全文获取类型
收费全文 | 172027篇 |
免费 | 2045篇 |
国内免费 | 722篇 |
专业分类
化学 | 92591篇 |
晶体学 | 2465篇 |
力学 | 7644篇 |
综合类 | 7篇 |
数学 | 17920篇 |
物理学 | 54167篇 |
出版年
2021年 | 1006篇 |
2020年 | 1129篇 |
2019年 | 1140篇 |
2018年 | 1168篇 |
2017年 | 1179篇 |
2016年 | 2370篇 |
2015年 | 1939篇 |
2014年 | 2523篇 |
2013年 | 7756篇 |
2012年 | 6320篇 |
2011年 | 8008篇 |
2010年 | 4924篇 |
2009年 | 5092篇 |
2008年 | 7369篇 |
2007年 | 7351篇 |
2006年 | 7073篇 |
2005年 | 6585篇 |
2004年 | 5897篇 |
2003年 | 5089篇 |
2002年 | 4998篇 |
2001年 | 6038篇 |
2000年 | 4569篇 |
1999年 | 3483篇 |
1998年 | 2631篇 |
1997年 | 2623篇 |
1996年 | 2575篇 |
1995年 | 2281篇 |
1994年 | 2171篇 |
1993年 | 2038篇 |
1992年 | 2352篇 |
1991年 | 2312篇 |
1990年 | 2023篇 |
1989年 | 2008篇 |
1988年 | 2033篇 |
1987年 | 1964篇 |
1986年 | 1875篇 |
1985年 | 2775篇 |
1984年 | 2767篇 |
1983年 | 2216篇 |
1982年 | 2394篇 |
1981年 | 2274篇 |
1980年 | 2228篇 |
1979年 | 2174篇 |
1978年 | 2244篇 |
1977年 | 2200篇 |
1976年 | 2131篇 |
1975年 | 2119篇 |
1974年 | 2024篇 |
1973年 | 2105篇 |
1972年 | 1199篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Claiser N Souhassou M Lecomte C Gillon B Carbonera C Caneschi A Dei A Gatteschi D Bencini A Pontillon Y Lelièvre-Berna E 《The journal of physical chemistry. B》2005,109(7):2723-2732
High-resolution X-ray diffraction and polarized neutron diffraction experiments have been performed on the Y-semiquinonate complex, Y(HBPz3)2(DTBSQ), in order to determine the charge and spin densities in the paramagnetic ground state, S = (1/2). The aim of these combined studies is to bring new insights to the antiferromagnetic coupling mechanism between the semiquinonate radical and the rare earth ion in the isomorphous Gd(HBPz3)2(DTBSQ) complex. The experimental charge density at 106 K yields detailed information about the bonding between the Y3+ ion and the semiquinonate ligand; the topological charge of the yttrium atom indicates a transfer of about 1.5 electrons from the radical toward the Y3+ ion in the complex, in agreement with DFT calculations. The electron density deformation map reveals well-resolved oxygen lone pairs with one lobe polarized toward the yttrium atom. The determination of the induced spin density at 1.9 K under an applied magnetic field of 9.5 T permits the visualization of the delocalized magnetic orbital of the radical throughout the entire molecule. The spin is mainly distributed on the oxygen atoms [O1 (0.12(1) mu B), O2(0.11(1) mu B)] and the carbon atoms [C21 (0.24(1) mu B), C22(0.20(1) mu B), C24(0.16(1) mu B), C25(0.12(1) mu B)] of the carbonyl ring. A significant spin delocalization on the yttrium site of 0.08(2) mu B is observed, proving that a direct overlap with the radical magnetic orbital can occur at the rare earth site and lead to antiferromagnetic coupling. The DFT calculations are in good quantitative agreement with the experimental charge density results, but they underestimate the spin delocalization of the oxygen toward the yttrium and the carbon atoms of the carbonyl ring. 相似文献
992.
We consider a lattice model of branched polymers in dilute solution in which the polymer is modelled as an animal, weakly embeddable in the (simple cubic) lattice. In order to model the effect on the thermodynamic properties of changing the temperature or the quality of the solvent, we include an energy associated with the number of nearneighbour contacts between pairs of vertices of the animals. We show that the configurational free energy of the animal is a continuous function of the temperature and derive rigorous upper and lower bounds on the temperature dependence of the free energy. Finally, we comment on similarities between these results and corresponding ones for a model in which the energy is associated with the cyclomatic index of the animal. 相似文献
993.
