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991.
Facile amine formation by intermolecular catalytic amidation of carbon-hydrogen bonds 总被引:2,自引:0,他引:2
Fructos MR Trofimenko S Díaz-Requejo MM Pérez PJ 《Journal of the American Chemical Society》2006,128(36):11784-11791
A simple copper-based catalytic system has been developed for the carbon-hydrogen amidation reaction. The copper-homoscorpionate complex Tp(Br3)Cu(NCMe) catalyzes the transfer of the nitrene unit NTs (Ts = p-toluenesulfonyl) and its subsequent insertion into the sp(3) C-H bonds of alkyl aromatic and cyclic ethers or the sp(2) C-H bonds of benzene using PhI=NTs as the nitrene source, affording the corresponding trisubstitued NR(1)HTs amines in moderate to high yields. The use of the environmentally friendly chloramine-T has also proven effective, with the advantage that sodium chloride is formed as the only byproduct. A tandem, one-pot consecutive nitrene-carbene insertion system has been developed to yield amino acid derivatives. 相似文献
992.
993.
Ultrasonic slurry sampling electrothermal vaporisation inductively coupled plasma mass spectrometry (USS-ETV-ICP-MS) was applied to the elemental analysis of silicate based minerals, such as talc or quartz, without any pre-treatment except the grinding of the sample. The electrothermal vaporisation device consists of a tungsten coil connected to a home-made power supply. The voltage program, carrier gas flow rate and sonication time were optimised in order to obtain the best sensitivity for elements determined. The relationship between the amount of sample in the slurry and the signal intensity was also evaluated. Unfortunately, in all cases, quantification had to be carried out by the standard additions method owing to the strong matrix interferences. The global precision of the proposed method was always better than 12%. The limits of detection, calculated as three times the standard deviation of the blank value divided by the slope of the calibration curve, were between 0.5 ng/g for As and 3.5 ng/g for Ba. The method was validated by comparing the concentrations found for Cu, Mn, Cr, V, Li, Pb, Sn, Mg, U, Ba, Sr, Zn, Sb, Rb and Ce using the proposed methodology with those obtained by conventional nebulisation ICP-MS after acid digestion of the samples in a microwave oven. The concentration range in the solid samples was between 0.2 μg/g for Cr and 60 μg/g for Ba. All results were statistically in agreement with those found by conventional nebulisation. 相似文献
994.
Juan M. Aceves-Hernández Esther Agacino-Valdés Manuel Paz 《Journal of Molecular Structure》2006,786(1):1-8
Hydrochlorothiazide was characterized in order to determine the possible structural modifications at different temperatures due to its importance as a drug to control heart diseases and a diuretic. This compound could present conformers due to the rotation of the NH2 group, which was studied by using different techniques such as Thermal Analysis, IR spectroscopy, X-Ray Diffraction and complemented by theoretical calculations. The theoretical and experimental results point to the conclusion that no polymorphic forms are present in the compound under study. The calculations confirm the apparent difference in values between theory and experiment for the vibration frequencies explained by the hydrogen bonds between near molecules. 相似文献
995.
Briselli E Garreffa G Bianchi L Bianciardi M Macaluso E Abbafati M Grazia Marciani M Maraviglia B 《Magnetic resonance imaging》2006,24(4):393-400
Interest about simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) data acquisition has rapidly increased during the last years because of the possibility that the combined method offers to join temporal and spatial resolution, providing in this way a powerful tool to investigate spontaneous and evoked brain activities. However, several intrinsic features of MRI scanning become sources of artifacts on EEG data. Noise sources of a highly predictable nature such as those related to the pulse MRI sequence and those determined by magnetic gradient switching during scanning do not represent a major problem and can be easily removed. On the contrary, the ballistocardiogram (BCG) artifact, a large signal visible on all EEG traces and related to cardiac activity inside the magnetic field, is determined by sources that are not fully stereotyped and causing important limitations in the use of artifact-removing strategies. Recently, it has been proposed to use independent component analysis (ICA) to remove BCG artifact from EEG signals. ICA is a statistical algorithm that allows blind separation of statistically independent sources when the only available information is represented by their linear combination. An important drawback with most ICA algorithms is that they exhibit a stochastic behavior: each run yields slightly different results such that the reliability of the estimated sources is difficult to assess. In this preliminary report, we present a method based on running the FastICA algorithm many times with slightly different initial conditions. Clustering structure in the signal space of the obtained components provides us with a new way to assess the reliability of the estimated sources. 相似文献
996.
997.
A recent study evaluates auditory brainstem responses (ABRs) evoked by chirps of different durations (sweeping rates) [Elberling et al. (2010). J. Acoust. Soc. Am. 128, 215-223]. The study demonstrates that shorter chirps are most efficient at higher levels of stimulation whereas longer chirps are most efficient at lower levels. Mechanisms other than the traveling wave delay, in particular, upward spread of excitation and changes in cochlear-neural delay with level, are suggested to be responsible for these findings. As a consequence, delay models based on estimates of the traveling wave delay are insufficient for the design of chirp stimuli, and another delay model based on a direct approach is therefore proposed. The direct approach uses ABR-latencies from normal-hearing subjects in response to octave-band chirps over a wide range of levels. The octave-band chirps are constructed by decomposing a broad-band chirp, and constitute a subset of the chirp. The delay compensations of the proposed model are similar to those found in the previous experimental study, which thus verifies the results of the proposed model. 相似文献
998.
Luciano Gonç alves Noleto Manuel N. D. Barcelos Jr. & Antonio C. P. Brasil Jr. 《advances in applied mathematics and mechanics.》2013,5(2):194-211
This work presents a moving mesh methodology based on the solution
of a pseudo flow problem. The mesh motion is modeled as a pseudo Stokes
problem solved by an explicit finite element projection method. The mesh
quality requirements are satisfied by employing a null divergent velocity
condition. This methodology is applied to triangular unstructured meshes
and compared to well known approaches such as the ones based on diffusion
and pseudo structural problems. One of the test cases is an airfoil with
a fully meshed domain. A specific rotation velocity is imposed as the
airfoil boundary condition. The other test is a set of two cylinders
that move toward each other. A mesh quality criterion is employed to
identify critically distorted elements and to evaluate the performance
of each mesh motion approach. The results obtained for each test case
show that the pseudo-flow methodology produces satisfactory meshes
during the moving process. 相似文献
999.
1000.
Otti D’Huys Javier Rodríguez-Laguna Manuel Jiménez Elka Korutcheva Wolfgang Kinzel 《The European physical journal. Special topics》2018,227(10-11):1129-1150
We study the dynamics of networks with coupling delay, from which the connectivity changes over time. The synchronization properties are shown to depend on the interplay of three time scales: the internal time scale of the dynamics, the coupling delay along the network links and time scale at which the topology changes. Concentrating on a linearized model, we develop an analytical theory for the stability of a synchronized solution. In two limit cases, the system can be reduced to an “effective” topology: in the fast switching approximation, when the network fluctuations are much faster than the internal time scale and the coupling delay, the effective network topology is the arithmetic mean over the different topologies. In the slow network limit, when the network fluctuation time scale is equal to the coupling delay, the effective adjacency matrix is the geometric mean over the adjacency matrices of the different topologies. In the intermediate regime, the system shows a sensitive dependence on the ratio of time scales, and on the specific topologies, reproduced as well by numerical simulations. Our results are shown to describe the synchronization properties of fluctuating networks of delay-coupled chaotic maps. 相似文献