Kinetic and theoretical studies on the mechanism of intramolecular catalysis in phenyl ester hydrolysis

The kinetic study of the hydrolysis reaction of Z-substituted phenyl hydrogen maleates (Z = H, m-CH3, p-CH3, m-Cl, p-Cl and m-CN) was carried out in aqueous solution, and the results were complemented with theoretical studies. Under some experimental conditions, two kinetic processes were observed. One of them was ascribed to maleic anhydride formation and the other to the anhydride hydrolysis. The Br?nsted-type plot for the leaving-group dependence was linear with slope beta(lg) = -1. The experimental results are consistent with a mechanism that involves significant bond breaking in the rate-limiting transition state (alpha(lg) = 0.64). Theoretical results for the reaction in the gas phase showed an excellent Br?nsted-type dependence with a beta(lg) of -1.03. A tetrahedral intermediate (TI) could not be found through DFT gas-phase studies (B3LYP/6-311+G*). Calculations carried out within a continuous solvation model or with discrete water molecules failed to find a stable TI. With both models, a flat region on the potential-energy surface is found and a tight optimization of the structures led back to starting materials. The theoretical results do not discard the possible existence of an unstable intermediate on the free-energy surface, but the analysis of the whole body of results compared with other acyl transfer reactions lead us to suggest that an enforced concerted mechanism is the most appropriate to describe these reactions.     

Nano‐scaled intrachain ordering and dynamics in two‐dimensional orientational polymer domains1)

Statistical and local relaxation properties of two‐dimensional finite polymer systems (domains) are considered. The domains consist of a large number of semirigid chains with the finite contour length at free, half‐free and fixed boundary conditions for chain ends. The intermolecular orientational order at short distances between chains in the thick domains is similar to the order in infinite two‐dimensional systems. The correlations of orientation between sufficiently distant elements of different chains decay by the exponential law, but the effective constant of interchain interactions in the domain is proportional to the molecular weight of the chain. At the given intra‐and interchain interactions an elongtation of the chains leads to a local ordering of chains in the domain (at free boundary conditions) or, on the contrary, to the decreasing of the parameter of short‐range orientational order (at fixed and half‐free boundary conditions). Independently of type of boundary conditions the parameter of large‐range orientational order tends to zero with increasing of the chain contour length. Dynamical equations and relaxation spectrums for times of local motions are obtained. From time correlation functions of local relaxation the times of nano‐scaled mobility of chains were calculated in depending on the bending rigidity of chains, the parameter of interchain interactions, and the contour length of chains. At the given intra‐and interchain interactions an elongtation of chains forming the domain leads to to the slowing‐down of local mobility of chains in the domain. The comparison with experimental date obtained by dielectric relaxation and polarized luminescence methods on investigation of nano‐scaled mobility in the dilute melts of comb‐shaped polymers has been carried out.     

Comparison of micromodels describing the elastic properties of diamond

A mechanical model of diatomic crystal lattice with force interaction between atoms and angular interaction between bonds taken into account is proposed. Some relations between the macroscopic moduli of elasticity and the microparameters of the longitudinal rigidity of interatomic bonds and of the angular interaction rigidity are obtained for crystals with diamond lattice. Comparison with experimental data and with other theories describing similar lattices is conducted by using two constants at the microlevel.     

Magnesium optical one-shot sensor based on a coumarin chromoionophore

The characterization of a new irreversible optical absorption-based one-shot sensor for magnesium is described. The magnesium photoactive probe is 7-diethylamino-3-(3,4-ethylendioxybenzoyl)coumarin immobilized in a plasticized polymeric membrane. The magnesium selectivity can be explained in terms of size and charge density of magnesium and charge-separated resonance forms contribution in the excited state of coumarin. The selectivity obtained for magnesium over a variety of naturally occurring species in natural waters meets the requirements for the determination of this ion in water. The one-shot sensor responds between 0.14 and 14mgL(-1) with a sensor-to-sensor reproducibility of 1.3% as [Formula: see text] , at the medium level of the range. The performance of the optical one-shot sensor was tested in the analysis of magnesium in different types of natural waters and soft drinks validating results against a reference procedure.     

