全文获取类型
收费全文 | 3872篇 |
免费 | 733篇 |
国内免费 | 768篇 |
专业分类
化学 | 3555篇 |
晶体学 | 52篇 |
力学 | 41篇 |
综合类 | 52篇 |
数学 | 316篇 |
物理学 | 1357篇 |
出版年
2024年 | 10篇 |
2023年 | 38篇 |
2022年 | 79篇 |
2021年 | 92篇 |
2020年 | 136篇 |
2019年 | 117篇 |
2018年 | 112篇 |
2017年 | 99篇 |
2016年 | 163篇 |
2015年 | 160篇 |
2014年 | 206篇 |
2013年 | 329篇 |
2012年 | 272篇 |
2011年 | 256篇 |
2010年 | 234篇 |
2009年 | 269篇 |
2008年 | 280篇 |
2007年 | 272篇 |
2006年 | 281篇 |
2005年 | 237篇 |
2004年 | 250篇 |
2003年 | 201篇 |
2002年 | 131篇 |
2001年 | 123篇 |
2000年 | 142篇 |
1999年 | 120篇 |
1998年 | 104篇 |
1997年 | 84篇 |
1996年 | 84篇 |
1995年 | 84篇 |
1994年 | 75篇 |
1993年 | 51篇 |
1992年 | 58篇 |
1991年 | 59篇 |
1990年 | 30篇 |
1989年 | 28篇 |
1988年 | 21篇 |
1987年 | 12篇 |
1986年 | 11篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 13篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有5373条查询结果,搜索用时 62 毫秒
991.
Atomic clusters characterized by finite size, low dimensionality, and reduced coordination number exhibit many novel properties that are very different from their bulk. As these clusters are assembled, their properties can be significantly altered due to the interaction of these clusters with each other as well as with their support. This paper provides a brief review of the cluster properties that are affected when clusters are deposited on metallic or organic substrates, isolated in matrices or in zeolite cages, coated with acetate ligands, or simply allowed to self-assemble without the presence of any reactive species. It is shown that the interface between the clusters and their support can play an important role on the properties of clusters as their unique characteristics do. 相似文献
992.
Roberto Della Pergola Luigi Garlaschelli Mario Manassero Mirella Sansoni Donatella Strumolo 《Journal of Cluster Science》2001,12(1):23-34
The cluster [HIr5(CO)12]2- (1) was prepared by condensation of [HIr4(CO)11]- and [Ir(CO)4]- (molar ratio 1:1) in refluxing THF, with almost quantitative yields. Its solid state structure was determined by X-ray diffraction at low temperature on the salt [PPh3CH2Ph]2[HIr5(CO)12]. The metal atoms define a trigonal bipyramidal arrangement. The hydride ligand was located indirectly as a bridge between apical and equatorial metal atoms. The phosphine-substituted cluster [HIr4(CO)10PPh3]- (2) was synthetized by CO displacement on [HIr4(CO)11]-, in THF at room temperature. This reaction is selective, with no traces of polysubstitution products. In the solid state, the hydride and the triphenylphosphine are axially bound on basal iridium atoms; the terminal hydrogen atom was directly located by X-ray analysis at a Ir–H distance of 1.57(9) Å. On the contrary, two isomers are present in THF solution, and they interconvert rapidly at room temperature, as shown by1H and 31P NMR spectra. 相似文献
993.
Hiroaki Umeda Shiro Koseki Umpei Nagashima Michael W. Schmidt 《Journal of computational chemistry》2001,22(12):1243-1251
The quasi‐degenerate multireference second‐order perturbation theory (MRMP2) routines in the GAMESS suite of program codes have been parallelized using a distributed data interface (DDI). Two typical kinds of molecules were chosen for examination of parallelization speedup using one to eight PCs gathered as a cluster and connected by Fast Ethernet. The first example, in which total energies of several low‐lying electronic states have been obtained for niobium monohydride, give parallelization speedup of 7.15 when eight PCs were used as a cluster. The second example is the ground‐state total energy for a medium sized molecule, 4a,4b,8a,9a‐tetrahydro‐pyridino[1′,2′‐4,3]imidazo‐lidino[1,5‐a]pyridine. When distributed memory is employed, the parallelization speedup improves to 6.84 for the MRMP2 calculations when an eight‐PC cluster is used. These results demonstrate that our efforts to achieve the parallelization of MRMP2 routines have been successful. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1243–1251, 2001 相似文献
994.
用硒酚铜簇合物和硫酚铜簇合物[Me4N]2[Cu4(EPh)6](E=Se和S)与CS2及少量S反应,分别获得了以过硫代碳酸根[CS4]^2-为配体的[Me4N]n[Cu(CS4)]n(I)和[Me4N]4[Cu4(CS4)4](Ⅱ)。并用X射线单晶衍射法测定了它们的晶体结构。I为二维层状聚合物,Ⅱ为四核铜簇阴离子化合物。并对[CS4]^2-形成的机理作了讨论。 相似文献
995.
