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991.
José Barluenga Pablo Barrio Rubén Vicente Luis A. López Miguel Tomás 《Journal of organometallic chemistry》2004,689(23):3793-3799
Fischer carbene complexes 1 underwent a clean ligand dimerization reaction yielding functionalized olefins and trienes 4 in the presence of copper (I) catalysts. If treated with trifluoroacetic acid (TFA), trienes 4c, d, f undergo a cyclization process (Nazarov reaction) which furnishes cyclopentenone derivatives 6c, d, 7c, d and 8 in good yields. Finally, the Fischer aminocarbene 9 efficiently cyclodimerizes to the substituted arene 10 in the presence of CuBr. 相似文献
992.
The characters, dynamics, and relaxation pathways of low-lying excited states of the complexes [W(CO)(5)L] [L = 4-cyanopyridine (pyCN) and piperidine (pip)] were investigated using theoretical and spectroscopic methods. DFT calculations revealed the delocalized character of chemically and spectroscopicaly relevant molecular orbitals and the presence of a low-lying manifold of CO pi-based unoccupied molecular orbitals. Traditional ligand-field arguments are not applicable. The lowest excited states of [W(CO)(5)(pyCN)] are W --> pyCN MLCT in character. They are closely followed in energy by W --> CO MLCT states. Excitation at 400 or 500 nm populates the (3)MLCT(pyCN) excited state, which was characterized by picosecond time-resolved IR and resonance Raman spectroscopy. Excited-state vibrations were assigned using DFT calculations. The (3)MLCT(pyCN) excited state is initially formed highly excited in low-frequency vibrations which cool with time constants between 1 and 20 ps, depending on the excitation wavelength, solvent, and particular high-frequency nu(CO) or nu(CN) mode. The lowest excited states of [W(CO)(5)(pip)] are W --> CO MLCT, as revealed by TD-DFT interpretation of a nanosecond time-resolved IR spectrum that was measured earlier in a low-temperature glass (Johnson, F. P. A.; George, M. W.; Morrison, S. L.; Turner, J. J. J. Chem. Soc., Chem. Commun. 1995, 391-393). MLCT(CO) excitation involves transfer of electron density from the W atom and, to a lesser extent, the trans CO to the pi orbitals of the four cis CO ligands. Optical excitation into MLCT(CO) transition of either complex in fluid solution triggers femtosecond dissociation of a W-N bond, producing [W(CO)(5)(solvent)]. It is initially vibrationally excited both in nu(CO) and anharmonicaly coupled low-frequency modes. Vibrational cooling occurs with time constants of 16-22 ps while the intramolecular vibrational energy redistribution from the v = 1 nu(CO) modes is much slower, 160-220 ps. No LF excited states have been found for the complexes studied in a spectroscopically relevant range up to 6-7 eV. It follows that spectroscopy, photophysics, and photochemistry of [W(CO)(5)L] and related complexes are well described by an interplay of close-lying MLCT(L) and MLCT(CO) excited states. The high-lying LF states play only an indirect photochemical role by modifying potential energy curves of MLCT(CO) states, making them dissociative. 相似文献
993.
Avenoza A Busto JH Canal N Peregrina JM 《Chemical communications (Cambridge, England)》2003,(12):1376-1377
The reactivity of 2-acylaminoacrylates with ketene diethyl acetal can be modulated by means of thermal conditions to yield cyclobutanes for the preparation of protected beta-hydroxycyclobutane-alpha-amino acids, or catalytic conditions that yield cyclohexanes by tandem condensations to obtain interesting building blocks that are alternatives to Danishefsky's diene. 相似文献
994.
