Thermochemical studies of phthalimide and two N-alkylsubstituted phthalimides (ALKYL=ETHYL AND n-PROPYL)

The standard (p⁰=0.1 MPa) molar enthalpies of formation, Δ_fH_m⁰, for crystalline phthalimides: phthalimide, N-ethylphthalimide and N-propylphthalimide were derived from the standard molar enthalpies of combustion, in oxygen, at the temperature 298.15 K, measured by static bomb-combustion calorimetry, as, respectively, – (318.0±1.7), – (350.1±2.7) and – (377.3±2.2) kJ mol^–1. The standard molar enthalpies of sublimation, Δ_cr^gH_m⁰, at T=298.15 K were derived by the Clausius-Clapeyron equation, from the temperature dependence of the vapour pressures for phthalimide, as (106.9±1.2) kJ mol^–1 and from high temperature Calvet microcalorimetry for phthalimide, N-ethylphthalimide and N-propylphthalimide as, respectively, (106.3±1.3), (91.0±1.2) and (98.2±1.4) kJ mol^–1. The derived standard molar enthalpies of formation, in the gaseous state, are analysed in terms of enthalpic increments and interpreted in terms of molecular structure.     

Estimation of precision and efficiency mass transfer steps for the determination of pesticides in vegetables aiming at the expression of results with reliable uncertainty

A 'bottom-up' approach for the expression of results obtained from analytical methods that include analytical steps with recovery inherently different from 100% [mass transfer steps (MTS): extraction, evaporation, clean-up procedures, digestion, etc.] is presented. The estimation of the combination of all MTS uncertainty involves the comparison of the experimental dispersion of replicated analyses of spiked samples with the estimation of the uncertainty obtained for the combination of all uncertainty sources except MTS ones ('incomplete' estimation). The estimation of MTS uncertainty by difference is performed after evaluating the statistical difference between the 'incomplete' estimation and the experimental dispersion (F-test). When the two estimations are statistically equivalent, the MTS uncertainty is considered to be negligible in relation to the other sources budget. The assumption of constancy of MTS performance within the analytical range is tested through single analyses at several concentration levels and is evaluated by the inclusion of the expected values at the intervals resulting from the combination of the MTS uncertainty estimation performed at one concentration level and the 'incomplete' estimation. The developed methodology can also be useful for method optimisation and validation and for the detection of small trends in results. The determination of pesticides in sweet peppers by GC-NPD was used to explore the above concepts.     

Chemometric interpretation of pesticide occurence in soil samples from an intensive horticulture area in north Portugal

An extensive monitoring programme of pesticides was carried out in soil samples from an intensive horticulture area in north of Portugal, putting into practice the needs for increased control of soil quality as far as organic pollution is concerned. The area under investigation was additionally defined as vulnerable to nitrates due to local soil and aquifer characteristics, which might be extended to pesticides contamination. Five sampling sites were selected and soils analysed at three depths in eight sampling campaigns, for the period of 2 years. A stepwise multivariate statistical approach was selected to uncover most relevant patterns inside a complex environmental data matrix. Cluster analysis was applied both to group pesticides and samples, giving a primary and unsupervised overlook of privileged relationships. Clusters of persistent pesticides and selected herbicides were identified, whereas sample classes were also formed and disposed geographically. Thirty eight percent of analysed soils samples fell into one class characterized by low contamination (class 1 in cluster analysis), which is entirely representative of the sampling site no. 1. Afterwards, linear discriminant analysis was applied to identify those pesticides, which had a higher impact in the definition of classes. Finally, factor analysis using a five component model was implemented in order to bring to light the constitution and data variance explained by each of the five main principal components, as well as, their relation to pest management practices. A factor was identified (PC1 – 22% variance) composed of chlorinated pesticides, which was representative of one of the investigated sites indicating its high contamination status. Qualitative main findings and class average concentration values were obtained through this multivariate statistical approach.     

