The influence of flow on symmetric and asymmetric splay state relaxations

We present a detailed discussion of the relaxation of splayed states in untwisted devices — the asymmetric H state (Ha), and the recently observed symmetric H state (Hs). Experimental evidence suggests that the Hs does not experience the optical bounce due to induced backflow usually associated with splay state relaxation. A dynamic model using Leslie-Eriksen-Parodi theory has been developed, and is used to model the flow within the device during switching. We show that there is no backflow during Hs relaxation, and that the flow profile is similar to that present during relaxation of the V state (the state used for pi-cell operation). This flow enhances the switching of the Hs, leading to a faster relaxation than might be expected. The influence of the different viscosity parameters is examined in detail, and a comparison between the experimental and simulated results is given.     

Microwave-assisted synthesis of symmetric and asymmetric viologens

Viologens are generally synthesized by N-alkylating 4,4′-bipyridine with alkyl halides. Under conventional heating conditions, however, their synthesis suffers from long reaction times and, often, low yields. In this work, symmetric and asymmetric viologens were synthesized under the assistance of microwave irradiation in good to excellent yields and in short reaction times.     

Reconciling slip measurements in symmetric and asymmetric systems

In the past decade, the slip of simple liquids on solid surfaces has been demonstrated by many groups. However, the slip of liquids on wettable surfaces is heavily debated. Using colloid probe atomic force microscopy (AFM), we found the slip length of di-n-octylphthalate in a symmetric wettable system (silica) to be around 11 nm, which raises the question of what the measured slip length in an asymmetric hydrophilic-hydrophobic system would be. To answer this question, we investigated liquid slip in one symmetric nonwettable system (hydrophobic DCDMS or OTS) and in one asymmetric hydrophilic (silica)-hydrophobic (DCDMS) system by the same method at driving velocities of between 10 and 80 μm/s. The slip results obtained from the three systems are in agreement with each other, and this comparison provides a means to self-assess the accuracy and reproducibility of the measured force curves and the fitted slip length in our systems. Furthermore, this method provides access to reliable values of the actual slip length on any investigated flat surface in an asymmetric system, avoiding the difficulty of preparing a symmetric probe/flat surface system in a colloid probe AFM force measurement.     

Quantifying growth of symmetric and asymmetric lipid bilayer domains

Here, we examine by atomic force microscopy (AFM) the kinetics and morphology of lipid domain growth during lipid phase separation by rapid thermal cooling of fully mixed two-component supported lipid bilayers. At the undercooled temperatures chosen, symmetric 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)-rich domains favored slower reaction-limited growth whereas asymmetric galactosylceramide (GalCer)-rich domains favored faster diffusion-limited growth, indicated by shape factors and kinetic exponents. Because kinetically limited conditions could be accessed, we were able to estimate the activation energy barrier (approximately 16kT) and lateral diffusion coefficient (approximately 0.20 microm2/s) of lipid molecular addition to a growing domain. We discuss these results with respect to transition states, obstructed diffusion, and the necessity for coordinating growth in both leaflets in a symmetric lipid domain.     

One-pot synthesis of new symmetric and asymmetric xanthene dyes

Symmetric and asymmetric xanthene dyes have been prepared by a convenient one-step procedure from aldehyde and diol or m-aminophenol precursors using concentrated phosphoric acid as a solvent. This protocol provides access to water-soluble dyes with large Stoke’s shifts and far-red fluorescence emission. These compounds are envisioned as components of fluorescence-based sensors for a variety of imaging applications.     

Hydration properties of symmetric chain and asymmetric chain sphingomyelin bilayers

Hydration properties of lipid bilayer systems are compared for symmetric chain sphingomyelin (N-palmitoylsphingomyelin) and asymmetric chain sphingomyelin (N-lignoceroylsphingomyelin). These sphingomyelins were semisynthesized by a deacylation- reacylation process with a natural sphingomyelin used as a starting material. The number of differently bound water molecules was estimated by a deconvolution analysis of the ice-melting curves obtained by a differential scanning calorimetry (DSC) and was used to construct a water distribution diagram for these water molecules. Similarly to a natural sphingomyelin used for comparison, the asymmetric chain sphingomyelin was found to form small size vesicles having an internal cavity and incorporate 15 water molecules per molecule of lipid into its cavity, in contrast with 5 H₂O/lipid for freezable interlamellar water observed for large size multilamellar vesicles formed by the symmetric chain sphingomyelin.     