[Ru3(CO)12]与Lawesson试剂[(MeOC6H4PS2)2]反应,合成、分离并用谱学表征了产物三核钌羰合簇[(μ-H)2Ru3(CO)9(μ3-P)](Ⅰ)和四核钌羰合簇[(Ru4(CO)10(μ3-S)(μ3-PC6H4OMe)](Ⅰ).X射线衍射测定了的晶体分子结构,表明含有1个裸磷原子作为面桥基配体,并具有颇短的Ru-Ru键距,该价电子数为49e的簇合物对氧和水稳定.谱学分析表明,化合物具有四面体型的Ru4簇心,其三角形面上分别具有面桥基μ3-S和μ3-PC6H4OMe基配体. 相似文献
996.
煤燃烧过程中表面结构变化的SEM图像分析 总被引:7,自引:2,他引:5
介绍了SEM图像分析的多种方法,并讨论了各自的描述能力,利用这些方法分析煤在燃烧各阶段SEM图像,通过研究得到煤在燃烧过程中表面分形维数的变化规律,同时可获得表面孔的某些参数变化历程。分析表明,淮南低变质煤在850℃燃烧过程中,煤焦颗粒表面的分维数有一个先变低后变高的过程,煤焦表面分维数在2.2至2.5区间内变化;同时孔径分布由近双曲线分布变化为近正态分布,孔油加权平均形状因子增加,在研究过程中发现,不同的计算方法会对图像的分形维数值产生影响,确定合适的计算方法是分形理论在煤燃烧领域中应用的关键。通过比较发现Xie法较为适合计算煤焦表面SEM图像。 相似文献
997.
中药黄芩及其混伪品的模糊聚类分析 总被引:1,自引:0,他引:1
采用模糊聚类分析法对黄芩及其混伪品进行了聚类分析,为中药材的质量鉴定提供了新方法。 相似文献
998.
Donald J. Darensbourg Floyd A. Beckford Joseph H. Reibenspies 《Journal of Cluster Science》2000,11(1):95-107
The syntheses of Ru3(CO)9(PTA)3 and Ir4(CO)7(PTA)5 were accomplished through the thermal reactions of Ru3(CO)12 or Ir4(CO)12 with the water-soluble phosphine, PTA(1,3,5-triaza-7-phosphaadamantane). The ruthenium derivative was shown by X-ray crystallography to consist of a triangular Ru3 core with three nearly equal Ru–Ru bonds, with each ruthenium atom bearing an equatorially positioned PTA ligand. In Ir4(CO)7(PTA)5 the iridium atoms define a tetrahedron which is bridged on three edges by CO ligands. One basal iridium atom contains two PTA ligands, while the other two basal and the apical iridium atoms each possess one PTA ligand in their coordination spheres. Although, Ru3(CO)9(PTA)3 is only sparingly soluble in pure water, it is very soluble in aqueous solution of pH<4. Indeed the triruthenium cluster can be extracted reversibly between an aqueous and an organic phase (e.g., CH2Cl2) by changing the pH of the aqueous phase. On the other hand the more highly PTA substituted cluster, Ir4(CO)7(PTA)5, exhibits good solubility in aqueous solution (pH 7 and below) and a variety of organic solvents. Both cluster derivatives are stable in deoxygenated, aqueous solutions for extended period of time (>24 h). 相似文献
999.
V. P. Fedin D. G. Samsonenko A. V. Virovets I. V. Kalinina D. Yu. Naumov 《Russian Chemical Bulletin》2000,49(1):19-25
Four new chalcogenide molybdenum and tungsten cubane clusters (NH4)6[M4Q4(CN)12]·6H2O (M=Mo or W; Q=S or Se) were prepared by high-temperature reactions of the triangular M3O7Br4 complexes with KCN at 430 °C followed by crystallization from aqueous solutions of ammonium acetate. The molecular and crystal
structures of (NH4)6[Mo4S4(CN)12]·6H2O, (NH4)6[W4S4(CN)12]·6H2O, and (NH4)6[W4Se4(CN)12]·6H2O were established by X-ray diffraction analysis. The mixed-valence cubane clusters are diamagnetic and isostructural and
have the symmetryT
d
. The clusters were characterized by IR and electronic spectroscopy. The data of cyclic voltammetry demonstrated that the
[M4Q4(CN)12]
n− clusters exist in three oxidation states from the most oxidized (n=6; 10 cluster electrons) to the most reduced electron-precise 12-electron species (n=8).
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 18–24, January, 2000. 相似文献
1000.
用直射式和反射式飞行时间质谱研究了氨分子团簇体系在 355 nm激光下的多光子电离,得到一系列的质子化团簇离子 (NH3)nH+,同时还观察到超价氨团簇离子 (NH3)n H2+。在反射式飞行时间质谱研究中观测到质子化氨团簇离子在自由飞行过程中的解离现象,表明在该实验条件下生成的质子化氨团簇离子是一些亚稳态团簇离子。对子离子产率的分析,得到质子化团簇离子解离速率常数,从而可以估计亚稳态团簇离子的寿命。团簇尺寸从 n=3增大到 20,其寿命从 21 ms减小到 120 μs,大约小了两个数量级。解离速率在 n=5到 6有一个阶跃式上升,这是由于 5个氨组成的质子化团簇离子(NH3)4NH4+ 的结构相对比较稳定。 相似文献