全文获取类型
收费全文 | 39151篇 |
免费 | 15758篇 |
国内免费 | 50篇 |
专业分类
化学 | 52350篇 |
晶体学 | 18篇 |
力学 | 1078篇 |
数学 | 1090篇 |
物理学 | 423篇 |
出版年
2024年 | 422篇 |
2023年 | 4241篇 |
2022年 | 1393篇 |
2021年 | 2394篇 |
2020年 | 4665篇 |
2019年 | 2208篇 |
2018年 | 2362篇 |
2017年 | 591篇 |
2016年 | 5524篇 |
2015年 | 5486篇 |
2014年 | 4904篇 |
2013年 | 5011篇 |
2012年 | 3003篇 |
2011年 | 925篇 |
2010年 | 3287篇 |
2009年 | 3229篇 |
2008年 | 961篇 |
2007年 | 644篇 |
2006年 | 81篇 |
2005年 | 45篇 |
1997年 | 43篇 |
1996年 | 46篇 |
1995年 | 108篇 |
1994年 | 67篇 |
1993年 | 186篇 |
1992年 | 66篇 |
1991年 | 56篇 |
1989年 | 44篇 |
1988年 | 75篇 |
1987年 | 60篇 |
1986年 | 43篇 |
1985年 | 46篇 |
1984年 | 57篇 |
1983年 | 64篇 |
1982年 | 71篇 |
1981年 | 84篇 |
1980年 | 104篇 |
1979年 | 96篇 |
1978年 | 95篇 |
1977年 | 163篇 |
1976年 | 181篇 |
1975年 | 185篇 |
1974年 | 195篇 |
1973年 | 110篇 |
1972年 | 153篇 |
1971年 | 122篇 |
1970年 | 208篇 |
1969年 | 127篇 |
1968年 | 133篇 |
1966年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Hydroarylation of bicyclic alkenes has been developed using a low‐valent ReI‐catalyzed, directing group‐assisted C?H bond activation strategy. The addition of sodium acetate significantly improves the reaction efficiency; moreover, bicyclic alkenes such as 7‐oxa and aza benzonorbornadienes worked efficiently under this reaction condition. Preliminary mechanistic studies suggest that, after the alkene insertion, the rhenacycle preferentially undergoes protonolysis rather than reductive elimination. 相似文献
992.
5,12‐Diacetyl‐5,12‐dihydroquinoxalino[2,3‐b]quinoxalines: Solid‐State Fluorescence,AIE Properties,and Orbital Switching by Substituent Effect 下载免费PDF全文
The compound 5,12‐diacetyl‐5,12‐dihydroquinoxalino[2,3‐b]quinoxaline 1 a and its derivatives were prepared, and their solid‐ and solution‐state spectroscopic properties were studied; 1 a shows stronger fluorescence in solution than in the solid state due to aggregation caused by self‐quenching. Phenyl‐ or alkoxy‐substituted derivatives 1 b – d show solid‐state fluorescence with moderate quantum yields of about Φ=0.12–0.15, although the corresponding values are 0.01–0.07 in solution. The spectroscopic properties of alkoxy‐substituted derivatives were hardly changed compared to 1 a and 1 b , although 1 a and 1 b have similar absorption and fluorescence maxima in solution and in the solid state. DFT calculations indicate that orbital switching occurs between HOMO and HOMO‐1 and HOMO‐2 due to orbital interactions with introduced substituents. Crystal structure analysis revealed that the molecules have bent structures around tertiary nitrogen atoms and form a characteristic dimeric structure. 相似文献
993.
994.
Facile Separation,Spectroscopic Identification,and Electrochemical Properties of Higher Trifluoromethylated Derivatives of [70]Fullerene 下载免费PDF全文
Maria P. Kosaya Alexey V. Rybalchenko Dr. Natalia S. Lukonina Olga N. Mazaleva Dr. Ilya N. Ioffe Dr. Vitaliy Yu. Markov Prof. Dr. Sergey I. Troyanov Prof. Dr. Lev N. Sidorov Dr. Nadezhda B. Tamm Dr. Alexey A. Goryunkov 《化学:亚洲杂志》2018,13(15):1920-1931
We survey the structure and electronic properties of the family of higher trifluoromethylated C70(CF3)n molecules with n=14, 16, 18, and 20. Twenty‐two available compounds, of which thirteen are newly obtained and characterized, demonstrate the broad diversity of π‐system topologies, which enabled us to study the interplay between the CF3 addition pattern and the electronic properties. UV/Vis spectroscopic and cyclic voltammetric studies demonstrate the importance of the exact addition pattern rather than the plain number of addends. Of particular interest is the skew pentagonal pyramid (SPP) addition pattern, which enables formation of closed‐shell cyclopentadienyl anions C70(CF3)n? 1 ? through CF3 detachment upon electron transfer. A detailed study of the process is presented for a SPP‐C70(CF3)16 where potentiostatic electrolysis at the second reduction potential gives C70(CF3)15? oxidizable to a persistent C70(CF3)15· radical. Together with the literature data for the lower C70(CF3)n compounds with n=2–12, the present results show good correlation between the experimental boundary level positions and the DFT predictions. The compounds turn out to be electron acceptor molecular semiconductors with experimental LUMO energies and HOMO–LUMO gaps within the ranges of ?4.3 to ?3.7 eV and 1.6 to 3.3 eV, respectively, depending on the shape of the conjugated fragments. The HOMO levels fall within the range of ?5.6 to ?6.9 eV and show linear correlation with the number of addends. 相似文献
995.
