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Luotonin A is an alkaloid structurally related to the natural anti-tumour agent camptothecin. The fluorescence behaviour of luotonin A and a series of six analogues is described in the present work. The influence of solvent polarity and pH on the native fluorescence properties of these alkaloids was studied, finding that in organic solvents or in aqueous solutions (pH 5.5–7.2) the neutral form of the luotonin derivatives emit in the region of 410–450 nm but, in both media, acidification to pH values below 3.0 causes a new emission band to appear at about 500 nm. An ESPT reaction occurs due to the protonation of the basic nitrogen atoms of the pentacyclic ring. Acid-base titrations of luotonin A and its derivatives in aqueous and acetonitrile media were carried out in order to determine their pKa? values which were around 2, showing these compounds to be very weak bases. In aqueous media, the absence of an iso-emissive point in the emission spectra suggests the existence of more than two species in the proton transfer equilibria. The basicity of the luotonin A derivatives is increased in organic media, and a good correlation between the pKa? values and the chemical structure was found. The protonation of luotonin A was also studied by 1H-NMR and 13C-NMR experiments, which proved the protonation of the nitrogen atoms at the positions 5 and 6 of the pentacyclic ring. The fluorescence quantum yields were determined in ethanol and in aqueous solutions under neutral and acidic conditions. The fluorescence quantum yields were higher in water for the case of the more polar compounds, and the opposite result was obtained for the more hydrophobic ones. The remarkable and interesting fluorescence properties of luotonin A prompted the development of its fluorimetric analytical quantitation, obtaining very good analytical features.  相似文献   
54.
A spectrophotometric method has been developed for the determination of traces of phosphorus in zirconium based alloys (Zr-2.5Nb and Zircaloy). It is achieved by selective fluoride complexation controlled by boric acid. The samples were dissolved in HF and fluoro-complexes of the matrices were formed by maintaining the concentration of HF while the excess HF was controlled by boric acid. After the formation of phosphomolybdate, extracted into n-butyl acetate, ion-associated with crystal violet and the absorbance was measured at 582 nm. The results obtained by this procedure were in close agreement with the certified reference material (CRM) values and further these values were compared with the values determined by Glow Discharge-Quadrupole Mass Spectrometry (GD-QMS). The potential interferences like fluoride, silicon, arsenic(V), niobium, titanium, tantalum, etc., were tolerable to large level. LOD (3 s) was found to be 0.055 mg kg(-1) with a precision (R.S.D.) of 2-3% and molar absorptivity was 2.7x10(5) L mol(-1) cm(-1).  相似文献   
55.
Elucidating single-atom effects on the fundamental properties of nanoparticles is challenging because single-atom modifications are typically accompanied by appreciable changes to the overall particle's structure. Herein, we report the synthesis of a [Cu58H20PET36(PPh3)4]2+ ( Cu58 ; PET: phenylethanethiolate; PPh3: triphenylphosphine) nanocluster—an atomically precise nanoparticle—that can be transformed into the surface-defective analog [Cu57H20PET36(PPh3)4]+ ( Cu57 ). Both nanoclusters are virtually identical, with five concentric metal shells, save for one missing surface copper atom in Cu57 . Remarkably, the loss of this single surface atom drastically alters the reactivity of the nanocluster. In contrast to Cu58 , Cu57 shows promising activity for click chemistry, particularly photoinduced [3+2] azide-alkyne cycloaddition (AAC), which is attributed to the active catalytic site in Cu57 after the removal of one surface copper atom. Our study not only presents a unique system for uncovering the effect of a single-surface atom modification on nanoparticle properties but also showcases single-atom surface modification as a powerful means for designing nanoparticle catalysts.  相似文献   
56.