The influence of fluorine on the asymmetric reduction of fluoromethyl ketones

A comparative study of the asymmetric reduction of representative aryl and alkyl α-fluoro- and α-chloromethyl ketones using (−)-diisopinocampheylchloroborane [(−)-DIP-Chloride™] and (−)-B-isopinocampheyl-9-borabicyclo[3.3.1]nonane [R-Alpine-Borane^®] has been made. It was observed that DIP-Chloride™ is superior in terms of the rate and enantioselectivity for both classes of halo-ketones. While the reduction of monofluoroacetone and trifluoroacetone with DIP-Chloride™ provided the product alcohols in 61% ee and 96% ee, respectively, the reduction of difluoroacetone yielded only 5% ee. The influence of a lone halogen atom was not observed for monochloroacetone, all of which point towards a chelating effect of monofluoroacetone on the Lewis acidic chloroborane.     

Pervaporation of water-ethanol mixtures through symmetric and asymmetric TPX membranes

In this work, the pervaporation performance and mechanism of water-ethanol mixtures through symmetric and asymmetric TPX [poly(4-methyl-1-pentene)] membranes were investigated. The results show that TPX is a highly water permselective material although it is strongly hydrophobic. It was found that, for a symmetric dense TPX membrane, the feed solution vaporizes first and then permeates through the membrane. The water selectivity stems from the huge difference in diffusivity between water and ethanol vapors. To improve the permeation flux, asymmetric TPX membranes were prepared by a wet inversion method. However, due to the swelling effect of ethanol on TPX, small pores occur when the dense skin contacts the feed solution, resulting in a loss of water selectivity. Stain experiments were carried out to verify this mechanism. In addition, it was found that a parallel model can describe the mechanism quite accurately. Good agreement between the theoretical calculation and experimental measurement has been obtained. Furthermore, we also found that the loss of selectivity can be avoided by turning the asymmetric membrane over; that is, let the dense skin face the permeate.     

Theoretical design of blue emitting materials based on symmetric and asymmetric spirosilabifluorene derivatives

Equilibrium ground state geometry configurations and their relevant electronic properties of four experimentally reported asymmetric spirosilabifluorene derivatives are calculated by the HF(DFT)/6-31G(d) method. Their excited state geometries are investigated using the CIS/6-31G(d) method. The absorption and emission spectra are evaluated using the TD-B3LYP/6-31G(d) and TD-PBE0/6-31+G(d) levels both in gas phase and CHCl₃ solvent. Our results show an excellent agreement with the experimental data on their optical properties. To predict the substitution effect, the H/R (R = –NO₂, –CN, –NH₂ and –OCH₃) substituted symmetric and asymmetric spirosilabifluorene derivatives are also investigated, and the optical properties of H/R substituted derivatives are predicted in gas phase and CHCl₃ solvent. In comparison with the parent compound, significant red-shift is predicted for the emission spectra of the di-substituted symmetric derivatives with –NH₂ (96 nm), –OCH₃ (61 nm) and the push–pull (containing both –NH₂ and –NO₂) derivative (56 nm). It is found that the performance and the optical properties of these derivatives can be improved by adding push–pull substitutents. The largest change in the electronic and optical properties of this system can be obtained upon symmetric di-substitution among mono-, di-, tri- and tetra-substitutions. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The influence of wavelength of light on cyanobacterial asymmetric reduction of ketone

Asymmetric reduction of ketone by a microalga, Synechocystis sp. PCC 6803, smoothly afforded to the corresponding (S)-alcohol in excellent enantiomeric excess by the aid of illumination of orange and red LED lights which are more effective than other LEDs such as blue and green lights. The condition under minimum energy flux (1.0 W/m²) of orange-red LEDs is enough for the reduction of ketone, and it seems that orange-red light rather effectively forwarded the regeneration of coenzyme.     

