessment of pharmaceuticals: differences,similarities, lessons from toxicology

The presence of human and veterinary pharmaceuticals in the environment has caused increasing concern due their effects on ecological receptors. Improving the risk assessment of these compounds necessitates a quantitative understanding of their metabolism and elimination in the target organism (toxicokinetics), particularly via the ubiquitous cytochrome P-450 (CYP) system and their mechanisms of toxicity (toxicodynamics). This review focuses on a number of pharmaceuticals and veterinary medicines of environmental concern, and the differences and similarities between ecological and human risk assessment. CYP metabolism is discussed with particular reference to its ubiquity in species of ecological relevance. The important issue of pharmaceutical mixtures is discussed to assess how emerging technologies such as ecotoxicogenomics may assist in moving towards a more mechanism-based environmental risk assessment of pharmaceuticals.     

Statistical thermodynamics of bond torsional modes: tests of separable, almost-separable, and improved Pitzer-Gwinn approximations

Practical approximation schemes for calculating partition functions of torsional modes are tested against accurate quantum mechanical results for H(2)O(2) and six isotopically substituted hydrogen peroxides. The schemes are classified on the basis of the type and amount of information that is required. First, approximate one-dimensional hindered-rotator partition functions are benchmarked against exact one-dimensional torsion results obtained by eigenvalue summation. The approximate one-dimensional methods tested in this stage include schemes that only require the equilibrium geometries and frequencies, schemes that also require the barrier heights of internal rotation, and schemes that require the whole one-dimensional torsional potential. Then, three classes of approximate full-dimensional vibrational-rotational partition functions are calculated and are compared with the accurate full-dimensional path integral partition functions. These three classes are (1) separable approximations combining harmonic oscillator-rigid rotator models with the one-dimensional torsion schemes, (2) almost-separable approximations in which the nonseparable zero-point energy is used to correct the separable approximations, and (3) improved nonseparable Pitzer-Gwinn-type methods in which approaches of type 1 are used as reference methods in the Pitzer-Gwinn approach. The effectiveness of these methods for the calculation of isotope effects is also studied. Based on the results of these studies, the best schemes of each type are recommended for further use on systems where a corresponding amount of information is available.     

Density-functional expansion methods: evaluation of LDA, GGA, and meta-GGA functionals and different integral approximations

We extend the Kohn-Sham potential energy expansion (VE) to include variations of the kinetic energy density and use the VE formulation with a 6-31G* basis to perform a "Jacob's ladder" comparison of small molecule properties using density functionals classified as being either LDA, GGA, or meta-GGA. We show that the VE reproduces standard Kohn-Sham DFT results well if all integrals are performed without further approximation, and there is no substantial improvement in using meta-GGA functionals relative to GGA functionals. The advantages of using GGA versus LDA functionals becomes apparent when modeling hydrogen bonds. We furthermore examine the effect of using integral approximations to compute the zeroth-order energy and first-order matrix elements, and the results suggest that the origin of the short-range repulsive potential within self-consistent charge density-functional tight-binding methods mainly arises from the approximations made to the first-order matrix elements.     

Comparing modern density functionals for conjugated polymer band structures: screened hybrid, Minnesota, and Rung 3.5 approximations

Semiconducting polymers with π-conjugated backbones show promise in fields such as photovoltaics. Practical applications of conjugated polymers require precise control over the polymer's electronic band structure. Several new classes of density functional approximation, including screened hybrids, semilocal Minnesota functionals, and Rung 3.5 functionals, show potential for improved predictions of conjugated polymer band structures. This work compares these methods to standard global hybrid density functionals for bandgaps and band structures of representative conjugated polymers. The new methods exhibit particular promise for modeling three-dimensionally periodic bulk polymers, which can be problematic for global hybrids.     

Structural phases in non-additive soft-disk mixtures: glasses, substitutional order, and random tilings

Relaxation of the additivity condition on the interaction length between unlike species in a binary mixture of soft disks opens up a rich variety of structures in both crystal and amorphous states with an associated diverse range of relaxation dynamics. We report on MD simulation studies of binary soft disks with negative deviations from additivity that include evidence of accumulation of crystal-like structures in metastable liquids prior to crystallization and the occurrence of a liquid to random-tiling transition.     

