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171.
Ignacio Viera Eduardo Manta Lucía González Graciela Mahler 《Tetrahedron: Asymmetry》2010,21(5):631-635
Novel α,α-disubstituted epoxy esters were prepared in enantiopure form by hydrolytic kinetic resolution (HKR) of the corresponding racemic mixtures using chiral salenCo(III) as catalyst. The methodology provides a convenient route to enantioenriched β,γ-epoxy esters 2a, 2c and 2d. 相似文献
172.
The structure of trans-1,4-dibromo-1,4-dicarboxymethylcyclohexane (I) has been determined by single crystal x-ray diffraction. The molecules, located on symmetry centers, are in the rigid chair conformation with the Br and COOCH3 axial and equatorial, respectively. The molecular geometry observed in the crystal is similar to that calculated for the isolated molecule using ab initio calculations performed at the HF/6-31(d) level. A peculiar arrangement of the carboxymethyl substituents, along with short contacts involving the Br atoms, seem to play a role in the stabilization of the crystalline structure. 相似文献
173.
Aira Miró Vera William Velásquez Alexander Briceño Alí Bahsas Bahsas Belkis Ramírez Valero Graciela Diaz de Delgado 《Journal of chemical crystallography》2007,37(8):543-548
Dimethyl-7-oxabicyclo[2.2.1]hept-5-ene exo,exo-2,3-dicarboxylate (1) was prepared by Fisher esterification of 7-oxabicyclo[2.2.1]-hept-5-ene exo-2,3-dicarboxylic anhydride (2) in methanol. The colorless plates obtained were characterized by FT-IR, 1H- and 13C-NMR, TGA-DSC, and single crystal X-ray diffraction. The material crystallizes in space group P21/c, with a = 9.2375(14) Å, b = 12.8757(18) Å, c = 9.4608(15) Å, β = 115.327(3)°, V = 1017.1(3) Å3, and Z = 4. Chains of hydrogen-bonded molecules along the c-axis interact in the b direction to form layers parallel to the bc plane. 相似文献
174.
Assuming that Y has a singular matrix variate elliptically contoured distribution with respect to the Hausdorff measure, the distributions of several matrices associated to QR, modified QR, SV and polar decompositions of matrix Y are determined, for central and non-central, non-singular and singular cases, as well as their relationship to the Wishart and pseudo-Wishart generalized singular and non-singular distributions. Some of these results are also applied to two particular subfamilies of elliptical distributions, the singular matrix variate normal distribution and the singular matrix variate symmetric Pearson type VII distribution. 相似文献
175.
176.
In this paper, a fully software-controlled multisyringe flow injection (MSFIA) spectrophotometric system is proposed for the determination of sulfide in environmental and waste waters. The implementation of ancillary solenoid valves into the flow network allows a multitude of injection modalities to be explored, the selected modality being directly dependent on the aim of the assays. The multicommuted sandwich-type approach is introduced in this work as an efficient means to warrant high sensitivity for the particular assay with excellent repeatabilities and a considerable reagent saving. Moreover, a high injection frequency may be easily attained by carrying out a multiple injection modality during a single forward displacement of the piston driver bar. The interfacing of the robust and versatile multisyringe piston pump with an optical fiber plug-in spectrophotometer furnished with a light emitting diode enables the miniaturization of the flow analyzer, which is thus readily adaptable to in-situ and real-time monitoring schemes. The flow method is based on the coupling Fischer’s reaction of sulfide with N,N-dimethyl-p-phenylenediamine in the presence of Fe(III) as oxidizing reagent in a 0.7 M HCl medium. Careful selection of the physical and chemical variables enabled coefficients of variations better than 1.5% (n = 10) at the 1 mg l−1 level for both injection modalities. Dynamic working ranges of 0.2–2.0 and 0.5–5 mg l−1 sulfide for sandwich and multiple injection techniques, and detection limits of 0.09 and 0.15 mg l−1, respectively, were obtained. Furthermore, the sandwich modality featured an average slope of 0.43 ± 0.02 l mg−1 calculated from 10 day-to-day calibration plots. This result reveals better sensitivity than other flowing stream methods described in the literature. The multiple injection technique allowed an improvement of the injection throughput up to 80 h−1, although a decrease of sensitivity was concomitantly observed (average slope of 0.17 ± 0.01 l mg−1).