A more convenient and general procedure for O-monobenzylation of diols via stannylenes: a critical reevaluation of the Bu2SnO method

A more consistent, straightforward, and economical protocol for generation of stannylene species and their reaction with BnBr leading to products of O-monobenzylation of diols has been set. It has shown to be specially indicated for substrates bearing vicinal trans 1,2-diol moieties on cyclohexane backbones, which are more resistant to these transformations. Such protocol has been successfully applied to myo-inositol derivatives and acyclic diols.     

A new production method for carrier-free201Tl using IEN'S cyclotron in Rio De Janeiro

A new and simplified method was developed for the separation of thallium 201 which will allow the shipment of a “one shot generator” to distant places and easy elution at the destination. Thallium is produced as usually, through the reaction $${}^{nat}Tl/p,xn/^{201} pb\xrightarrow{{\beta ^ + }}{}^{201}Tl$$ with 24-MeV protons from IEN'S CV-28 cyclotron. The separation is based on the properties of a chelating carboxylic acid cation exchange resin column which at pH 4.5 retains lead while thallim is easily eluted. This column can also be used as a regular generator to produced greater quantities of²⁰¹Tl but at the expense of more elutions.     

Mesomorphism,isomerization, and dynamics in a new series of pyramidic liquid crystals

Nona-alkanoyloxy tribenzocyclononene (CTV-n, where n is the number of carbons in the side chains) were prepared for n = 2 to 14. The homologues of this series appear in two stable isomeric forms, rigid crown and flexible saddle. We report on their isomerization equilibria and dynamics in solution and on their mesomorphic properties in the neat state. The crown-saddle equilibrium and interconversion kinetics of the CTV-8 isomers were studied in dimethyl formamide solutions using high-resolution (1)H NMR in the temperature range from 50 to 130 degrees C. At lower temperatures, the isomerization is too slow to measure. In this range the equilibrium saddle fraction increases from approximately 0.40 to approximately 0.65, whereas the isomerization rate increases from approximately 10(-)(4) to approximately 1 s(-)(1). The saddle isomer undergoes fast pseudorotation at room temperature, but below about -50 degrees C, it becomes slow enough to affect the NMR line width. The rate parameters for this process were estimated from the carbon-13 spectra in methylene chloride solutions to be, k(p)(-100 degrees C) approximately 1.7 x 10(3) s(-)(1) and E(a) approximately 9.6 kJ/mol. The slow crown-saddle isomerization at room temperature (half-life of about one year) allows quantitative separation (by chromatography) of the two isomers and their separate investigation. When the alkanoyloxy side chains are sufficiently long both isomers are mesogenic (n >or= 4 for the saddle and n >or= 5 for the crown), exhibiting hexagonal columnar mesophases. The structure, dynamics, and mesomorphic properties of these mesophase were investigated by X-ray diffraction, optical polarizing microscopy, differential scanning calorimetry, and NMR. The lattice parameters of the crown and saddle mesophases of corresponding homologues are almost identical and increase monotonically with increasing length of the side chains. The clearing temperatures of the saddle isomers are consistently lower than those of the corresponding crowns. Within each series, the clearing temperatures are almost independent of the length of the side chains (156 to 170 degrees C for the crown and 115 to 148 degrees C for the saddle). The thermal and kinetic properties of the neat compounds lead to peculiar phase sequences, as observed in the polarizing microscope and in the DSC thermogram, involving repeated, back and forth, interconversion between the two isomers. Carbon-13 MAS NMR measurements of the crown and saddle mesophases of several homologues were carried out. The spectra of the crown mesophase exhibit dynamic features consistent with planar 3-fold molecular jumps about the column axes. A quantitative analysis for the CTV-8 crown homologue yielded the following Arrhenius parameters, A = 3.1 x 10(22)s(-)(1) and E(a) = 130.1kJ/mol. These unusually high values suggest that the barrier to the jump process is temperature dependent, decreasing with increasing temperature. The rate of this 3-fold jump process is slower for the lower homologues and faster for the higher ones. In contrast, the saddle isomers in the mesophase do not show dynamic effects in their carbon-13 MAS spectra. They do not undergo pseudorotation, and it appears that the molecules remain locked within the columns in a saddle conformation, up to the clearing temperature. However, on (super-)cooling to room temperature and below, selective line broadening is observed in their carbon-13 MAS spectra. This suggests that the saddle conformation is twisted in the mesophase and undergoes fast high-amplitude jumps between the twisted forms. On cooling, these high-amplitude librations freeze out to give an orientationally disordered state. On a very long time scale (of the order of days at 100 degrees C), the saddle mesophase transforms into that of the crown, apparently by sublimation.     