Diffusion,elasticity, and shear flow in self-assembled block copolymers: A molecular dynamics study

Self-assembled ordered structures composed of block copolymers are simulated by molecular dynamics under stress-free conditions and under shear. We address several methodological points. The system must be allowed to adjust its size to accommodate natural periods of self-assembled structures. In addition, these structures need to be capable of rotating freely under shear. An examination of the diffusion of polymer molecules in the lamellar phase reveals subdiffusion along translationally invariant directions between the ballistic and diffusive regime. The diffused distance d increases with time t as d ∝ t^1/3. We also examine the possibility of mapping structures such as cylindrical phases onto particle–field types of models. Using measurements of the wavevector-dependent dynamic matrix, we show that this cannot be done with only two-body potentials. We then examine the molecular origin of shear alignment of lamellar phases. Lamellae oriented parallel to the shear direction become unstable at high shear rates when the major axis of the tensor of gyration of individual polymers forms an average angle of 45° with the lamellae. This instability can be understood in analogy to similar transitions in liquid crystals. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 970–982, 2005     

Diffusion, relaxation, and chemical exchange in casein gels: a nuclear magnetic resonance study

Water in protein/water mixtures can be described in terms of bound water and free water, by exchange between these two states, and by its exchange with appropriate sites on the protein. 1H-NMR diffusion and relaxation measurements provide insights into the mobility of these states. T2 relaxation-time dispersions (i.e., T2 relaxation times at different echo pulse spacings) reveal additional information about exchange. We present a comprehensive set of diffusion and T2 dispersion measurements on casein gels for which the protein/water ratio ranges from 0.25 to 0.5. The combination of these methods, taken in conjunction with concentration dependence, allows a good estimate of the parameters required to fit the data with Luz/Meiboom and Carver/Richards models for relaxation and chemical exchange. We compare the exchange (a) between water and protein and (b) between free water and bound water. Further, we attempt to distinguish chemical site exchange and diffusion/susceptibility exchange.     

Association and segregation in aqueous polymer/polymer,polymer/surfactant,and surfactant/surfactant mixtures: similarities and differences

Recent experimental findings on the phase behaviour of aqueous polymer/surfactant mixtures are reviewed and compared with the phase behaviour of “analogous” polymer/polymer or surfactant/surfactant mixtures, which is also reviewed. Polyelectrolyte effects are given special consideration. Attention is drawn to the polymer aspect of a surfactant aggregate, and, also, to the surfactant aspect of an hydrophobe-modified polymer. It is proposed that a consideration of these aspects should be helpful in predicting the phase behaviour of polymer/surfactant mixtures.     

Nanostructured ZnS with random close-packed structure: Synthesis,formation rate,and crystal structure study

The chemical precipitation of nanocrystalline ZnS from aqueous alkaline solutions containing zinc ethylenediamine complex salt Zn(En)²⁺ and thiocarbonic acid diamide N₂H₄CS was studied in the course of time in terms of formal chemical kinetics. The ZnS formation rate was found to depend on the concentration of precipitation agent (N₂H₄CS) and to decrease with increasing the ligand (ethylenediamine) concentration, whereas the pH dependence is less pronounced. The size of resulting ZnS nanoparticles does not exceed 5 nm, and their crystal structure was identified as a random close packed structure.     

Interactions between Silica Colloids with Magnetite Cores: Diffusion, Sedimentation and Light Scattering

The magnetic and Van der Waals attraction between uncharged silica spheres with a magnetic core has been studied using sedimentation and static and dynamic light scattering techniques. Specifically the effect of the interactions on the concentration dependence of the sedimentation velocity, diffusion, and the apparent radius of gyration was investigated. It was found experimentally that the concentration dependence is decreased significantly as a result of the Van der Waals and magnetic attractions and even may change sign in comparison to the hard-sphere case. Calculations of the (linear) concentration dependence for weak interactions predict this decrease and also indicate that for the silica-magnetite particles the second virial coefficient passes a maximum with increasing silica layer thickness. Copyright 1999 Academic Press.     