A new C3-symmetric drum-shaped homoditopic haxaamino bicyclic cyclophane and its hexachloride and hexaiodide complexes have been synthesized and characterized and dual recognition of guests has been demonstrated. Single-crystal X-ray analysis illustrates that bicyclic cyclophane has a cavity and side pockets for acetone molecules. The hexaprotonated state of this bicycle shows encapsulation of an iodide inside its cavity, and in hexachloride complex, chloride is recognized as Cl(-)...H2O in each of the three side pockets which are in extensive hydrogen bonding interactions with the water and chlorides. (1)H NMR experiments have also been carried out on hexatosylated cyclophane with the halides to study solution state binding.  相似文献   
57.
Coordinatively unsaturated double‐stranded helicates [(H2L)2Eu2(NO3)2(H2O)4](NO3)4, [(H2L)2Tb2(H2O)6](NO3)6, and [(H2L)2Tb2(H2O)6]Cl6 (H2L=butanedioicacid‐1,4‐bis[2‐(2‐pyridinylmethylene)hydrazide]) are easily obtained by self‐assembly from the ligand and the corresponding lanthanide(III) salts. The complexes are characterized by X‐ray crystallography showing the helical arrangement of the ligands. Co‐ligands at the metal ions can be easily substituted by appropriate anions. A specific luminescence response of AMP in presence of ADP, ATP, and other anions is observed. Specificity is assigned to the perfect size match of AMP to bridge the two metal centers and to replace quenching co‐ligands in the coordination sphere.  相似文献   
58.
M. Arunachalam  Pradyut Ghosh 《Tetrahedron》2007,63(46):11371-11376
Tiny azacryptand 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane (L) upon reaction with 48% hydrobromic acid (containing <0.05% chloride contamination) forms hexabromide salt (1). Single crystal X-ray crystallographic investigation of the hexaprotonated bromide (1) shows no guest encapsulation inside the tiny cage. This bromide salt 1 with an empty proton cage has been utilized as the receptor for encapsulation of chloride (2) and fluoride (3). Crystallographic results of mixed chloride/bromide (2) and fluoride/bromide (3) complexes of L are examined, which show monotopic recognition of chloride in the case of 2 and fluoride in the case of 3 inside the proton cage with five bromide and three water molecules outside the cavity. Single crystals obtained from an experiment on mixed anionic system (chloride and fluoride), 1 shows selective encapsulation of fluoride, which supports the formation of complex 3 and crystals obtained upon treatment of 2 with tetrabutyl ammonium fluoride also yields complex 3. In a separate reaction between L and 49% hydrobromic acid containing higher chloride contamination (<0.2%) forms chloride/bromide salt (2). 1H NMR studies of 1 with sodium chloride and fluoride support the encapsulation of the respective anions inside the proton cage.  相似文献   
59.
The oxidation of 4‐substituted phenyl phenyl sulfides was carried out with several oxo(salen)manganese(V) complexes in MeCN/H2O 9 : 1. The kinetic data show that the reaction is first‐order each in the oxidant and sulfide. Electron‐attracting substituents in the sulfides and electron‐releasing substituents in salen of the oxo(salen)manganese(V) complexes reduce the rate of oxidation. A Hammett analysis of the rate constants for the oxidation of 4‐substituted phenyl phenyl sulfides gives a negative ρ value (ρ=?2.16) indicating an electron‐deficient transition state. The log k2 values observed in the oxidation of each 4‐substituted phenyl phenyl sulfide by substituted oxo(salen)manganese(V) complexes also correlate with Hammett σ constants, giving a positive ρ value. The substituent‐, acid‐, and solvent‐effect studies indicate direct O‐atom transfer from the oxidant to the substrate in the rate‐determining step.  相似文献   
60.
Researchers world-wide have employed diverse strategies to achieve various anion binding hosts and anion induced supramolecular architectures due to the increasing appreciation of anion receptor chemistry. Intellectual discovery of molecular capsules for the recognition of different guest species has opened up a new field of research in the area of supramolecular chemistry. This feature article aims to provide an overview of the current status and recent achievements made by us and others in the last decade in the area of anion induced construction of supramolecular capsules and anion binding in molecular capsules.  相似文献   
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