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21.
Historical records of short lived (140Ba, 131I, 103Ru and 95Zr+95Nb) and long-lived (137Cs and 90Sr) fission products by fallout measurements performed in Argentina since 1959 were analyzed in order to define the main characteristics of 137Cs fallout time evolution in the Nahuel Huapi National Park, Patagonia, Argentina. Sedimentary cores were sampled from Lake Nahuel Huapi and Lake Morenito, which are located within Nahuel Huapi National Park. 137Cs specific activity profiles were measured and 210Pb dating was performed in each core. The time evolution of 137Cs fallout shows different characteristics than records taken in the Northern Hemisphere.137Cs specific activity profiles of the cores studied reproduce the fallout time sequence observed in the historical records, and the chronology obtained shows excellent agreement with 210Pb dating. 相似文献
22.
García-Río L Leis JR Moreira JA Araujo E Norberto F Ribeiro L 《The Journal of organic chemistry》2003,68(11):4330-4337
A kinetic study was carried out on the hydrolysis of two N-nitrosoguanidines, 1-nitroso-1-methyl-3-tolylsulfonylguanidine (TSGNO) and 1-nitroso-1-methyl-3-benzoylguanidine (BCGNO). We observed an absence of buffer catalysis using H(2)PO(4)(-)/HPO(4)(2)(-), H(3)BO(3)/H(2)BO(3)(-), and HCO(3)(-)/CO(3)(2)(-) regulators and a complex dependency of the rate constant on the pH. We discovered the existence of three simultaneous reaction paths: spontaneous decomposition of the neutral form of the N-nitrosoguanidine, decomposition of the monoanion, and decomposition through the form of the dianion. The analysis of the kinetic data has allowed us to obtain the acidity constant for the formation of the monoanion of the N-nitrosoguanidine, with values of p = 11.5. The reaction rate for the process through the monoanion, k(2), decreases as the acidity increases. The application of the principle of nonperfect synchronization shows that the basicity and reactivity do not correlate when there exists a possibility of stabilization of the negative charge by resonance. This behavior is consistent with the mechanism E1cB whereby the stabler the negative charge, the slower the elimination reaction. When dealing with the case of the elimination through the neutral form we observe that the reaction rate increases together with the capacity of stabilization of the positive charge on the nitrogen atom adjacent to the imino group. For the reaction through the dianion we used a maximum value of k(3) = 10(10) s(-)(1) to estimate the value of p for the formation of the dianion of the N-nitrosoguanidine, obtaining values of p < 24. 相似文献
23.
Cyclohexene transformation was used as test reaction to study the effect of lanthanum exchange on the catalytic behavior of HY zeolite cracking catalysts. Lanthanum has an overall positive effect increasing both acitivity and stability of HY zeolite and maximum selectivity towards isomerization products is obtained at higher conversion.
HY. , , HY. .相似文献
24.
do Nascimento Filho I Schossler P Santos Freitas L Melecchi MI Rodrigues Vale MG Bastos Caramão E 《Journal of chromatography. A》2004,1027(1-2):167-170
In this work a simple method was described for selective extraction of benzoic acid from landfill leachate samples. The samples were submitted to solid-phase extraction (SPE) with XAD-4 resin as the stationary phase and ion-exchange chromatography (IEC) using the ion-exchange resin Amberlyst A-27. The instrumental analysis was performed by gas chromatography with mass spectrometric detection (GC-MSD). Benzoic acid was isolated, identified and quantified. The extraction process is rapid, simple and of low cost. It was also environmental friendly, that is, it was used a minimum amounts of hazardous organic solvents and produced also minimum quantities of residues. 相似文献
25.
Jarmila Duskova Jiri Labsky Ivana Cisarova Tereza Skalova Jan Dohnalek Jindrich Hasek 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(9):o563-o566
The asymmetric unit of the title compound, C11H5D16N2O2·0.33H2O, is formed by three crystallographically independent piperidin‐1‐yloxyl molecules and a molecule of water. The molecules are crosslinked by nine hydrogen bonds into layers parallel with the ac plane. The water molecule contributes to the stability of the low‐symmetry arrangement by four hydrogen bonds. 相似文献
26.
