Kinetic study of the phenolysis of bis(4-nitrophenyl) carbonate, bis(4-nitrophenyl) thionocarbonate, and methyl 4-nitrophenyl thionocarbonate

The reactions of a homogeneous series of phenols with bis(4-nitrophenyl) carbonate (BNPC), bis(4-nitrophenyl) thionocarbonate (BNPTOC), and methyl 4-nitrophenyl thionocarbonate (MNPTOC) are subjected to a kinetic investigation in water, at 25.0 degrees C and ionic strength of 0.2 M (KCl). Under excess of phenol over the substrate, all the reactions obey pseudo-first-order kinetics and are first order in phenoxide anion. The reactions of BNPC show a linear Br?nsted-type plot with slope beta = 0.66, consistent with a concerted mechanism (one step). In contrast, those of BNPTOC and MNPTOC show biphasic Br?nsted-type plots with slopes beta = 0.30 and 0.44, respectively, at high pK(a), and beta = 1.25 and 1.60, respectively, at low pK(a), consistent with stepwise mechanisms. For the reactions of both thionocarbonates, the pK(a) value at the center of the Br?nsted plot (pK(a)(0)) is 7.1, which corresponds to the pK(a) of 4-nitrophenol. This confirms that the phenolyses of the thionocarbonates are stepwise processes, with the formation of an anionic tetrahedral intermediate. By the comparison of the kinetics and mechanisms of the title reactions with similar reactions, the following conclusions can be drawn: (i) Substitution of S(-) by O(-) in an anionic tetrahedral intermediate (T(-)) destabilizes it. (ii) The change of MeO by 4-nitrophenoxy in T(-) results in an increase of both the rate constant and equilibrium constant, for the formation of T(-), and also in an enlargement of the rate coefficient for the expulsion of 4-nitrophenoxide from T(-). (iii) Substitution of an amino group in a tetrahedral intermediate by ArO destabilizes it. (iv) Secondary alicyclic amines and other amines show greater reactivity toward MNPTOC than isobasic phenoxide anions.     

A solid-phase extraction procedure for the clean-up of thiram from aqueous solutions containing high concentrations of humic substances

A simple solid-phase extraction (SPE) procedure with an octadecyl bonded phase silica (C₁₈) was developed for clean-up of the fungicide thiram from aqueous solutions containing high concentrations of humic substances, for future studies of thiram adsorption onto solid humic substances or soils. Suspensions of humic acids and soil, in aqueous 0.01 M CaCl₂ solution, were prepared and used as samples. These extracts were spiked with thiram and immediately applied to a C₁₈-SPE cartridge. Thiram was eluted with chloroform and its concentration measured by spectrophotometry at 283 nm. Non-spiked aqueous extracts (blanks) and a control sample of thiram in 0.01 M CaCl₂ aqueous solution were also prepared and submitted to the same SPE procedure. The results show that humic substances are extensively retained by the C₁₈ cartridge but are not eluted with CHCl₃. Recoveries of 100-104% were obtained for thiram in the presence of humic substances. The SPE procedure described in this work is an efficient clean-up step to remove the interference of humic substances absorbance and to be coupled to any spectrophotometric or HPLC-UV method, usually used for thiram analysis in food extracts.     

Mono- and bimetallic chromium(III) complexes containing 2,3,5,6-tetrakis-(α-pyridyl)pyrazine as a tridentate ligand: synthesis and spectroscopic characterization

We have investigated the reactions between 2,3,5,6-tetrakis(α-pyridyl)pyrazine (TPP) and CrCl₃ · 6H₂O in 1:2 and 2.6:1 (Cr^III:TPP) stoichiometric ratios. The resulting products, [Cr(TPP)Cl₃] (1) and [(CrCl₃)₂-μ-TPP] (2), respectively, have been characterized by elemental and thermogravimetric analyses, molar conductivities, vibrational and electronic spectroscopy, and powder e.p.r. spectroscopy at room temperature. In both (1) and (2), TPP behaves as a tridentate ligand. Whereas (1) is a monometallic species with three uncoordinated nitrogen atoms, (2) is bimetallic with TPP as a bridging ligand.     

