全文获取类型
收费全文 | 394篇 |
免费 | 141篇 |
国内免费 | 250篇 |
专业分类
化学 | 452篇 |
晶体学 | 5篇 |
力学 | 3篇 |
数学 | 11篇 |
物理学 | 314篇 |
出版年
2023年 | 3篇 |
2022年 | 12篇 |
2021年 | 13篇 |
2020年 | 12篇 |
2019年 | 18篇 |
2018年 | 20篇 |
2017年 | 18篇 |
2016年 | 28篇 |
2015年 | 13篇 |
2014年 | 27篇 |
2013年 | 48篇 |
2012年 | 53篇 |
2011年 | 60篇 |
2010年 | 60篇 |
2009年 | 59篇 |
2008年 | 65篇 |
2007年 | 39篇 |
2006年 | 61篇 |
2005年 | 49篇 |
2004年 | 31篇 |
2003年 | 19篇 |
2002年 | 10篇 |
2001年 | 5篇 |
2000年 | 13篇 |
1999年 | 10篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1996年 | 9篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1980年 | 1篇 |
排序方式: 共有785条查询结果,搜索用时 0 毫秒
241.
The cobalt(II) complex C20H34O20Co has been synthesized and characterized by elemental analysis, IR, TG and single-crystal X-ray diffraction. The crystal belongs to triclinic, space group P with a = 5.1374(10), b = 10.519(2), c = 12.913(3) (A), α = 86.89(3), β = 79.94(3), γ = 86.74(3)o, V = 685.3(3) (A)3, Z = 1, Mr = 653.40, Dc = 1.583 g/cm3, μ = 0.717 mm-1, F(000) = 341, the final R = 0.0657 and wR = 0.1141 for 1900 observed reflections with I > 2σ(I). The cobalt ion is six-coordinated by two O atoms of different 1,3-bdoaH- ligands and four water molecules, residing in an octahedral environment. The intermolecular hydrogen bonds form a 3-D supramolecular network structure. 相似文献
242.
CdSe量子点的合成及标记胃蛋白酶的研究 总被引:4,自引:0,他引:4
以硒粉及镉盐为反应前体,通过固相/液相沉淀反应及室温乙二胺溶剂体系两种方法合成了平均粒径为8~10nm的CdSe量子点,以巯基乙酸修饰量子点使之可溶,并标记胃蛋白酶。CdSe量子点在350和380nm附近有明显尖锐的紫外吸收,有481.6nm的荧光带边发射,标记胃蛋白酶后,紫外吸收峰位不变而吸光度增强,荧光峰位蓝移至467.2nm,荧光强度降低。在最佳实验条件下,胃蛋白酶浓度在5~50mg/L范围内与荧光降低值之间呈线性关系;检出限(3σ)为0.36mg/L(n=10),方法已用于人体胃液胃蛋白酶的测定。 相似文献
243.
从实验内容和教学方法上对界面缩聚合成尼龙实验进行了综合性改进。用5倍物质的量的氯化亚砜和癸二酸回流反应,可把癸二酰氯单体制备时间由4 h缩短到0.5 h,用环己烷代替四氯化碳,可大大降低溶剂的毒性;在教学方法上,从研究性实验的角度进行问题驱动式的讲解。该实验在高分子化学基础实验教学中取得了较好的效果。 相似文献
244.
Four new lupane triterpenoid saponins, along with one known lupane and eight hederagenin saponins, were isolated from the EtOH extract of the buds of Lonicera similis Hemsl. The structures of the new compounds were established as 3-O-β-D-glucopyranosyl-(1→2)-β-D-glucopyranosyl 23-hydroxybetulinic acid 28-O-β-D-glucopyranosyl ester (lonisimilioside A, 1), 3-O-β-D-glucopyranosyl-(1→2)-β-D-glucopyranosyl 23-hydroxybetulinic acid 28-O-β-D-glucopyranosyl-(1→6)-β-D-glucopyranosyl ester (lonisimilioside B, 2), 3-O-β-D-glucopyranosyl-(1→2)-β-D-glucopyranosyl betulinic acid 28-O-β-D-glucopyranosyl-(1→6)-β-D-glucopyranosyl ester (lonisimilioside C, 3) and 3-O-β-D-glucopyranosyl-(1→2)-β-D-glucopyranosyl-(1→6)-β-D-glucopyranosyl betulinic acid 28-O-β-D-glucopyranosyl ester (lonisimilioside D, 4), respectively. The cytotoxic activities of the isolates against human cancer cell lines HepG2, MCF-7 and A-549 were evaluated. Only the monodesmosidic saponin with a free carboxyl group at C-28 (12) exhibited significant cytotoxicities against HepG2, MCF-7 and A-549 cell lines with the IC50 values of 8.98 ± 0.19, 12.48 ± 0.45 and 11.62 ± 0.54 μM, respectively. Furthermore, Hoechst fluorescence 33342 staining was used to demonstrate that 12 could induce HepG2 and A-549 cells apoptosis significantly. 相似文献
245.
