Rosa Valero

A new algorithm for the kinetic data analysis

Abstract In this paper, a new algorithm for the kinetic data analysis is presented. The main objective of the algorithm is to retrieve the maximum information concerned with a multi-response complex chemical system evolving in time, in order to retrieve the rate constants (calibration problem) or the initial concentration of species. As a difference with other data treatments found in the literature, the algorithm is able to estimate the uniqueness and reliability of the calculated rate constants. This task is carried out by analyzing of the principal components of the sensitivity coefficients with regard to the rate constants. The analysis allows understanding whether the located stationary points consist of a single point, or a surface relating a set of rate constants on which the least-squares (LS) function takes a constant value. In the latter case, the rate constants will not be uniquely determined for the chosen experimental design. The algorithm has been materialized in the opkine 2 software, which has been provided with a wide range of mathematical resources related to the minima location, and to the integration of stiff ordinary differential equations (ODE) systems. Finally, the statistical criteria used to compare the experimental data with a model are not only restricted to the LS method, and it is possible to use the determinant criterion, or to fit the principal components of the experimental responses. The application of the algorithm to first-order networks allows saving up a great quantity of calculation time compared to classical numerical integration.

Spectroscopic study of intermolecular complexes between FAD and some β-carboline derivatives

Abstract The formation of molecular complexes between flavine adenine dinucleotide (FAD) and some β-carboline derivatives [antidepressant drugs that have a pronounced inhibition of monoamine oxidase (MAO)] has been studied by using electronic absorption and fluorescence spectroscopic methods. Thermodynamic parameters have been determined from the values of association constants for the molecular complexes at various temperatures. The influence of substituents in the β-carboline molecule on the stability of the complexes formed was also investigated.

Kinetics and mechanism of the addition of triphenylphosphoniocyclopentadienide to tetrachloro-p-benzoquinone

The kinetics and mechanism of the reaction of triphenylphosphoniocyclopentadienide 1 with tetrachloro-p-benzoquinone (chloranil 2a) to yield 2,3,5-trichloro-6-(3-triphenylphosphoranylidene-cyclopenta-1,4-dienyl)-p-benzoquinone 3 in dichloromethane solution are reported. The rate limiting step was found to be electrophilic attack of tetrachloro-p-benzoquinone on the aromatic cyclopentadiene ring of the ylide. A thermochemical study of the system showed a small enthalpy and a large negative entropy of activation in agreement with the existence of a highly polar betaine as ah intermediate in the reaction. The proposed mechanism involves bimolecular transfer of a proton to a molecule of ylide. No evidence was found either for the formation of a π-complex or for acid catalysis of the reaction.

Apparent dipole moments and molar volumes of β-carbolines and carbazole in dioxane solution

The apparent molecular polarization at infinite dilution, P2∞, molecular refraction, RD, dipole moment, µ2, and partial molar volume, V2°, of carbazole and some β-carboline derivatives have been determined from dielectric permittivity, specific volume, and refractive index measurements in 1,4-dioxane solutions at 25 °C. Evidence of hydrogen-bonding interaction with solvent has been found from i.r. spectroscopy and the apparent molar volumes. A relation has been found between literature pKa values of β-carboline derivatives studied in the ground state and their corresponding dipole moments in solution.

THE KINETICS AND MECHANISM OF THE REACTION OF ONIUM CYCLOPENTADIENYLIDES WITH TETRAHALO-p-BENZOQUINONES

Abstract The reaction of triphenylphosphonium cyclopentadienylide (1) with halogen-substituted p-benzoquinones (4) is shown to give a new class of dipolar (zwitterionic) dyes (5) containing phosphorus. The general structure of these molecules has been investigated by a combination of mass spectrometry and multinuclear (7H, 13C and 31P) nmr using the specialist techniques of DEFT spectroscopy, homonuclear (COSY) and heteronuclear 2-D nmr. In addition, stopped-flow (uv/visible) techniques have been used to study the kinetics of the reactions and hence demonstrate that the rate-limiting step is nucleophilic addition of the ylid nucleophile to the quinone, followed by a rapid loss of halide ion. This mechanism follows the classical pattern associated with nucleophilic aromatic substitution in activated aryl halides.

Kinetics and mechanism between chloranil and s-butylamine in cyclohexane solution

The interaction of chloranil (CA) with s-butylamine (A), in cyclohexane solution has been studied by measuring visible and ultraviolet absorption spectra at various temperatures in the range 279–308 K. Kinetic and thermodynamic parameters have been derived and a mechanism has been proposed for this reaction. The results indicate the existence of a charge-transfer complex in the presence of an excess of s-butylamine, which decomposes to give a final product (D: the disubstituted product of CA). The reaction path has been found to be autocatalysed by the presence of the amine.

Kinetics and Mechanism of the Reactions between Chloranil and n-Butylamine in Cyclohexane Solution

Interaction between chloranil (CA) and n-butylamine (A) at several temperatures has been studied by u.v.–visible spectroscopy. Kinetic and thermodynamic parameters, as well as a mechanism, have been proposed for this reaction. The results indicate the initial formation of charge-transfer complexes between CA and A, in the presence of an excess of amine, which decomposes to give a final product (D)(disubstituted product of CA). The reaction path is autocatalysed by the presence of amine.

Determination of the complexes between dimethylphenols and pyridine by a dielectric constant method in non-polar solvents

The apparent equilibrium constants for the association of pyridine with dimethylphenols in benzene and in cyclohexane at 25 °C have been measured by the method of dielectric constant variation, in order to check and extend the results obtained by spectroscopic methods. Solvent effects were detected. Excess of dipole moments have been derived and related with pKa values from the literature. The influence of the relative positions of the methyl substituents on complex formation are discussed qualitatively.

Kinetics and mechanism of the addition of triphenylphosphoniocyclopentadienide to tetrahalo-p-benzoquinones. Part III. The disubstitution of chloranil and bromanil

Chloranil and bromanil react with an excess of triphenylphosphoniocyclopentadienide to yield the 2,5- and 2,6-disubstituted quinone derivatives through two parallel, second-order reactions. Kinetic data suggest that the reaction proceeds in two steps, involving addition to form a polar betaine intermediate followed by elimination of hydrogen halide. The activation parameters strongly suggest that the loss of halide is of the E1 type. The empirical rate law has been established carrying out a multi-response non-linear least-squares analysis followed by a Gaussian de-convolution of the visible spectra of the reaction mixtures.

Kinetics and mechanism of the addition of triphenylphosphoniocyclopentadienide to tetrahalo-p-benzoquinones. Part 2. Reaction with bromanil and iodanil

The reaction of tetrahalo-p-benzoquinones with triphenylphosphoniocyclopentadienide yields 6-(triphenylphosphonio-3′-cyclopentadienyl)-2,3,5-trihalocyclohexa-2,5-diene-1,4-dione (4a–d), a new class of zwitterionic dyes containing phosphorus. The rate-limiting step has been found to be the addition of the ylide to the quinone through a highly polar betaine intermediate. The elimination of hydrogen halide from the betaine, is of the E2 or E1cB type for the bromanil system and of the E1 type for iodanil. SCF–AM1 calculations suggest that the E1cB path is energetically favoured relative to the E1 elimination path.