全文获取类型
收费全文 | 1218篇 |
免费 | 62篇 |
国内免费 | 11篇 |
专业分类
化学 | 841篇 |
晶体学 | 4篇 |
力学 | 29篇 |
数学 | 166篇 |
物理学 | 251篇 |
出版年
2023年 | 12篇 |
2022年 | 18篇 |
2021年 | 59篇 |
2020年 | 49篇 |
2019年 | 45篇 |
2018年 | 29篇 |
2017年 | 36篇 |
2016年 | 67篇 |
2015年 | 39篇 |
2014年 | 49篇 |
2013年 | 75篇 |
2012年 | 85篇 |
2011年 | 131篇 |
2010年 | 68篇 |
2009年 | 59篇 |
2008年 | 74篇 |
2007年 | 81篇 |
2006年 | 83篇 |
2005年 | 60篇 |
2004年 | 27篇 |
2003年 | 21篇 |
2002年 | 28篇 |
2001年 | 17篇 |
2000年 | 9篇 |
1999年 | 5篇 |
1998年 | 2篇 |
1997年 | 5篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 7篇 |
1992年 | 4篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有1291条查询结果,搜索用时 15 毫秒
961.
Calculations of the 13C NMR chemical shifts for the methoxy and ethoxy groups adsorbed on Y and ZSM-5 zeolites were computed at GIAO/B3LYP/6-31+G*//MM+ level of theory, using a cluster representing a real part of the zeolites. The Y zeolite was represented by a cluster with 168 atoms, while ZSM-5 was represented by a cluster with 144 atoms. The calculated chemical shifts agreed well with reported experimental values, showing that the difference in chemical shifts is associated with differences in the geometry of the alkoxides on the two zeolites. 相似文献
962.
Rodrigo Beltrán-Suito Angela Pinedo-Flores Fabiola Bravo-Hualpa Jorge Ramos-Muñoz 《Journal of Dispersion Science and Technology》2019,40(6):901-908
Water-resistant MOF-235 was used to adsorb N,N-dimethylamine (DMA) from aqueous solutions. It was synthesized from terephthalic acid and FeCl3.6H2O under air-free conditions and characterized by its crystalline structure, functional groups and temperature resistance. The kinetic data results were best adjusted to the pseudo-second order model (R2>0.963). The best-fit isotherm, Langmuir model, suggested the adsorption of DMA is localized on homogenously distributed active sites on the surface. This fit was confirmed by the value of β?=?1 on the Redlich-Peterson model. Our study suggests that the manipulation of novel materials such as MOF-235 promises new avenues for water treatment solutions.
Schematic structure of MOF-235 and its application as adsorbent. 相似文献
963.
J. Manuel López-Romero Rodrigo Rico Jesús Hierrezuelo Chengzhi Cai Isabel López-Tocón 《Tetrahedron letters》2007,48(35):6075-6079
We report an efficient synthesis of a series of penta-p-phenylene derivatives with four side chains of various lengths, including deca(ethylene glycol) groups. The key feature of the synthesis is that the side chains are efficiently introduced in the last step, facilitating optimization of the side chains for different applications. Raman spectroscopy study indicates a similarly high rigidity for all these compounds, even those with long oligo(ethylene glycol) side chains. The deca(ethylene glycol)-substituted penta-p-phenylene derivatives are versatile building blocks for construction of nanometric, tripod-shaped adsorbates for biological applications. 相似文献
964.