K. Jungmann P. E. G. Baird J. R. M. Barr C. Bressler P. F. Curley R. Dixson G. H. Eaton A. I. Ferguson H. Geerds V. W. Hughes J. Kenntner S. N. Lea F. Maas M. A. Persaud G. zu Putlitz P. G. H. Sandars W. Schwarz W. T. Toner M. Towrie G. Woodman L. Zhang Z. Zhang 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,21(3):241-243
The 12 S 1/2(F=1)?22 S 1/2(F=1) energy interval in muonium has been investigated by Doppler-free two-photon laser spectroscopy. The transition was observed by uniquely identifying and counting the muons released after the photoionization of the 2S state by the same laser field. The measured transition frequency of 2455 528016(58)(43) MHz is in good agreement with QED calculations. The experiment can be interpreted as a test of the Lamb shift contributions at the 1% level. The method is also well suited for a precise determination of the mass of the positive muon in a muoniumhydrogen isotope shift measurement. 相似文献
994.
Treatment of the μ-alkylidyne clusters [Fe2W(μ3-CC6H4Me-4)(μ-CO)- (CO)8(η-C5H5)] and [Co2W(μ3-CMe)(CO)8(η-C5H5)] with PPh2H affords a series of new μ-phosphido-μ-hydrido alkylidyne complexes which undergo protonation with HBF4·Et2O to give cationic derivatives. The X-ray structure of [Co2W(μ-H)(μ3-CMe)(μ-PPh2)(CO)6(η-C5H5)] has been determined. 相似文献
995.
996.
Evanescent fluorobiosensor for the detection of polyaromatic hydrocarbon based on DNA intercalation 总被引:1,自引:0,他引:1
A flow-injection analysis (FIA) system coupled with an evanescent wave (EW) Biosensor employing total internal reflection
of fluorescence radiation (TIRF) for the detection of polyaromatic hydrocarbon that intercalates into DNA is reported. A highly
fluorescent intercalator, “ethidium bromide,” has been used as the reference compound for detection. The EW Biosensor was
developed according to the procedure described earlier (1,2). Data on the analysis of Naphthalene, 3-methy cholanthrene, 7,12-dimethylbenz(a)anthracene,
1,2-benzanthracene, and some standard reference materials supplied by the National Institute of Standards and Technology are
reported. The relative ability of the polyaromatic hydrocarbon to displace ethidium bromide, based on the relative binding
ratio, is found to be on the order of 7,12-dimethylbenz[a]anthracene > 3-methylcholanthrene > 1,2-benzanthracene > napthalene. 相似文献
997.
Changes in the physicochemical properties of transferrin upon chemical derivatization with ceramides
M. A. Egea M. L. García M. A. Alsina C. Mestres F. Reig 《Colloid and polymer science》1994,272(5):570-576
The interaction of native and derivatized transferrin with lecithin, cholesterol and their mixtures was studied using monomolecular layers of phospholipids. Moreover, the thermodynamic parameters corresponding to these interactions: the mixing excess free energy, interaction energy, and interaction parameter were also calculated.The ability of these two proteins to insert into monolayers was studied working at constant area.The formation of aggregates or secondary structures in solution was determined by the fluorescence and polarization values of ANS (anilinonaphthalene sulfonate), measured at different protein/ANS relationships.Abbreviations TF
Transferrin
- DTF
derivatized transferrin
- PC
lecithin
- CHOL
cholesterol
- CER
ceramides
- ANS
anilinonaphthalene sulfonate
- TNBS
trinitro benzenesulfonate method 相似文献
998.
999.
Simms PJ Jeffries CT Huang Y Zhang L Arrhenius T Nadzan AM 《Journal of combinatorial chemistry》2001,3(5):427-433
A micellar electrokinetic chromatography (MEKC) method has been developed that can evaluate the purity of samples generated in combinatorial chemistry libraries. This method uses an open tube capillary (27 cm x 50 microm) along with a run buffer composed of sodium dodecyl sulfate (SDS), hydroxypropyl-beta-cyclodextrin, and sodium tetraborate coupled with UV detection. Neutral compounds and compounds that were insoluble in aqueous buffers could be analyzed under these conditions in approximately 3 min. The concentration of SDS and the concentration of hydroxypropyl-beta-cyclodextrin effected the separation. The affect on selectivity resulting from the addition of an organic modifier to the run buffer was examined. The low background absorbency of the run buffer made for easy detection of compounds that absorbed at low UV wavelengths. The quick analysis time made this suitable for analysis of combinatorial chemistry samples. 相似文献
1000.
Alcohol dehydrogenase (ADH) has been immobilized on a nickel hexacyanoferrate modified microband gold electrode surface by a glutaraldehyde/bovine serum albumin (BSA) cross-linking procedure to provide a new amperometric sensor for the assay of ethanol. The resulting enzyme electrode exhibits excellent electrocatalysis for the oxidation of reduced nicotinamide-adenine dinucleotide (NADH). The amperometric determination is based on the electrochemical detection of NADH which is generated in the enzymatic reaction of ethanol with NAD(+) under catalysis of ADH. The influence of various experimental conditions was examined for the determination of the optimum analytical performance. The sensor responds rapidly to ethanol with a detection limit of (5.0 +/- 0.3) x 10(-7) mol 1(-1). The response current increases linearly with ethanol concentration up to 5 mmol 1(-1). The sensor remains relatively stable for about 1 week. 相似文献