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841.
Dijana Pešić Ivana Ozimec Landek Ana Čikoš Biserka Metelko Vesna Gabelica Barbara Stanić Mladen Merćep Milan Mesić 《Journal of heterocyclic chemistry》2007,44(5):1129-1133
842.
Pace A Pibiri I Piccionello AP Buscemi S Vivona N Barone G 《The Journal of organic chemistry》2007,72(20):7656-7666
The experimental investigation of the base-catalyzed rearrangements of 3-acylamino-1,2,4-oxadiazoles evidenced a new reaction pathway which competes with the well-known ring-degenerate Boulton-Katritzky rearrangement (BKR). The new reaction consists of a one-atom side-chain rearrangement that is base-activated, occurs at a higher temperature than the BKR, and irreversibly leads to the corresponding 2-acylamino-1,3,4-oxadiazoles. An extensive DFT study is reported to elucidate the proposed reaction mechanism and to compare the three possible inherent routes: (i) the reversible three-atom side-chain ring-degenerate BKR, (ii) the ring contraction-ring expansion route (RCRE), and (iii) the one-atom side-chain rearrangement. The results of the computational investigation point out that the latter route is kinetically preferred over the RCRE and can be considered as the ground-state analogue of a previously proposed C(3)-N(2) migration-nucleophilic attack-cyclization (MNAC) photochemically activated pathway. The MNAC consists of the formation of a diazirine intermediate, involving the exocyclic nitrogen, that eventually evolves into a carbodiimide intermediate (migration); the latter undergoes a single intramolecular nucleophilic attack-cyclization step leading to the final 2-acylamino-1,3,4-oxadiazole. 相似文献
843.
Ivana Slamić 《Journal of Fourier Analysis and Applications》2014,20(4):766-783
Various properties of the system \({\mathcal {B}}_{\psi }\) of integer translates of a square integrable function \(\psi \in L^2({\mathbb {R}})\) can be completely described in terms of the periodization function \(p_{\psi }(\xi )=\sum _{k\in {\mathbb {Z}}}|\widehat{\psi }(\xi +k)|^2\) . In this paper, we consider the problem of \(\ell ^p\) -linear independence, where \(p>2\) . The results we present include the method of construction for one type of counterexamples to several naturally taken conjectures, a new sufficient condition for \(\ell ^p\) -linear independence and a characterization theorem having an additional assumption on \({\mathcal {B}}_{\psi }\) . In the latter, we obtain the characterization in terms of the sets of multiplicity of Lebesgue measure zero. 相似文献
844.
845.
Vector potentials are known to have a direct significance to quantum particles moving in the magnetic field. This is called
the Aharonov–Bohm effect and is known as one of the most remarkable quantum phenomena. Here we study this quantum effect through
the resonance problem. We consider the scattering system consisting of two scalar potentials and one magnetic field with supports
at large separation in two dimensions. The system has trajectories oscillating between these supports. We give a sharp lower
bound on the resonance widths as the distances between the three supports go to infinity. The bound is described in terms
of the backward amplitude for scattering by each of the scalar potentials and by the magnetic field, and it also depends heavily
on the magnetic flux of the field. 相似文献
846.
Aleksandra Č. Mišan Neda M. Mimica-Dukić Anamarija I. Mandić Marijana B. Sakač Ivan Lj. Milovanović Ivana J. Sedej 《Central European Journal of Chemistry》2011,9(1):133-142
Rapid Resolution HPLC/DAD method, on a 1.8 μm, 4.6×50 mm column, was developed to enable a rapid separation of a mixture of
17 compounds, which consisted of hydroxybenzoic acids, hydroxycinnamic acids, flavones, flavonols, flavanone, flavonol-glycoside
and antraquinone, in a single run, within 22 minutes. The developed method is precise, accurate and sensitive enough for simultaneous
quantitative evaluation of major compounds in crude and hydrolyzed extracts of parsley, buckthorn, mint, caraway and birch.
In order to overcome the inability to quantify all the phenolic compounds present in the samples caused by lack of external
standards, HPLC approaches for the total phenolic content estimation based on sum of all integrated peak areas were made.
These results were compared with the total phenolic content determined by Folin-Cioacalteu method. Although the correlation
between the series of data was not significant (p<0.05), the difference between the results of total phenolic content obtained spectrophotometrically and by HPLC was not high
in the case of parsley, buckthorn and mint extract. Regarding the obtained results, the HPLC approach could serve as an excellent
tool for total phenolic content estimation, without the need for complete identification of the individual compounds.
相似文献
847.
