Vortex particle‐in‐cell method for three‐dimensional viscous unbounded flow computations

A new vortex particle‐in‐cell (PIC) method is developed for the computation of three‐dimensional unsteady, incompressible viscous flow in an unbounded domain. The method combines the advantages of the Lagrangian particle methods for convection and the use of an Eulerian grid to compute the diffusion and vortex stretching. The velocity boundary conditions used in the method are of Dirichlet‐type, and can be calculated using the vorticity field on the grid by the Biot–Savart equation. The present results for the propagation speed of the single vortex ring are in good agreement with the Saffman's model. The applications of the method to the head‐on and head‐off collisions of the two vortex rings show good agreement with the experimental and numerical literature. Copyright © 2000 John Wiley & Sons, Ltd.     

Flow boundary conditions for chain-end adsorbing polymer blends

Using the phenol-terminated polycarbonate blend as an example, we demonstrate that the hydrodynamic boundary conditions for a flow of an adsorbing polymer melt are extremely sensitive to the structure of the epitaxial layer. Under shear, the adsorbed parts (chain ends) of the polymer melt move along the equipotential lines of the surface potential whereas the adsorbed additives serve as the surface defects. In response to the increase of the number of the adsorbed additives the surface layer becomes thinner and solidifies. This results in a gradual transition from the slip to the no-slip boundary condition for the melt flow, with a nonmonotonic dependence of the slip length on the surface concentration of the adsorbed ends.     

A Joint Theoretical and Experimental Study of the Behavior of the DIDS Inhibitor and its Derivatives

4,4′‐Diisothiocyanostilbene‐2,2′‐disulfonic acid (DIDS) is a well‐known ion‐exchange inhibitor targeting cardiac functions and indirectly impeding both radio‐ and chemo‐resistance. A joint computational and experimental study is presented to provide deeper insights into DIDS and other members of this family of compounds. To this end, we applied state‐of‐the‐art density functional theory (DFT) and time‐dependent DFT methods, in addition to measuring the optical properties. The experimental data show that such compounds are highly sensitive to their environment and that the optical properties change within as little time as 7 h. However, the optical properties of DIDS are similar in various acidic/basic environments, which were confirmed by pK_a computations on both cis and trans isomers. The protonation analysis also highlights that the singly protonated form of DIDS behaves like a proton sponge compound. The experimentally observed redshift that can be seen when going from water to DMSO was reproduced solely by using the solvation model based on density, although the polarization continuum model and implicit/explicit hybrid schemes were also tested. The characteristic broadening of the absorption peak in water and the vibronic fine structure in DMSO were also reproduced thanks to vibronic coupling simulations associated with the solvent reorganization energy. For other stilbene derivatives, a correlation is found between the maximum absorption wavelength and the Hammett parameters.     

Enantiomeric and enantiotopic analysis of cone-shaped compounds with C(3) and C(3)(v) symmetry using NMR spectroscopy in chiral anisotropic solvents

We describe the enantiomeric and enantiotopic analysis of the NMR spectra of compounds derived from the functionalized cone-shaped core, cyclotriveratrylenes (CTV), dissolved in weakly oriented lyotropic chiral liquid crystals (CLCs) based on organic solutions of poly-gamma-benzyl-L-glutamate. The CTV core lacks prostereogenic as well as stereogenic tetrahedral centers. However, depending on the pattern of substitution, chiral and achiral compounds with different symmetries can be obtained. Thus, symmetrically nonasubstituted CTVs (C(3) symmetry) are optically active and exhibit enantiomeric isomers, while symmetrically hexasubstituted (C(3v) symmetry) derivatives are prochiral and possess enantiotopic elements. In the first part we use (2)H and (13)C NMR to study two nonasubstituted (-OH or -OCH(3)) CTVs, where the ring methylenes are fully deuterated, and show for the first time that the observation of enantiomeric discrimination of chiral molecules with a 3-fold symmetry axis is possible in a CLC. It is argued that this discrimination reflects different orientational ordering of the M and P isomers, rather than specific chiral short-range solvent-solute interactions that may affect differently the magnetic parameters of the enantiomers or even their geometry. In the second part we present similar measurements on hexasubstituted CTV with flexible side groups (-OC(O)CH(3) and the, partially deuterated bidentate, -OCH(2)CH(2)O-), having on the average C(3v) symmetry. No spectral discrimination of enantiotopic sites was detected for the -OC(O)CH(3) derivative. This is consistent with a recent theoretical work (J. Chem. Phys. 1999, 111, 6890) that indicates that in C(3v) molecules no chiral discrimination between enantiotopic elements, based on ordering, is possible. In contrast, a clear splitting was observed in the (2)H spectra of the enantiotopic deuterons of the side groups in the tri(dioxyethylene)-CTV. It is argued that this discrimination reflects different ordering characteristics of the various, rapidly (on the NMR time scale) interconverting conformers of this compound. Assuming two twisted structures for each of the dioxyethylene side groups, four different conformers are expected, comprising two sets of enantiomeric pairs with, respectively, C(3) and C(1) symmetries. Differential ordering and/or fractional population imbalance of these enantiomeric pairs leads to the observed spectral discrimination of sites in the side chains that on average form enantiotopic pairs.     

