全文获取类型
收费全文 | 5554篇 |
免费 | 332篇 |
国内免费 | 36篇 |
专业分类
化学 | 3797篇 |
晶体学 | 67篇 |
力学 | 204篇 |
数学 | 882篇 |
物理学 | 972篇 |
出版年
2023年 | 40篇 |
2022年 | 166篇 |
2021年 | 168篇 |
2020年 | 114篇 |
2019年 | 203篇 |
2018年 | 171篇 |
2017年 | 121篇 |
2016年 | 272篇 |
2015年 | 170篇 |
2014年 | 273篇 |
2013年 | 580篇 |
2012年 | 324篇 |
2011年 | 340篇 |
2010年 | 252篇 |
2009年 | 233篇 |
2008年 | 250篇 |
2007年 | 249篇 |
2006年 | 227篇 |
2005年 | 181篇 |
2004年 | 169篇 |
2003年 | 145篇 |
2002年 | 198篇 |
2001年 | 77篇 |
2000年 | 118篇 |
1999年 | 68篇 |
1998年 | 49篇 |
1997年 | 51篇 |
1996年 | 46篇 |
1995年 | 41篇 |
1994年 | 39篇 |
1993年 | 39篇 |
1992年 | 34篇 |
1991年 | 27篇 |
1990年 | 18篇 |
1989年 | 34篇 |
1988年 | 37篇 |
1987年 | 25篇 |
1986年 | 20篇 |
1985年 | 34篇 |
1984年 | 33篇 |
1983年 | 25篇 |
1982年 | 27篇 |
1981年 | 28篇 |
1980年 | 20篇 |
1979年 | 23篇 |
1978年 | 17篇 |
1976年 | 15篇 |
1975年 | 18篇 |
1974年 | 10篇 |
1973年 | 10篇 |
排序方式: 共有5922条查询结果,搜索用时 11 毫秒
961.
Rosemary H. Crampton Samir El Hajjaji Martin E. Fox Simon Woodward 《Tetrahedron: Asymmetry》2009,20(21):2497-2503
Chiral shift 31P NMR spectroscopy allows the identification of ligand leads in asymmetric catalyst systems for ZnMe2 addition to ArCHNP(O)Ph2. Subsequent GC-based optimisation shows [RhCl(CH2CH2)2]2 and (R,R)-MeDuPhos to be the optimal pre-catalyst combination (product in 78–93% ee). Transmetallation of [(MeDuPhos)Rh{N(P(O)Ph2–CHMeAr}] with ZnMe2 appears to be the rate limiting step of the catalytic cycle as competing coordination by the imine starting material leads to Ph2P(O)NHCH2Ar via MVP hydrogen-transfer. This limitation can largely be overcome by the slow addition of the imine. 相似文献
962.
In this work, complexes of Zr(IV) and Al(III) cations with 2,7,12,17-tetra-tert-butyl-5,10,15,20-tetraazaporphine (TAP) were tested as ionophores in plasticized PVC membranes of ion-selective electrodes. It was found that both tested ionophores show enhanced affinity towards fluoride anion. High fluoride selectivity was observed in the presence of anionic or cationic additives in the membrane, which indicates that proposed compounds work according to charged or neutral carrier mechanism, depending on membrane composition and pretreatment.tert-Butyl substituents, present in the structure of tested compounds, were supposed to prevent formation of ionophore dimers within the membrane phase. This process was found to be responsible for some unfavorable potentiometric properties of electrodes based on complexes of Zr(IV) and Al(III) cations with porphyrins (compounds closely related to tetra-tert-butyl-5,10,15,20-tetraazaporphine). As it was shown using spectrophotometrical measurements, Al(III)-TAP was not susceptible to dimerization, while dimer formation was observed for Zr(IV)-TAP. In full agreement with these observations, electrodes with membranes containing Al(III)-TAP responded in near-Nernstian and fast manner towards fluoride anion, while the employment of Zr(IV)-TAP as ionophore resulted in super-Nernstian and sluggish response. Plasticized PVC membranes doped with Al(III)-TAP and 20 mol% of lipophilic anionic additives shown remarkable F− selectivity, with selectivity coefficients, , as follows: −4.4 (Y−Br−), −4.3 (Cl−), −4.2 (NO3−), −3.6 (SCN−), −2.9 (ClO4−). 相似文献
963.
