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51.
Anthony J. Burke Elisabete da Palma Carreiro Nuno M.M. Moura Ana Isabel Rodrigues 《Journal of organometallic chemistry》2007,692(22):4863-4874
In our quest to find new ligands for highly stereoselective reactions, we tested a variety of chiral non-racemic pseudo-C2 symmetric bis-oxazolines derived from malonic acid containing an arylidene bridge unit (and appropriately termed Arylid-Box) in the catalytic asymmetric cyclopropanation (CAP) reaction of styrene and ethyl diazoacetate using between only 1-2 mol% of a Cu(I) pre-catalyst. Some very good e.e.s (up to 86%), were obtained. It was possible to isolate 10a′-[Cu(CH3CN)4]PF6 which existed as a bench stable solid that proved to be more efficient than the catalyst prepared in situ. Cu(I) pre-catalysts were used for the first time in the CAP reaction with the Isbut-Box ligands 13a and 13b and, although, the e.e.s were better for ligand 13a using these pre-catalysts, in the case of ligand 11b this was not the case. Spectroscopic studies (UV-Vis and 1H NMR) were carried out to gain an insight into the nature of the catalytic species at work so that the conditions could be optimised giving better results. Some theoretical studies were conducted to try to explain the better enantioselectivities obtained using Evans’ tert-Box-Cu(I) complex over our complex. 相似文献
52.
A Szanto de Toledo N Carlin N Added E M Szanto M G Munhoz J Takahashi A Suaide M M de Moura 《Pramana》1999,53(3):483-483
Considerable interest has been devoted to fusion reactions between light heavy ions specially between weakly bound ones, due
to the anomalous decrease of the fusion cross sections when compared to the total reaction cross section in the energy region
around the barrier [1–4].
While the exact nature of the process responsible for the fusion cross section limitation at barrier energies is still unclear,
this study shows an inhibition of the yield as the system mass decreases, resulting from the progressive increase of the barrier
height and decrease of the effective barrier radius [3]. Furthermore, extensive efforts have been made recently in the study
of energy-damped binary yields from light heavy-ion collisions [2,4]. Based on the substantial amount of data accumulated
so far, it is now generally accepted and supported by the transition state model [4], that the observed yields arise mostly
from a fusion-fission process. Data on complete fusion, fusion-fission and ‘elastic fission’ for the 9Be, 10,11B+10,11B; 16,17,18O + 10,11B; 19F+12C; 6,7Li+9Be, 12C reactions among others, are presented. For the loosely bound nuclei it was found that the severe fusion cross section limitation
is due to a low survival probability of the weakly bound nuclei until the instant of the collision [1]. 相似文献
53.
T. N. Castro Dantas T. Y. F. Vale A. A. Dantas Neto H. Scatena Jr. M. C. P. A. Moura 《Colloid and polymer science》2009,287(1):81-87
A sulfonate (2,4-sodium dissulfonate-5-n-pentadecylphenol) was synthesized from hydrogenated cardanol and the micellization study was carried out using temperature
and electrolyte concentration as variables. The adsorption parameters were obtained using surface tension data by the Frumkin
adsorption model and the Simplex Nelder–Mead method. Values of critical micelle concentration (cmc) and surface excess (Γ) were obtained in three different temperatures (303 K, 313 K, and 323 K) and two electrolyte concentrations (NaCl solution—0.1 M
and 0.25 M). It was verified that cmc decreased with increasing electrolyte concentration and temperature. The Gibbs free
energy showed that the micellization process was spontaneous for all studied systems and temperatures, and also that the presence
of several CH2 groups was significant for micelle formation. 相似文献
54.
Eufrânio N. da Silva Júnior Kelly C.G. de Moura Claudia J. Nascimento Antônio V. Pinto 《Tetrahedron letters》2009,50(14):1575-2418
The structure of ‘lapachol peroxide’ was published by Hooker in 1936; however, we recently discovered a mistake in the originally proposed structure; the correct structure, according to X-ray spectroscopy, is given here. 相似文献
55.
N. M. F. S. A. Cerqueira P. J. Gonzalez C. D. Brondino M. J. Romão C. C. Romão I. Moura J. J. G. Moura 《Journal of computational chemistry》2009,30(15):2466-2484
The catalytic mechanism of nitrate reduction by periplasmic nitrate reductases has been investigated using theoretical and computational means. We have found that the nitrate molecule binds to the active site with the Mo ion in the +6 oxidation state. Electron transfer to the active site occurs only in the proton‐electron transfer stage, where the MoV species plays an important role in catalysis. The presence of the sulfur atom in the molybdenum coordination sphere creates a pseudo‐dithiolene ligand that protects it from any direct attack from the solvent. Upon the nitrate binding there is a conformational rearrangement of this ring that allows the direct contact of the nitrate with MoVI ion. This rearrangement is stabilized by the conserved methionines Met141 and Met308. The reduction of nitrate into nitrite occurs in the second step of the mechanism where the two dimethyl‐dithiolene ligands have a key role in spreading the excess of negative charge near the Mo atom to make it available for the chemical reaction. The reaction involves the oxidation of the sulfur atoms and not of the molybdenum as previously suggested. The mechanism involves a molybdenum and sulfur‐based redox chemistry instead of the currently accepted redox chemistry based only on the Mo ion. The second part of the mechanism involves two protonation steps that are promoted by the presence of MoV species. MoVI intermediates might also be present in this stage depending on the availability of protons and electrons. Once the water molecule is generated only the MoVI species allow water molecule dissociation, and, the concomitant enzymatic turnover. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 相似文献
56.