The multisyringe flow method was successfully applied to the determination of sulfide in different spiked water matrices (namely, mineral, tap, freshwater, seawater and wastewater) with recoveries ranging from 96 to 104%. Good agreement was also found in water samples between the MSFIA results and those of the batch APHA-standard method. 相似文献
177.
Gustavo?EcheverríaEmail author Adolfo?Fantoni Julio?Mara?ón Graciela?Punte 《Structural chemistry》2005,16(6):571-579
Molecular geometry, crystal packing and thermal stability of trans-1,4-cyclohexanedicarboxylic acid, 1,4-dibromo (I) are examined, in order to analyse the relative influence of the different intermolecular interactions at play. From a comparison
with the corresponding properties of trans1,4-cyclohexanedicarboxylic acid (II), some insights on the results of the competition between Br and carboxylic groups are obtained. In the crystalline state,
the molecules of both compounds present chair centrosymmetric geometry and build infinite chains through ring hydrogen bonds
involving the carboxylic groups. These groups are axial in I and equatorial in II, giving rise to different assembly of the chains. In both compounds there is in general good agreement between the in-crystal
geometry and the B3LYP/LanL2DZ optimized geometry of the corresponding isolated molecule. The only significant differences
are related to the carboxylic groups. The intermolecular contacts in I have been rationalized on the basis of a graph set analysis. A molecular dynamics based method has been used to analyse the
temperature dependence of the mean life time of the different kinds of hydrogen bonds characterized in I. It has been found that the O–H⋅ < eqid2 > ⋅O and C–H⋅ < eqid3 > ⋅O(carbonyl) interactions are the main source of thermal stability. The melting point of I determined within this approach agrees very well with the experimental value. 相似文献
178.
An automatic multisyringe flow injection analysis (MSFIA) system coupling a flow-through optical fiber diffuse reflectance sensor with in-line gas-diffusion (GD) separation is proposed for the isolation, preconcentration and determination of traces of volatile and gas-evolving compounds in samples containing suspended solids, with no need for any preliminary batch sample treatment. The flowing methodology overcomes the lost of sensitivity of the in-line separation technique, when performed in a uni-directional continuous-flow mode, through the implementation of disk-based solid-phase extraction schemes. The high selectivity and sensitivity, the low reagent consumption and the miniaturization of the whole assembly are the outstanding features of the automated set-up. The proposed combination of techniques for separation, flow analysis, preconcentration and detection was applied satisfactorily to sulfide determination in environmental complex matrixes. The method based on multicommutation flow analysis involves the stripping of the analyte as hydrogen sulfide from the donor channel of the GD-module into an alkaline receiver segment, whereupon the enriched plug merges with well-defined zones of the chormogenic reagents (viz., N,N-dimethyl-p-phenylenediamine (DMPD) and Fe(III)). The in-line generated methylene blue dye is subsequently delivered downstream to the dedicated optrode cell furnished with a C18 disk, while recording continuously the diffuse reflectance spectrum of the pre-concentrated compound. This procedure provides a linear working range of 20-500 μg l−1 sulfide with a relative standard deviation of 2.2% (n = 10) at the 200 μg l−1 level, and a detection limit of 1.3 μg l−1. 相似文献
179.
Coldham I Patel JJ Sanchez-Jimenez G 《Chemical communications (Cambridge, England)》2005,(24):3083-3085
Asymmetric substitution of the organolithium derived either from N-Boc-2-tributylstannylpyrrolidine by tin-lithium exchange or from N-Boc-pyrrolidine by deprotonation occurs in the presence of a commercially available chiral diamine ligand with high levels of enantioselectivity by a dynamic kinetic resolution pathway. 相似文献
180.
Quiclet-Sire B Sanchez-Jimenez G Zard SZ 《Chemical communications (Cambridge, England)》2003,(12):1408-1409
Treatment of a geminal pivaloxy xanthate, prepared by an intermolecular radical addition of a xanthate to vinyl pivalate, gives a 1,3-dithietanone, a little known class of compounds. 相似文献