Scission shapes of pair fragments in asymmetric and symmetric fission modes

Scission shapes composed of two touching spheroids in asymmetric and symmetric fission modes have been deduced from static potential calculations with the experimental fragment total kinetic energy and excitation energies for a typical fragment pair of ¹⁰³Nb and ¹³⁰Sn in the proton-induced fission of ²³²Th. It was found that the fragment deformation of the heavy fragment ¹³⁰Sn in the symmetric fission mode was extremely large compared with that in the asymmetric fission and those of the complementary fragment ¹⁰³Nb. Such scission shapes also provide internal excitation energies of pair fragments in the two fission modes. The deduced total internal excitation energies of complementary fragments for the two fission modes are nearly the same as the excitation energy of the fissioning nucleus. The results suggest that the two fission modes are strongly characterized by the degrees of fragment deformation of the heavy fragments, not by the total internal excitation energies at scission.     

Ground and excited state double hydrogen transfer in symmetric and asymmetric potentials: comparison of 2,7,12,17-tetra-n-propylporphycene with 9-acetoxy-2,7,12,17-tetra-n-propylporphycene

Analysis of time-resolved anisotropy of transient absorption enabled determination of room temperature ground and excited state rate constants for intramolecular double hydrogen transfer in two similar porphycenes, one of them with symmetric and the other, with asymmetric character of a double minimum potential for hydrogen motion. The perturbation preserves a quasi-symmetric minimum in S(0), but the rate decreases approximately two times. In S(1), the perturbed potential becomes strongly asymmetric, and the downhill hydrogen transfer occurs with a rate higher than that observed for a symmetrical compound.     

The mechanism of asymmetric reduction catalyzed by a C2 symmetric bis-amino alcohol catalyst

By means of¹H,¹³C,¹¹B NMR, polar transfer DEPT135, DEPT90 and 2D NMR experiments, IR, etc, the structure of the diaminodihydroxyl ligand and its derivatives used in the asym-metric reduction of prochiral ketones were detected. The catalysts derived from the ligand and borane formedin situ were also studied and the structure transformations in solution were monitored. The structure of the catalyst was proved to be a new type of dual-centered catalyst — bisoxazaborolidine.     

Single chain conformations in the system of symmetric and asymmetric diblock copolymers

A theory which describes a local structure and global properties of a diblock copolymer melt has been developed in the framework of the one‐loop self‐consistent approximation. We have derived expressions for the sizes of a single diblock macromolecule and its parts. Two different behaviors of single macromolecule conformations in the disordered melt have been obtained depending on the asymmetry of chains and morphologies occurring in ordered states after the order‐disorder transition (ODT). In the nearly symmetric melt, 0.35 < f ⪇ 0.5 (f is a composition), the blocks of both types shrink a little initially as the temperature decreases and then, at some temperature, they begin to swell. In strongly asymmetric melts, f < 0.35, the block of a macromolecule which consists of the monomers of minority type shrinks monotonically, while the other block monotonically swells. We have found nearly Gaussian behavior of the individual blocks and stretching near the chemical bond joining the blocks. Near the ODT the chains are stretched with a magnitude which is of the order of a few percent of their Gaussian sizes. We have calculated the peak position in the scattering curve as a function of the Flory‐Huggins interaction parameter, composition and degree of polymerization. Less then 5% change in the size of copolymer molecules lead to a 25% shift of the scattering peak in comparison to the Gaussian limit. We have found a good quantitative agreement of our theoretical results with the experimental neutron scattering data.     

