Rapid, one-pot derivatization and distillation of chlorophenols from solid samples with their on-line enrichment

A microwave-assisted steam distillation (MASD) sample preparation technique for extracting chlorophenols from solid samples was studied. This open vessel microwave system based study developed as an integrated method since it has incorporated extractive distillation, derivatization and on-line enrichment. Gas chromatography (GC) with electron-capture detection was used for the analysis. The study involved optimization of MASD parameters and on-line enrichments using spiked solid samples. MASD achieved recoveries for spiked soil samples in the range of 94-101% within 20min with a collection of only 20ml distillate facilitating on-line enrichment. Some real samples were analyzed that included soil, wood, leather, textiles, dyes and certified reference materials of soil and wood samples. Limit of detection values of 12ng/g for pentachlorophenol and 194ng/g for monochlorophenol were found. Recoveries of 96.6% in the case of soil certified reference material (CRM) with RSD 2.7% and 80.6% in the case of wood CRM with RSD 3.3% were observed in this study. MASD studied found to produce very clean extracts in comparison to reference techniques.     

Synthesis of an advanced intermediate enroute to (−)-clavosolide A

A stereoselective route to an advanced intermediate toward the synthesis of clavosolide A is disclosed. The key steps include Wadsworth-Emmons cyclopropanation, utilization of a sulfinyl moiety as an internal nucleophile to open a cyclopropane ring activated by Hg(II)- to create the C3-C5 stereogenic centers, CC bond formation employing an α-chloro sulfide, asymmetric transfer hydrogenation, regioselective hydrosilylation and Tamao-Fleming oxidation.     

Ab initio and density functional theory evidence on the rate-limiting step in the Morita-Baylis-Hillman reaction

The first ab initio and DFT studies on the mechanism of the MBH reaction show that the rate-limiting step involves an intramolecular proton transfer in the zwitterionic intermediate generated by the addition of enolate to electrophile. The activation barrier for the C-C bond-formation is found to be 20.2 kcal/mol lower than the proton-transfer step for the MBH reaction between methyl vinyl ketone and benzaldehyde catalyzed by DABCO.     

Bicyclic proline analogues as organocatalysts for stereoselective aldol reactions: an in silico DFT study

Density functional theory has been employed in investigating the efficiency of a series of bicyclic analogues of proline as stereoselective organocatalysts for the aldol reaction. Three classes of conformationally restricted proline analogues, as part of either a [2.2.1] or [2.1.1] bicyclic framework, have been studied. Transition states for the stereoselective C-C bond formation between enamines derived from [2.2.1] and [2.1.1] bicyclic amino acids and p-nitrobenzaldehyde, leading to enantiomeric products, have been identified. Analysis of the transition state geometries revealed that the structural rigidity of catalysts, improved transition state organization as well as other weak interactions influence the relative stabilities of diastereomeric transition states and help contribute to the overall stereoselectivity in the aldol reaction. These bicyclic catalysts are predicted to be substantially more effective in improving the enantiomeric excess than the widely used organocatalyst proline. Enantiomeric excesses in the range 82-95% are predicted for these bicyclic catalysts when a sterically unbiased substrate such as p-nitrobenzaldehyde is employed for the asymmetric aldol reaction. More interestingly, introduction of substituents, as simple as a methyl group, at the ortho position of the aryl aldehyde bring about an increase in the enantiomeric excess to values greater than 98%. The reasons behind the vital energy separation between diastereomeric transition states has been rationalized with the help of a number of weak interactions such as intramolecular hydrogen bonding and Coulombic interactions operating on the transition states. These predictions could have wider implications for the rational design of improved organocatalysts for stereoselective carbon-carbon bond-forming reactions.     

Effects of La-doping on structural,electrical and multiferroic properties of Bi6Fe2Ti3O18 thin films

Journal of Sol-Gel Science and Technology - Bi6Fe2Ti3O18 (BFTO) and La-doped Bi6?xLaxFe2Ti3O18 (BLFT) (x = 0.03; BLFT3, x = 0.06; BLFT6 and 0.09; BLFT9)...     

