全文获取类型
收费全文 | 21975篇 |
免费 | 1036篇 |
国内免费 | 1265篇 |
专业分类
化学 | 8682篇 |
晶体学 | 192篇 |
力学 | 393篇 |
综合类 | 63篇 |
数学 | 11246篇 |
物理学 | 3700篇 |
出版年
2023年 | 360篇 |
2022年 | 187篇 |
2021年 | 279篇 |
2020年 | 440篇 |
2019年 | 554篇 |
2018年 | 483篇 |
2017年 | 392篇 |
2016年 | 466篇 |
2015年 | 550篇 |
2014年 | 778篇 |
2013年 | 1368篇 |
2012年 | 910篇 |
2011年 | 1622篇 |
2010年 | 1250篇 |
2009年 | 1615篇 |
2008年 | 1786篇 |
2007年 | 1575篇 |
2006年 | 1221篇 |
2005年 | 935篇 |
2004年 | 857篇 |
2003年 | 780篇 |
2002年 | 704篇 |
2001年 | 457篇 |
2000年 | 418篇 |
1999年 | 504篇 |
1998年 | 380篇 |
1997年 | 319篇 |
1996年 | 387篇 |
1995年 | 362篇 |
1994年 | 320篇 |
1993年 | 248篇 |
1992年 | 223篇 |
1991年 | 175篇 |
1990年 | 150篇 |
1989年 | 166篇 |
1988年 | 109篇 |
1987年 | 114篇 |
1986年 | 99篇 |
1985年 | 130篇 |
1984年 | 106篇 |
1983年 | 62篇 |
1982年 | 98篇 |
1981年 | 96篇 |
1980年 | 49篇 |
1979年 | 36篇 |
1978年 | 32篇 |
1977年 | 43篇 |
1976年 | 32篇 |
1975年 | 9篇 |
1974年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
《Mendeleev Communications》2022,32(3):327-330
Carbon–carbon composites (C/C) were produced from carbon fiber reinforced phthalonitrile (CFRP) matrix composites in a two-step impregnation–carbonization procedure. After graphitization at 1800 °C, the obtained C/C composites demonstrated highly crystalline structure and properties characteristic of composites derived from phenolic matrix CFRP by the industrial procedure: d = 1.73 g cm?3, interlaminar shear strength was 14.1 MPa, compression strength was 139.8 MPa, and coefficient of friction was in the range 0.32–0.34. 相似文献
92.
Vianey Vasquez-Ruiz M. Ángeles Ramírez-Cisneros Maria Yolanda Rios 《Magnetic resonance in chemistry : MRC》2022,60(3):275-358
Cedrela genus, a member of the Meliaceae family, presents both chemical characteristics associated with and those that distinguish it from the rest of its members. The presence of triterpenes and limonoids is the characteristic of the Meliaceae family, but the class and type of these chemical constituents are distinctive for each genus. Cedrela includes cycloartane, ursane, oleanane, tirucallane, butyrospermane, and apotirucallane triterpenes, and its limonoids belongs to six class and nine types, known as class Ia-type havanensines, class Ib-type delevoyin, class II-type gedunin, class IIIb-type andirobin, class IIIg-type mexicanolide, class IVa-type evoludone, class Va-type obacunol, class V-type limonin, and class VIII. Each of these structural arrangements includes specific traits, defined by their biosynthetic origin, which can be established by means of structural elucidation techniques, particularly 1H and 13C NMR, which assisted by 2D NMR techniques, allowing to deduce their structures unequivocally. The constant presence of these skeletal arrangements in Cedrela ensures that they are its chemophenetic markers and their recurrence is an important criterion for their identity. This review is a compilation of the occurrence of triterpenes and limonoids in Cedrela genus, detailing their biosynthetic association and collecting and organizing their NMR data, with the purpose of facilitating its location, analysis, and use in the phytochemical study of species from this genus. 相似文献
93.
Dr. Toshiro Takao Dr. Yuta Takahashi Masataka Kai 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(29):e202200327
A dicationic triruthenium complex containing a μ3-η3-C3 ring, [(Cp*Ru)3(μ3-η3-C3MeH2−)(μ3-CH)(μ-H)]2+ ( 1 a , Cp*=η5-C5Me5), reacted with ammonia to yield a μ-amido complex, [(Cp*Ru)3(μ3-η3-CHCMeCH) (μ3-CH)(μ-NH2)]2+ ( 5 ), via N−H bond scission. Subsequent treatment with base resulted in C−N bond formation to yield a μ3-η2:η2-1-azabutadien-4-yl complex, [(Cp*Ru)3(μ3-CH)(μ3-η2:η2-NH=CH−CMe=CH−)]+ ( 6 a ). The azaruthenacyclopentadiene skeleton was alternatively synthesized by the photolysis of mono-cationic complex [(Cp*Ru)3(μ3-η3-C3RH2−)(μ3-CH)]+ ( 2 a ; R=Me, 2 b ; R=H) in the presence of ammonia. The C3 ring skeleton was broken via the electron transfer to the π*(C−C) orbital in the C3 ring, and a transiently generated unsaturated μ3-allylic species can take up ammonia, resulting in N−H bond scission followed by C−N bond formation. 相似文献
94.
