Kinetics and mechanism of the benzenethiolysis of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates

The reactions of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates (DNPC and TNPC, respectively) and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates (DNPTC and TNPTC, respectively) with a series of benzenethiolate anions were subjected to a kinetic investigation in water, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). These reactions obey pseudo-first-order kinetics, under excess of benzenethiolate, and are first order in the latter reactant. However, comparable reactant concentrations were used in the reactions of 4-nitrobenzenethiolate anion with TNPC and TNPTC, which showed second-order kinetics. The nucleophilic rate constants are pH independent, except those for the reactions of TNPC with 4-methoxy- and pentafluorobenzenethiolates, and TNPTC with benzenethiolate and 4-chloro- and 3-chlorobenzenethiolates, which show acid dependence. The Br?nsted-type plots for the nucleophilic rate constants are linear with slopes beta = 0.9, 1.0, 0.9, and 0.9 for the reactions of DNPC, TNPC, DNPTC, and TNPTC, respectively. No break in the Br?nsted plot was found for the reactions of DNPC and DNPTC at pK(a) ca. 4.1 and 3.4, respectively, consistent with concerted mechanisms. TNPC is more reactive toward benzenethiolate anions than DNPC, and TNPTC more than DNPTC due to the better leaving groups involved. Comparison of the kinetic results obtained in this work with those for the concerted phenolysis of the same substrates shows that benzenethiolate anions are better nucleophiles toward carbonates than isobasic phenoxide anions. This is explained by Pearson's "hard and soft acids and bases" principle.     

Pore size distribution analysis of selected hexagonal mesoporous silicas by grand canonical Monte Carlo simulations

We combine here a regularization procedure with individual adsorption isotherms obtained from grand canonical Monte Carlo simulations in order to obtain reliable pore size distributions. The methodology is applied to two hexagonal high-ordered silica materials: SBA-15 and PHTS, synthesized in our laboratory. Feasible pore size distributions are calculated through an adaptable procedure of deconvolution over the adsorption integral equation, with two necessary inputs: the experimental adsorption data and individual adsorption isotherms, assuming the validity of the independent pore model. The application of the deconvolution procedure implies an adequate grid size evaluation (i.e., numbers of pores and relative pressures to be considered for the inversion, or kernel size), the fulfillment of the discret Picard condition, and the appropriate choice of the regularization parameter (L-curve criteria). Assuming cylindrical geometry for both porous materials, the same set of individual adsorption isotherms generated from molecular simulations can be used to construct the kernel to obtain the PSD of SBA-15 and PHTS. The PSD robustness is measured imposing random errors over the experimental data. Excellent agreement is found between the calculated and the experimental global adsorption isotherms for both materials. Molecular simulations provide new insights into the studied systems, pointing out the need of high-resolution isotherms to describe the presence of complementary microporosity in these materials.     

Interpretation of thorium bioassay data

This study is a comparison between bioassay data of thorium-exposed workers from two different facilities. The first of these facilities is a monazite sand extraction plant. Isotopic equilibrium between²³²Th and²²⁸Th was not observed in excreta samples of these workers. The second facility is a gas mantle factory. An isotopic equilibrium between²³²Th and²²⁸Th was observed in excreta samples. Whole body counter measurements have indicated a very low intake of thorium through inhalation. As the concentration of thorium in feces was very high we concluded that the main pathway of entrance of the nuclide was ingestion, mainly via contamination through dirty hands.The comparison between the bioassay results of workers from the two facilities shows that the lack of Th isotopic equilibrium observed in the excretion from the workers at the monazite sand plant possibly occurred due to an additional Th intake by ingestion of contaminated fresh food. This is presumably because²²⁸Ra is more efficiently taken up from the soil by plants, in comparison to²²⁸Th or²³²Th, and subsequently,²²⁸Th grows in from its immediate parent,²²⁸Ra.     

Characterization of polychloroprenes and cationically modified polychloroprenes by thermal dehydrochlorination

Thermal dehydrochlorination of polychloroprenes and modified polychloroprenes was studied and was found to be rapid, readily reproducible, and reliable for the characterization of these rubbers. The rates of thermal dehydrochlorination of polychloroprenes and modified polychloroprenes are first order at 190°C under nitrogen. Significantly, the plot of dehydrochlorination rate (V_HCl) versus dehydrochlorination extent (ζHCl) for polychloroprenes shows three regions. These data may be attributed to three dehydrochlorination regimes associated with four contributing repeat structures that have been identified by ¹³C-NMR spectroscopy: In contrast to polychloroprene, V_HCl vs. ζ_HCl plots for modified (cyclopentadienylated and grafted) polychloroprenes, i.e., polychloroprenes that do not contain allylic chlorines, exhibit only one region suggesting only one dehydrochlorination mechanism in these materials. The slopes of the V_HCl vs. ζ_HCl plots of the modified polychloroprenes are virtually identical. Moreover, the slopes of the V_HCl vs. ζ_HCl plots for modified polychloroprenes and that of the latest of the three regions for unmodified polychloroprene are indistinguishable. This similarity in the V_HCl vs. ζ_HCl curves suggests dehydrochlorination by a fundamentally similar mechanism in these materials, most likely one involving the ? CH₂? C(Cl)?CH? CH₂? structure common to all of them. Concentration of labile chlorines in polychloroprene may be estimated by the differences in the dehydrochlorination kinetics of polychloroprene and modified polychloroprenes.     

A gas chromatography quartz crystal microbalance for speciation of nitroaromatic compounds in landfill gas

A new method based on separation with gas chromatography and detection with a quartz crystal microbalance was used for quantification of nitrobenzene, 2-, 3- and 4-nitrotoluene in landfill gas. The analytical error, the analysis time and general analytical performance were identical for gas chromatography-mass spectrometry (GC-MS) and gas chromatography-quartz crystal microbalance (GC-QCM). GC-QCM constitutes a very inexpensive alternative to GC-MS for monitoring nitroaromatic compounds in landfill gas.     

Development and application of a capillary electrophoresis based method for the simultaneous screening of six antibiotics in spiked milk samples

An easy to use and low time consuming capillary electrophoresis (CE) method was developed and applied to the simultaneous determination of six antibiotics (ampicillin, amoxicillin, cloxacillin, penicillin, tetracycline and chloramphenicol) in spiked milk samples. Samples of milk were cleaned up by solid-phase extraction (with a C₁₈ cartridge) after protein precipitation. Analysis was performed by CE and results compared with the obtained via HPLC, both coupled to a UV-vis detector (210 nm). CE employed a 58.5 cm long fused-silica capillary (50 cm to detector), 75 μm i.d., a 2.7 × 10⁻² M KH₂PO₄, 4.3 × 10⁻² M Na₂B₄O₇ separation buffer, pH 8; an applied voltage of 18 kV; a hydrostatic injection of 0.5 psi during 3 s; and a run temperature of 25 °C. Under the described conditions, amoxicillin was not separated by HPLC, while CE was able to separate, and, therefore, allow detection. Regardless of amoxicillin, comparable results were obtained by HPLC and CE. The average recoveries of antibiotics, from milk fortified at 2.5 and 5 μg/mL, was over 72% with R.S.D.s within 5%. Recovery levels were essentially dictated by the used SPE cartridge.     

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.     

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.