Synthesis of Hydroxymethyl Side-Chained α-Aminoxy Diamide

Unnatural polar α-aminoxy acid residue with proteingenous hydroxymethyl side chain, a building block of the peptidomimetic foldamer of α-aminoxy peptide, was synthesized starting from natural amino acid L-serine. The starting material, L-serine, undergoes a reaction sequence to produce compound 1 in three steps: (1) the neighboring carboxyl group participates in diazotization/bromination to transform the amino group to a bromo group, (2) the C-terminal carboxyl group is protected, and (3) bromide is S_N2-displaced by N-hydroxyl phthalimide to introduce a N?O bond. After several conventional deprotection/coupling reactions, compound 1 is easily transformed to an α-aminoxy diamide, which can be widely used in peptidomimetics design.     

New pressure‐assisted sweeping on‐line preconcentration for polar environmentally relevant nitrosamines: Part 1. Sweeping for polar compounds and application of auxiliary pressure

Typically sweeping reversed migration EKC (RM‐EKC) is used for online enrichment and separation of neutral compounds in CE, however sweeping is not usually suitable for highly polar neutral compounds due to the lack of strong interaction with micellar phase. Since acidic BGE or coated capillaries (BGE pH 2–8) are used to virtually eliminate the EOF, migration of neutral analytes is only through association with the micelles with relatively slow electrophoretic mobility. To decrease the long analysis times that result, an auxiliary pressure can be applied, which also serves to avoid the associated band broadening. In this study, we have modified a commercially available CE instrument to perform pressure‐assisted sweeping. The apparatus described can be used to precisely control the application of pressure, and therefore direction and magnitude of bulk flow in the capillary. This modification allows us to employ longer capillaries and capillaries with larger internal diameter to increase the sensitivity. An optimized method was used for the analysis of a group of seven N‐nitrosamines that have been widely reported in environmental samples and good concentration factors of up to 34 were achieved. When a coated capillary is employed, this method is effective even at neutral pH, making it broadly applicable.     

On the choice of a reference state for one‐step perturbation calculations between polar and nonpolar molecules in a polar environment

One‐step perturbation is an efficient method to estimate free energy differences in molecular dynamics (MD) simulations, but its accuracy depends critically on the choice of an appropriate, possibly unphysical, reference state that optimizes the sampling of the physical end states. In particular, the perturbation from a polar moiety to a nonpolar one and vice versa in a polar environment such as water poses a challenge which is of importance when estimating free energy differences that involve entropy changes and the hydrophobic effect. In this work, we systematically study the performance of the one‐step perturbation method in the calculation of the free enthalpy difference between a polar water solute and a nonpolar “water” solute molecule solvated in a box of 999 polar water molecules. Both these polar and nonpolar physical reference states fail to predict the free enthalpy difference as obtained by thermodynamic integration, but the result is worse using the nonpolar physical reference state, because both a properly sized cavity and a favorable orientation of the polar solute in a polar environment are rarely, if ever, sampled in a simulation of the nonpolar solute in such an environment. Use of nonphysical soft‐core reference states helps to sample properly sized cavities, and post‐MD simulation rotational and translational sampling of the solute to be perturbed leads to much improved free enthalpy estimates from one‐step perturbation. © 2012 Wiley Periodicals, Inc.     

Internal‐to‐Cartesian back transformation of molecular geometry steps using high‐order geometric derivatives

In geometry optimizations and molecular dynamics calculations, it is often necessary to transform a geometry step that has been determined in internal coordinates to Cartesian coordinates. A new method for performing such transformations, the high‐order path‐expansion (HOPE) method, is here presented. The new method treats the nonlinear relation between internal and Cartesian coordinates by means of automatic differentiation. The method is reliable, applicable to any system of internal coordinates, and computationally more efficient than the traditional method of iterative back transformations. As a bonus, the HOPE method determines not just the Cartesian step vector but also a continuous step path expressed in the form of a polynomial, which is useful for determining reaction coordinates, for integrating trajectories, and for visualization. © 2013 Wiley Periodicals, Inc.     

Azomesogens with polar chloro,nitro and phenolic –OH substituents

Two new mesogenic homologous series containing chloro, nitro and phenolic hydroxy groups were synthesised and their molecular structures characterised by a combination of element analysis and standard spectroscopic methods. The mesomorphic behaviour of the compounds of both series was investigated by polarising optical microscopy and, in some cases, differential scanning calorimetry. All the compounds of both the series exhibit an enantiotropic nematic mesophase. In series I higher members also exhibit an enantiotropic smectic A (SmA) mesophase, whereas in series II the enantiotropic SmA mesophase commences somewhat earlier for the middle members. The mesomorphic properties of both series are compared with each other and also with the properties of other structurally related series to evaluate the effects of different polar substituents on mesomorphism.     

