全文获取类型
收费全文 | 6042篇 |
免费 | 186篇 |
国内免费 | 1篇 |
专业分类
化学 | 3883篇 |
晶体学 | 27篇 |
力学 | 199篇 |
数学 | 780篇 |
物理学 | 1340篇 |
出版年
2023年 | 32篇 |
2022年 | 34篇 |
2021年 | 49篇 |
2020年 | 69篇 |
2019年 | 75篇 |
2018年 | 100篇 |
2017年 | 71篇 |
2016年 | 138篇 |
2015年 | 177篇 |
2014年 | 150篇 |
2013年 | 333篇 |
2012年 | 383篇 |
2011年 | 373篇 |
2010年 | 251篇 |
2009年 | 151篇 |
2008年 | 377篇 |
2007年 | 337篇 |
2006年 | 316篇 |
2005年 | 304篇 |
2004年 | 264篇 |
2003年 | 249篇 |
2002年 | 206篇 |
2001年 | 124篇 |
2000年 | 137篇 |
1999年 | 71篇 |
1998年 | 85篇 |
1997年 | 58篇 |
1996年 | 83篇 |
1995年 | 45篇 |
1994年 | 52篇 |
1993年 | 49篇 |
1992年 | 54篇 |
1991年 | 32篇 |
1990年 | 37篇 |
1989年 | 33篇 |
1988年 | 46篇 |
1986年 | 34篇 |
1985年 | 50篇 |
1984年 | 60篇 |
1983年 | 31篇 |
1982年 | 42篇 |
1981年 | 54篇 |
1980年 | 41篇 |
1979年 | 37篇 |
1978年 | 47篇 |
1977年 | 48篇 |
1976年 | 55篇 |
1975年 | 39篇 |
1974年 | 31篇 |
1973年 | 29篇 |
排序方式: 共有6229条查询结果,搜索用时 30 毫秒
141.
Balandina A Kalinin A Mamedov V Figadère B Latypov S 《Magnetic resonance in chemistry : MRC》2005,43(10):816-828
(1)H, (13)C and (15)N NMR chemical shifts for a variety of novel quinoxalines were determined by different 2D methods and were calculated using the GIAO DFT approach. Comparison with experimental data shows good correlations in the case of (1)H, (13)C and (15)N chemical shifts. Different combinations of basis sets were tested. In non-polar solvents quinoxalines exist as dimers owing to strong hydrogen bonding. Calculations for dimers improve the correlation between experiment and theory. Additive empirical methods for estimating chemical shifts have drawbacks and have to be used with a great care for this type of compound. 相似文献
142.
143.
P. De Bièvre 《Accreditation and quality assurance》2000,5(6):224-230
Traceability is a property of the result of a measurement. Since values carried by (reference) materials must also have been
obtained, of necessity, by measurement, the definition of traceability also applies to reference materials. It is extremely
helpful to give the traceability (of the origin) of a reference material a separate name, i.e. 'trackability'. An analysis
of the function of values carried by reference materials, shows that they can fulfill different functions, depending on the
intended use. One of the functions located outside the traceability chain – and hence not very relevant for establishing traceability
– is evaluating the approximate size of the uncertainty of the measurement of an unknown sample by performing a similar measurement
on a reference material, used as a 'simulated sample'. Another function is located inside the traceability chain, where the
reference material is used as an added 'internal standard'. Then, the value carried by the reference material is essential
for establishing the traceability of the measured value of an unknown sample. In the latter application, the reference material
acts as an 'amount standard' (the certified value for amount is used).
Received: 11 November 1999 / Accepted: 24 February 2000 相似文献
144.
Valérie Bernat Catherine Ringard-Lefebvre Geneviève Le Bas Sylviane Lesieur 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):113-119
Interactions of α-cyclodextrin (α-CD) with dimyristoylphosphatidylcholine (DMPC) and Egg phosphatidylcholine (Egg-PC) were studied (i) by analyzing surface
pressure-area isotherms and surface tension of phospholipid monolayers formed at the interface between air and α-CD aqueous solutions and (ii) by X-ray diffraction performed on fully hydrated α-CD/phospholipid binary mixtures. The cyclodextrin molecules strongly interact with the two-dimension phospholipid assembly.
Their addition into the aqueous sub-phase leads to the removal of part of the phospholipids from the air-water interface:
the higher the α-CD concentration, the higher the phospholipid depletion. This should preferentially involve interactions between cyclodextrin
and the phosphatidylcholine head group as α-CD is water-soluble and not surface-active. At the three-dimension level, the bilayer packing of the phospholipid lamellar
phase appears not affected by the presence of cyclodextrin as shown by X-ray scattering at small angles whereas wide-angle
diffraction patterns reveal the formation of a crystalline phase organized in a pseudo-hexagonal lattice usually characteristic
of α-CD dimers. These results point out that α-CD should interact with bilayer-forming phospholipid molecules but likely according to a process that would preserve intact
at least a part of the multilamellar assembly. 相似文献
145.
146.
Livio Racanè Vesna Trali?-Kulenovi? Richard P. Kitson Grace Karminski-Zamola 《Monatshefte für Chemie / Chemical Monthly》2006,18(1):1571-1577
Series of cyano, dicyano, amidino, and diamidino substituted 2-phenylbenzothiazoles were prepared. Mono- and dicyano substituted
benzothiazoles were obtained by condensation of appropriate substituted benzaldehydes with 2-aminothiophenol or 4-amino-3-mercaptobenzonitrile.
