全文获取类型
收费全文 | 62041篇 |
免费 | 13630篇 |
国内免费 | 5661篇 |
专业分类
化学 | 59748篇 |
晶体学 | 780篇 |
力学 | 2183篇 |
综合类 | 416篇 |
数学 | 5400篇 |
物理学 | 12805篇 |
出版年
2024年 | 41篇 |
2023年 | 409篇 |
2022年 | 704篇 |
2021年 | 1013篇 |
2020年 | 2105篇 |
2019年 | 3403篇 |
2018年 | 1817篇 |
2017年 | 1507篇 |
2016年 | 4471篇 |
2015年 | 4629篇 |
2014年 | 4915篇 |
2013年 | 6004篇 |
2012年 | 5283篇 |
2011年 | 4730篇 |
2010年 | 4676篇 |
2009年 | 4620篇 |
2008年 | 4346篇 |
2007年 | 3500篇 |
2006年 | 3208篇 |
2005年 | 3155篇 |
2004年 | 2667篇 |
2003年 | 2349篇 |
2002年 | 3186篇 |
2001年 | 2202篇 |
2000年 | 2023篇 |
1999年 | 995篇 |
1998年 | 504篇 |
1997年 | 419篇 |
1996年 | 383篇 |
1995年 | 320篇 |
1994年 | 264篇 |
1993年 | 232篇 |
1992年 | 215篇 |
1991年 | 184篇 |
1990年 | 159篇 |
1989年 | 139篇 |
1988年 | 122篇 |
1987年 | 76篇 |
1986年 | 75篇 |
1985年 | 82篇 |
1984年 | 40篇 |
1983年 | 36篇 |
1982年 | 25篇 |
1981年 | 22篇 |
1980年 | 17篇 |
1979年 | 14篇 |
1978年 | 15篇 |
1976年 | 4篇 |
1974年 | 4篇 |
1957年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Three new alkaloids, longistylumphyllines A–C (1–3), together with the six known alkaloids deoxycalyciphylline B, deoxyisocalyciphylline B, methyl homosecodaphniphyllate, daphnicyclidin A, daphnicyclidin B, and daphnicyclidin F, were isolated from the stems and leaves of Daphniphyllum longistylum. Their structures and relative configuration were elucidated on the basis of spectroscopic data, especially 1D and 2D NMR techniques. 相似文献
992.
993.
Xu‐Cheng Fu Xiao‐Yan Wang Ming‐Tian Li Xiao‐Tao Deng Cheng‐Gang Wang 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(8):m343-m345
The title complex, {[Cu2(C8H4O4)2(C3H4N2)4(H2O)]·H2O}n, is a three‐dimensional polymer formed through bridging by phthalate dianions of two different CuII cations and a network of O(N)—H⋯O hydrogen bonds. The Cu—O and Cu—N interaction distances are in the ranges 2.0020 (16)–2.4835 (17) and 1.968 (2)–1.9855 (19) Å, respectively. The structure is composed of alternating polymer chains parallel to the c axis, with a shortest Cu⋯Cu distance of 6.3000 (5) Å. 相似文献
994.
Li Zhang Xin‐An Chang Xue‐An Chen He‐Gui Zang Wei‐Qiang Xiao 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(9):i76-i78
The title compound represents a new structure type, in which distorted VO6 octahedra are bridged by iodate groups to form infinite two‐dimensional [VO2(IO3)2]− layers that are separated by octahedrally coordinated Li+ cations. 相似文献
995.
Using the mixture of cetyltrimethylammonium bromide (CTAB) and p‐Octyl polyethylene glycol phenyl ether (OP‐10) as templates, siliceous MCM‐48 materials can be synthesized with low molar ratio of CTAB to silica (0.139:1) and low concentration of mixed surfactants (ca.5%) and within a wide range of OP‐10/CTAB ratio (0.08?0.25). The materials were characterized by X‐ray powder diffraction, N2 adsorption/desorption isotherm, TEM, TG‐DSC and 29Si MAS NMR. Measurements indicated that the use of mixed surfactants allowed better condensation and higher ordering of the cubic mesostructure; at the same time, some properties of these materials were sensitive to the OP‐10/CTAB ratio. It was also found that the reduced pH of the gel which had been crystallized for a certain time gave a highly reproducible synthesis with a high silica yield (about 95%). Furthermore, the reaction mechanism of the synthesis is discussed in detail. 相似文献
996.
Antifreeze proteins (AFPs) protect organisms from freezing damage by inhibiting the growth of seed-ice crystals. It has long been hypothesized that irreversible binding of AFPs to ice surfaces is responsible for inhibiting the growth of seed-ice crystals as such a mechanism supports the popularly accepted Kelvin effect for the explanation of local freezing-point depression. However, whether the binding is reversible or irreversible is still under debate due to the lack of direct experimental evidence. Here, we report the first direct experimental result, by using the newly developed multiple quantum (MQ) filtering-spin exchange NMR experiment, that shows that the binding of HPLC6 peptides to ice surfaces is reversible. It was found that the reversible process can be explained by the model of monolayer adsorption. These results suggest that the Kelvin effect is not suitable for explaining the antifreeze mechanism, and direct interactions between the peptides and the ice-surface binding sites are the driving forces for the binding of AFPs to ice surfaces. We propose that there exists a concentration gradient of AFP from an ice-binding surface to the solution due to the affinity of ice surfaces to AFPs. This concentration gradient creates a dense layer of AFP in contact with the ice-binding surface, which depresses the local freezing point because of the colligative property, but not the Kelvin effect. 相似文献
997.
An efficient synthetic sequence toward the C8-C19 region of peloruside A has been developed. The route is highlighted by a selective electrophilic cyclization reaction, a single-step epoxide ring-opening/methylation sequence, and a stereoselective Mukaiyama aldol reaction. [reaction: see text] 相似文献
998.
磷酸铝类分子筛是近年来继Y型和ZSM七分子筛之后的第三代合成分子筛材料,其骨架由A10。和P0。四面体均匀交替地组成问.由于带负电行的A10。和带正电荷的PO。四面体数目相等,整个骨架呈电中性,所以这类分子筛本身无离子交换能力和催化活性.当用硅或某些金属离子(如Mg、Fe、Co、Zn等)对磷酸铝分子筛进行同晶取代,就会在骨架上造成电行不平衡,从而使这类分子筛具有一定的离子交换能力和酸性【习.因而对硅或金属离子在磷酸错类分子筛中取代机理的研究一直是人们感兴趣的问题问.Vpl-5是最近才合成出来的具有18圆环一维孔结构的… 相似文献
999.
Xin‐Zhou Yang Li‐Gen Lin Chun‐Ping Tang Yong‐Qiang Liu Yang Ye 《Helvetica chimica acta》2007,90(2):318-325
Five new nonalkaloid constituents, a neolignan, japonin A ( 1 ), a macrocyclic lactone, japonin B ( 2 ), a (phenylethyl)benzoquinone, japonin C ( 3 ), a phenanthraquinone, japonin D ( 4 ), as well as a dihydrostilbene, stilbostemin M ( 5 ), were isolated from the roots of Stemona japonica, together with eight known compounds. Their structures were established by spectroscopic analyses. 相似文献
1000.