This paper describes a study of the different methods of sample preparation for the determination of calcium in grape juice, wines, and other alcoholic beverages by flame atomic absorption spectrometry; results are also reported for the practical application of these methods to the analysis of commercial samples produced in Spain. The methods tested included dealcoholization, dry mineralization, and wet mineralization with heating by using different acids and/or mixtures of acids. The sensitivity, detection limit, accuracy, precision, and selectiviy of each method were established. Such research is necessary because of the better analytical indexes obtained after acid digestion of the sample, as recommended by the European Union, which advocates the direct method. In addition, although high-temperature mineralization with an HNO3-HCIO4 mixture gave the best analytical results, mineralization with nitric acid at 80 degrees C for 15 min gave the most satisfactory results in all cases, including those for wines with high levels of sugar and beverages with high alcoholic content. The results for table wines subjected to the latter treatment had an accuracy of 98.70-99.90%, a relative standard deviation of 2.46%, a detection limit of 19.0 microg/L, and a determination limit of 31.7 microg/L. The method was found to be sufficiently sensitive and selective. It was applied to the determination of Ca in grape juice, different types of wines, and beverages with high alcoholic content, all of which are produced and widely consumed in Spain. The values obtained for Ca were 90.00 +/- 20.40 mg/L in the grape juices, 82.30 +/- 23.80 mg/L in the white wines, 85.00 +/- 30.25 mg/L in the sweet wines, 84.92 +/- 23.11 mg/L in the red wines, 85.75 +/- 27.65 mg/L in the rosé wines, 9.51 +/- 6.65 mg/L in the brandies, 11.53 +/- 6.55 mg/L in the gin, 7.3 +/- 6.32 mg/L in the pacharán, and 8.41 +/- 4.85 mg/L in the anisettes. The method is therefore useful for routine analysis in the quality control of these beverages. 相似文献
995.
Arenas-Alatorre J Gómez-Cortés A Avalos-Borja M Díaz G 《The journal of physical chemistry. B》2005,109(6):2371-2376
The surface properties of bimetallic Ni-Pt/SiO2 catalysts with variable Ni/Ni + Pt atomic ratio (0.75, 0.50, and 0.25) were studied using N2O decomposition and N2O reduction by hydrogen reactions as probes. Catalysts were prepared by incipient wetness impregnation of the silica support with aqueous solutions of the metal precursors to a total metal loading of 2 wt %. For both model reactions, Pt/SiO2 catalyst was substantially more active than Ni/SiO2 catalyst. Mean particle size by TEM was about the same (in the range 6-8 nm) for all catalysts and truly bimetallic particles (more than 95%) were evidenced by EDS in the Ni-Pt/SiO2 catalysts. CO adsorption on the bimetallic catalysts showed differences in the linear CO absorption band as a function of the Ni/Pt atomic ratio. Bimetallic Ni-Pt/SiO2 catalysts showed, for the N2O decomposition, a catalytic behavior that points out an ensemble-size sensitive behavior for Ni-rich compositions. For the N2O + H2 reaction, the bimetallic catalysts were very active at low temperature. The following activity order at 300 K was observed: Ni75Pt25 > Ni25Pt75 approximately Ni50Pt50 > Pt. TOF values for these catalysts increased 2-5 times compared to the most active reference catalyst (Pt/SiO2). The enhancement of the activity in the Ni75Pt25 bimetallic catalysts is explained in terms of the presence of mixed Ni-Pt ensembles. 相似文献
996.
Strong pulse sequences can be used to control the position and width of the molecular wave packet. In this paper we propose a new scheme to maximally compress the wave packet in a quasistatic way by freezing it at a peculiar adiabatic potential shaped by two laser pulses. The dynamic principles of the scheme and the characteristic effect of the different control parameters are presented and analyzed. We use two different molecular models, electronic potentials modeled by harmonic oscillators, with the same force constants, and the Na(2) dimer, to show the typical yield that can be obtained in compressing the initial (minimum width) molecular wave function. 相似文献
997.