The appropriate amidines or diamidines were prepared by Pinner reaction. The compounds were tested against breast, prostate, and lung cancer cell lines in a 72 h cytotoxicity assay. Many
of the compounds had at 10 μM activity equivalent to 2-(4-aminophenyl)benzothiazole, while four compounds had significantly better activity, particularly
in the breast cancer model. 相似文献
147.
Brunetto R Gutiérrez L Delgado Y Gallignani M Burguera JL Burguera M 《Analytical and bioanalytical chemistry》2003,375(4):534-538
A column-switching high-performance liquid chromatographic method with UV detection for the determination of cocaine (COC) and benzoylecgonine (BZE) in human blood plasma samples is described. The method uses an alkyl-diol-silica ADS-C18 extraction precolumn. A 50- micro L plasma sample was introduced to the ADS precolumn in order to separate the analytes from proteins and endogenous compounds. The fraction containing COC and BZE was back-flushed and transferred to an Alltech mixed-mode C(18)/cation-exchange analytical column for final separation. The validation of the method revealed quantitative recoveries from 95.0 to 99.0% for COC at three different concentrations (0.5, 1.0 and 2.0 micro g mL(-1)), and from 96.0 to 99.0% for BZE at the same concentration levels with coefficients of variation <4.00% (n=5). The detection limit (signal to noise ratio (S/N)>3) was 0.03 micro g mL(-1) for all the compounds with an injection volume of 50 micro L. However, it was possible to enhance the sensitivity further by injecting larger plasma volumes, up to 200 micro L, at the same optimal conditions. The overlap of sample preparation, analysis and reconditioning of the extraction column, increase the overall sample throughput to 5 samples h(-1). The developed method has been applied to human blood plasma samples from subjects suspected of cocaine abuse. 相似文献
148.
The [Ru(bipy)(2)(1)](PF(6))(2) (bipy refers to 2,2'-bipyridine) complex, comprising a ruthenium(II) tris(2,2'-bipyridine) luminophore covalently linked to a di[(o-triethyleneglycoxy)phenyl]amine crown ether 1, has been synthesized and fully characterized. The photophysical properties of this metal complex have been examined in solution at ambient temperature. Luminescence from the metal complex is enhanced significantly in the presence of various adventitious cations, including protons. In particular, Li(+) cations bind to the crown ether, as evidenced by (1)H NMR and luminescence spectroscopy. Cation binding serves to decrease the rate of reductive quenching of the triplet state of the metal complex, thereby increasing the extent of luminescence. The solution-phase conformation of [Ru(bipy)(2)(1)](PF(6))(2), with and without encapsulated Li(+), has been examined by 2-D NMR and by molecular dynamics simulations. 相似文献
149.
Isobe H Tanaka T Nakanishi W Lemiègre L Nakamura E 《The Journal of organic chemistry》2005,70(12):4826-4832
The reaction of [60]fullerene with a variety of a secondary aliphatic amines in 20% v/v dimethyl sulfoxide in chlorobenzene under an atmospheric pressure of molecular oxygen allows regioselective introduction of four amino groups and one epoxide group around one pentagon of the fullerene molecule in good to high yield. This new synthesis of tetraaminofullerene expoxide can be carried out with a simple procedure on a multigram scale at room temperature and affords a variety of functionalized fullerene derivatives. Near-infrared analysis of a mixture of [60]fullerene and piperidine in a deaerated dimethyl sulfoxide/chlorobenzene mixture indicated equilibrium formation of [60]fullerene radical anion (C60*-) that persists at least for 2 weeks at room temperature but reacts immediately with molecular oxygen to give the tetraaminofullerene expoxide. The Benesi-Hildebrand analysis of the concentration dependency of the near-infrared absorption indicated that a [C60*- piperidine*+] radical ion pair is formed with an equivalent constant of K = 0.62 +/- 0.02 M(-1) at 25 degrees C. This and other lines of evidence suggest that the oxygenative amination reaction involves C60-mediated reduction of molecular oxygen by the amine. 相似文献
150.
Troutier AL Delair T Pichot C Ladavière C 《Langmuir : the ACS journal of surfaces and colloids》2005,21(4):1305-1313
A model study was investigated to develop colloidal supramolecular assemblies consisting of particles coated with lipid layers. The interactions between monodisperse sulfate-charged poly(styrene) submicrometer particles and zwitterionic/cationic lipid vesicles composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-3-trimethylammonium propane were considered. The influence of relevant experimental parameters on the final associations was examined by quasi-elastic light scattering to point out some new phenomena occurring in these colloidal systems. The major role of electrostatic interactions as driving forces to control the organization between cationic lipids and oppositely charged poly(styrene) particles was clearly evident, whereas this influence was less pronounced when considering the zwitterionic lipids. The characterization of these original complex assemblies was completed by a thorough study of the surface modification. The combination of zeta potential measurements, X-ray photoelectron spectroscopy analyses, and microscopy observations proved that the envisioned model can really correspond to polymer particles surrounded by lipids. 相似文献