全文获取类型
收费全文 | 13912篇 |
免费 | 2661篇 |
国内免费 | 2281篇 |
专业分类
化学 | 10501篇 |
晶体学 | 293篇 |
力学 | 734篇 |
综合类 | 157篇 |
数学 | 1264篇 |
物理学 | 5905篇 |
出版年
2024年 | 57篇 |
2023年 | 307篇 |
2022年 | 591篇 |
2021年 | 704篇 |
2020年 | 726篇 |
2019年 | 722篇 |
2018年 | 603篇 |
2017年 | 611篇 |
2016年 | 779篇 |
2015年 | 851篇 |
2014年 | 1032篇 |
2013年 | 1292篇 |
2012年 | 1445篇 |
2011年 | 1446篇 |
2010年 | 1047篇 |
2009年 | 946篇 |
2008年 | 1074篇 |
2007年 | 847篇 |
2006年 | 744篇 |
2005年 | 567篇 |
2004年 | 462篇 |
2003年 | 328篇 |
2002年 | 272篇 |
2001年 | 220篇 |
2000年 | 191篇 |
1999年 | 178篇 |
1998年 | 126篇 |
1997年 | 89篇 |
1996年 | 112篇 |
1995年 | 73篇 |
1994年 | 74篇 |
1993年 | 66篇 |
1992年 | 50篇 |
1991年 | 51篇 |
1990年 | 30篇 |
1989年 | 32篇 |
1988年 | 18篇 |
1987年 | 14篇 |
1986年 | 27篇 |
1985年 | 18篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1959年 | 1篇 |
1957年 | 5篇 |
1937年 | 1篇 |
1936年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
A dicyano-containing [Fe(bpb)(CN)2]- building block has been employed for the synthesis of cyano-bridged heterometallic Ni(II)-Fe(III) complexes. The presence of steric bpb(2-) ligand around the iron ion results in the formation of low-dimensional species: five are neutral NiFe2 trimers and three are one-dimensional (1D). The structure of the 1D complexes consists of alternating [NiL]2+ and [Fe(bpb)(CN)2]- generating a cyano-bridged cationic polymeric chain and the perchlorate as the counteranion. In all complexes, the coordination geometry of the nickel ions is approximately octahedral with the cyano nitrogen atoms at the trans positions. Magnetic studies of seven complexes show the presence of ferromagnetic interaction between the metal ions through the cyano bridges. Variable temperature magnetic susceptibility investigations of the trimeric complexes yield the following J(NiFe) values (based on the spin exchange Hamiltonian H = -2J(NiFe) S(Ni) (S(Fe(1)) + S(Fe(2))): J(NiFe) = 6.40(5), 7.8(1), 8.9(2), and 6.03(4) cm(-1), respectively. The study of the magneto-structural correlation reveals that the cyanide-bridging bond angle is related to the strength of magnetic exchange coupling: the larger the Ni-N[triple bond]C bond angle, the stronger the Ni- - -Fe magnetic interaction. One 1D complex exhibits long-range antiferromagnetic ordering with T(N) = 3.5 K. Below T(N) (1.82 K), a metamagnetic behavior was observed with the critical field of approximately 6 kOe. The present research shows that the [Fe(bpb)(CN)2]- building block is a good candidate for the construction of low-dimensional magnetic materials. 相似文献
962.
Dixit AA Lei Y Lee KW Quiñones E Houston PL 《The journal of physical chemistry. A》2005,109(9):1770-1775
Multiphoton excitation and dissociation of SO(2) have been investigated in the wavelength range from 224 to 232 nm. Strong evidence is found for two-photon excitation to the H Rydberg state, followed by dissociation to SO + O and ionization of the SO product by absorption of a third photon. The two-photon excitation is resonantly enhanced via the C (1)B(2) intermediate state, and the two-photon yield spectrum thus bears a strong resemblance to the spectrum of this intermediate. Imaging of the O((3)P(2)), S((1)D(2)), and SO products suggests that, following dissociation of SO(2) from the H state, SO is produced in the A and B electronic states. S((1)D(2)) is produced both from two-photon dissociation of SO(2) to give S((1)D(2)) + O(2) and by single-photon dissociation of SO(+). In the former process, the O(2) is likely formed in all of its lowest three electronic states. 相似文献
963.