Thorsteinn Loftsson Hákon H. Sigurdsson Dagný Hreinsdóttir Fífa Konrádsdóttir Einar Stefánsson 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):585-589
Due to anatomic barriers and lacrimal drainage it is difficult to obtain therapeutic drug concentrations in the posterior part of the eye after topical drug administrations. Lipophilic cyclodextrins, such as randomly methylated β-cyclodextrin (RMβCD), are known to act both as solubilizers of water-insoluble drugs in aqueous solutions and as penetration enhancers that reduce the barrier function of lipophilic membranes. The purpose of this study was to investigate the effects of RMβCD on dexamethasone delivery from aqueous eye drop solution into rabbit eyes. Dexamethasone (0.5 and 1.5% w/v) drops (50 μl) were administered to the left eye of rabbits (n = 6) and the drug levels measured in different eye tissues 2 h after administration. In aqueous humor dexamethasone levels were 1,190 ± 110 and 1,670 ± 630 ng/g (mean ± SD) after administration of the 0.5 and 1.5% dexamethasone eye drops, respectively. In the retina the levels were 33 ± 7 and 66 ± 49 ng/g, and in optic nerve 41 ± 12 and 130 ± 50 ng/g, respectively. In a previous study the dexamethasone concentration in aqueous humor after topical administration of 1.3% (w/v) dexamethasone eye drops in aqueous 2-hydroxypropyl-β-cyclodextrin (HPβCD) solution was determined to be 320 ± 230 ng/g and 66 ± 20 ng/g after administration of Maxidex® eye drops. Both the hydrophilic HPβCD and the lipophilic RMβCD enhance topical dexamethasone delivery into the eye, but of the two, the lipophilic RMβCD results in higher dexamethasone concentrations. 相似文献
998.
C. Mansilla F. Gracia A. I. Martin‐Concepción J. P. Espinós J. P. Holgado F. Yubero A. R. González‐Elipe 《Surface and interface analysis : SIA》2007,39(4):331-336
The initial steps in the formation of thin films have been investigated by analysis of the peak shape (both inelastic background and elastic contributions) of X‐ray photoelectron spectra. Surface coverage and averaged height of the deposited particles have been estimated for several overlayers (nanometre range) after successive deposition cycles. This study has permitted the assessment of the type of nucleation and growth mechanisms of the films. The experiments have been carried out in situ in the preparation chamber of an XPS spectrometer. To check the performance of the method, several materials (i.e. cerium oxide, vanadium oxide and cadmium sulfide) have been deposited on different substrates using a variety of preparation procedures (i.e. thermal evaporation, ion beam assisted deposition and plasma enhanced chemical vapour deposition). It is shown that the first deposited nuclei of the films are usually formed by three‐dimensional particles whose heights and degree of surface coverage depend on the chemical characteristics of the growing thin film and substrate materials, as well as the deposition procedure. It is concluded that XPS peak shape analysis can be satisfactorily used as a general method to characterize morphologically the first nanometric moieties that nucleate a thin film. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
999.
Kassai Cs. Illés R. Pokol G. Sztatisz J. Fogassy E. Kozma D. 《Journal of Thermal Analysis and Calorimetry》2000,62(3):647-655
The resolution of three chiral alcohols with O,O'-dibenzoyl-(2R,3R)-tartaric acid (DBTA) via diastereoisomeric supramolecular
compound formation was investigated with thermoanalytical methods. On the basis of TG measurements the DBTA:chiral alcohol
molar ratio in the compounds is 1:1 which agrees with the results of single-crystal X-ray diffraction analysis. The DBTA –
chiral alcohol supramolecular compounds have different supramolecular structure than the DBTA – achiral alcohol supramolecular
compounds. The supramolecular compounds containing cyclohexanols have higher thermal stability than the compounds containing
acyclic aliphatic alcohols. The amount of unreacted DBTA monohydrate in the solid phase can be determined both with DSC and
with TG measurements.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
1000.
Preconcentration and speciation of chromium in waters using solid-phase extraction and atomic absorption spectrometry 总被引:1,自引:0,他引:1
A method for the preconcentration and speciation of chromium was developed. After formation of an anionic compound with ethylenediaminetetraacetic acid (CrY(-)), Cr (VI) and Cr (III) are retained on a strong anionic phase (SAX) and controlled elution with 0.5 M NaCl permits their speciation. The retention and elution conditions were optimised, and interferences due to the presence of other ions such as Mg(II), Mn(II), Sn(II), Fe(III), Ba(II), Al(III), Ca(II), chloride, iodine, bromide, fluoride, sulphate, phosphate, bicarbonate and nitrate were studied. The detection limits were 0.4 mug l(-1) and 1.1 mug l(-1) for Cr(III) and Cr(VI), respectively, and reproducibility was 9%. The results obtained for speciation of chromium by the proposed method in wastewaters are in agreement with the values obtained by a reference method for a 95% confidence level. 相似文献