An investigation of flame zones and burning velocities of laminar unconfined methane-oxygen premixed flames

Numerical and experimental investigations of unconfined methane-oxygen laminar premixed flames are presented. In a lab-scale burner, premixed flame experiments have been conducted using pure methane and pure oxygen mixtures having different equivalence ratios. Digital photographs of the flames have been captured and the radial temperature profiles at different axial locations have been measured using a thermocouple. Numerical simulations have been carried out with a C2 chemical mechanism having 25 species and 121 reactions and with an optically thin radiation sub-model. The numerical results are validated against the experimental and numerical results for methane-air premixed flames reported in literature. Further, the numerical results are validated against the results from the present methane-oxygen flame experiments. Visible regions in digital flame photographs have been compared with OH isopleths predicted by the numerical model. Parametric studies have been carried out for a range of equivalence ratios, varying from 0.24 to 1.55. The contours of OH, temperature and mass fractions of product species such as CO, CO₂ and H₂O, are presented and discussed for various cases. By using the net methane consumption rate, an estimate of the laminar flame speed has been obtained as a function of equivalence ratio.     

Investigation of interaction between methanol fed tandem porous spheres burning in a mixed convective environment

Flame interaction during the burning of two porous spheres in tandem arrangement fed with methanol and subjected to a mixed convective environment, has been studied experimentally and numerically. Porous sphere technique is employed for experimentally simulating the burning characteristics of methanol transpired spheres of different sizes, separated by fixed distances. The mass burning rates from both the spheres and visible flame stand-off distances from the sphere surfaces have been measured in the experiments. In the numerical simulations, transient, axisymmetric, mass, momentum, species and energy conservation equations are solved using a finite volume technique based on non-orthogonal semi-collocated grids. Features of the numerical model include finite rate chemistry and temperature and mixture composition dependent thermo-physical properties. Burning of tandem porous spheres in an air stream flowing vertically upwards, at atmospheric pressure has been simulated for different sphere sizes, separation distances and free stream velocities. The numerical predictions have been compared with experimental results. Results reveal that when two spheres burn one over the other, the transition from envelope to wake flame is delayed when compared with that of an isolated sphere. For two spheres of same diameter burning one over the other, depending on the separation distance, flame blows-off after the occurrence of transition from envelope to wake flame in the bottom sphere. For the case of larger sphere at the top, either the flame stabilises in the recirculation zone formed in between the spheres or the flame from the smaller sphere lifts off and stabilises near the front portion of the larger sphere, depending on the separation distance. At higher separation distances, around four times the diameter of the sphere, both the spheres burn independently. The burning rate undergoes complex variations with air stream velocity depending on the sphere sizes and separation distances.     

Palladium(II)-catalyzed direct alkoxylation of arenes: evidence for solvent-assisted concerted metalation deprotonation

Density functional theory investigations on the mechanism of palladium acetate catalyzed direct alkoxylation of N-methoxybenzamide in methanol reveal that the key steps involve solvent-assisted N-H as well as C-H bond activations. The transition state for the critical palladium-carbon bond formation through a concerted metalation deprotonation (CMD) process leading to a palladacycle intermediate has been found to be more stable in the methanol-assisted pathway as compared to an unassisted route.     

Exploring the random genesis of co-occurrence networks

Using the network random generation models from Gustedt (2009) [23], we simulate and analyze several characteristics (such as the number of components, the degree distribution and the clustering coefficient) of the generated networks. This is done for a variety of distributions (fixed value, Bernoulli, Poisson, binomial) that are used to control the parameters of the generation process. These parameters are in particular the size of newly appearing sets of objects, the number of contexts in which new elements appear initially, the number of objects that are shared with ‘parent’ contexts, and, the time period inside which a context may serve as a parent context (aging). The results show that these models allow to fine-tune the generation process such that the graphs adopt properties as can be found in real world graphs.     

High‐resolution stress mapping of polycrystalline alumina compression using synchrotron X‐ray diffraction

The ability to achieve uniform stress in uniaxial compression tests of polycrystalline alumina is of significance for the calibration of piezospectroscopic coefficients as well as strength studies in ceramics. In this study high‐energy X‐rays were used to capture powder diffraction profiles over a half‐section of a polycrystalline alumina parallelepiped sample under an increasing uniaxial compressive load. The data were converted to strain and results were used for stress mapping of the sample. Stress maps from the study quantify the higher stresses at the sample–platen contact interface and reveal the evolution of the stress distribution in these specimens with load. For the geometry of the samples used, at the center section of the specimen the overall magnitudes of the compressive stresses were found to be 20% higher compared with the average expected theoretical stress based on the applied load and cross‐sectional area. The observed compressive stresses at the corners of the parallelepiped specimen were 62% higher and shear stresses were observed at the specimen interface to the load mechanism. The effects, seen at the interface, can lead to premature failure at these locations and can affect the accuracy of calibration of spectral peaks with stress as well as compression strength measurements. The results provide important information that can be used to establish guidelines on material and geometry considerations in developing compression tests on high‐strength ceramics.