A symmetric aqueous redox flow battery based on viologen derivative

Due to the diversity and feasibility of structural modification for organic molecules,organic-based redox flow batteries(ORFBs) have been widely investigated,especially in aqueous solution under neutral circumstance.In this work,a symmetric aqueous redox flow battery(SARFB) was rationally designed by employing a bipolar redox active molecule(N,N'-dimethyl-4,4-bipyridinium diiodide,MVI_2) as both cathode and anode materials and combining with an anion exchange membrane.For one MVI_2 flow battery,MV~(2+)/MV~(·+) and I~-/I_3~-serve as the redox couples of anode and cathode,respectively.The MVI_2 battery with a working voltage of 1.02 V exhibited a high voltage efficiency of 90.30% and energy efficiency of 89.44% after 450 cycles,and crossover problem was prohibited.The comparable conductivity of MVI_2 water solution enabled to construct a battery even without using supporting electrolyte.Besides,the bipolar character of MVI_2 battery with/without supporting electrolyte was investigated in the voltage range between-1.2 V and 1.2 V,showing excellent stable cycling stability during the polarity-reversal test.     

Reconstructions and relaxations on metal surfaces

On metal surfaces the top layer of atoms usually relaxes inwards, and several metal surfaces reconstruct, particularly W and Mo (001) and Ir, Pt and Au (001) and (110). This paper reviews the electronic origins of these atomic displacements, and the physical effects which they lead to. The surface relaxations are explained in terms of the change in balance between electronic forces at the surface. The W (001) surface reconstructs from (1 × 1) to (?2 × ?2)R45° in a continuous phase transition on cooling below room temperature, and the structure of these phases and the nature of the phase transition are discussed. The driving force for the reconstruction is discussed in terms of calculations and photoemission studies of the surface electronic structure. Phenomenological Landau theory is used to describe the effect of surface steps on the transition, and the origin of the long wavelength modulation in the Mo (001) reconstruction. Finally the reconstructions of Ir, Pt and Au (001) and (110) are briefly described; these involve a more substantial movement of atoms than in the W and Mo reconstructions.     

An efficient route to the synthesis of symmetric and asymmetric diastereomerically pure fullerene triads

A new synthetic route based on the stepwise functionalisation of fullerene cages allows the facile formation of linear, diastereomerically pure triads incorporating two different fullerene cages linked by an organic spacer group. The critical coupling step of two fullerene cages via activation by N,N′-dicyclohexylcarbodiimide was systematically investigated to reveal that the yield of the coupling is maximised in o-dichlorobenzene at high concentrations of the reactant fullerene nucleophile, while in more polar solvents or at lower concentrations of reactants the formation of unwanted side-products (such as guanidine-, N-acylurea- and anhydride-functionalised fullerenes) is favoured. The resultant triads possess an atypically good solubility for this class of compound, which enabled full detailed characterisation by ¹H and ¹³C NMR, IR and UV–vis spectroscopies and by MALDI-TOF mass spectrometry.     

Determination of asymmetric and symmetric dimethylarginines in human plasma by HPLC with electrochemical detection

A new HPLC method was developed and validated for the determination of asymmetric and symmetric dimethylarginines and l ‐arginine in human plasma. After SPE and evaporation of the eluate, the samples were derivatised with an o‐phthaldialdehyde reagent containing 3‐mercaptopropionic acid. The derivatives formed were analysed by isocratic RP‐HPLC with electrochemical detection at +320 mV. The mobile phase consisted of 50 mM phosphate buffer (pH 6.1) containing 10% v/v acetonitrile, the flow rate was 1 mL/min. The retention times of all compounds including monomethylarginine (internal standard) were <24 min. The LODs (S/N 3:1) were 0.012 μM for both dimethylarginines and 0.013 μM for l ‐arginine; the linearity of the method was from 0.1 to 20 μM for both dimethylarginines and from 1 to 200 μM for l ‐arginine. Absolute extraction recoveries measured for all analytes ranged from 85 to 88%.     

The influence of the cyclic product's structure on the Diels-Alder transition state

The concerted cycloaddition of ethylene and cyclopentadiene has been found with full geometry optimization and the STO-3G basis set to be synchronous with E _a =35 kcal/mol, while the previously reported, partially optimized, concerted cycloaddition of butadiene and ethylene has been found to have a tendency towards asynchronism due to the nonsymmetrical structure of its transition state; this is due to the two conformations of the cyclohexene product, which also have been optimized, with the boat form being a transition state structure 5.3 kcal/mol above the half-chair structure.