全文获取类型
收费全文 | 1871篇 |
免费 | 64篇 |
国内免费 | 7篇 |
专业分类
化学 | 1194篇 |
晶体学 | 15篇 |
力学 | 63篇 |
数学 | 231篇 |
物理学 | 439篇 |
出版年
2023年 | 11篇 |
2022年 | 21篇 |
2021年 | 58篇 |
2020年 | 38篇 |
2019年 | 40篇 |
2018年 | 46篇 |
2017年 | 34篇 |
2016年 | 53篇 |
2015年 | 66篇 |
2014年 | 63篇 |
2013年 | 134篇 |
2012年 | 136篇 |
2011年 | 136篇 |
2010年 | 82篇 |
2009年 | 93篇 |
2008年 | 120篇 |
2007年 | 103篇 |
2006年 | 91篇 |
2005年 | 90篇 |
2004年 | 80篇 |
2003年 | 56篇 |
2002年 | 62篇 |
2001年 | 32篇 |
2000年 | 19篇 |
1999年 | 29篇 |
1998年 | 18篇 |
1997年 | 18篇 |
1996年 | 21篇 |
1995年 | 23篇 |
1994年 | 18篇 |
1993年 | 17篇 |
1992年 | 17篇 |
1991年 | 15篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 10篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1944年 | 2篇 |
排序方式: 共有1942条查询结果,搜索用时 500 毫秒
991.
We deal with a very useful numerical method for both controlled and uncontrolled queuing and multiplexing type systems. The basic idea starts with a heavy traffic approximation, but it is shown that the results are very good even when working far from the heavy traffic regime. The underlying numerical method is a version of what is known as the Markov chain approximation method. It is a powerful methodology for controlled and uncontrolled stochastic systems, which can be approximated by diffusion or reflected diffusion type systems, and has been used with success on many other problems in stochastic control. We give a complete development of the relevant details, with an emphasis on multiplexing and particular queueing systems. The approximating process is a controlled or uncontrolled Markov chain which retains certain essential features of the original problem. This problem is generally substantially simpler than the original physical problem, and there are associated convergence theorems. The non-classical associated ergodic cost problem is derived, and put into a form such that reliable and good numerical algorithms, based on multigrid type ideas, can be used. Data for both controlled and uncontrolled problems shows the value of the method.Supported by ARO contract DAAL-03-92-G-0115, AFOSR contract F49620-92-J-0081, and DARPA contract AFOSR-91-0375.Formerly at Brown University. Supported by DARPA contract AFOSR-91-0375. 相似文献
992.
César IC Bastos LF Godin AM Coelho Mde M Araujo DP de Fátima  Guidine PA Pianetti GA 《Journal of mass spectrometry : JMS》2011,46(11):1125-1130
A liquid chromatography-electrospray ionization tandem mass spectrometry method was developed and validated for the simultaneous quantitation of nicorandil and its denitrated metabolite, N-(2-hydroxyethyl)-nicotinamide, in rat plasma. After a liquid-liquid extraction step, chromatographic separation was performed on a ShinPack C(18) column with an isocratic mobile phase composed of methanol and 2 mM aqueous ammonium acetate containing 0.03% (v/v) formic acid (33:67 v/v). Procainamide was used as an internal standard (IS). Selected reaction monitoring was performed using the transitions m/z 212 → m/z 135, m/z 166 → m/z 106 and m/z 236 → m/z 163 to quantify nicorandil, its denitrated metabolite and IS, respectively. Calibration curves were constructed over the range of 5-15,000 ng.ml(-1) for both nicorandil and its metabolite. The mean relative standard deviation (RSD%) values for the intra-run precision were 5.4% and 7.3% and for the inter-run precision were 8.5% and 7.3% for nicorandil and its metabolite, respectively. The mean accuracy values were 100% and 95% for nicorandil and its metabolite, respectively. No matrix effect was detected in the samples. The validated method was successfully applied to a pharmacokinetic study after per os administration of nicorandil in rats. 相似文献
993.
Souza AB de Souza MG Moreira MA Moreira MR Furtado NA Martins CH Bastos JK dos Santos RA Heleno VC Ambrosio SR Veneziani RC 《Molecules (Basel, Switzerland)》2011,16(11):9611-9619
The antimicrobial activity of four labdane-type diterpenes isolated from the oleoresin of Copaifera langsdorffii as well as of two commercially available diterpenes (sclareol and manool) was investigated against a representative panel of microorganisms responsible for periodontitis. Among all the evaluated compounds, (-)-copalic acid (CA) was the most active, displaying a very promising MIC value (3.1 μg mL-1; 10.2 μM) against the key pathogen (Porphyromonas gingivalis) involved in this infectious disease. Moreover, CA did not exhibit cytotoxicity when tested in human fibroblasts. Time-kill curve assays performed with CA against P. gingivalis revealed that this compound only inhibited the growth of the inoculums in the first 12 h (bacteriostatic effect). However, its bactericidal effect was clearly noted thereafter (between 12 and 24 h). It was also possible to verify an additive effect when CA and chlorhexidine dihydrochloride (CHD, positive control) were associated at their MBC values. The time curve profile resulting from this combination showed that this association needed only six hours for the bactericidal effect to be noted. In summary, CA has shown to be an important metabolite for the control of periodontal diseases. Moreover, the use of standardized extracts based on copaiba oleoresin with high CA contents can be an important strategy in the development of novel oral care products. 相似文献
994.