Ribeiro da Silva MA Gomes JR Ferreira AI 《The journal of physical chemistry. B》2005,109(27):13356-13362
The standard (p degrees = 0.1 MPa) molar enthalpies of formation of 2-, 3-, and 4-chloroaniline were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. The Calvet high-temperature vacuum sublimation technique was used to measure the enthalpies of vaporization or sublimation of the three isomers. These two thermodynamic parameters yielded the standard molar enthalpies of formation of the three isomers of chloroaniline, in the gaseous phase, at T = 298.15 K, as 53.4 +/- 3.1 kJ.mol(-1) for 2-chloroaniline, 53.0 +/- 2.8 kJ.mol(-1) for 3-chloroaniline, and 59.7 +/- 2.3 kJ.mol(-1) for 4-chloroaniline. These values, which correct previously published data, were used to test the computational methodologies used. Therewith, gas-phase acidities, proton affinities, electron donor capacities, and N-H bond dissociation enthalpies were calculated and found to compare well with available experimental data for these parameters. 相似文献
27.
Ionic dynamics in room temperature molten salts (ionic liquids) containing 1-alkyl-3-methylimidazolium cations is investigated by molecular-dynamics simulations. Calculations were performed with united atom models, which were used in a previous detailed study of the equilibrium structure of ionic liquids [S. M. Urahata and M. C. C. Ribeiro, J. Chem. Phys. 120, 1855 (2004)]. The models were used in a systematic study of the dependency of several single particle time correlation functions on anion size (F-, Cl-, Br-, and PF6-) and alkyl chain length (1-methyl-, 1-ethyl-, 1-butyl-, and 1-octyl-). Despite of large mass and size of imidazolium cations, they exhibit larger mean-square displacement than anions. A further detailed picture of ionic motions is obtained by using appropriate projections of displacements along the plane or perpendicular to the plane of the imidazolium ring. A clear anisotropy in ionic displacement is revealed, the motion on the ring plane and almost perpendicular to the 1-alkyl chain being the less hindered one. Similar projections were performed on velocity correlation functions, whose spectra were used to relate short time ionic rattling with the corresponding long time diffusive regime. Time correlation functions of cation reorientation and dihedral angles of the alkyl chains are discussed, the latter decaying much faster than the former. A comparative physical picture of time scales for distinct dynamical processes in ionic liquids is provided. 相似文献
28.
D. T. Ribeiro C. Madzak A. Sarasin P. DI Mascio H. Sies C. F. M. Menck † 《Photochemistry and photobiology》1992,55(1):39-45
The effects of singlet oxygen (1O2), generated by the thermal decomposition of water soluble NDPO2 (endoperoxide of the disodium 3,3'-(1,4-naphthylidene) dipropionate), on a single-stranded shuttle vector were analysed. 1O2 induces a much higher level of breaks in the phosphodiester backbone of single-stranded than double-stranded DNA. This may be due to a higher accessibility of guanine residue, primarily damaged by 1O2. The damaged vector was transfected into monkey COS7 cells where single-stranded DNA was converted to the double-stranded replicative form DNA. After 3 days, extrachromosomal DNA was extracted and the plasmids rescued in E. coli to study mutagenesis. There is a significant increase in mutation frequency of damaged single-stranded DNA in comparison to untreated DNA. It is concluded that 1O2 induces breaks in the backbone of single-stranded DNA and that the 1O2-damaged molecules are mutated after passage through mammalian cells. 相似文献
29.
30.
Eliane Przytyk Jung Beatriz Pereira de Freitas Claudete Norie Kunigami Davyson de Lima Moreira Natlia Guimares de Figueiredo Leilson de Oliveira Ribeiro Ricardo Felipe Alves Moreira 《Molecules (Basel, Switzerland)》2022,27(17)
This study aimed to evaluate Bauhinia forficata infusions prepared using samples available in Rio de Janeiro, Brazil. As such, infusions at 5% (w/v) of different brands and batches commercialized in the city (CS1, CS2, CS3, and CS4) and samples of plant material botanically identified (BS) were evaluated to determine their total phenolic and flavonoid contents (TPC and TFC), antioxidant capacity (ABTS•+, DPPH•, and FRAP assays), phytochemical profile, volatile compounds, and inhibitory effects against the α-amylase enzyme. The results showed that infusions prepared using BS samples had lower TPC, TFC and antioxidant potential than the commercial samples (p < 0.05). The batch averages presented high standard deviations mainly for the commercial samples, corroborating sample heterogeneity. Sample volatile fractions were mainly composed of terpenes (40 compounds identified). In the non-volatile fraction, 20 compounds were identified, with emphasis on the CS3 sample, which comprised most of the compounds, mainly flavonoid derivatives. PCA analysis demonstrated more chemical diversity in non-volatile than volatile compounds. The samples also inhibited the α-amylase enzyme (IC50 value: 0.235–0.801 mg RE/mL). Despite the differences observed in this work, B. forficata is recognized as a source of bioactive compounds that can increase the intake of antioxidant compounds by the population. 相似文献