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91.
Propylene was copolymerized with the linear α‐olefins 1‐octene, 1‐decene, 1‐tetradecene, and 1‐octadecene. The metallocene catalyst Me2Si(2‐Me Benz[e]Ind)2ZrCl2, in conjunction with methylalumoxane as a cocatalyst, was used to synthesize the copolymers. The copolymers were characterized by 13C and 1H NMR with a solvent mixture of 1,2,4‐trichlorobenzene (TCB) and benzene‐d6 (9/1) at 100 °C. Thermal analyses were carried out to determine the melting and crystallization temperatures, whereas the molecular weights and molecular weight distributions were determined by gel permeation chromatography with TCB at 140 °C. Glass‐transition temperatures were determined with dynamic mechanical analysis. Relationships among the comonomer type and amount of incorporation and the melting/crystallization temperatures, glass‐transition temperature, crystallinity, and molecular weight were established. Moreover, up to 3.5% of the comonomer was incorporated, and there was a decrease in the molecular weight with increased comonomer content. Also, the melting and crystallization temperatures decreased as the comonomer content increased, but this relationship was independent of the comonomer type. In contrast, the values for the glass‐transition temperature also decreased with increased comonomer content, but the extent of the decrease was dependent on the comonomer type. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4110–4118, 2000  相似文献   
92.
β,β′‐Bisporphyrins are intrinsically chiral porphyrin dimers with fascinating properties. The configurational stability at their axes can be directed by variation of the central metal atoms. Herein, we present a regioselective functionalization of the monomeric 2‐amino‐tetraphenyl‐porphyrin as a versatile substrate for dimerization by oxidative coupling. By simple variation of the reaction conditions (solvent and oxidant), the oxidation selectively gave either the axially chiral C,C‐coupled diaminobisporphyrin in high yields or, under Ullmann conditions, the twofold N,C‐linked achiral dimer, also in good yields. A generalized mechanism for the coupling reaction is proposed based on DFT calculations. The axially chiral β,β′‐coupled porphyrin dimers were isolated as racemic mixtures, but can be resolved by HPLC on a chiral phase. TDDFT and coupled‐cluster calculations were used to explain the spectroscopic properties of the aminoporphyrins and their dimers and to elucidate the absolute configurations of the C,C‐coupled bisporphyrins.  相似文献   
93.
Flavonols, a class of flavonoids, are present in flowers, fruits and vegetables. They are jointly responsible for antioxidant activity as free radical acceptors. The redox behaviour of myricetin, quercetin, isorhamnetin, fisetin, morin and kaempferol is investigated using cyclic and differential pulse voltammetry. Quantum chemical calculations of proton affinities and electron transfer enthalpies were performed to identify possible reactive sites and radical species to compare them with measured oxidation potentials of the flavonols. Regarding to their chemical structure, these flavonols showed an oxidation order: myricetin > quercetin > isorhamnetin > fisetin > morin > kaempferol.  相似文献   
94.
A model for sensing with semiconducting metal oxide (SMOX)-based gas sensors was developed which takes the effect of the shape of the grains in the sensing layers into account. Its validity is limited to materials in which the grains of the SMOX sensing layer are large enough to have an undepleted bulk region (large grains). This means that in all experimental conditions, the SMOX properties ensure that the influence of surface phenomena is not extended to the whole grain. The model takes the surface chemistry and its impact on the electrical properties of the sensing material into consideration. In this way, it relates the sensor signal—defined as the relative change of the sensor’s conductance—directly to the concentration of the target gas and also exhibits meaningful chemical parameters, such as the type of reactive oxygen species, the reaction constants, and the concentration of adsorption sites. The validity of the model is confirmed experimentally by applying it to data gathered by measuring homemade sensors in relevant conditions.  相似文献   
95.
Despite of great advances of phospholipids and liposomes in clinical therapy, very limited success has been achieved in the preparation of smart phospholipids and controlled-release liposomes for in vivo drug delivery and clinical trials. Here we report a supramolecular approach to synthesize novel supramolecularly engineered phospholipids based on complementary hydrogen bonding of nucleosides, which greatly reduces the need of tedious chemical synthesis, including reducing the strict requirements for multistep chemical reactions, and the purification of the intermediates and the amount of waste generated relative more traditional approaches. These upgraded phospholipids self-assemble into liposome-like bilayer structures in aqueous solution, exhibiting fast stimuli-responsive ability due to the hydrogen bonding connection. In vitro and in vivo evaluations show the resulted supramolecular liposomes from nucleoside phospholipids could effectively transport drug into tumor tissue, rapidly enter tumor cells, and controllably release their payload in response to an intracellular acidic environment, thus resulting in a much higher antitumor activity than conventional liposomes. The present supramolecularly engineered phospholipids represent an important evolution in comparison to conventional covalent-bonded phospholipid systems.  相似文献   
96.
