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91.
An efficient synthesis of the key building block for 17-epi-calctriol from the Hajos-Parrish dione involving a sequence of diastereoselective transformation of the azulene core and the side-chain construction is presented. 相似文献
92.
Woźnica M Butkiewicz A Grzywacz A Kowalska P Masnyk M Michalak K Luboradzki R Furche F Kruse H Grimme S Frelek J 《The Journal of organic chemistry》2011,76(9):3306-3319
In the present work, the validity of the helicity rule relating the absolute configuration of the bridgehead carbon atom in bicyclic β-lactams to the sign of the 220 nm band observed in their electronic circular dichroism (ECD) spectra is examined for ring-expanded cephalosporin analogues. To this end, a series of model compounds with a seven-membered ring condensed with the β-lactam unit was synthesized. A key step of their synthesis was either the ring-closing metathesis (RCM) or the free radical cyclization leading to the seven-membered ring with an S, O, or C atom at the 6 position in the bicyclic skeleton. To investigate the scope and limitations of the simple, empirically established helicity rule, a combination of ECD spectroscopy, variable-temperature ECD measurements, X-ray analysis, and time-dependent density functional theory (TD-DFT) calculations was used. A comparison of the experimental ECD spectra with the spectra simulated by TD-DFT calculations gives a reasonable interpretation of the Cotton effects observed in the 240-215 nm spectral range. The results suggest that the helicity rule does not apply to the investigated compounds because of the planarity of their amide chromophore. Thus, these compounds do not constitute an exception to the rule that was established for bi- and polycyclic β-lactams with the nonplanar amide chromophore only. 相似文献
93.
Zuzana Cibulková Andrea Černá Peter Šimon Ján Uhlár Karol Kosár Peter Lehocký 《Journal of Thermal Analysis and Calorimetry》2011,105(2):607-613
Effects of several heterocyclic compounds containing nitrogen, trisubstituted amines and diamines and molecules based on N,N,N′,N′,N′′,N′′-substituted-[1,3,5]triazine-2,4,6-triamine in preventing thermooxidative degradation of styrene–butadiene rubber (SBR)
have been studied using non-isothermal DSC measurements. The aim of this study was to determine and compare the stabilizing
effect of individual compounds and to select the structures with the best antioxidative properties. In order to compare the
stabilizing effect of the compounds, induction periods and protection factors have been calculated. The treatment of the experimental
data was carried out using a method based on a non-Arrhenian temperature function. The results show that better antioxidative
properties can be assigned to heterocyclic compounds. On the other hand, molecules containing triazine structures exhibited
only negligible stabilizing effect on SBR at lower temperatures and even destabilizing effect at higher temperatures. 相似文献
94.
Antognazza MR Scotognella F Miszta K Dorfs D Zanella M Zavelani-Rossi M Manna L Lanzani G Tassone F 《Physical chemistry chemical physics : PCCP》2011,13(33):15326-15330
Colloidal branched nanocrystals have been attracting increasing attention due to evidence of an interesting relationship between their complex shape and charge carrier dynamics. Herein, continuous wave photoinduced absorption (CW PIA) measurements of CdSe/CdS octapod-shaped nanocrystals are reported. CW PIA spectra show strong bleaching due to the one-dimensional (1D) CdS pod states (480 nm) and the zero-dimensional (0D) CdSe core states (690 nm). The agreement with previously reported ultrafast pump-probe experiments indicates that this strong bleaching signal may be assigned to state filling. Additional bleaching features at 520 and 560 nm are characterized by a longer lifetime and are thus ascribed to defect states, localized at the pod-core interface of the octapod, showing that some of the initially photogenerated carriers get quickly trapped into these long-lived defect states. However, we remark that a relevant part of electrons remain untrapped: this opens up the opportunity to exploit octapod shaped nanocrystals in photovoltaics applications, as electron acceptor materials, considering that several efficient hole extracting materials are already available for the realization of a composite bulk heterojunction. 相似文献
95.
Churchard AJ Banach E Borgschulte A Caputo R Chen JC Clary D Fijalkowski KJ Geerlings H Genova RV Grochala W Jaroń T Juanes-Marcos JC Kasemo B Kroes GJ Ljubić I Naujoks N Nørskov JK Olsen RA Pendolino F Remhof A Románszki L Tekin A Vegge T Zäch M Züttel A 《Physical chemistry chemical physics : PCCP》2011,13(38):16955-16972
The widespread adoption of hydrogen as an energy carrier could bring significant benefits, but only if a number of currently intractable problems can be overcome. Not the least of these is the problem of storage, particularly when aimed at use onboard light-vehicles. The aim of this overview is to look in depth at a number of areas linked by the recently concluded HYDROGEN research network, representing an intentionally multi-faceted selection with the goal of advancing the field on a number of fronts simultaneously. For the general reader we provide a concise outline of the main approaches to storing hydrogen before moving on to detailed reviews of recent research in the solid chemical storage of hydrogen, and so provide an entry point for the interested reader on these diverse topics. The subjects covered include: the mechanisms of Ti catalysis in alanates; the kinetics of the borohydrides and the resulting limitations; novel transition metal catalysts for use with complex hydrides; less common borohydrides; protic-hydridic stores; metal ammines and novel approaches to nano-confined metal hydrides. 相似文献
96.
Aequationes mathematicae - Given a probability space $$ (\Omega , {\mathcal {A}}, P) $$ , a complete and separable metric space X with the $$ \sigma $$ -algebra $$ {\mathcal {B}} $$ of all its... 相似文献
97.
Periodica Mathematica Hungarica - In this note we construct an algorithm generating any discrete distribution with an arbitrary coin (and, as a result, with arbitrary initial distribution). The... 相似文献
98.
Karol Jackowski Micha? Jaszuński W?odzimierz Makulski 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,127(2):139-143
New experimental and theoretical results are presented for the NMR shielding of oxygen in the carbonyl group. The experimental values clearly demonstrate that the solvent effects are very significant. The new results for the chemical shifts are in better agreement than the previous literature data with the corresponding ab initio values calculated for isolated molecules. 相似文献
99.
B. Kojić-Prodić M. Tonković O. Hadžija V. Kovač 《Analytical and bioanalytical chemistry》1976,279(5):359-362
For the cyanide determination in waste waters containing more than 2.5×10?6 Mol of CN?/l a conductometric method has been proposed. The sulphuric acid containing samples were distilled and the obtained HCN was passed into a 5×10?4 Mol/l AgNO3 solution, of which the conductivity was measured. The method offers the possibility of a precise observation of the completion of the HCN distillation and also the direct measurement of the cyanide concentration by the change of conductivity. The analysis time is about 40 min. 相似文献
100.
Gregor Kovačič Thomas A. Wettergren 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》1996,47(2):221-264
We present a plethora of homoclinic and heteroclinic orbits that exist in the phase space of a weakly nonlinear model describing small oscillations of two resonantly driven coupled pendula. These orbits connect equilibria in a resonance band, which is contained in an invariant plane, and is born under perturbation out of a circle of equilibria. Their existence is shown by using a combination of the Melnikov method and singular perturbation techniques. 相似文献