Chlorodifluoroacetyl Azide, ClF(2)CC(O)N(3): Preparation, Properties, and Decomposition

Chlorodifluoroacetyl azide, ClF(2)CC(O)N(3), was prepared and characterized by IR (gas, Ar matrix), Raman (liquid), UV-vis (gas), and (19)F, (13)C NMR spectroscopy. The vibrational spectra were analyzed in terms of a single conformer, gauche-syn, where the Cl-C and the N(α)═N(β) bonds are gauche and syn to the C═O bond, respectively. The photo and thermal decomposition reactions of the azide were studied with the aid of matrix isolation. In both cases, a new isocyanate species ClF(2)CNCO was produced and characterized by matrix IR spectroscopy. The conformational properties and the Curtius rearrangement pathways of this new carbonyl azide were theoretically explored, which suggest the preference of a concerted over stepwise decomposition for the global minimum gauche-syn conformer.     

Oxoneptunium(v) as part of the framework of a polyoxometalate

(NH4)14Na4[(Np3W4O15)(H2O)3(BiW9O33)3].62H2O (1) and (NH4)14.5Na3.5[(Np3W4O15)(H2O)3(SbW9O33)3].40.5H2O (2) each contain three neptunyl(v) moieties encapsulated within heteropolyoxotungstate frameworks in which axial {NpO2}+ oxygens form one face of a WO6 octahedron.     

Dynamics of surface catalyzed reactions; the roles of surface defects, surface diffusion, and hot electrons

The mechanism that controls bond breaking at transition metal surfaces has been studied with sum frequency generation (SFG), scanning tunneling microscopy (STM), and catalytic nanodiodes operating under the high-pressure conditions. The combination of these techniques permits us to understand the role of surface defects, surface diffusion, and hot electrons in dynamics of surface catalyzed reactions. Sum frequency generation vibrational spectroscopy and kinetic measurements were performed under 1.5 Torr of cyclohexene hydrogenation/dehydrogenation in the presence and absence of H(2) and over the temperature range 300-500 K on the Pt(100) and Pt(111) surfaces. The structure specificity of the Pt(100) and Pt(111) surfaces is exhibited by the surface species present during reaction. On Pt(100), pi-allyl c-C6H9, cyclohexyl (C6H11), and 1,4-cyclohexadiene are identified adsorbates, while on the Pt(111) surface, pi-allyl c-C6H9, 1,4-cyclohexadiene, and 1,3-cyclohexadiene are present. A scanning tunneling microscope that can be operated at high pressures and temperatures was used to study the Pt(111) surface during the catalytic hydrogenation/dehydrogenation of cyclohexene and its poisoning with CO. It was found that catalytically active surfaces were always disordered, while ordered surface were always catalytically deactivated. Only in the case of the CO poisoning at 350 K was a surface with a mobile adsorbed monolayer not catalytically active. From these results, a CO-dominated mobile overlayer that prevents reactant adsorption was proposed. By using the catalytic nanodiode, we detected the continuous flow of hot electron currents that is induced by the exothermic catalytic reaction. During the platinum-catalyzed oxidation of carbon monoxide, we monitored the flow of hot electrons over several hours using a metal-semiconductor Schottky diode composed of Pt and TiO2. The thickness of the Pt film used as the catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal-semiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. We found the chemicurrent was well-correlated with the turnover rate of CO oxidation separately measured by gas chromatography.     

Reactions of silylmercury compounds with sodium and potassium in the aromatic solvents

Bis(triethylsilyl)mercury (I) and triethylsilyl(pentaethyldisilanyl)mercury (II) react with metallic sodium or potassium in benzene solution to give phenyltriethylsilane (III) and a mixture of III and 1-phenyl-2,2,3,3,3-pentaethyldisilane, respectively, instead of the expected silylmetallic compounds (e.g. Et₃SiM and Et₅Si₂M, where M = Na or K). These results may be explained by the intermediate formation of silylmetallic compounds which reacted with the solvent. The reaction of I with potassium in toluene proceeds analogously.     

Thermal stability of paracetamol and its pre-formulates obtained by spray drying

Organobentonites were synthesized by treating Na-bentonites (Wyomingand Re?adiye) with cetylpyridinium chloride (CPC) using ion exchangemethod. The difference in the basal spacing by 0.65 Å and a higher massloss by 13.32% of organobentonite (Wyoming) in the temperature range 200–550°Ccomparing with that of organobentonite (Re?adiye) were in conformitywith the CEC values of organobentonites. The HOH stretching and bending peaksof bentonites became sharper following the treatment with the organo-cation.The intensity decrease of the Al–OH band for organobentonite (Re?adiye)and in particular, the significantly higher amount of m-cresolretained by the organobentonite of lower CEC than that by the organobentonite(Wyoming) indicate the influence of the CP cation on the adsorption process.