Tuning the Electronic Properties and Acid‐Response Behavior of N‐Heteroacene‐Based π‐Conjugated Liquids by Changing the Number of π‐Conjugated Substituents 下载免费PDF全文
Yuika Sato Dr. Yuichiro Mutoh Dr. Daisuke Matsukuma Makoto Nakagawa Prof. Takeshi Kawai Dr. Kyosuke Isoda 《化学:亚洲杂志》2018,13(18):2619-2625
We have designed and synthesized two room‐temperature‐fluorescent π‐conjugated liquids based on the N‐heteroacene framework ( 1 and 2 ). These two π‐conjugated liquids, which contained one and two thiophene rings, respectively, exhibited different electronic properties and rheology behaviors. Single‐crystal X‐ray analysis of dithiophene‐appended compound 4 revealed that two thiophene rings hindered the interactions of the imino N atoms with acids through the formation of interactions between the S atoms of the thiophene rings and the imino N atoms of the pyrazine group. On the other hand, monothiophene‐appended molecules 1 and 3 each contained an unhindered imino N atom on the opposite site to the thiophene ring. Upon dissolving various acids with different pKa values in compounds 1 and 2 , these slight structural differences gave rise to marked differences in their acid‐response behaviors, thereby resulting in the emission of variously colored fluorescence in the liquid state. Furthermore, when acids with lower pKa values was dissolved in compounds 1 and 2 , phase transition occurred from an isotropic liquid state to a self‐organized liquid‐crystalline phase. 相似文献
996.
A Porous Organic Poly(triphenylimidazole) Decorated with Palladium Nanoparticles for the Cyanation of Aryl Iodides 下载免费PDF全文
Haiwen Yu Siqi Xu Dr. Yijiang Liu Prof. Hongbiao Chen Prof. Huaming Li 《化学:亚洲杂志》2018,13(18):2708-2713
A new porous organic poly(triphenylimidazole), PTPI‐Me, was prepared through a Yamamoto self‐coupling reaction of 2,4,5‐tris‐(4‐bromophenyl)‐1‐methyl‐1H‐imidazole (TPI‐Me) in the presence of bis(1,5‐cyclooctadiene)nickel(0). The polymer was subsequently decorated with Pd nanoparticles (NPs) to afford a heterogeneous cyanation catalyst, Pd@PTPI‐Me. Pd NPs with an average diameter of 2.7 nm were grown within the PTPI‐Me framework, owing to the coordination of the imidazole rings to the Pd species. The resultant Pd@PTPI‐Me catalyst, with a Pd loading of 0.13 mmol g?1, exhibited superior catalytic activity for the cyanation of aryl iodides. More importantly, the heterogeneous catalyst was also readily recycled and displayed negligible deactivation after five cycles. 相似文献
997.
Efficient Synthesis of Five Types of Heterocyclic Compounds via Intramolecular Elimination Using Ultrasound‐Static Heating Technique 下载免费PDF全文
Dr. Hongfei Jiang Xueyang Dong Xin Jin Danyang Zhu Dr. Ruijuan Yin Dr. Rilei Yu Prof. Shengbiao Wan Prof. Lijuan Zhang Prof. Tao Jiang 《化学:亚洲杂志》2018,13(16):2009-2013
An experimental technique, ultrasound‐static heating, has been developed for the efficient synthesis of heterocyclic compounds. The technique involves ultrasonic irradiation and static heating processes. First, the ultrasonic irradiation process is performed to form an intermediate of the heterocyclic compound under mild conditions and the subsequent static heating process (heating the intermediate under solvent‐free conditions without stirring) produces the target heterocyclic compounds via intramolecular elimination. 相似文献
998.
[Bis(imidazolyl)–BH2]+[Bis(triazolyl)–BH2]− Ionic Liquids with High Density and Energy Capacity 下载免费PDF全文
Nianming Jiao Prof. Yanqiang Zhang Hao Li Dr. Long Liu Prof. Suojiang Zhang 《化学:亚洲杂志》2018,13(15):1932-1940
[Bis(imidazolyl)–BH2]+[bis(triazolyl)–BH2]? and [bis(imidazolyl)–BH2]+[tris(triazolyl)–BH]? were synthesized, the cations and anions of which were functionalized with B?H groups and azoles. As B?H groups contribute to the hypergolic activity and azole groups improve the energy output, the resulting ionic liquids exhibited ignition delay times as low as 20 ms and energy outputs as high as 461.1 kJ mol?1. In addition, densities (1.07–1.22 g cm?3) and density‐specific impulse (≈360 s g cm?3) values reached a relatively high level. These ionic liquids show great promise as sustainable rocket fuels. 相似文献
999.
1000.