The oblique chiral nematic phase in calamitic bimesogens

ABSTRACT

The discovery of the oblique chiral (or, the twist-bend, N_TB) nematic phase predicted for bent-core mesogens has engendered much interest due to its unique structure and physical properties, and the possibility of use in the next generation of fast electro-optic technology. Bimesogenic calamitic as well as bent-core mesogens are found to form the N_TB phase. Here, we report direct measurements of the temperature dependence of the conical tilt and the evidence of volcano-like orientational distribution of molecules in the N_TB phase. Optical and x-ray scattering investigations of two single-component calamitic bimesogens and their mixtures show that, while the Maier–Saupe orientational distribution function (ODF) is valid for the higher temperature nematic phase, a generalised expansion in terms of even Legendre functions is needed for the N_TB phase. Temperature dependence of the ODFs and the order parameters 〈P₂(cosβ)〉, 〈P₄(cosβ)〉, and 〈P₆(cosβ)〉 has been measured in both phases. The parameters 〈P₂(cosβ)〉 and 〈P₄(cosβ)〉 increase/decrease in the N/N_TB phase with decreasing temperature, while 〈P₆(cosβ)〉 remains vanishingly small for all samples. The value of 〈P₄(cosβ)〉 becomes negative in the N_TB phase confirming a conical distribution of molecules as they follow a helical trajectory keeping the local director tilted at an angle α wrt the macroscopic director. The heliconical tilt calculated from ODFs, exhibits a power law behaviour with temperature, vanishing at the transition to the N phase.     

Effect of gemini surfactant additives on pour point depressant of crude oil

Efficiencies of cationic gemini surfactant additives in improving the pour point depressant of crude oil were investigated. The length of alkyl chain is a major factor affecting the improvement of the pour point depression. The adsorption behavior of these gemini surfactants at air/solution and oil/solution interfaces were investigated by measuring the surface tension and interfacial tension as functions of concentration. It is found that there is a good relation between surface properties especially interfacial tension of the gemini surfactants and their efficiency in depressing the pour point. Also, the surface parameters and free energies of micellization and adsorption confirm the decreasing and improving of pour point depression. Crystallization study in crude oil revealed the relationship between the structure and activity of gemini surfactant additives. It is found that the x-ray diffraction patterns of waxes with additives are remarkably different from those without additives. The mechanism of the depressants action has been suggested according the adsorption of each additive. Adsorption of the additive on the surface of the wax particles inhibits their growth and alters the crystal habits through micelle core. Pretreatment of the crude oil with pour point depressants has received the greatest acceptance due to its simplicity and economy.     

INFLUENCE OF THERMAL TREATMENTS ON THE POLYMORPHISM IN STEREOIRREGULAR ISOTACTIC POLYPROPYLENE: EFFECT OF STEREO-DEFECT DISTRIBUTION

The influence of cooling rate from the melt on the polymorphism and crystallinity is investigated as a function of isotacticity and stereo-defect distribution in polypropylenes. Detailed analysis of wide angle x-ray diffraction patterns shows that crystallinity in the materials used is nearly independent of the experimental cooling rates (0.5–40°C/min). At high cooling rates, the materials exist mainly in the α-phase, whereas the amount of the γ-phase increases at the lower cooling rates. With an increasing amount of stereo-defects, this cooling-rate dependence of the polymorphism is enhanced. The effect of different stereo-defect distributions, as observed in metallocene-(random) and Ziegler–Natta (blocklike) derived isotactic polypropylenes, was investigated. The formation of the γ-phase is more prevalent in materials with a random defect distribution compared to the materials in which the stereo-defects have a blocklike distribution. The crystallinity decreases more rapidly as a function of the tacticity in the random defect-distributed materials.     

The Conformation of Sterically Congested Seven-Membered Rings Containing Tetracoordinate Germanium(IV): A First X-Ray Crystallographic Structure Determination

The X-ray crystallographic analysis of 6,6-dimethyl-2,4,8,10-tetra-tert-octyl-dibenzo[d,f][1,3,2]dioxagermepin, 1 is reported. In the solid-state conformation of 1, the dihedral angle about the C─C sp²-sp ² σ bond connecting the two aryl rings is 50.1°. The observed C₂ symmetry in the solid-state conformation of 1 is consistent with the previously suggested solution conformation.     