Water on the Surfaces of Clay Minerals: Orientation,Diffusion and Proton Exchange

Abstract

The surface of clay minerals such as montmorillonite and vermiculite offersan unique environment for the study of water molecules arrangement and of their interaction with cations since     

Diffusion rayleigh depolarisee en serie organosilicique : II. Composes a liaisons C---Si, N---Si, O---Si

The authors establish that Z---Si and Z---SiMe₃ (Z = C, N, O) bond optical anisotropies are positive. This report involving conformational aspect of the compounds Me₃MCH₂CHMe₂ (M = C, Si), Me₃SiOCHMe₂ and Me₃SiN(Me)CHMe₂ brings a complementary argument in favour of the positive sign of these anisotropies.     

CASSCF calculations for photoinduced processes in large molecules: Choosing when to use the RASSCF, ONIOM and MMVB approximations

In this article, we compare and contrast the RASSCF, ONIOM and MMVB electronic structure methods for calculating relaxation paths on potential energy surfaces of the excited states of large molecules, and for locating any resulting conical intersections at which nonadiabatic decay can take place. Each method is treated here as an approximation to CASSCF, which we choose as our reference level of theory, but which becomes prohibitively expensive computationally for large molecules. Both MMVB and ONIOM are hybrid computational methods – combining different levels of theory in an energy plus derivatives calculation at a particular molecular geometry – but they differ fundamentally in that MMVB is a hybrid-atom method, whereas ONIOM is a hybrid-molecule method. We explain this distinction through four representative applications: the photostability of pyracylene (studied with CASSCF, RASSCF, MMVB); large geometry changes in the singlet excited states of triangulene (studied with MMVB); a model for interstitial nickel defects in a synthetic diamond lattice (studied with ONIOM CAS:UFF); and the photochemical [4 + 4] cycloaddition of cyclohexadiene to naphthalene (studied with ONIOM CAS:MMVB). We show that each method is more appropriate for a particular type of photochemical problem. This article is part perspective, part review, and contains new results for three multi-state or photoinduced processes in complex systems.     

Exciton Diffusion in Polyfluorene Copolymer Thin Films: Kinetics,Energy Disorder and Thermally Assisted Hopping

A series of {(9,9‐dioctylfluorene)_0.7?x‐(dibenzothiophene‐S,S‐dioxide)_0.3‐[4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole]_x} (PFS₃₀‐TBTx), where x represents the minor percentage of the red emitter 4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole (TBT) randomly incorporated into the copolymer backbone, is investigated in order to follow the energy transfer from PFS₃₀ to TBT moieties. The emission of the donor poly[(9,9‐dioctylfluorene)_0.7‐(dibenzothiophene‐S,S‐dioxide)_0.3 identified by PFS₃₀ and peaking at 450 nm, is clearly quenched by the presence of the red TBT chromophore emitting at 612 nm, with an isoemissive point observed when the spectra are collected as a function of temperature. A plot of the ratio between the TBT and PFS₃₀ emissions as a function of the reciprocal of temperature gives a clear linear trend between 290 and 200 K, with an activation energy of 20 meV and showing a turn over to a non‐activated regime below 200 K. Picosecond time‐resolved fluorescence decays collected at the PFS₃₀ and TBT emission wavelengths, show a decay of the PFS₃₀ emission and a fast build‐in, followed by a decay, of the TBT emission, confirming that the population of the TBT excited state occurs during the PFS₃₀ lifetime(～600 ps). The population of the TBT excited state occurs on a time regime around 150 ps at 290 K, showing an energy barrier of 20 meV that turns over to a non‐activated regime below 200 K in clear agreement with the steady‐state data. The origin of the activation barrier is attributed to the presence of physical and energetic disorder, affected by fast thermal fluctuations that dynamically change the energy landscape and control the exciton migration through the polymer density of states.     

Introducing Barium in Transition Metal Oxide Frameworks: Impact upon Superconductivity,Magnetism, Multiferroism and Oxygen Diffusion and Storage

The role of barium in the structural chemistry of some transition metal oxides of the series “Cu, Mn, Fe,Co” is reviewed, based on its size effect and its particular chemical bonding. Its impact upon various properties, superconductivity, magnetism, multiferroism, oxygen storage is emphasized.