Effect of solvent and structure on the kinetics and mechanism of the aquation of bromopentaamine cobalt(III) complex in binary aqueous mixtures

The kinetics of aquation of bromopentaamine cobalt(III) complex have been investigated spectrophotometrically in aqueous‐organic solvent media using acetonitrile, urea, and dimethyl sulfoxide as co‐solvents at 45 ≤ T (°C) ≤ 65. The logarithms of rate constant of the aquation reaction vary nonlinearly with the reciprocal of the dielectric constant for all cosolvent mixtures, indicating a specific solute–solvent interaction. Also, the rate constants are correlated with the total number of moles of water and the organic solvents. However, the solvent effects on the solvation components of the enthalpy of activation, ΔH^?, and the entropy of activation, ΔS^?, have been studied. Analysis of the solvent effect confirmed a common I_d mechanism for the aquation of the cobalt(III) complex. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36:494–499, 2004     

The Synthesis of Triazolothiadiazines and Thiadiazoles From 1,2-Bis-(4-amino-5-mercapto-1,2,4-triazol-3-yl)- Ethanol and Ethane

The reaction of DL-malic and succinic acids with thiocarbohydrazide afforded 1,2-bis[4-amino-5-mercapto-1,2,4-triazol-3-yl]-ethane derivatives 3a and 3b. The reaction of 3a,b with phenacyl bromide and benzoin afforded 1,2-bis-1,2,4-triazolo [3,4-b][1,3,4]thiadiazine derivatives 4 and 5. The carboethoxymethylation of 3a and 3b gave 6a and 6b, respectively, and their reactions with carbon disulfide and benzoylisothiocyanate gave the 1,2-bis-1,2,4-triazolo[3,4-b][1,3,4]thiadiazole 7 and 9, and with p-nitrobenzaldehyde gave a Schiff's base and dihydrothiadiazole 8. The structures were confirmed by using ¹ H and ¹³ C NMR spectra. Selected members of these compounds were screened for antimicrobial activity.     

Numerical modeling of surface reaction kinetics in electrokinetically actuated microfluidic devices

We outline a comprehensive numerical procedure for modeling of species transport and surface reaction kinetics in electrokinetically actuated microfluidic devices of rectangular cross section. Our results confirm the findings of previous simplified approaches that a concentration wave is created for sufficiently long microreactors. An analytical solution, developed for the wave propagation speed, shows that, when normalizing with the fluid mean velocity, it becomes a function of three parameters comprising the channel aspect ratio, the relative adsorption capacity, and the kinetic equilibrium constant. Our studies also reveal that the reactor geometry idealized as a slit, instead of a rectangular shape, gives rise to the underestimation of the saturation time. The extent of this underestimation increases by increasing the Damkohler number or decreasing the dimensionless Debye–Hückel parameter. Moreover, increasing the values of the Damkohler number, the dimensionless Debye–Hückel parameter, the relative adsorption capacity, and the velocity scale ratio results in lower saturation times.     

Theoretical study on keto–enol tautomerism and isomerization in pyruvic acid

Isomerization and tautomerism of 12 isomers of pyruvic acid including 4 keto and 8 enol forms were studied at the MP2 and B3LYP levels of theory using 6‐311++G(2df,p) basis set, separately. Activation energy (E_a), imaginary frequency (υ), and Gibbs free energy (ΔG^#) of the considered isomerization and tautomerism reactions were calculated. Interconversion of the enol forms proceeds through two paths: (i) proton transfer and (ii) internal rotation. Activation energies for the proton transfer paths were in the range of 125–145 kJ/mol and for the internal rotation paths were in the range of 5–45 kJ/mol. Keto–enol tautomerism of pyruvic acid proceeds only through proton transfer route and their activation energies were in the range of 200–300 kJ/mol. Effect of microhydration on the transition state structures and activation energies was also investigated. It was found that the presence of a water molecule catalyzes the isomerization and tautomerism reactions of pyruvic acid so that the activation energies decrease. © 2013 Wiley Periodicals, Inc.     

Theoretical study on the isomerization and tautomerism in barbituric acid

Isomerization and tautomerism of 16 isomers of barbituric acid (BA) were studied at the MP2 and B3LYP levels of theory. Activation energies (E _a), imaginary frequencies (υ), and Gibbs free energies (ΔG ^#) of the amine-imine and keto-enol tautomerisms and O–H internal rotations were calculated. The activation energies of amine-imine tautomerisms were in the range of 110–200 kJ/mol and for keto-enol tautomerisms were larger than 200 kJ/mol. The calculated activation energies of internal O–H rotations were smaller than 60 kJ/mol. Effect of micro-hydration on the transition state structures and activation energies of the tautomerisms were also investigated. Water molecule catalyzed the tautomerisms and decreased the activation energies of both the amine-imine and keto-enol tautomerisms about 100–120 kJ/mol.     

DFT study on the isomerization in vitamin B6

Isomerization and tautomerism reactions of the active form of vitamin B6, pyridoxal phosphate, are studied at B3LYP level of theory using 6-311++G(2df,p) basis set in gas and aqueous phases. Twenty-three transition state (TS) structures for vitamin B6 isomerization are optimized, including 13 TS structures for O–H and C–C rotations, 8 TS structures for imine–enamine tautomerism, and 2 TS structures for keto–enol tautomerism. Activation energy (E _a), imaginary frequency (υ), and Gibbs free energy of activation (ΔG ^#) for the isomerization reactions are calculated. The activation energies of the imine–enamine tautomerism are in the range of 190–280 kJ/mol and of O–H and C–C rotations are mainly less than 60 kJ/mol. Also, our calculation shows that the imine forms of B6 are mainly more stable than the enamine forms. Effect of microhydration on the TS structures and activation energies is also investigated. It is found that the presence of water molecules catalyzes only the imine–enamine tautomerism.     

Error indicators for the Navier-Stokes equations in stream function and vorticity formulation

This paper deals with a posteriori estimates for the finite element solution of the Stokes problem in stream function and vorticity formulation. For two different discretizations, we propose error indicators and we prove estimates in order to compare them with the local error. In a second step, these results are extended to the Navier-Stokes equations. Received March 25, 1996 / Revised version received April 7, 1997     

Waveguides and gratings fabrication in zirconium-based organic/inorganic hybrids

Sol–gel derived poly(oxyethylene)/siloxane organic/inorganic di-ureasil hybrids containing different amounts of methacrylic acid (McOH, CH₂=C(CH₃)COOH)) modified zirconium oxo-clusters (Zr-OMc) were processed as thin films deposited in glassy substrates via spin coating and as transparent and shape controlled monoliths. Channel monomode waveguides and diffraction gratings were UV patterned using the Talbot interferometer and the Lloyd mirror interferometer experimental setups. The time dependence of the diffraction gratings efficiency was studied for hybrids containing different amounts of Zr-OMc. Finally, the number of propagating modes and the refractive index gradient within the waveguide region, determined as a Gaussian section located below the patterned channel, was evaluated and modeled, a maximum index contrast of 2.43 × 10^?5 being estimated.