Tareq Youssef 《Photochemistry and photobiology》2009,85(4):921-926
Hypericin has been reported as a potent photosensitizing agent exhibiting antiviral, antibacterial, antineoplastic activities. Although its photophysics and mode of action are strongly modulated by the binding protein, detailed information about its mechanism of interaction with possible cellular targets, including proteins, is still lacking. Previous in vitro studies demonstrated that hypericin can be uptaken by intact lens and is able to bind to the major lens protein "α-crystallin." In this study, the mechanism of interaction of this potent drug with α-crystallin was studied using the chemical denaturant guanidine hydrochloride (GdnHCl) and the hydrophobic surface probe, 8-anilino-1-naphthalenesulfonic acid (ANS). Fluorescence measurements showed that the increased exposure of tryptophan resulting from partial unfolding of α-crystallin incubated with 1.0 mol L−1 of GdnHCl corresponds to the maximum accessibility of hydrophobic sites to ANS at the same GdnHCl concentration. Interestingly at this additional hydrophobicity of the protein, hypericin exhibited its maximum fluorescence intensity. This in vitro study implied that hydrophobic sites of α-crystallin play a significant role in its interaction with hypericin. The binding between α-crystallin and hypericin was found to be enhanced by partial perturbation of the protein. 相似文献
964.
H. El Shinawi J.F. Marco F.J. Berry C. Greaves 《Journal of solid state chemistry》2009,182(8):2261-2268
The M4+-containing K2NiF4-type phases La0.8Sr1.2Co0.5Fe0.5O4 and La0.8Sr1.2Co0.5Mn0.5O4 have been synthesized by a sol–gel procedure and characterized by X-ray powder diffraction, thermal analysis, neutron powder diffraction and Mössbauer spectroscopy. Oxide ion vacancies are created in these materials via reduction of M4+ to M3+ and of Co3+ to Co2+. The vacancies are confined to the equatorial planes of the K2NiF4-type structure. A partial reduction of Mn3+ to Mn2+ also occurs to achieve the oxygen stoichiometry in La0.8Sr1.2Co0.5Mn0.5O3.6. La0.8Sr1.2Co0.5Fe0.5O3.65 contains Co2+ and Fe3+ ions which interact antiferromagnetically and result in noncollinear magnetic order consistent with the tetragonal symmetry. Competing ferromagnetic and antiferromagnetic interactions in La0.8Sr1.2Co0.5Fe0.5O4, La0.8Sr1.2Co0.5Mn0.5O4 and La0.8Sr1.2Co0.5Mn0.5O3.6 induce spin glass properties in these phases. 相似文献
965.
Samir Mohamed El‐Moghazy Mohamed Abd El‐Azem Mohamed Marwa Fadel Mohamed Nadia Fayek Youssef 《中国化学会会志》2009,56(2):360-367
Reversed phase‐high performance liquid chromatography (RP‐HPLC), thin layer chromatography (TLC) densitometry and first derivative spectrophotometry (1D) techniques are developed and validated as a stability‐indicating assay of ezetimibe in the presence of alkaline induced degradation products. RP‐HPLC method involves an isocratic elution on a Phenomenex Luna 5μ C18 column using acetonitrile: water: glacial acetic acid (50:50:0.1 v/v/v) as a mobile phase at a flow rate of 1.5 mL/min. and a UV detector at 235 nm. TLC densitometric method is based on the difference in Rf‐values between the intact drug and its degradation products on aluminum‐packed silica gel 60 F254 TLC plates as stationary phase with isopropanol: ammonia 33% (9:1 v/v) as a developing mobile phase. On the fluorescent plates, the spots were located by fluorescence quenching and the densitometric analysis was carried out at 250 nm. Derivative spectrophotometry, the zero‐crossing method, ezetimibe was determined using first derivative at 261 nm in the presence of its degradation products. Calibration graphs of the three suggested methods are linear in the concentration ranges 1–10 mcg/mL, 0.1–1 mg/mL and 1–16 mcg/mL with a mean percentage accuracy of 99.05 ± 0.54%, 99.46 ± 0.63% and 99.24 ± 0.82% of bulk powder, respectively. The three proposed methods were successfully applied for the determination of ezetimibe in raw material and pharmaceutical dosage form; the results were statistically analyzed and compared with those obtained by the reported method. Validation parameters were determined for linearity, accuracy and precision; selectivity and robustness and were assessed by applying the standard addition technique. 相似文献
966.
