Oxidative degradations of the side chain of unsaturated ent-labdanes. Part I

A selective route for the degradation of the unsaturated side chain of ent-labdanes has been devised, giving two useful synthons: 2beta-acetoxy-14,15,17-trinor-ent-labdane-8,13- dione (5) and 2beta-acetoxy-14,15-dinor-ent-labd-8(17)-en-13-one (7), the use of which for the preparation of terpenylquinone derivatives shall be reported elsewhere.     

Electrochemical and microsomal production of free radicals from 1,2,5-oxadiazole N-oxide as potential antiprotozoal drugs

The electron spin resonance (ESR) spectra of free radicals obtained by electrolytic or microsomal reduction of several potential antiprotozoal 1,2,5-oxadiazoles were characterized and analyzed. Ab initio molecular orbital calculations were performed to obtain the optimized geometries and the theoretical hyperfine constant was carried out using ZINDO semiempirical methodology. Density functional theory was used to rationalize the reduction potentials of these compounds.     

Dependence of the Lifetime upon the Excitation Energy and Intramolecular Energy Transfer Rates: The (5) D(0) Eu(III) Emission Case

In many Eu(III) -based materials, the presence of an intermediate energy level, such as ligand-to-metal charge transfer (LMCT) states or defects, that mediates the energy transfer mechanisms can strongly affect the lifetime of the (5) D(0) state, mainly at near-resonance (large transfer rates). We present results for the dependence of the (5) D(0) lifetime on the excitation wavelength for a wide class of Eu(III) -based compounds: ionic salts, polyoxometalates (POMs), core/shell inorganic nanoparticles (NPs) and nanotubes, coordination polymers, β-diketonate complexes, organic-inorganic hybrids, macro-mesocellular foams, functionalized mesoporous silica, and layered double hydroxides (LDHs). This yet unexplained behavior is successfully modelled by a coupled set of rate equations with seven states, in which the wavelength dependence is simulated by varying the intramolecular energy transfer rates. In addition, the simulations of the rate equations for four- and three-level systems show a strong dependence of the emission lifetime upon the excitation wavelength if near-resonant non-radiative energy transfer processes are present, indicating that the proposed scheme can be generalized to other trivalent lanthanide ions, as observed for Tb(III) /Ce(III) . Finally, the proper use of lifetime definition in the presence of energy transfer is emphasized.     

Response surface methodology for optimisation of 4,6-Dinitro-o-cresol quantification using hanging mercury drop electrode

A method for the pesticide DNOC (4,6-Dinitro-o-cresol) quantification using hanging mercury drop electrode (HMDE) and stripping square wave voltammetry (SSWV) optimisation is proposed. As a continuous cathodic current decrease was observed during the experiments, a waiting time, together with solution pH and SWV instrumental variables were optimised by factorial designs. From the two reduction current peaks values, only one was considered as dependent variable in the optimisation process. While the cathodic current peak and standard deviation were used in the waiting time and solution pH optimisation process, the instrumental parameters for SWV were optimised by using only current peak values. With the optimal parameters, a calibration curve from (0.01–0.55) × 10^?6 mol L^?1 with LOD of 2 × 10^?8 mol L^?1 was obtained. The proposed method was checked for DNOC quantification in different water samples obtained from Cordoba area and very good results, with recovery values around 102% were observed.     

Liquid-vapor isotherm in a closed single-component system with curved interfaces

A thermodynamic model to obtain the radius of bubbles or droplets in a single-component system for a given temperature, total volume, and phase distribution is developed. The general formulation of the model includes bubbles or droplets in the form of spheres, truncated spheres on a flat solid surface or inside conical walls. In these three geometries the liquid-vapor curvature radius is positive but in the case of conical walls it can be also negative. States with different dispersed-phase distributions are compared using the total free energy of the system. When the curvature radius is positive, it has a minimum nonvanishing value and the occurrence of the Ostwald ripening is energetically favorable. On the other hand, when the curvature radius is negative, it is energetically more favorable to find the dispersed phase even in the expected single-phase region, and the occurrence of an anti-ripening phenomenon. The PV isotherms obtained from the model and the applicability of the results to the nucleation process are discussed.     

The natural abundance of C with different agricultural management by NIRS with fibre optic probe technology

In the present study the natural abundance of ¹³C is quantified in agricultural soils in Mexico which have been submitted to different agronomic practices, zero and conventional tillage, retention of crop residues (with and without) and rotation of crops (wheat and maize) for 17 years, which have influenced the physical, chemical and biological characteristics of the soil. The natural abundance of C13 is quantified by near infrared spectra (NIRS) with a remote reflectance fibre optic probe, applying the probe directly to the soil samples.Discriminate partial least squares analysis of the near infrared spectra allowed to classify soils with and without residues, regardless of the type of tillage or rotation systems used with a prediction rate of 90% in the internal validation and 94% in the external validation. The NIRS calibration model using a modified partial least squares regression allowed to determine the δ¹³C in soils with or without residues, with multiple correlation coefficients 0.81 and standard error prediction 0.5‰ in soils with residues and 0.92 and 0.2‰ in soils without residues. The ratio performance deviation for the quantification of δ¹³C in soil was 2.5 in soil with residues and 3.8 without residues. This indicated that the model was adequate to determine the δ¹³C of unknown soils in the −16.2‰ to −20.4‰ range. The development of the NIR calibration permits analytic determinations of the values of δ¹³C in unknown agricultural soils in less time, employing a non-destructive method, by the application of the fibre optic probe of remote reflectance to the soil sample.