Preparation of Metallaiminophosphoranes and Their Reactivity

Treatment of [Cp*(CO) 2 M{P(NHPh)(OMe) 2 }]PF 6 (M = Fe and Ru) with NaNH 2 gives metallaiminophosphoranes, Cp*(CO) 2 M{P(NPh)(OMe) 2 }. The x-ray structures and reactivity of the complexes are reported.     

REDUCTIVE DEIMINATION OF SULFOXIMIDES AND SULFIMIDES WITH P-TOLUENESULFONYL NITRITE AND T-BUTYL THIONITRATE1

Abstract

Reduction of sulfimides and sulfoximides with p-toluenesulfonyl nitrite, a new nitrosating agent, gave nearly quantitatively the corresponding deimination products, sulfides and sulfoxides, respectively. In the reaction of dialkyl and aryl alkyl sulfoximides with t-butyl thionitrate, N-t-butylthiosulfoximides were obtained besides the usual deimination products, although diaryl sulfoximides were readily deiminated to the corresponding sulfoxides in good yields in the same treatment. t-Butyl thionitrate was also found to deiminate diphenyl sulfimide to give diphenyl sulfide in good yield. Sulfoximides reacted sluggishly with t-butyl thionitrite, however, eventually affording a small amount of sulfoxides.     

On the Reaction of 3-Acetyl Coumarin with Alkyl Phosphites. Structural Confirmations Based on X-Ray Crystallography

3-Acetyl coumarin ( 1 ) reacts with dialkyl phosphites at 100°;C for 8 h to give the phosphonates 2 . Under the same experimental conditions, 1 reacts with trialkyl phosphites to form a mixture of the phosphonates 2 and 3 . The molecular structures of these products were elucidated by x-ray crystallography.     

Synthesis of Novel Spiro[indol-2,2′-pyrroles] Using Isocyanide-Based Multicomponent Reaction

An efficient and facile method for the synthesis of novel spiro[indole-2,2′-pyrroles] from N-methyl-3-isatin imines, t-butyl isocyanide, and dialkyl acetylenedicarboxylate has been achieved by [3 + 2] cyclo addition reaction. All the products were purified by column chromatography as yellow solids and confirmed with ¹H NMR, ¹³C NMR, fast atom bombardment, mass, and infrared. Compound 11 was further confirmed with x-ray analysis.     

Exploiting the Potential of Meroterpenoid Cyclases to Expand the Chemical Space of Fungal Meroterpenoids

Fungal meroterpenoids are a diverse group of hybrid natural products with impressive structural complexity and high potential as drug candidates. In this work, we evaluate the promiscuity of the early structure diversity-generating step in fungal meroterpenoid biosynthetic pathways: the multibond-forming polyene cyclizations catalyzed by the yet poorly understood family of fungal meroterpenoid cyclases. In total, 12 unnatural meroterpenoids were accessed chemoenzymatically using synthetic substrates. Their complex structures were determined by 2D NMR studies as well as crystalline-sponge-based X-ray diffraction analyses. The results obtained revealed a high degree of enzyme promiscuity and experimental results which together with quantum chemical calculations provided a deeper insight into the catalytic activity of this new family of non-canonical, terpene cyclases. The knowledge obtained paves the way to design and engineer artificial pathways towards second generation meroterpenoids with valuable bioactivities based on combinatorial biosynthetic strategies.     

Recent advances in surface x-ray diffraction and the potential for determining structure-sensitivity relations in single-crystal electrocatalysis

Determining how electrode structure governs the performance of an electrocatalyst requires techniques capable of probing structure at the atomic scale, often in situ and operando. In recent years, there have been numerous advances in the main experimental techniques for determining the structure of the electrochemical interface. In situ/operando synchrotron surface x-ray diffraction measurements are key to investigate the atomic structure of the electrode surfaces as well as understand the structure-reactivity relations in electrocatalysis. Here we discuss some recent improvements that have taken place in surface x-ray diffraction and how we expect them to lead to an enhanced understanding of electrocatalysis.