97.
The reaction of 1,3‐diisopropylimidazolin‐2‐ylidene (iPr2Im) with diphenyldichlorosilane (Ph2SiCl2) leads to the adduct (iPr2Im)SiCl2Ph2 1 . Prolonged heating of isolated 1 at 66 °C in THF affords the backbone‐tethered bis(imidazolium) salt [(aHiPr2Im)2SiPh2]2+ 2 Cl? 2 (“a” denotes “abnormal” coordination of the NHC), which can be synthesized in high yields in one step starting from two equivalents of iPr2Im and Ph2SiCl2. Imidazolium salt 2 can be deprotonated in THF at room temperature using sodium hydride as a base and catalytic amounts of sodium tert‐butoxide to give the stable N‐heterocyclic dicarbene (aiPr2Im)2SiPh2 3 , in which two NHCs are backbone‐tethered with a SiPh2 group. This easy‐to‐synthesize dicarbene 3 can be used as a novel ligand type in transition metal chemistry for the preparation of dinuclear NHC complexes, as exemplified by the synthesis of the homodinuclear copper(I) complex [{a(ClCu?iPr2Im)}2SiPh2] 4 .  相似文献   
98.
An electrolyte based on the tris(acetylacetonato)iron(III)/(II) redox couple ([Fe(acac)3]0/1?) was developed for p‐type dye‐sensitized solar cells (DSSCs). Introduction of a NiO blocking layer on the working electrode and the use of chenodeoxycholic acid in the electrolyte enhanced device performance by improving the photocurrent. Devices containing [Fe(acac)3]0/1? and a perylene–thiophene–triphenylamine sensitizer (PMI–6T–TPA) have the highest reported short‐circuit current (JSC=7.65 mA cm?2), and energy conversion efficiency (2.51 %) for p‐type DSSCs coupled with a fill factor of 0.51 and an open‐circuit voltage VOC=645 mV. Measurement of the kinetics of dye regeneration by the redox mediator revealed that the process is diffusion limited as the dye‐regeneration rate constant (1.7×108 M ?1 s?1) is very close to the maximum theoretical rate constant of 3.3×108 M ?1 s?1. Consequently, a very high dye‐regeneration yield (>99 %) could be calculated for these devices.  相似文献   
99.
Extensive ab initio calculations complemented by a photodissociation experiment at 193 nm elucidate the nature of hydrogen halide molecules bound on free ice nanoparticles. Electronic absorption spectra of small water clusters (up to 5 water molecules) and water clusters doped with hydrogen fluoride, hydrogen chloride and hydrogen bromide were calculated. The spectra were modeled at the time-dependent density functional (TDDFT) level of theory with the BHandHLYP functional using the reflection principle. We observe the emergence of a charge-transfer-to-solvent (CTTS) band in the absorption spectra upon the acidic dissociation of the hydrogen halides. The CTTS band provides a spectroscopically observable feature for the acidic dissociation. The calculated spectra were compared with our new experimental photodissociation data for larger water clusters doped with HCl and HBr. We conclude that HCl and HBr dissociate to a large extent on the surface of ice nanoparticles at temperatures near 120 K and photoactive products are formed. The acidic dissociation of HX leads to an enhancement by about 4 orders of magnitude of the HCl photolysis rate in the 200-300 nm region, which is potentially relevant for the halogen budget in the atmosphere.  相似文献   
100.
The ring-opening reactions of lithium bromocyclopropylidenoids to allenes have been investigated computationally at the B3LYP/6-31G(d) level of theory. Formally, two pathways can be considered: the reaction may either proceed in a concerted fashion or stepwise with the intermediacy of a free cyclopropylidene. In both cases, the loss of the bromide ion determines the kinetic of the reaction. The stability of the reactive intermediate, i.e., the carbene, is dependent on the substituent. Cyclopropylidenes bearing an electron-donating group (+M) are extremely unstable and ring-open readily to the allene. In contrast, bromocyclopropylidenoids with electron-withdrawing groups are particularly stable species. Here, a high energy barrier needs to be overcome in order to split off bromide and to generate the corresponding carbene or allene. Still, for most of the monosubstituted cyclopropylidenes investigated during this study, the activation energy for the cyclopropylidene to allene rearrangement is lower than the energy required for parent compound (X = H) except for X = -SiH3 and -CF3.  相似文献   
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