1-D and 2-D comprehensive (LC×LC) liquid chromatography methods have been developed and compared for the separation and quantification of flavanones in various Citrus juices. 1-D analyses were carried out on a superficially porous C18 column, whereas the 2-D LC approach was composed of a polyethylene glycol silica narrow-bore column packed with totally porous particles in the first dimension (D1) and a superficially porous C18 column in the second dimension (D2). Low-selectivity correlations were ensured by the complementary separation mechanisms offered by the D1 and D2 columns. Quantification was carried out both manually and by means of a software capable of detecting and quantifying each peak from the 2-D plot. Limit of detection (LOD) values as low as 0.023 μg/mL were obtained for hesperidin used as reference material for 1-D LC analyses, whereas values as high as 0.432 μg/mL were obtained by comprehensive LC. This discrepancy can be traced back to the minor sensitivity experienced in comprehensive LC due to both sample dilution in D1 and the high flow rates employed in D2. On the other hand, the separation capabilities of the LC×LC approach allowed to reduce the interferences coming from the matrix and to achieve the separation of some critical pairs, e.g. hesperidin/naringin difficult to accomplish in 1-D LC. 相似文献
848.
Two flavonoids, rutin and hesperidin, were investigated in vitro for anticoagulant activity through coagulation tests: activated partial thromboplastin time (aPTT), prothrombin time (PT) and thrombin time (TT). Only an ethanolic solution of rutin at the concentration of 830 μM prolonged aPTT, while TT and PT were unaffected. In order to evaluate whether the prolongation of aPTT was due to the decrease of coagulation factors, the experiment with deficient plasma was performed, showing the effects on factors VIII and IX. Since pharmacological activity of flavonoids is believed to increase when they are coordinated with metal ions, complexes of these flavonoids with Al(III) and Cu(II) ions were also tested. The results showed that complexes significantly prolonged aPTT and had no effects on PT and TT. Assay with deficient plasma (plasma having the investigated factor at less then 1%) revealed that complexes could bind to the coagulation factors, what may lead to a non-specific inhibition and aPTT prolongation. An effort was made to correlate stability of complexes with their anticoagulant properties. 相似文献
849.
To address fundamental questions in bioinorganic chemistry, such as metal ion selectivity, accurate computational protocols for both the gas-phase association of metal-ligand complexes and solvation/desolvation energies of the species involved are needed. In this work, we attempt to critically evaluate the performance of the ab initio and DFT electronic structure methods available and recent solvation models in calculations of the energetics associated with metal ion complexation. On the example of five model complexes ([M(II)(CH(3)S)(H(2)O)](+), [M(II)(H(2)O)(2)(H(2)S)(NH(3))](2+), [M(II)(CH(3)S)(NH(3))(H(2)O)(CH(3)COO)], [M(II)(H(2)O)(3)(SH)(CH(3)COO)(Im)], [M(II)(H(2)S)(H(2)O)(CH(3)COO)(PhOH)(Im)](+) in typical coordination geometries) and four metal ions (Fe(2+), Cu(2+), Zn(2+), and Cd(2+); representing open- and closed-shell and the first- and second-row transition metal elements), we provide reference values for the gas-phase complexation energies, as presumably obtained using the CCSD(T)/aug-cc-pVTZ method, and compare them with cheaper methods, such as DFT and RI-MP2, that can be used for large-scale calculations. We also discuss two possible definitions of interaction energies underlying the theoretically predicted metal-ion selectivity and the effect of geometry optimization on these values. Finally, popular solvation models, such as COSMO-RS and SMD, are used to demonstrate whether quantum chemical calculations can provide the overall free enthalpy (ΔG) changes in the range of the expected experimental values for the model complexes or match the experimental stability constants in the case of three complexes for which the experimental data exist. The data presented highlight several intricacies in the theoretical predictions of the experimental stability constants: the covalent character of some metal-ligand bonds (e.g., Cu(II)-thiolate) causing larger errors in the gas-phase complexation energies, inaccuracies in the treatment of solvation of the charged species, and difficulties in the definition of the reference state for Jahn-Teller unstable systems (e.g., [Cu(H(2)O)(6)](2+)). Although the agreement between the experimental (as derived from the stability constants) and calculated values is often within 5 kcal·mol(-1), in more complicated cases, it may exceed 15 kcal·mol(-1). Therefore, extreme caution must be exercised in assessing the subtle issues of metal ion selectivity quantitatively. 相似文献
850.
We study the quantum resonances in magnetic scattering in two dimensions. The scattering system consists of two obstacles by which the magnetic fields are completely shielded. The trajectories trapped between the two obstacles are shown to generate the resonances near the positive real axis, when the distance between the obstacles goes to infinity. The location is described in terms of the backward amplitudes for scattering by each obstacle. A difficulty arises from the fact that even if the supports of the magnetic fields are largely separated from each other, the corresponding vector potentials are not expected to be well separated. To overcome this, we make use of a gauge transformation and develop a new type of complex scaling method. We can cover the scattering by two solenoids at large separation as a special case. The obtained result heavily depends on the magnetic fluxes of the solenoids. This indicates that the Aharonov–Bohm effect influences the location of resonances. 相似文献