Liu Z Huang C Fan K Wei P Chen H Liu S Pei J Shi L Li B Yang K Liu Y Lai L 《Journal of chemical information and modeling》2005,45(1):10-17
The SARS coronavirus 3C-like proteinase is considered as a potential drug design target for the treatment of severe acute respiratory syndrome (SARS). Owing to the lack of available drugs for the treatment of SARS, the discovery of inhibitors for SARS coronavirus 3C-like proteinase that can potentially be optimized as drugs appears to be highly desirable. We have built a "flexible" three-dimensional model for SARS 3C-like proteinase by homology modeling and multicanonical molecular dynamics method and used the model for virtual screening of chemical databases. After Dock procedures, strategies including pharmocophore model, consensus scoring, and "drug-like" filters were applied in order to accelerate the process and improve the success rate of virtual docking screening hit lists. Forty compounds were purchased and tested by HPLC and colorimetric assay against SARS 3C-like proteinase. Three of them including calmidazolium, a well-known antagonist of calmodulin, were found to inhibit the enzyme with an apparent K(i) from 61 to 178 microM. These active compounds and their binding modes provide useful information for understanding the binding sites and for further selective drug design against SARS and other coronavirus. 相似文献
964.
利用电动势法得到了牛血清白蛋白(BSA)与十二烷基硫酸钠(SDS)相互作用的结合等温线. 通过四阶导数紫外光谱法和荧光光谱法研究了相互作用过程中芳香族氨基酸残基微环境极性的变化. 通过研究发现, 随着SDS浓度的逐渐增大, SDS在BSA上的平均结合数(v)逐渐增大, 色氨酸(Trp)残基所处微环境的极性在减弱后保持基本不变, 酪氨酸残基所处微环境的极性在明显增强后稍有减弱, 苯丙氨酸残基所处微环境的极性略有增强. 结果表明, 当v由0增大到14时, SDS主要结合在BSA的Trp-213附近并逐渐形成聚集体, 从而诱导BSA由结构域ⅡA 开始逐渐展开. 此后, SDS呈正协同作用的特点与BSA 结合, v急剧增大. 当v约为302 时, SDS在BSA上的结合基本达到饱和, BSA的构象趋于稳定. 相似文献
965.
A new hydroxo‐bridged dimeric Cr(III) complex [Cr(saltn)OH]2·4H2O [H2saltn=N,N′‐bis(salicylidene)trimethylenediamine] has been synthesized and its structural and magnetic properties have been investigated. The complex crystallizes in the triclinic space group P‐1 with one dimeric formula unit in a cell of dimensions a=0.95828(19) nm, b=0.95926(19) nm, c=1.0437(2) nm, α=86.77(3)°, β=82.48(3)°, and γ=64.93(3)°. The geometry around each chromium(III) center is six‐coordinate, distorted‐octahedral. The bridging Cr2O2 unit is strictly planar, as required by the crystallographic symmetry. The Cr? O? Cr′ bridging angle is 99.94(16)°, and the distance between Cr…Cr′ is 0.3019 nm. The magnetic susceptibility of the complex has been examined in the range of 2‐300 K. By using the spin‐spin coupled model for an S1=S2=3/2 dimeric system , the magnetic data were fitted to give the parameters of g=2.01(1), J=‐0.85(2) cm‐1, and zJ' =0.18(3)cm‐1, indicating the presence of a weak antiferromagnetic spin‐exchange interaction between the Cr(III) ions in the binuclear complex. 相似文献
966.