Determination of copper in medicinal plants used as dietary supplements by atomic absorption spectrometry with direct flame solid analysis

An alternative device for the direct solid analysis (DSA) for copper determination by flame atomic absorption spectrometry (FAAS) is proposed. Copper was directly determined in commercial medicinal plants used as dietary supplements. The determination of copper in solid samples by DSA–FAAS was made by using a conventional air–acetylene flame. Between 0.05 and 1.5 mg of each test, sample was weighed directly into a small polyethylene vial connected to the device used for solid introduction into the flame. Test samples were introduced into the flame as a dry aerosol using a T-quartz cell set between the burner and the optical path. The T-quartz cell has a slit in the superior part by which the solid aerosol passes to the flame. A transient signal, evaluated as integrated absorbance, is produced and it is totally integrated in 2 s. Background signals always presented absorbance values less than 0.1. It was found a characteristic mass of 0.8 ng Cu and absolute limit of detection of 1.2 ng (3 s), or 1.2 μg g⁻¹ if a sample mass of 1 mg was used. Optimized conditions for air flow rate, flame stoichiometry, and so on were established as well. No excessive grinding of the samples was needed and samples with particle of size less than 80 μm were used throughout. No statistical difference between the results from the proposed system and those obtained by sample digestion and determination by conventional FAAS was observed. With the proposed procedure, more than 50 test samples can be analyzed in 1 h and it can be easily adapted to conventional spectrometers for FAAS.     

High-precision measurements of 17O/16O and 18O/16O of O2 and O2/Ar ratio in air

A method for high-precision and high-accuracy mass spectrometric measurements of the ratios among the three oxygen isotopes, and of the O(2)/Ar ratio, is presented. It involves separation of the O(2)-Ar mixture from air and includes a fully automated system that ensures highly reliable sample processing. Repeated measurements of atmospheric oxygen yield the repeatability (+/-SE x t, standard error of the mean (n = 12) multiplied by Student's t-factor for a 95% confidence limit) of 0.004, 0.003 and 0.2 per thousand for delta(18)O, delta(17)O and delta O(2)/Ar, respectively.     

预处理条件对Zn/Al-CLM的丙烷芳构化性能的影响

在脉冲微反装置上考察了不同的预处理条件对Zn/Al-CLM催化剂的丙烷芳构化反应的影响。结果表明，丙烷在Zn/Al-CLM催化剂上具有一定的的反应活性和芳烃选择性，而且芳构化主要转化为苯；载体Al-CLM的焙烧温度、金属锌负载量、活化温度等对Zn/Al-CLM催化剂的丙烷芳构化性能具有重要影响。载体经500℃预焙烧制备的Zn/Al-CLM催化剂具有最好的柱结构保留度，从而表现出最佳的芳构化性能；随着锌含量的增加，Zn/Al-CLM催化剂的酸量增大，从而使丙烷转化率增大，而选择性则是锌含量质量分数为5．8％时具有极大值；400℃活化处理可使Zn与Al-CLM之间具有适中的相互作用，从而使得Zn/Al-CLM具有较高的芳烃收率。     

Chemical synthesis and firefly luciferase produced dehydroluciferyl-coenzyme A

Dehydroluciferyl-coenzyme A (L-CoA) was chemically synthesized and characterized by MS, UV-vis spectrometry and RP-HPLC. The identity of the chemically synthesized compound with the one that was produced by firefly luciferase was confirmed. Moreover, the reversibility of the enzymatic conversion of dehydroluciferin ? dehydroluciferyl-adenylate ? L-CoA was also confirmed. The chemical synthesis of L-CoA, described here, may help the clarification of the activator effect of CoA on luciferase bioluminescent assays, in which the enzyme catalyzed formation of L-CoA and the consequent destruction of L-AMP is one of the possible explanations for that effect.