Manuel A.V. Ribeiro da Silva Ana I.M.C. Lobo Ferreira Joana I.T.A. Cabral Ana Filipa L.O.M. Santos Ana Rita G. Moreno Tiago L.P. Galvão Inês M. Rocha Paula M.V. Fernandes Sílvia Q. Salgueiro Vanessa A.F. de Moura Isabel M.S.C. Oliveira Paula C. Cotelo Mariana R.A. Ribeiro 《The Journal of chemical thermodynamics》2009,41(9):984-991
57.
Fátima Duarte Freire C. Flávio Soares Aragão T. Flávio Accioly de Lima e Moura Fernanda Nervo Raffin 《Journal of Thermal Analysis and Calorimetry》2009,97(1):355-357
Chlorpropamide ((4-chloro-N-(propylamino)-carbonyl)-benzenesulfonamide) belongs to compounds having sulfonylurea group and is widely used as an oral
antidiabetic agent. In this work differential scanning calorimetry (DSC) was used during pre-formulation of chlorpropamide
tablets to determine the drug-excipients compatibility. The DSC curves of chlorpropamide and binary mixtures with excipients
(sodium croscarmellose, sodium lauryl sulfate, microcrystalline cellulose, magnesium stearate and calcium carbonate) showed
that chlorpropamide exhibited interaction with magnesium stearate and sodium lauryl sulfate. The binary mixtures of chlorpropamide–magnesium
stearate presented a single endothermic process at 96–108 °C and chlorpropamide–sodium lauryl sulfate showed a wide endotherm
at 99–120 °C. 相似文献
58.
We study the thermodynamics of the classical anisotropic antiferromagnetic Heisenberg model in a checkerboard lattice. The checkerboard lattice is distinguished from the antiferromagnetic square lattice (with coupling constant J) by the presence of a diagonal crossing (coupling constant ) in half of the sites. This lattice model is the direct analog of the three-dimensional pyrochlore lattice on a two-dimensional surface. Besides, we considered a single-ion anisotropy D that breaks the symmetry and contributes to planar spin fields. Since the model is two-dimensional endowed with an symmetry, a Berezinskii–Kosterlitz–Thouless (BKT) transition is expected to take place. We also investigated the BKT temperature as a function of the coupling constants and D. The problem is developed through a continuous representation given by the Nonlinear Sigma Model (NLSM). Computer simulations were also carried out, and the results were in accordance with the analytical model. 相似文献
59.
L. D. da Silva A. Ranciaro Neto J. L. L. dos Santos F. A. B. F. de Moura 《Brazilian Journal of Physics》2017,47(6):583-588
We consider the problem of a harmonic lattice in which masses’ distribution is a superposition of a random and a periodic distribution. Classical equations for the mass displacement and velocities are solved using a second-order Euler formalism. Energy flow was investigated on two distinct experiments: (i) We injected an initial wave-packet with energy E 0 and analyzed the dynamics of the resulting energy pulse; (ii) we pumped one of the sides of the lattice with a external signal and then we observed the propagation of the pulse until the other side of chain. Our calculations suggest that the diluted disordered mass distribution promotes energy dynamics at high frequency limit. 相似文献
60.
Sun Y Yang D Ye Q Williams M Moura JM Boada F Liang ZP Ho C 《Magnetic resonance imaging》2003,21(6):593-598
This paper addresses the problem of enhancing spatiotemporal resolution of ultra-small superparamagnetic iron oxide (USPIO)-enhanced dynamic MRI of rat kidneys. To alleviate the limited resolution problem of conventional full-scan Fourier imaging methods, we use a generalized series-based imaging scheme to reduce coverage of kappa-space. Experimental results demonstrate that the generalized series imaging method with basis functions constructed using two references (pre- and post-contrast) can reduce the number of phase encodings measured during the dynamic contrast wash-in process by a factor of 4 with a negligible or minimal loss of image quality. The method is expected to make 3D studies possible using USPIO-enhanced dynamic imaging of rat kidneys, and prove valuable for early detection of renal rejection after kidney transplantation. 相似文献