Guilherme M.O Barra Rodrigo L Oréfice Jose R.G Carneiro 《European Polymer Journal》2004,40(9):2017-2023
Incorporation of polyaniline (PAni) into thermoplastic elastomers can be used to produce materials that potentially combine the good mechanical properties and processability of thermoplastic elastomers with electrical, magnetic and optical characteristics of PAni. In this work, a polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene copolymer (SEBS) was chemically modified by grafting a sulfonic group onto the chain backbone in order to promote higher levels of compatibility between the thermoplastic elastomer and polyaniline. The sulfonation process was performed by reacting SEBS with acetyl sulfate. Infrared spectroscopy and titration were used to monitor the amount of sulfonic groups successfully grafted on SEBS. Mechanical tests performed in sulfonated SEBS showed that sulfonation levels lower than 15% did not reduce substantially the mechanical properties of SEBS. PAni doped with dodecylbenzenesulfonic acid (PAni·DBSA), used in the preparation of the blends, was prepared by the “in situ doping polymerization” method. PAni·DBSA was then blended in solution with SEBS having different levels of sulfonation. The introduction of sulfonic group into the structure of SEBS improved coulombic interactions between the phases in the blend and enhanced compatibility. As a consequence, higher values of electrical conductivity (measured by the four-probe method) were achieved in blends with sulfonic groups grafted onto polymer chains. Concentrations as low as 20 wt% of PAni were able to lead to electrical conductivities of PAni·DBSA/sulfonated SEBS blends close to 1.2 S/cm. Optical micrographs of the blends showed that PAni·DBSA/sulfonated SEBS microstructure is composed of a very disperse group of small conducting particles. This type of microstructure would then be responsible for the enhanced electrical conductivity and low percolation threshold of PAni·DBSA/sulfonated SEBS, when compared to PAni·DBSA/SEBS blends. 相似文献
965.
Marcio José Tiera Gerson Rodrigues dos Santos Vera A. de Oliveira Tiera Neide Aparecida Blaz Vieira Elisabete Frolini Rodrigo C. da Silva Watson Loh 《Colloid and polymer science》2005,283(6):662-670
A series of N-isopropylacrylamide (NIPAM)-acrylic acid–ethyl methacrylate terpolymers with varied monomer compositions was prepared by radical polymerization. The solution behavior of these polymers was studied in dilute aqueous solution using spectrophotometry, fluorescence spectroscopy and high-sensitivity differential scanning calorimetry. The results obtained revealed that the lower critical solution temperatures depend strongly on the copolymer composition, solution pH and ionic strength. At a high pH, the ionization of acrylic acid (AA) units leads to an increase in solution cloud points (Tc). Solutions of polymers containing 10% or less of AA display a constant Tc for pH above 5.5, with 15% there is a continuous increase in Tc with pH and, for higher AA contents, no clouding was observed within the studied temperature range. Fluorescence probe studies were conducted by following the I 1/I 3 ratio of pyrene vibronic bands and the emission of anilinonaphtalene sulfonic acid, sodium salt (ANS), both approaches revealing the existence of hydrophobic domains for polymers with higher ethyl methacrylate content at temperatures lower than Tc, suggesting some extent of aggregation and/or a coil-to-globule transition. Scanning calorimetry measurements showed an endothermic transition at temperatures agreeing with the previously detected cloud points. Moreover, the transition curves became broader and with a smaller transition enthalpy, as both the AA content and the solution pH were increased. These broader transitions were interpreted to be the result of a wider molecular distribution upon polymer ionization, hence, displaying varied solution properties. The decrease in transition enthalpy was rationalized as a consequence of reminiscent hydration of NIPAM units, even after phase separation, owing to the presence of electric charges along the polymer chain. 相似文献
966.
Froimowitz M Wu KM Rodrigo J George C 《Journal of computer-aided molecular design》2000,14(2):135-146
Molecular mechanics calculations using MM3-92 and ab initio quantum mechanical calculations using SPARTAN 5.0 were performed on the structurally similar PCP and BTCP, in which only the latter has a cocaine-like pharmacological profile as a dopamine reuptake blocker. Calculations were also performed on BTCP analogs with a methyl group in various positions of the cyclohexane ring. The results for the cis-2-methyl compound, which retains good pharmacological activity, allowed us to determine that an aryl-axial conformer is the biologically active form for at least some of the compounds in this series. However, an aryl-equatorial conformer presents the identical pharmacophore, as shown by superposition of the two conformers. X-ray crystallographic structures were also obtained for BTCP and related compounds with a 2-methyl group on the cyclohexane ring, with reasonable agreement between the computational and experimental results. Superposition studies were performed with two rigid analogs of cocaine which illustrate the optimal orientations of the ammonium hydrogen for monoamine transporters. There is excellent agreement between a `back-bridged' cocaine analog that is optimal as a dopamine reuptake blocker and the previously proposed biologically active conformer of methylphenidate. However, BTCP is found to be a better fit to the `front-bridged' cocaine analog that is optimal for a serotonin reuptake blocker. 相似文献
967.
