A theoretical investigation of the enantioselective hydrogenation mechanism of α-ketoesters

The enantioselective hydrogenation of α-ketoesters to α-hydroxyesters over Pt/Al₂O₃ catalysts modified by cinchona alkaloids is an interesting model reaction for the investigation of heterogeneous catalysis capable of producing optically active products. The aim of the present theoretical study is to rationalize the interaction between protonated cinchona alkaloids (modifiers) and methyl pyruvate (substrate) by investigating the possible weak complexes formed by these two species. For this purpose we use molecular mechanics and the AM1 semiempirical method. The optimization leads to two stable forms of the complexes, where the substrate is bound to the modifier via hydrogen bonding between the oxygen of the α-carbonyl of pyruvate and the quinuclidine nitrogen of the alkaloid. In such complexes the methyl pyruvate is transformed into a half-hydrogenated species which can be adsorbed on the platinum surface and, after hydrogenation, leads to methyl lactate product. The results show that adsorption of the complex leading to (R)-methyl lactate is more favorable than that of the corresponding system yielding (S)-methyl lactate, which may be the key for the enantio-differentiation.     

Thermodynamic properties of alkali halides. IV. Apparent molal volumes,expansibilities, compressibilities,and heat capacities in urea-water mixtures

The apparent molal volume φ_v, expansibility φ_E, compressibility φ_K, and heat capacity φ_c of NaCl were measured in urea-water mixtures, as a function of salt (<1.5m) and urea (<13m) concentrations at 25°C. At a fixed urea concentration, the transfer functions from H₂O to 3m urea are linear functions of the NaCl aquamolality. At a fixed salt aquamolality, (0.1m), the sign of the transfer functions is in the direction of a decrease in the structure-breaking effect, and the absolute values of the transfer functions tend to level off at high urea concentrations (13m). The functions φ_v, φ_E, φ_K, φ_c, and (?φ_v/?T)_p were measured for the sodium halides and alkali, bromides (chlorides in the case of φ_K) at a fixed salt aquamolality of 0.1m and fixed urea molality of 3m. The corresponding transfer functions from H₂O to 3m urea are opposite those from H₂O to D₂O and similarly are relatively independent of ionic size. This suggests that urea, shows no specific interaction affinity for ions and that the overall number of water molecules influenced by the ions is relatively constant for all alkali halides. The lithium halides are an exception in that Li⁺ seems to have hardly any structure-breaking effect.     

Two unexpected effects concerning perturbations of the B3Π0u+ state of molecular iodine

We present in this paper two new effects observed in the state of molecular iodine: a very high diamagnetic correction in the vibrational level υ′ = 62 and an important orientation of the optically excited molecules due to predissociation. Both effects can be interpreted by taking into consideration the perturbation of the B state by other electronic states.     

Determination de la configuration absolue de quelques derives du bicyclo[2.2.2]octane: Utilisation comparee de la regle des octants et de la methode d'horeau

A series of bicyclo[2.2.2]octane derivatives whose absolute configurations are known through chemical correlations with confirmed reference products (bicyclo[2.2.2]octanol, bicyclo[2.2.2]octene carboxylic acid) have been prepared.     

Supercritical fluid extraction-capillary gas chromatography: On-line coupling with a programmed temperature vaporizer

A simple and versatile system is described for the on-line coupling of SFE to capillary GC. The interfacing consists of a programmed temperature vaporizer (PTV) injector. With this injector it is possible to combine solute trapping, elimination of a high flow of extraction fluid, and quantitative transfer of solutes to the seperation column. The problems caused by impurities in the extraction fluid in on-line SFE-GC are discussed. Simple methods are described for the purification of commercially available carbon dioxide. The trapping efficiency of the PTV injector is studied. Applications of the SFE-PTV-GC system are given for the analysis of polymer anti-degradants, polar compounds, and samples with environmental relevance.     

Chemical equilibrium model for the thermodynamic properties of mixed aqueous micellar systems: Application to thermodynamic functions of transfer

Mixed micelles can be formed in water between various pairs of hydrophobic solutes such as surfactants, alcohols and hydrocarbons. These systems can often be studied through the thermodynamic functions of transfer of one of the solutes, usually kept near infinite dilution, from water to an aqueous solution of the other solute. When mixed micelles are formed, these functions change significantly, and often go through extrema, in the region where the binary system micellizes or undergoes some microphase transition.Three main effects are responsible for the observed trends: pair-wise interactions between both solutes in the monomeric form, a distribution of the reference solute between the aqueous and micellar phases and a shift in the monomer-micelle equilibrium in the vicinity of the reference solute. Simple equations can be derived for these three effects which can account for the sign and magnitude of the observed trends using parameters which are derived for the most part from the two binary systems.     