I. Najeh N. Ben Mansour M. Mbarki A. Houas J. Ph. Nogier L. El Mir 《Solid State Sciences》2009,11(10):1747-1751
Electrical conducting carbon (ECC) porous structures were explored by changing the pyrolysis temperature of organic xerogel compounds prepared by sol–gel method from resorcinol–formaldehyde (RF) mixtures in acetone using picric acid as catalyst. The effect of this preparation parameter on the structural and electrical properties of the obtained ECCs was studied. The analysis of the obtained results revealed that the polymeric insulating xerogel phase was transformed progressively with pyrolysis temperature into carbon conducting phase; this means the formation of long continuous conducting path for charge carriers to move inside the structure with thermal treatment and the samples exhibited tangible percolation behaviour where the percolation threshold can be determined by pyrolysis temperature. The temperature-dependent conductivity of the obtained ECC structures shows a semi-conducting behaviour and the I(V) characteristics present a negative differential resistance. The results obtained from STM micrographs revealed that the obtained ECC structures consist of porous electrical conducting carbon materials. 相似文献
967.
Jamil Rima Soula Kyriacos Maurice Abou Rida 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2009,74(1):10-15
The solubilization of pyrene in aqueous solution of β-cyclodextrin (β-CD) or its derivatives such as β-CD-hexanoyl, β-CD-benzoyl and β-CD-dodecylsulfonate was investigated by spectrophotometry. Linear and non-linear regression methods were used to estimate the association constants (K1). A 1:1 stoichiometric ratio and different effects of the hexanoyl, benzoyl and dodecylsulfonate groups on the association constant were observed for the binary inclusion complex between pyrene and β-CD. The formation constant was shown to decrease when β-CD was modified by a dodecylsulfonate chain. The value of K1 was 190 ± 10 L mol−1 for the [pyrene/β-CD] complex and 145 L mol−1 for the [pyrene/β-CD-dodecylsulfonate] complex. Partitioning of the pyrene molecules between the dodecylsulfonate chains and cyclodextrin cavities can explain the decrease in the association constant value. In the cases of β-CD-hexanoyl and β-CD-benzoyl derivatives, no association constants were detected. Results suggest that the high hydrophobicity of the hexanoyl and benzoyl groups prevents the inclusion of pyrene molecules inside the cyclodextrin cavity. 相似文献
968.
Confocal laser scanning microscopy (CLSM) was used to study single‐ and two‐component protein uptake for α‐lactalbumin (ALA) and β‐lactoglobulin (BLG), as models for whey proteins, to SP Sepharose FF at pH 3.7 during batch experiments in a finite bath. By coupling a fluorescent dye with the protein molecule, the penetration into individual adsorbent particles at different times during batch uptake was visualised. In a single‐component system, BLG penetrated fast into the adsorbent beads and gradually filled them in a shell‐wise fashion, while adsorption of ALA was mostly confined to the outer shells of the adsorbent. For the two‐component studies, the results showed that ALA was able to displace BLG despite its lower affinity to the adsorbent under the employed conditions. CLSM results were then compared both qualitatively and quantitatively to their counterparts obtained in traditional experiments by indirect measurements of the protein concentration in the fluid phase. A novel quantitative approach was undertaken by modifying the simple kinetic rate model traditionally used to determine the kinetic rate constant, k1, for batch uptake experiments, in order to describe batch uptake kinetics based on CLSM data. Although BLG results were in good agreement, there was a discrepancy in ALA results. 相似文献
969.
Phase relations up to the solidus line in part of the Sb-Zn-O system have been investigated over the entire concentration
range of the α-Sb2O4-ZnO system in air (= 0.21 atm) using XRD and DTA/TG. The components of this system in air form ZnSb2O6 and Zn7Sb2O12. The results allow division of the system into three subsystems, i.e. α-Sb2O4-ZnSb2O6; ZnSb2O6Zn7Sb2O12 and Zn7Sb2O12-ZnO. The temperature ranges over which the ZnSb2O6 and Zn7Sb2O12 remain at equilibrium with other solid compounds depend on the gaseous atmosphere.
相似文献
970.
Hajja S. Alonazy Hassan M.A. AL-Hazimi Makarem M.S. Korraa 《Arabian Journal of Chemistry》2009,2(2):101-108
A series of 5,6-bis(4-substitutedphenyl)-2H(3)-pyridazinones 2a–f have been synthesized from the condensation of the corresponding benzil monohydrazones 1 either with ethyl cyanoacetate or diethyl malonate in ethanol. The synthesized pyridazinones were converted to the corresponding 3-chloro derivatives 3a–f by the action of phosphoryl chloride. Reaction of the latter halogenated pyridazines with various aromatic amines led to the formation of new 3-aminoaryl pyridazines (4) in moderate yield. The structures of all new compounds 2b,c,e,f, 3b–e, 4 were fully identified by the analysis of their 1H and 13C NMR and mass spectra. Some of these synthetic heterocyclic compounds were screened for their antimicrobial activities but they were almost negative. 相似文献