Lago JH Souza ED Mariane B Pascon R Vallim MA Martins RC Baroli AA Carvalho BA Soares MG dos Santos RT Sartorelli P 《Molecules (Basel, Switzerland)》2011,16(12):9827-9837
The chemical composition and antimicrobial activity of essential oils obtained from leaves of two Myrtaceae species-Eugenia uniflora L. and Plinia trunciflora (O. Berg) Kausel-were determined. Analysis by GC/MS as well as determination of Kovatz indexes indicated atractylone (26.78%) and curzerene (17.96%) as major constituents of E. uniflora oil and α-cadinol (19.15%), apiole (11.15%) and cubenol (5.43%) as main components in P. trunciflora oil. Both essential oils were tested for antimicrobial activity against yeasts and bacteria. E. uniflora and P. trunciflora essential oils were active towards two Gram-positive bacteria, Streptococcus equi and Staphylococcus epidermis. In addition, biological activity of both essential oils was detected for pathogenic yeasts of the genus Candida and Cryptococcus. E. uniflora was active towards all yeast tested and exhibited interesting minimal inhibitory concentrations (0.11 to 3.75 mg/mL) across a broad spectrum of activity. 相似文献
995.
Busto N García B Leal JM Gaspar JF Martins C Boggioni A Secco F 《Physical chemistry chemical physics : PCCP》2011,13(43):19534-19545
The interaction of ACMA (9-amino-6-chloro-2-methoxy acridine) (D) with DNA (P) has been studied by absorbance, fluorescence, circular dichroism, spectrophotometry, viscometry and unwinding electrophoresis. A T-jump kinetic study has also been undertaken. The experimental data show that, totally unlike other drugs, ACMA is able to form with DNA three complexes (PD(I), PD(II), PD(III)) that differ from each other by the characteristics and extent of the binding process. The main features of PD(I) fulfil the classical intercalation pattern and the formation/dissociation kinetics have been elucidated by T-jump techniques. PD(II) and PD(III) are also intercalated species but, in addition to the dye units lodged between base pairs, they also bear dye molecules externally bound, more in PD(III) relative to PD(II). A reaction mechanism is put forward here. Comparison between absorbance, fluorescence and kinetic experiments has enabled us to determine the binding constants of the three complexes, namely (6.5 ± 1.1) × 10(4) M(-1) (PD(I)), (5.5 ± 1.5) × 10(4) M(-1) (PD(II)) and (5.7 ± 0.03) × 10(4) M(-1) (PD(III)). The Comet assay reveals that the ACMA binding to DNA brings about genotoxic properties. The mutagenic potential studied by the Ames test reveals that ACMA can produce frameshift and transversion/transition mutations. ACMA also is able to produce base-pair substitution in the presence of S9 mix. Moreover, the MTT assays have revealed cytotoxicity. The biological effects observed have been rationalized in light of these features. 相似文献
996.
Kopylovich MN Nunes AC Mahmudov KT Haukka M Mac Leod TC Martins LM Kuznetsov ML Pombeiro AJ 《Dalton transactions (Cambridge, England : 2003)》2011,40(12):2822-2836
Reactions of copper(II) with 3-phenylhydrazopentane-2,4-diones X-2-C(6)H(4)-NHN=C{C(=O)CH(3)}(2) bearing a substituent in the ortho-position [X = OH (H(2)L(1)) 1, AsO(3)H(2) (H(3)L(2)) 2, Cl (HL(3)) 3, SO(3)H (H(2)L(4)) 4, COOCH(3) (HL(5)) 5, COOH (H(2)L(6)) 6, NO(2) (HL(7)) 7 or H (HL(8)) 8] lead to a variety of complexes including the monomeric [CuL(4)(H(2)O)(2)]·H(2)O 10, [CuL(4)(H(2)O)(2)] 11 and [Cu(HL(4))(2)(H(2)O)(4)] 12, the dimeric [Cu(2)(H(2)O)(2)(μ-HL(2))(2)] 9 and the polymeric [Cu(μ-L(6))](n)] 13 ones, often bearing two fused six-membered metallacycles. Complexes 10-12 can interconvert, depending on pH and temperature, whereas the Cu(II) reactions with 4 in the presence of cyanoguanidine or imidazole (im) afford the monomeric compound [Cu(H(2)O)(4){NCNC(NH(2))(2)}(2)](HL(4))(2)·6H(2)O 14 and the heteroligand polymer [Cu(μ-L(4))(im)](n)15, respectively. The compounds were characterized by single crystal X-ray diffraction (complexes), electrochemical and thermogravimetric studies, as well as elemental analysis, IR, (1)H and (13)C NMR spectroscopies (diones) and ESI-MS. The effects of the substituents in 1-8 on the HOMO-LUMO gap and the relative stability of the model compounds [Cu(OH)(L(8))(H(2)O)]·H(2)O, [Cu(L(1))(H(2)O)(2)]·H(2)O and [Cu(L(4))(H(2)O)(2)]·H(2)O are discussed on the basis of DFT calculations that show the stabilization follows the order: two fused 6-membered > two fused 6-membered/5-membered > one 6-membered metallacycles. Complexes 9, 10, 12 and 13 act as catalyst precursors for the peroxidative oxidation (with H(2)O(2)) of cyclohexane to cyclohexanol and cyclohexanone, in MeCN/H(2)O (total yields of ca. 20% with TONs up to 566), under mild conditions. 相似文献
997.