Measurement of Tumor Vascular Damage in Mice with 99m Tc-MIBI Following Photodynamic Therapy

Abstract— The clinical perfusion agent ^99mTc-MIBI was used to monitor changes in tumor vascular perfusion (TVP) induced by Photofrin® (Pll)-mediated photodynamic therapy (PDT). BALB/c mice bearing an EMT-6 tumor on each hind thigh were given an intravenous injection of 1, 2 or 5 mg kg⁻¹ PII. Twenty-four hours later, one tumor was illuminated (600–650 run, 200 mW cm⁻² 400 J cm⁻²) while the other served as a control. At various time intervals after PDT (0, 2 and 24 h) mice received an intravenous injection of ^99mTc-hexakismethoxy(sobutyusonitri-le (MIBI) (0.18 MBq g⁻¹) and were sacrificed 2 min later. The light-treated and the untreated tumors were then dissected, the radioactivity was counted and the percentage of the injected dose per gram of tumor (%ID g⁻¹) was calculated as a measure of TVP. We observed that TVP is drug dose dependent, develops progressively with time post-PDT and is inversely related to PDT efficacy. Our data show that early tumor retention of ^99mMIBI is a simple method to assess TVP and vascular damage induced by PDT.     

Copper sorption on a straw lignin: experiments and EPR characterization

Spectroscopic and physicochemical data, X-ray photoelectron spectroscopy (XPS), solid-state (13)C cross-polarization magic-angle-spinning NMR, GC/MS, specific surface area, site density, and surface acidity constants have been recorded or determined for a ligno-cellulosic substrate (LS) extracted from straw. Its copper(II)-bound ability has also been studied. The LS solid that exhibits two types of binding sites, carboxylic and phenolic groups, has a great affinity for copper(II), with a maximum of adsorption at around 4 mg g(-1) as early as the pH reaches 6. The structural characterization of the Cu(II) surface complexes has been investigated using EPR spectroscopy. The CuO(4) chromophore of the inner-sphere surface complexes is a slightly distorted square. A surface oxidation occurred simultaneously, which leads to semiquinonic radicals. Furthermore, our empirical approach, which used a correlation between thermodynamics data and EPR parameters, has led to a surface complexation constant log beta equal to 12.6. This indicates that copper(II) surface complexes are relatively stable.     

Acidobasic properties of molten potassium tetrahalogenogallates (X=Cl,I)

Molten potassium tetrachlorogallate and potassium tetraiodogallate were studied in terms of halogenoacidity, based on X^? ion-exchange. Titration of KX solution with GaX₃ were achieved and characterized by the shift of cathodic voltammetric curves. Autodissociation constants K_i,X/mol² kg^?1 were determined: ?log K_i,Cl=4.25±0.05 and ?log K_i,I=2.6±0.05, as well as the solubility values of KX: 0.41±0.02 and 0.80±0.02 mol kg^?1 for KCl and KI respectively.     

Catalytic wet air oxidation of oleic acid on ceria-supported platinum catalyst.effect of pH

Summary Catalytic wet air oxidation (CWAO) of oleic acid was carried out in a batch reactor on platinum supported ceria catalyst (Pt/CeO₂). Oleic acid is a water insoluble linear unsaturated fatty acid of 18 carbon atoms. To increase the homogeneity of the solution by saponification, the influence of NaOH additions in oleic acid CWAO mechanism and catalyst performances have been investigated. The oxidation of such molecule occurs by two types of mechanisms: successive carboxy-decarboxylation which leads essentially to CO₂and/or C-C bonds splitting in the alkyl chain inducing a high formation of acetic acid. With or without NaOH, the 5%Pt/CeO₂catalyst is active in the conversion of oleic acid and selective to carbon dioxide. In alkaline medium, oleic acid is initially saponified which increases the solubility of the reactant before it to be oxidized. Finally the oxidation is slightly delayed by the presence of NaOH. The catalyst characterizations show no significant difference before and after reaction.</o:p>