A oxovanadium(Ⅴ) complex of [VOL(C9H6NO)]·CH2Cl2[H2L=2-hydroxy-1-naphthaldehyde 4-methoxy-benzoylhydrazone, C9H6NO-=8-hydroxyquinoline anion] has been synthesized and characterized by elemental analyses, IR and single crystal X-ray diffraction. The crystal crystallizes in triclinic system, the space group is P1, with the crystal cell parameters a=1.087 0(2) nm, b=1.107 1(2) nm, c=1.269 3(3) nm, α=67.84(3)°, β=79.26(3)°, γ=73.85(3)° and V=1.353 1(6) nm3 , Mr=614.34, R=0.0503, wR=0.1409. The V(Ⅴ) atom is six-coordinated by one O atom of oxo group, two O atoms and one N atom of the tridentate hydrazone ligand, as well as one N atom and one O atom of 8-hydroxyquinoline ligand, thus defining a distorted octahedral VO(ONO)(NO) geometry. A two-dimensional supramolecular layer structure is formed through π-π stacking interactions. CCDC: 268672. 相似文献
246.
近几年来,静电自组装成膜材料的合成、薄膜的制备、表征和功能研究已得到了迅速发展[1~6],但对成膜材料分子结构与薄膜性质间构效关系的研究报道则较少[6].杂多化合物具有特殊的结构特征和氧化还原性质,而半菁化合物是备受关注的二阶非线性光学和光电转换材料[7,8],二者形成的 相似文献
247.
Nellutla S van Tol J Dalal NS Bi LH Kortz U Keita B Nadjo L Khitrov GA Marshall AG 《Inorganic chemistry》2005,44(26):9795-9806
The dimeric, pentacopper(II)-substituted tungstosilicate [Cu(5)(OH)(4)(H(2)O)(2)(A-alpha-SiW(9)O(33))(2)](10)(-) (1) has been characterized by single-crystal X-ray diffraction, elemental analysis, IR, electrochemistry, magnetic measurements, electron paramagnetic resonance (EPR), and mass spectrometry (MS). Magnetization and high-field EPR measurements reveal that the pentameric copper core {Cu(5)(OH)(4)(H(2)O)(2)}(6+) of 1 exhibits strong antiferromagnetic interactions (J(a) = -51 +/- 6 cm(-)(1), J(b) = -104 +/- 1 cm(-)(1), and J(c) = -55 +/- 3 cm(-)(1)) resulting in a spin S(T) = (1)/(2) ground state. EPR data show that the unpaired electron spin density is localized on the spin-frustrated apical Cu(2+) ion with g(zz) = 2.4073 +/- 0.0005, g(yy) = 2.0672 +/- 0.0005, g(xx) = 2.0240 +/- 0.0005, and A(zz) = -340 +/- 20 MHz (-0.0113 cm(-)(1)). 1 can therefore be considered as a model system for a five-spin, electronically coupled, spin-frustrated system. Polyanion 1, which is stable over a wide pH domain (pH 1-7), was characterized by cyclic voltammetry (CV) in a pH 5 medium. Its CV was constituted by an initial two-step reduction of the Cu(2+) centers to Cu(0) through Cu(+), followed at more negative potential by the redox processes of the W centers. Controlled potential coulometry of 1 allows for the reduction of the five Cu(2+) centers, as seen by consumption of 10.05 +/- 0.05 electrons per molecule. Polyanion 1 triggers efficiently the electrocatalytic reduction of nitrate and nitrite, and it also catalyzes the reduction of N(2)O. To our knowledge, this is the first example of N(2)O catalytic reduction by a polyoxoanion. Fourier transform ion cyclotron resonance MS was used to unambiguously assign the molecular weight of the solution-phase species 1 and the oxidation states of the Cu atoms in the central {Cu(5)(OH)(4)(H(2)O)(2)}(6+) core. Infrared (IR) multiphoton dissociation MS/MS of 1 showed evidence of a condensation process similar to bronze formation at low irradiation intensity. Higher IR intensity resulted in the formation of stable fragments consistent with those previously observed in the solution chemistry of polyoxoanions. 相似文献
248.
249.
We obtain the equation for the steady state joint probability distribution of the system driven by external and internal noises simultaneously, where external noise is the dichotomous noise and internal noise is the Gaussian white noise. As to the application of our equation, we discuss two examples for which the correlation functions and associated relaxation times are calculated. We find the coupling effect of the external and the internal noises for the relaxation time. 相似文献
250.
On the basis of a charged BTZ black hole, we add an extra term in the metric function to describe the contribution from nonlinear electrodynamics. In this way we artificially construct a (2 + 1)-dimensional black hole in general relativity coupled with a nonlinear electrodynamics source. The thermodynamic quantities and Smarr formula are derived. It is found that this black hole has T−S criticality like that of an RN-AdS black hole. Further modifying the metric function, we obtain a (2 + 1)-dimensional black hole possessing P−V critical behaviors similar to that of van der Waals fluid. To our knowledge, this is the first example of (2 + 1)-dimensional black holes having this kind of critical behavior. 相似文献