A study on the antipicornavirus activity of flavonoid compounds (flavones) by using quantum chemical and chemometric methods

The AM1 semiempirical method is employed to calculate a set of molecular properties (variables) of 45 flavone compounds with antipicornavirus activity, and 9 new flavone molecules are used for an activity prediction study. Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA), Stepwise Discriminant Analysis (SDA), and K-Nearest Neighbor (KNN) are employed in order to reduce dimensionality and investigate which subset of variables should be more effective for classifying the flavone compounds according to their degree of antipicornavirus activity. The PCA, HCA, SDA, and KNN methods showed that the variables MR (molar refractivity), B(9) (bond order between C(9) and C(10) atoms), and B(25) (bond order between C(11) and R(7) atoms) are important properties for the separation between active and inactive flavone compounds, and this fact reveals that electronic and steric effects are relevant when one is trying to understand the interaction between flavone compounds with antipicornavirus activity and the biological receptor. In the activity prediction study, using the PCA, HCA, SDA, and KNN methodologies, three of the 9 new flavone compounds studied were classified as potentially active against picornaviruses.     

Bianchi type-I solutions for modified Brans-Dicke cosmology

Cosmological dust solutions have been obtained in Bianchi type-I homogeneous space for Brans-Dicke modified theory by the condition of the cosmological parameter ().     

Spectrum of stationary electromagnetic radiation in a static gravitational field

From the equation of continuity, an equation is derived for the change in the spectrum of radiation stationary in a static gravitational field, as measured by local observers in their proper frame. It is shown that an isotropic solution always exists in which the energy density per unit frequency interval is ³ times an arbitrary function of .     

NMR spectroscopy study of 2-methylbenzoxazolium salts hydroxylation in DMSO-d6 solution

2-Methylbenzoxazolium salts showed an unexpected transformation at room temperature, promoted by residual water present in dimethyl sulfoxide. This transformation was followed up inside of an NMR sample tube using DMSO-d₆ as solvent, in a series of representative quaternary ammonium salts with carboxymethyl, ethyl, pentyl and decyl N-alkyl chains, resulting in an exclusive product. The corresponding benzothiazolium and benzoselenazolium salts presented a similar behavior after heating at a temperature of 50 °C with the addition of one drop of D₂O for the first of these two salts. This reaction could play a very important role in explaining certain secondary reactions occurring in the preparation of compounds where these salts are precursors, classically in cyanine dye synthesis. Four new benzoxazolols were characterized by NMR spectroscopy and HR FAB-MS.     

Imide--amide rearangement of cylic phosphorimidates: a mechanistic study

Studies aimed at the development of new synthetic pathways for the preparation of chiral cyclic oxaza and diaza phosphoramides suitable for use in asymmetric chemistry led us to the investigation of the imide -amide rearrangement of cyclic phosphorimidates. As a result of this work new types of oligomeric organophosphorus compounds, formed by a novel 1,4-addition type ring opening polymerisation, were identified. These compounds are the stable intermediates of the imide-amide rearrangement, which upon heating yield the previously reported rearranged product. A detailed study of the mechanism of the Lewis acid catalysed imide-amide rearrangement and stereochemical control of the final products is reported. As a result, the full mechanism was elucidated and evidence of retention of configuration at the rearranged carbon atom is presented. Substituent effects were rationalised based on molecular modelling calculations.     

Electrochemical study of interfacial composite nanostructures: polyelectrolyte/gold nanoparticle multilayers assembled on phospholipid/dextran sulfate monolayers at a liquid-liquid interface

The build up and electrochemical characterization of interfacial composite nanostructures containing a cationic polyelectrolyte and negatively charged mercaptosuccinic acid stabilized gold nanoparticles (AuNPs) is reported. The nanostructures were formed at the interface between two immiscible electrolyte solutions in which the organic phase is an immobilized 2-nitrophenyl octyl ether/PVC gel. The growth of the multilayer was verified with UV-vis spectra, and approximately a linear increase in UV-vis absorbance with increasing number of layers was observed. The interfacial capacitance of the multilayers was measured as a function of the potential and a theoretical model was developed to explain the results. The excellent agreement between theoretical and experimental capacitance curves allows us to conclude that nanocomposites behave similarly to polyelectrolyte multilayers, with the outmost layer determining the alternating sign of the outer surface charge density. Cyclic voltammograms were used to evaluate the transfer rate constant across the multilayers of a model drug, metoprolol, and the standard probe tetraethylammonium cation. The apparent rate constants were slightly larger than in other studies in the literature and decrease with the increasing number of layers.