Magnetic‐Structure‐Stabilized Polarization in an Above‐Room‐Temperature Ferrimagnet

Above‐room‐temperature polar magnets are of interest due to their practical applications in spintronics. Here we present a strategy to design high‐temperature polar magnetic oxides in the corundum‐derived A₂BB′O₆ family, exemplified by the non‐centrosymmetric (R3) Ni₃TeO₆‐type Mn²⁺₂Fe³⁺Mo⁵⁺O₆, which shows strong ferrimagnetic ordering with T_C=337 K and demonstrates structural polarization without any ions with (n?1)d¹⁰ns⁰, d⁰, or stereoactive lone‐pair electrons. Density functional theory calculations confirm the experimental results and suggest that the energy of the magnetically ordered structure, based on the Ni₃TeO₆ prototype, is significantly lower than that of any related structure, and accounts for the spontaneous polarization (68 μC cm^?2) and non‐centrosymmetry confirmed directly by second harmonic generation. These results motivate new directions in the search for practical magnetoelectric/multiferroic materials.     

Synthesis and crystal structures of RE7Zn21+xSi2–x [RE = Ce,Pr, and Nd; 0.09 (1) < x < 0.95 (1)]

The focus of this paper is on the synthesis and crystal structures of three Zn‐rich compounds with the general formula RE₇Zn_21+xSi_2−x, where RE = Ce [x = 0.95 (1); heptacerium docosazinc silicon], Pr [x = 0.09 (1); heptapraseodymium henicosazinc disilicon], and Nd [x = 0.53 (1); heptaneodymium docosazinc silicon]. The compounds were obtained by high‐temperature reactions, using the respective elements as starting materials. The structures were determined by single‐crystal X‐ray diffraction. The title compounds crystalize in the orthorhombic space group Pbam (No. 55, Pearson symbol oP60) and are isostructural with about a dozen RE₇Zn_21+xTt_2−x (RE = La–Nd; Tt = Ge, Sn, and Pb) compounds previously reported by our group. The results from the present refinements confirm the previously published data on RE₇Zn_21+xSi_2−x (RE = La and Ce; x≃ 1.45) [Malik et al. (2013). Intermetallics, 36 , 118–126]. Additionally, magnetic susceptibility measurements on the corresponding bulk samples show Curie–Weiss paramagnetic behavior from 5 to 300 K, consistent with RE³⁺ ground states and local‐moment magnetism due to the core 4f electrons.     

Copolymerizations of ethylene and polar comonomers with bis(phenoxyketimine) group IV complexes: Effects of the central metal properties

With triisobutylaluminum as a protection reagent, copolymerizations of ethylene with 10‐undecen‐1‐ol, 10‐undecenoic acid, and 5‐hexen‐1‐ol have been performed with bis[(3,5‐di‐^tBu? C₆H₂? 2‐O)? PhC?N(2‐F? C₆H₄)]ZrCl₂ ( a )/methylaluminoxane or bis[(3,5‐di‐^tBu? C₆H₂? 2‐O)? PhC?N(2‐F? C₆H₄)]TiCl₂ ( b )/methylaluminoxane as the catalyst. Both catalysts exhibit high activities of copolymerization in the presence of polar groups. The properties of the copolymers are strongly affected by the central metal properties of the catalysts. In comparison with complex a , titanium complex b appears to be less sensitive to polar monomers and more favorable for the preparation of higher molecular weight functionalized polyethylenes containing higher contents of polar groups. Studies on the polymerization temperature indicate that the catalytic activities decrease greatly with both complex a and complex b . The comonomer contents incorporated into the copolymers are slightly dependent on the polymerization temperature in the case of complex a , whereas in the case of complex b , the effect of the polymerization temperature is more distinct: an increase in the polymerization temperature can efficiently facilitate the incorporation of polar comonomers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 59–68, 2007     

Singular random matrix decompositions: Jacobians

For a singular random matrix X, we find the Jacobians associated to the following decompositions: QR, Polar, Singular Value (SVD), L′U, L′DM and modified QR (QDR). Similarly, for the cross-product matrix S=X′X we find the Jacobians of the Spectral, Cholesky's, L′DL and symmetric nonnegative definite square root decompositions.     

Dispersion of cellulose nanocrystals in polar organic solvents

In an attempt to prepare stable dispersions of cellulose nanocrystals in dipolar aprotic solvents, dilute aqueous suspensions of cellulose nanocrystals were prepared by sulfuric acid hydrolysis of cotton. The aqueous suspensions were freeze-dried, and then sonicated in the solvent of interest. Dispersions of 1 and 3% w/v concentration were prepared in polar organic solvents DMSO and DMF. The dispersions showed flow birefringence. The redispersion was incomplete, and there was some evidence for aggregation in the suspensions. A small amount of water appeared to be critical to suspension stability. Birefringent cellulose films were prepared from the dispersions by drying under vacuum and at ambient conditions.