Speciation of chromium by ion chromatography and laser-enhanced ionization: optimization of the excitation-ionization scheme

Ion chromatography and laser-enhanced ionization were combined for the speciation of chromium at the trace level. Several one- and two-step excitation schemes were examined, in the UV and visible regions of the spectrum. It was found that a near-resonant two-step/one-color excitation scheme, using a single strongly saturating laser for the simultaneous excitation of both steps of the (3d⁵4s) ⁷S₃→(3d⁵4p) ⁷P₃⁰→(3d⁴4s5s) ⁷D₄ sequence, provides the best analytical results. This scheme was applied to the determination of total chromium in solutions directly aspirated into the air–acetylene flame, and to the determination of Cr(III) and Cr(VI) separated by ion chromatography. A detection limit of 0.5 ng/mL was obtained by direct solution aspiration, with a dynamic range covering four orders of magnitude. Detection limits of 5 ng/mL for Cr(III) and 4 ng/mL for Cr(VI) were obtained by ion chromatography and laser-enhanced ionization.     

Effects of Some Hydrotropic Agents on the Formation of Indomethacin/β-Cyclodextrin Inclusion Compounds

The influence of the water structure promoters, mannitol and sucrose, on the indomethacin/-cyclodextrin inclusion process at different pH values was evaluated using the phase solubility method and circular dichroism spectroscopy. The effect of mannitol and sucrose on the total solubilizing activity of -cyclodextrin as well as on the stabilization of the complex was moderate, and depended strongly on the pH value of the medium. The most pronounced effect on the association constant of the indomethacin inclusion compound was observed in KCl/HCl buffer of pH 1.6, because mannitol and sucrose, in the absence of -cyclodextrin, approximately double the indomethacin solubility so that the constant value decreases. On the other hand, a very strong complex formation between indomethacin and -cyclodextrin was observed in acid medium (KCl/HCl buffer, pH = 1.6), a fact not much discussed in the literature. The apparent association constant of the inclusion compound formed in the absence of mannitol or sucrose was found to be of the same order of magnitude as for the complex formed at pH 6.8. Changes in the optical activity of the indomethacin/-cyclodextrin inclusion complex were registered in the presence of sucrose, which interacts more strongly than mannitol with water molecules, and could probably adhere to the surface of the inclusion complex, thus changing its configuration and environment.     

White light scanning interferometry adapted for large-area optical analysis of thick and rough hydroxyapatite layers

Understanding the mechanisms of biomineralization continues to be an important area of research in physics, chemistry, materials science, medicine, and dentistry due to its importance in the formation of bones, teeth, cartilage, etc. Stimulated by these fascinating natural examples, as well as by certain others such as shells and corals, attempts are being made to develop synthetic, biomimetic nanocomposites by simulating the basic principles of biomineralization. We have grown bio-like hydroxyapatite layers in vitro on substrates of stainless steel, silicon, and silica glass by using a biomimetic approach (i.e., immersion in a supersaturated simulated body fluid). Hydroxyapatite is one of the most common natural biomaterials and an important structural component of bones and teeth. Metal substrates are of interest for hard tissue implants, while semiconductors and glasses are under investigation for their use as biosensors. Using classical techniques such as stylus profiling, atomic force microscopy (AFM), and scanning and transmission electron microscopy (SEM and TEM), it was found difficult, ambiguous, destructive, or time-consuming to measure the topography, thickness, and profile of the grown heterogeneous, thick, and rough hydroxyapatite layers. On the other hand, coherence probe microscopy based on white light scanning interferometry and image processing provides rapid, contactless measurements of surface roughness and does not need any sample preparation. The results obtained have shown a typical layer thickness of up to 20 microm and an average root-mean-square (rms) roughness of about 4 mum. The hydroxyapatite investigated in this work presents nonetheless a challenge for this technique because of its semi-translucency, high surface roughness, and the presence of cavities formed throughout its volume. This results in a variable quality of fringe pattern, ranging from classical fringes (on a smooth surface) to complex fringes displaying properties of white light speckle (on a rough surface), together with multiple fringe signals along the optical axis in the presence of buried layer interfaces, which in certain configurations affect the axial and lateral precision of the measurement. In this paper we present the latest results for optimizing the measurement conditions in order to reduce such errors and to provide additional useful information concerning the layer.     