Taking in consideration the global analysis of complex samples, proposed by the metabolomic approach, the chromatographic
fingerprint encompasses an attractive chemical characterization of herbal medicines. Thus, it can be used as a tool in quality
control analysis of phytomedicines. The generated multivariate data are better evaluated by chemometric analyses, and they
can be modeled by classification methods. “Stone breaker” is a popular Brazilian plant of Phyllanthus genus, used worldwide to treat renal calculus, hepatitis, and many other diseases. In this study, gradient elution at reversed-phase
conditions with detection at ultraviolet region were used to obtain chemical profiles (fingerprints) of botanically identified
samples of six Phyllanthus species. The obtained chromatograms, at 275 nm, were organized in data matrices, and the time shifts of peaks were adjusted
using the Correlation Optimized Warping algorithm. Principal Component Analyses were performed to evaluate similarities among
cultivated and uncultivated samples and the discrimination among the species and, after that, the samples were used to compose
three classification models using Soft Independent Modeling of Class analogy, K-Nearest Neighbor, and Partial Least Squares for Discriminant Analysis. The ability of classification models were discussed
after their successful application for authenticity evaluation of 25 commercial samples of “stone breaker.” 相似文献
998.
William B. Fernandes Angelo Q. Arago Felipe T. Martins Caridad Noda‐Perez Carlito Lariucci Hamilton B. Napolitano 《Acta Crystallographica. Section C, Structural Chemistry》2011,67(7):o226-o229
Two arylsulfonamide derivatives, N‐(4‐acetylphenyl)benzenesulfonamide, C14H13NO3S, and N‐(4‐acetylphenyl)‐2,5‐dichlorobenzenesulfonamide, C14H11Cl2NO3S, differing by the absence or presence of two chloro substituents on one of the phenyl rings, were synthesized and characterized in order to establish structural relationships and the role of chloro substitution on the molecular conformation and crystal assembly. Both arylsulfonamides form inversion‐related dimers through C—H...π and π–π interactions. These dimers pack in a similar way in the two structures. The substitution of two H atoms at the 2‐ and 5‐positions of one phenyl ring by Cl atoms did not substantially alter the molecular conformation or the intermolecular architecture displayed by the unsubstituted sulfonamide. The structural information controlling the assembly of such compounds in their crystal phases is in the (phenyl)benzenesulfonamide molecular framework. 相似文献
999.
Structural basis for a Kolbe-type decarboxylation catalyzed by a glycyl radical enzyme 总被引:2,自引:0,他引:2
Martins BM Blaser M Feliks M Ullmann GM Buckel W Selmer T 《Journal of the American Chemical Society》2011,133(37):14666-14674
4-Hydroxyphenylacetate decarboxylase is a [4Fe-4S] cluster containing glycyl radical enzyme proposed to use a glycyl/thiyl radical dyad to catalyze the last step of tyrosine fermentation in clostridia. The decarboxylation product p-cresol (4-methylphenol) is a virulence factor of the human pathogen Clostridium difficile . Here we describe the crystal structures at 1.75 and 1.81 ? resolution of substrate-free and substrate-bound 4-hydroxyphenylacetate decarboxylase from the related Clostridium scatologenes . The structures show a (βγ)(4) tetramer of heterodimers composed of a catalytic β-subunit harboring the putative glycyl/thiyl dyad and a distinct small γ-subunit with two [4Fe-4S] clusters at 40 ? distance from the active site. The γ-subunit comprises two domains displaying pseudo-2-fold symmetry that are structurally related to the [4Fe-4S] cluster-binding scaffold of high-potential iron-sulfur proteins. The N-terminal domain coordinates one cluster with one histidine and three cysteines, and the C-terminal domain coordinates the second cluster with four cysteines. Whereas the C-terminal cluster is buried in the βγ heterodimer interface, the N-terminal cluster is not part of the interface. The previously postulated decarboxylation mechanism required the substrate's hydroxyl group in the vicinity of the active cysteine residue. In contrast to expectation, the substrate-bound state shows a direct interaction between the substrate's carboxyl group and the active site Cys503, while His536 and Glu637 at the opposite side of the active site pocket anchor the hydroxyl group. This state captures a possible catalytically competent complex and suggests a Kolbe-type decarboxylation for p-cresol formation. 相似文献
1000.