Dr. Joseph N. Capilato Stefan A. Harry Dr. Maxime A. Siegler Prof. Thomas Lectka 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(8):e202103922
As appreciation for nonclassical hydrogen bonds has progressively increased, so have efforts to characterize these interesting interactions. Whereas several kinds of C−H hydrogen bonds have been well-studied, much less is known about the R3N+−C−H⋅⋅⋅X variety. Herein, we present crystallographic and spectroscopic evidence for the existence of these interactions, with special relevance to Selectfluor chemistry. Of particular note is the propensity for Lewis bases to engage in nonclassical hydrogen bonding over halogen bonding with the electrophilic F atom of Selectfluor. Further, the first examples of 1H NMR experiments detailing R3N+−C−H⋅⋅⋅X (X=O, N) hydrogen bonds are described. 相似文献
95.
Chikako Matsuda Ryo Igarashi Prof. Dr. Hiroshi Katagiri Dr. Takashi Murase 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(19):e202200132
Tri- and tetra-fluorinated [7]helicenes are photolabile and undergo a double fluorine atom transfer. Herein, we show that the transferred product further undergoes a skeletal transformation on silica gel. The transformation begins with activation of the allylic C−F bond on the silanol surface. Then, the resulting carbocation readily undergoes a regioselective nucleophilic aromatic substitution with water, depending on the position of the fluorine substituents. Hexafluoro-2-propanol also activated the allylic C−F bond and acted as a nucleophile. These findings support the generation of a highly reactive cationic electrophilic intermediate in the successive transformations involving fluorine atoms. 相似文献
96.
Marcel Sickert Harald Weinstabl Brendan Peters Xiao Hou Mark Lautens 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2014,126(20):5247-5251
A palladium‐catalyzed intermolecular cross‐coupling of two aryl iodides is reported, giving polycyclic ring systems with a high level of convergence and efficiency. 相似文献
97.
Dr. Zheng Liu Weiguo Han Prof. Dr. Jingbo Lan Lingyan Sun Dr. Junbin Tang Prof. Dr. Cheng Zhang Prof. Dr. Jingsong You 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2023,135(3):e202211412
Disclosed herein is a RhCl3-catalyzed peri-selective C−H/C−H oxidative homo-coupling of 1-substituted naphthalenes, which provides a highly efficient and streamlined approach to chalcogen-embedded anthanthrenes from readily available starting materials. Introducing O, S, and Se into the anthanthrene skeleton leads to gradually increased π–π stacking distances but significantly enhanced π–π overlaps with the growth of the hetero-atom radius. Moderate π–π distance, overlap area, and intermolecular S−S interactions endow S-embedded anthanthrene ( PTT ) with excellent 2D charge-transport properties. Moreover, the transformation of p-type to n-type S-embedded anthanthrenes is realized for the first time via the S-atom oxidation from PTT to PTT-O4 . In organic field-effect transistor devices, PTT derivatives exhibit hole transport with mobilities up to 1.1 cm2 V−1 s−1, while PTT-O4 shows electron transport with a mobility of 0.022 cm2 V−1 s−1. 相似文献
98.
《印度化学会志》2023,100(6):100999
Cyclodextrins are natural oligosaccharides used to increase the solubility of drugs. It has numerous applications in drug discovery, food storage and other fields. Loftsson et al. explained about applications of Cyclodextrins(CDs) in administrating the drugs through various ways. Jansook et al. has given insights into the structure, physicochemical properties and pharmaceutical applications of CDs. In the present work, cyclodextrin and its derivatives such as α, β, γ CDs are studied for which various degree and neighborhood degree-based topological indices are computed through M-polynomial and NM-polynomial respectively and the comparison of the indices for all three types of cyclodextrins are presented which are of great importance in QSPR/QSAR studies. 相似文献
99.
Formation of active phases of Fe/C,Cr/C and Fe–Cr/C catalysts in oxidative dehydrogenation of ethane
《Mendeleev Communications》2023,33(3):422-424
The oxidative dehydrogenation of ethane into ethylene using CO2 as an oxidant at temperatures of 650–750 °C was carried out over Fe/C, Cr/C and Fe–Cr/C catalysts deposited on a carbon support. Before and after the reaction the catalysts were investigated by X-ray powder diffraction (XRD), in situ magnetometry and transmission electron microscopy methods. The correlation between activity of Fe/C, Cr/C and Fe–Cr/C catalytic systems and their phase composition was established. 相似文献
100.
工业发展与人类活动导致大气中CO2浓度逐年升高, 引发一系列生态环境问题. 将CO2光催化转化为高附加值化学物质不仅有利于缓解环境压力,也可以带来额外经济价值. 然而, 由于多电子利用效率低和C―C偶联动力学缓慢, 光还原CO2制多碳产品面临产率低和选择性差等挑战. 光催化剂活性位点调控能够有效解决上述问题. 我们综述了近几年用于光还原CO2催化剂表面活性位点设计的研究进展, 主要包括缺陷位点、 金属位点以及掺杂位点等, 从活性位点的角度为光还原CO2催化剂设计提供新视角, 并对开发高效光催化剂具有启发意义. 相似文献