Leonardo S.G. Teixeira Rodrigo B.S. Rocha Paulo R.B. Guimarães Josanaide S.R. Teixeira 《Talanta》2007,72(3):1073-1076
This paper proposes an alternative analytical method using energy dispersive X-ray fluorescence (EDXRF) to determine Fe and Cu in gasoline samples. In the proposed procedure, samples were distilled and the distillation residues were spotted on cellulose paper disk to form a uniform thin film and to produce a homogeneous and reproducible interface to the XRF instrument. The disks were dried at 60 °C for 20 min and copper and iron were determined directly in the solid phase at 6.40 and 8.04 keV, respectively. The calibration curves showed linear response in the 20-800 μg L−1 concentration range of each metal. The precisions (repeatability) calculated from 15 consecutive measurements and defined as the coefficient of variation of solutions containing 100 μg L−1 of Fe and Cu were 7.8 and 8.1%, respectively. The limits of detection (LOD), defined as the analyte concentration that gives a response equivalent to three times the standard deviation of the blank (n = 10), were found to be 10 and 15 μg L−1 for Fe and Cu, respectively. The proposed method was applied to copper and iron determination in gasoline samples collected from different gas stations. 相似文献
968.
Christopher Glidewell John N. Low Justo Cobo Manuel Nogueras Adolfo Snchez Emerson Rengifo Rodrigo Abonia 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(6):o314-o317
In the title compound, C20H22N6, the molecules are linked into a chain of rings by N—H?N [H?N 2.16 Å, N?N 2.950 (3) Å and N—H?N 149°] and C—H?N [H?N 2.55 Å, C?N 3.481 (3) Å and C—H?N 165°] hydrogen bonds, and these chains are linked into sheets by means of C—H?π(pyrazole) interactions. 相似文献
969.
[reaction: see text] o-Toluate anions generated via transmetalation from the corresponding tributyl stannane underwent a Michael addition-Dieckmann condensation sequence with various cyclohexenones. This protocol provides an efficient entry into complex polycyclic systems without the use of beta-alkoxy enones hitherto required for the reaction. 相似文献
970.
Souza GH Catharino RR Ifa DR Eberlin MN Hyslop S 《Journal of mass spectrometry : JMS》2008,43(5):594-599
Fingerprinting by mass spectrometry has been increasingly used to study venom variations and for taxonomic analyses based on venom components. Most of these studies have concentrated on components heavier than 3 kDa, but Bothrops snake venoms contain many biologically active peptides, principally C-type natriuretic peptides and bradykinin-potentiating peptides (BPPs). In this work, we have examined the peptide profile of Bothrops venoms (B. alternatus, B. erythromelas, B. insularis, B. jararaca, B. jararacussu, B. leucurus and B. moojeni) using direct infusion nano-electrospray ionization mass spectrometry (nano-ESI-MS) subjecting the data further to principal components analysis (PCA) to assess whether the peptide distributions are reliable in distinguishing the venoms. ESI-MS of a low molar mass fraction obtained by ultrafiltration of each venom (5 kDa nominal cutoff filters) revealed that the venoms have a variety of peptides in common but that each venom also contains taxonomic marker peptides not shared with other venoms. One BPP peptide, QGGWPRPGPEIPP, was found to be common to the seven Bothrops species examined. This peptide may represent a specific marker for this genus since it was not found in the venom of the South American rattlesnake, Crotalus durissus terrificus. PCA on the ESI-MS data reveals a close relationship between B. jararaca, B. jararacussu and B. moojeni venoms, with B. leucurus and B. erythromelas being more distant from these three; B. alternatus and B. insularis were also located distant from these five species, as was C. d. terrificus. These results agree partially with established phylogenetic relationships among these species and suggest that ESI-MS peptide fingerprinting of snake venoms coupled with PCA is a useful tool for identifying venoms and for taxonomic analyses. 相似文献