全文获取类型
收费全文 | 3990篇 |
免费 | 246篇 |
国内免费 | 19篇 |
专业分类
化学 | 3212篇 |
晶体学 | 30篇 |
力学 | 153篇 |
数学 | 242篇 |
物理学 | 618篇 |
出版年
2023年 | 15篇 |
2022年 | 48篇 |
2021年 | 77篇 |
2020年 | 70篇 |
2019年 | 77篇 |
2018年 | 53篇 |
2017年 | 48篇 |
2016年 | 141篇 |
2015年 | 121篇 |
2014年 | 171篇 |
2013年 | 241篇 |
2012年 | 355篇 |
2011年 | 344篇 |
2010年 | 247篇 |
2009年 | 195篇 |
2008年 | 306篇 |
2007年 | 268篇 |
2006年 | 250篇 |
2005年 | 221篇 |
2004年 | 190篇 |
2003年 | 142篇 |
2002年 | 156篇 |
2001年 | 77篇 |
2000年 | 57篇 |
1999年 | 47篇 |
1998年 | 26篇 |
1997年 | 32篇 |
1996年 | 42篇 |
1995年 | 22篇 |
1994年 | 24篇 |
1993年 | 20篇 |
1992年 | 12篇 |
1991年 | 13篇 |
1990年 | 12篇 |
1989年 | 9篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 4篇 |
1985年 | 11篇 |
1984年 | 4篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 9篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 6篇 |
排序方式: 共有4255条查询结果,搜索用时 257 毫秒
81.
The kinetics of cation exchange reaction of europium propylenediaminetetraacetate with ytterbium ion
The kinetics of cation exchange reaction of Eu(III) propylenediaminetetraacetate with Yb(III) has been studied in an aqueous solution by the polarographic procedure. The measurements were made at 25°C and at an ionic strength of 0.5 M KCl. The dissociation of the europium complex has been found to be catalyzed by hydrogen ions. The influences of the inductive effect and the steric hindrance of a C-methyl group on the ethylene of PDTA are discussed by comparing it to analogous EDTA and CyDTA system. 相似文献
82.
Joseph F. Chiang Raymond Chiang K.C. Lu Eun-Mo Sung Marlin D. Harmony 《Journal of Molecular Structure》1977,41(1):67-77
The molecular structure of norbornene has been investigated in the gas phase by combining electron diffraction data with microwave spectroscopic rotational constants. The interatomic distances (rg) and bond angles were obtained by applying a least squares program to the refined experimental molecular diffraction intensities. The CC bond length was found to be 1.336 ± 0.002 Å while the ) bond length was 1. 529 ± 0.007 Å. Other bond lengths and angles included (IUPAC numbering system was used for norbornene): C1-C6 = 1.550 ± 0.020 Å, C1-C7 = 1.566± 0.005 Å, C5-C6 = 1.556 ± 0.005 Å, C-Have. = 1.103 ± 0.003 Å, ∠C1C2C4 = 95.3°. The dihedral angle between planes C1C2C3C4 and C1C6C5C4 is 110.8 ± 1.5° while that between C1C2C3C4 and C1C7C4 is 122.3°. The moments of inertia calculated from ED structure are in good agreement with microwave spectroscopic values. 相似文献
83.
Kim SH Tamrazi A Carlson KE Daniels JR Lee IY Katzenellenbogen JA 《Journal of the American Chemical Society》2004,126(15):4754-4755
We present the first example of a nuclear hormone receptor microarray, using for illustration the ligand-binding domains of the two estrogen receptors, ERalpha-LBD and ERbeta-LBD. The proteins are printed and allowed to attach to aldehyde slides; the efficiency of attachment depends on whether the LBD is liganded with agonists (low attachment) versus liganded with antagonists or unliganded (high attachment). This suggests that attachment is orientation specific and involves principally a single lysine residue. The attached ERs retain good ligand-binding activity that can be assessed using an estradiol-fluorophore conjugate, and specific and ER subtype-selective binding of ligands can be determined conveniently in competitive binding assays. This powerful new, high-throughput technique to study ligand binding to ER-LBDs can be extended to other nuclear hormone receptors and adapted to assay the recruitment of coregulator proteins. 相似文献
84.
[reaction: see text] In situ generated allylindium reagents from the reaction of 1 equiv of indium with 1.5 equiv of allyl halides could be effective cross-coupling partners in palladium-catalyzed cross-coupling reactions to aryl halides. The best results were obtained with 2% Pd(2)dba(3)CHCl(3) and 16% Ph(3)P in the presence of 3 equiv of LiCl in DMF at 100 degrees C. 相似文献
85.
Summary Procedures for the quantitative analysis of industrial effluents which involve concentration by solvent extraction or the
purge-and-trap method are time-consuming, labor-intensive, and prone to error. Direct aqueous injection gas chromatography
using an electron-capture detector for the analysis of volatile halocarbons at the ppb level is in routine use in many laboratories.
We now discuss the development of a similar protocol for the analysis of volatile polar organics such as acetone, methyl ethyl
ketone, methyl isobutyl ketone, and tretrahydrofuran using a flame-ionization detector. 相似文献
86.
Interpretation of the chemical and spectral (IR., UV., 1H- and 13C-NMR.) properties of the antitumor antibiotic hedamycin (C41H50N2O11) suggests that the molecule contains a methyl substituted 1-hydroxyanthraquinone nucleus, an α, β-unsaturated ketone, two sugar-like tetrahydropyran rings ( 4 and 8 ) and an aliphatic chain 2 , presumably with an epoxy group (see the Scheme). 相似文献
87.