YUEBin JIANGLei HUChang-wen CHENJian-min HEHe-yong 《高等学校化学研究》2005,21(4):386-390
Paratungstate-loaded titania catalysts were prepared via the addition of a series of aqueous solutions of paratungstate( denoted as W7 ) into an isopropanol solution of Ti[ OCH( CH3 )2 ]4 by means of the sol-gel method. The catalysts were characterized by EDX, BET, FTIR, UV-Vis DRS, XRD and the results indicate that such paratungstateloaded catalysts maintained their heptatungstate structure in the anatase titania matrix up to 400℃. The catalysts were tested for the heterogeneous photodegradation of chlorobenzene in aqueous media and showed a better catalytic activity than P-25 TiO2 because paratungstate can prevent the recombination of the holes and electrons produced during irradiation. Moreover, the paratungstate-loaded titania catalysts can resist the disaggregation during the photoirradiation and can be easily recycled from the aqueous suspensions after reactions. 相似文献
967.
ZHANG Zhengbin LIN Cai LIU Chunying XING Lei WU Zhenzhen & SUN Feng Institute of Marine Chemistry Ocean University of China Qingdao China 《中国科学B辑(英文版)》2005,48(4):376-384
With supernatural bioactivation, nitric oxide (NO), which was first regarded as an endothelium- derived relaxing factor, was recognized by Science as the “Molecule of the Year” in 1992. The Nobel Prize of Physiology & Medicine in 1998 was awarded jointly to Furchgatt, Ignarro, and Murad for their discoveries concerning the NO effects in the cardiovascular sys- tem. From then on, researchers have paid more and more attention to the special and diverse functions of NO in organism[1―18].… 相似文献
968.
The reactions of the mixed metal cluster complexes PtRu5(CO)16(μ6-C)[Pt(PBu
)], 5 and PtRu5(CO)16(μ6-C)[Pt(PBu
]2, 6 with selected alkynes have been investigated. Compound 5 adds one and two equivalents of PhC2H to yield the new compounds PtRu5(CO)15(μ6-C)(μ3-PhC2H)[Pt(PBu
)], 8 and PtRu5(CO)
-C)(
-PhC2H)2[Pt(PBu
)], 9 at 40 and 68°C, respectively. Compound 6 was found to react with PhC2H at 40°C to yield the new compound PtRu5(CO)
-C)(
-PhC2H)[Pt(PBu
)]2, 10. The reaction of 6 with PhC2Ph at 97°C yielded the new compound PtRu5(CO)
-C)(
-PhC2Ph)2[Pt(PBu
)]2, 11. All products were characterized crystallographically by single crystal X-ray diffraction techniques. The structure of 8 consists of a pseudo-octahedral PtRu5 cluster with a second platinum atom bridging a basal edge of the Ru5 square pyramid. A triply bridging PhC2H ligand is bonded to the two platinum atoms and one of the ruthenium atoms. The structure of 9 consists of a nido-dodecahedral Pt2Ru5 cluster with a carbido ligand in the interior that is not bonded to all seven of the metal atoms. It also contains two triply
bridging PhC2H ligands. The structure of 10 consists of a central octahedral cluster of five ruthenium atoms and one platinum atom. Two additional platinum atoms are
bonded to the platinum atom in this cluster but these atoms are not bonded to any other metal atoms of the PtRu5 cluster. A triply bridging PhC2H ligand is coordinated to the group of three platinum atoms. The structure of 11 consists of an octahedral PtRu5 cluster with two additional platinum atoms capping two PtRu2 triangular faces. There are two PhC2Ph ligands bridging two Ru3 triangular faces of the central octahedron.
This report is dedicated to Professor Ilya Moiseev on the occasion of his 75th birthday for his many pioneering contributions
to the chemistry of metal clusters. 相似文献
969.
田菁胶负载钯催化Heck反应的研究 总被引:19,自引:0,他引:19
通过较简单的方法合成了田菁胶负载钯化合物,利用XPS、IR、TG、DTA和TEM等手段对其进行了表征.该化合物在空气氛围中就能很好地催化丙烯酸、苯乙烯和丙烯酰胺与芳基碘的Heck反应,高产率(≥85 % )且立体选择性地生成取代的反式肉桂酸、1,2- 二苯乙烯和肉桂酰胺.化合物具有较好的重复使用性能.催化剂中起催化作用的是金属态的钯. 相似文献
970.