Search results for "gate"
showing 10 items of 1811 documents
A spectroelectrochemical study of poly(dithienothiophenes)
1997
Abstract The use of n- and p-dopable conjugated polymers as both the working and counter electrodes in an electrochemical device has always attracted chemists and has stimulated the design of new molecules with a low energy gap that can be p- and n-doped efficiently. The present paper reports the spectroelectrochemical study of polymers obtained from several dithienothiophene isomers, molecules with three thiophene rings fused in different positions. Of the six isomers, poly(dithieno[3,4-b:3′,2′-d]thiophene) and poly(dithieno[3,4-b:2′,3′-d] thiophene) were tested against poly(dithieno[3,2-b:2′,3′-d]thiophene) and poly(dithieno[3,4-b:3′,4′-d]thiophene). The differences in their performance, …
Long-term cropping systems and tillage management effects on soil organic carbon stocks and steady state level of C sequestration rates in a semiarid…
2010
A calcareous and clayey xeric Chromic Haploxerept of a long-term experimental site in Sicily (Italy) was sampled (0–15 cm depth) under different land use management and cropping systems (CSs) to study their effect on soil aggregate stability and organic carbon (SOC). The experimental site had three tillage managements (no till [NT], dual-layer [DL] and conventional tillage [CT]) and two CSs (durum wheat monocropping [W] and durum wheat/faba bean rotation [WB]). The annually sequestered SOC with W was 2·75-times higher than with WB. SOC concentrations were also higher. Both NT and CT management systems were the most effective in SOC sequestration whereas with DL system no C was sequestered. …
Synthesis of γ-Amino Acid Esters by 1,4-Addition of Deprotonated α-Aminonitriles and α-(Alkylideneamino)nitriles to α,β-Unsaturated Esters.
2007
α-Aminonitriles and α-(alkylideneamino)nitriles can serve as readily available α-aminocarbanion equivalents. Their conjugate addition to α,β-unsaturated esters followed by reduction furnishes polysubstituted γ-amino acid esters in moderate to high yield.
Bile acid–amino acid ester conjugates: gelation, structural properties, and thermoreversible solid to solid phase transition
2010
Design, synthesis, and gelation properties of three novel biocompatible bile acid–L-methionine methyl ester conjugates are presented. Two of the conjugates have been shown to undergo self-assembly leading to organogelation in certain aromatic solvents. The properties of these gels have been investigated by conventional methods typical for molecular gel studies along with 13C CPMAS NMR spectroscopic studies of the native gel. In addition, properties in solid and solution states for all three compounds have been investigated, and single crystal X-ray structures of all compounds determined. Furthermore, powder X-ray diffraction studies have revealed that compound 1 undergoes a dynamic and reve…
Conjugate Alkynylation of Electrophilic Double Bonds. From Regioselectivity to Enantioselectivity
2018
This review surveys the historical efforts addressed toward the development of the conjugate alkynylation reaction. The regio- and enantioselective conjugate alkynylation of electron-deficient double bonds, most commonly unsaturated carbonyl compounds, has been an elusive reaction for a long time. Intensive research during the last decades has resulted in the identification of a number of effective reagents and catalysts to perform this reaction. Non-stereoselective conjugate alkynylation of unsaturated carbonyl compounds was first achieved by using preformed alkynyl organometallics and later with terminal alkynes under catalytic conditions. These methods paved the way for the development o…
Determination of the absolute configuration of (−)-abietic acidviaits (4R,5R,9R,10R)-7,13-abietadien-18-ylp-bromobenzoate derivative
2006
The absolute configuration of the title bromo derivative of abietic acid, C27H35BrO2, has been determined. The structural analysis confirms the absolute stereochemistry for (−)-abietic acid proposed by Bose & Struck [(1959). Chem. Ind. (London), pp. 1628–1630] on the basis of optical rotatory dispersion measurements. The molecule exhibits a trans anti 6/6/6 tricyclic hydrocarbon skeleton, with the cyclohexane ring in the expected chair form and the two cyclohexene rings, the double bonds of which are conjugated, in half-chair conformations.
From linear quaterthiophene to sulflower: A comparative theoretical study
2009
Abstract In this paper, we report a theoretical study of four types of thiophene-based oligomers showing the same number of C C double bonds and very different molecular structures. The comparative study has been performed on the basis of B3LYP/6-31G∗∗ calculations. The way the thiophene rings are linked together has a remarkable influence on the molecular and electronic properties. Linear quaterthiophene and heptathienoacene show similar aromatic structures but a loss of π-conjugation is detected for the latter due to the condensation of thiophene rings. A blue shift of the most intense electronic transition is predicted for fused heptathienoacene compared with non-fused quaterthiophene. C…
Experimental and theoretical studies on corals. I. Toward understanding the origin of color in precious red corals from Raman and IR spectroscopies a…
2009
An attempt to explain the origin of the vivid red color in precious pink and red corals was undertaken. Raman and IR spectroscopies were applied to characterize white, pink and red corals. The position of the Raman signal near 1500 cm −1 of some corals and pearls was associated by several authors with the presence of the mixture of all-trans-polyenic pigments, containing 6 – 16 conjugated C C bonds or β-carotenoids. This hypothesis was examined theoretically by performing extensive B3LYP-DFT calculations of vibrational spectra of the model polyenic compounds. The B3LYP/6-311++G ∗∗ predicted positions of the dominating Raman mode depend on the number of C Cu nits (Cn parameter) and can be ac…
Interaction Mechanisms between guaiacols and lignin: the conjugated double bond makes the difference.
2011
Lignin is considered to be responsible for a selective sorption of phenolic compounds on wood. In order to investigate the mechanisms involved, two similar guaiacol compounds--only differing by the nature of the para side chain--were adsorbed on oak wood extracted lignin. Vapor sorption-desorption isotherms indicated that about 3.5 wt % of 4-vinylguaiacol is adsorbed near saturation whereas it is only 0.8% for 4-ethylguaiacol. For both compounds, the isotherms displayed a hysteresis though significantly greater for 4-vinylguaiacol. Analyses of the hydroxyl stretching region of FTIR spectra of the lignin/4-ethylguaiacol and lignin/4-vinylguaiacol complexes indicated that physisorption via hy…
Asymmetric diastereodivergent Michael addition of 2-chloromalonate esters to conjugated imines enabled by catalyst metal change
2019
Despite recent progress in asymmetric diastereodivergent reactions leading to products bearing two stereogenic centers, little research has been devoted to processes were a stereogenic center and a double bond are formed. Here, we report the asymmetric diastereodivergent Michael addition of 2-chloromalonate esters to N-tosyl β,γ-unsaturated α-ketimino esters to give chiral α,β-dehydro-α-aminoesters bearing either a Z or E enamine moiety, using pyBOX-metal complexes. Diastereodivergency is achieved by simply changing the metal ion from a trivalent La(III) to a divalent Ca(II) ion, providing the Z or E enamines. Computational studies reveal the crucial role of London interactions between the …