Joo J Kwon SG Yu T Cho M Lee J Yoon J Hyeon T 《The journal of physical chemistry. B》2005,109(32):15297-15302
A simple method of synthesizing a large quantity of TiO(2) nanorods was developed. A nonhydrolytic sol-gel reaction between titanium(IV) isopropoxide and oleic acid at 270 degrees C generated 3.4 nm (diameter) x 38 nm (length) sized TiO(2) nanocrystals. The transmission electron microscopic image showed that the particles have a uniform diameter distribution. X-ray diffraction and selected-area electron diffraction patterns combined with high-resolution transmission electron microscopic image showed that the TiO(2) nanorods are highly crystalline anatase crystal structure grown along the [001] direction. The diameters of the TiO(2) nanorods were controlled by adding 1-hexadecylamine to the reaction mixture as a cosurfactant. TiO(2) nanorods with average sizes of 2.7 nm x 28 nm, 2.2 nm x 32 nm, and 2.0 nm x 39 nm were obtained using 1, 5, and 10 mmol of 1-hexadecylamine, respectively. The optical absorption spectrum of the TiO(2) nanorods exhibited that the band gap of the nanorods was 3.33 eV at room temperature, which is 130 meV larger than that of bulk anatase (3.2 eV), demonstrating the quantum confinement effect. Oleic acid coordinated on the nanorod surface was removed by the reduction of the carboxyl group of oleic acid, and the Brunauer-Emmett-Teller surface area of the resulting naked TiO(2) nanorods was 198 m(2)/g. The naked TiO(2) nanorods exhibited higher photocatalytic activity than the P-25 photocatalyst for the photocatalytic inactivation of E. coli. 相似文献
88.
A new approach to a CD45 protein tyrosine phosphatase inhibitor, pulchellalactam, is described. The key step of the sequence involves addition and elimination of an enolic lactam in a single step and 70% yield, employing an organocuprate reagent. The resulting alpha,beta-unsaturated lactam could be condensed with isobutyraldehyde to produce Z-pulchellalactam or converted into siloxypyrrole, which was subjected to the BF(3) x Et(2)O-promoted coupling reaction with isobutyraldehyde to afford E-pulchellalactam after E1-cB elimination and TFA deprotection. This first total synthesis afforded Z-pulchellalactam in six steps and 32% overall yield from Boc-glycine. The same sequence of reactions could also be applied to the liquid- or solid-phase synthesis of trifunctionalized pulchellalactam derivatives. 相似文献
89.
Structurally characterized tungstoenzymes contain mononuclear active sites in which tungsten is coordinated by two pterin-dithiolene ligands and one or two additional ligands that have not been identified. In this and prior investigations (Sung, K.-M.; Holm, R. H. Inorg. Chem. 2000, 39, 1275; J. Am. Chem. Soc. 2001, 123, 1931), stable coordination units of bis(dithiolene)tungsten(IV,V,VI) complexes potentially related to enzyme sites have been sought by exploratory synthesis. In this work, additional members of the sets [WL(S2C2Me2)2](2-,-) and [WLL'(S2C2Me2)2](2-,-) have been prepared and structurally characterized. Tungsten(IV) complexes obtained by substitution are carbonyl displacement products of [W(CO)2(S2C2Me2)2] and include those with the groups W(IV)S (4), W(IV)(O2CPh) (5), and W(IV)(2-AdQ)(CO) (Q = S (6), Se (7); Ad = adamantyl). Those obtained by oxidation reactions contain the groups W(V)O (9), W(V)(QPh)2 (Q = S (10), Se (11)), W(VI)S(OPh) (12), and W(VI)O2 (14). The latter two complexes were obtained from W(IV) precursors using sulfur and oxygen atom transfer reactions, respectively. Complexes 4 and 9 are square pyramidal; 6, 7, 10, and 11 are distorted trigonal prismatic with cis ligands LL'; and 12 and 14 are distorted octahedral. Complexes 4, 10, and 11 support three-membered electron transfer series. Attempts to oxidize 4 to the W(V)S complex results in the formation of binuclear [W2(mu2-S)2(S2C2Me2)4](2-) having distorted octahedral coordination. The 21 known functional groups WL and WLL' in mononuclear bis(dithiolene) complexes prepared in this and prior investigations are tabulated. Of those with physiological-type ligands, it remains to be seen which (if any) of these ligation modes are displayed by enzyme sites. 相似文献
90.
A sequential injection analysis (SIA) system was developed to monitor the concentration of l-cysteine in biological processes on-line. It is based on the redox reaction of l-cysteine with iron(III) in the presence of 1,10-phenanthroline (phen) and the detection of the red-iron(II)-phen complex with a spectrophotometry. The system was fully automated using software written in the LabVIEW™ development environment. A number of system variables such as the flow rate of the carrier buffer solution, the volume ratio of the sample to the reagents, and the reaction coil length, etc., were evaluated to increase the sensitivity and performance of the SIA system. Under partially optimized operating conditions the performance of the SIA system was linear up to a concentration of l-cysteine of 1 mM (R2 = 0.998) with a detection limit of 0.005 mM and a sample frequency of 15 hr−1. The SIA system was employed to monitor the concentration of l-cysteine on-line in a continuously stirred reactor and a fermentation process of Saccharomyces cerevisiae. The on-line monitored data were in good agreement with the off-line data measured by a HPLC with a fluorescence detector (n = 15, R2 = 09899). 相似文献