Different contrast information obtained from CARS and nonresonant FWM images

We demonstrate that coherent anti‐Stokes Raman scattering (CARS) microscopy can be used not to obtain vibrational contrast related to a particular chemical species only, but furthermore, it directly yields topological contrast. Topological contrast is due to the spatially dependent refractive index of the sample and a number of linear effects as interference, refraction and absorption contribute to the topological contrast inherent to CARS images. In order to distinguish between different contrast mechanisms we propose an image correction method based on the analysis of the nonresonant four‐wave mixing (FWM) signal distribution. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical characterization of the low-lying electronic states of NbC

We have studied the potential-energy curves and the spectroscopic constants of the ground and low-lying excited states of NbC by employing the complete active space self-consistent field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified 23 low-lying electronic states of NbC with different spin multiplicities and spatial symmetries within 40,000 cm(-1). At the multireference single and double configuration interaction level of theory the 2sigma+ and 2delta states are nearly degenerated, with the 2delta state located 187 cm(-1) lower than the 2sigma+ state. The estimated spin-orbit splitting for the 2delta state results in a 2delta(3/2) ground state and A 2sigma+ which is placed 650 cm(-1) above the ground state, in reasonable agreement with the experimental result, 831 cm(-1). Our computed spectroscopic constants are in good agreement with experimental values although our results differ from those of a previous density-functional investigation of the excited states of NbC, mainly due to the strong multiconfigurational character of NbC. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally.     

The First Conducting Spin-Crossover Compound Combining a MnIII Cation Complex with Electroactive TCNQ Demonstrating an Abrupt Spin Transition with a Hysteresis of 50 K

We present herein the synthesis, crystal structure, and electric and magnetic properties of the spin-crossover salt [Mn(5-Cl-sal-N-1,5,8,12)]TCNQ_1.5 ⋅ 2 CH₃CN ( I ), where 5-Cl-sal-N-1,5,8,12=N,N′-bis(3-(2-oxy-5-chlorobenzylideneamino)propyl)-ethylenediamine, containing distinct conductive and magnetic blocks along with acetonitrile solvent molecules. The Mn^III complex with a Schiff-base ligand, [Mn(5-Cl-sal-N-1,5,8,12)]⁺, acts as the magnetic unit, and the π-electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ⁻) is the conducting unit. The title compound ( I ) exhibits semiconducting behavior with room temperature conductivity σ_RT≈1×10⁻⁴ ohm⁻¹ cm⁻¹ and activation energy Δ ≈0.20 eV. In the temperature range 73–123 K, it experiences a hysteretic phase transition accompanied by a crossover between the low-spin S=1 and high-spin S=2 states of Mn^III and changes in bond lengths within the MnN₄O₂ octahedra. The pronounced shrinkage of the basal Mn−N bonds in I at the spin crossover suggests that the d orbital is occupied/deoccupied in this transition. Interestingly, the bromo isomorphic counterpart [Mn(5-Br-sal-N-1,5,8,12)]TCNQ_1.5 ⋅ 2 CH₃CN ( II ) of the title compound evidences no spin-crossover phenomena and remains in the high-spin state in the temperature range 2–300 K. Comparison of the chloro and bromo compounds allows the thermal and spin-crossover contributions to the overall variation in bond lengths to be distinguished. The difference in magnetic behavior of these two salts has been ascribed to intermolecular supramolecular effects on the spin transition. Discrete hydrogen bonding exists between cations and cations and anions in both compounds. However, the hydrogen bonding in the crystals of II is much stronger than in I . The relatively close packing arrangement of the [Mn(5-Br-sal-N-1,5,8,12)]⁺ cations probably precludes their spin transformation.