全文获取类型
收费全文 | 220342篇 |
免费 | 4692篇 |
国内免费 | 2811篇 |
专业分类
化学 | 117320篇 |
晶体学 | 3591篇 |
力学 | 11545篇 |
综合类 | 169篇 |
数学 | 23645篇 |
物理学 | 71575篇 |
出版年
2021年 | 2601篇 |
2020年 | 2753篇 |
2019年 | 3214篇 |
2018年 | 4239篇 |
2017年 | 4185篇 |
2016年 | 5621篇 |
2015年 | 3153篇 |
2014年 | 5287篇 |
2013年 | 9784篇 |
2012年 | 8424篇 |
2011年 | 9668篇 |
2010年 | 7545篇 |
2009年 | 7653篇 |
2008年 | 8797篇 |
2007年 | 8612篇 |
2006年 | 7724篇 |
2005年 | 6773篇 |
2004年 | 6309篇 |
2003年 | 5727篇 |
2002年 | 5712篇 |
2001年 | 6062篇 |
2000年 | 4522篇 |
1999年 | 3574篇 |
1998年 | 3009篇 |
1997年 | 3049篇 |
1996年 | 2878篇 |
1995年 | 2495篇 |
1994年 | 2596篇 |
1993年 | 2424篇 |
1992年 | 2706篇 |
1991年 | 2795篇 |
1990年 | 2684篇 |
1989年 | 2731篇 |
1988年 | 2638篇 |
1987年 | 2617篇 |
1986年 | 2470篇 |
1985年 | 3075篇 |
1984年 | 3131篇 |
1983年 | 2622篇 |
1982年 | 2575篇 |
1981年 | 2545篇 |
1980年 | 2342篇 |
1979年 | 2710篇 |
1978年 | 2671篇 |
1977年 | 2870篇 |
1976年 | 2872篇 |
1975年 | 2664篇 |
1974年 | 2602篇 |
1973年 | 2711篇 |
1972年 | 2085篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
121.
Dr. Zhoulong Fan Katherine L. Bay Dr. Xiangyang Chen Zhe Zhuang Han Seul Park Dr. Kap-Sun Yeung Prof. Dr. K. N. Houk Prof. Dr. Jin-Quan Yu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(12):4800-4807
A simple and efficient nitrile-directed meta-C−H olefination, acetoxylation, and iodination of biaryl compounds is reported. Compared to the previous approach of installing a complex U-shaped template to achieve a molecular U-turn and assemble the large-sized cyclophane transition state for the remote C−H activation, a synthetically useful phenyl nitrile functional group could also direct remote meta-C−H activation. This reaction provides a useful method for the modification of biaryl compounds because the nitrile group can be readily converted to amines, acids, amides, or other heterocycles. Notably, the remote meta-selectivity of biphenylnitriles could not be expected from previous results with a macrocyclophane nitrile template. DFT computational studies show that a ligand-containing Pd–Ag heterodimeric transition state (TS) favors the desired remote meta-selectivity. Control experiments demonstrate the directing effect of the nitrile group and exclude the possibility of non-directed meta-C−H activation. Substituted 2-pyridone ligands were found to be key in assisting the cleavage of the meta-C−H bond in the concerted metalation–deprotonation (CMD) process. 相似文献
122.
123.
Physics of Particles and Nuclei Letters - The problem of spontaneous isotopic symmetry breaking in Nambu–Jona-Lasinio quark models is considered. It is shown that, in models with light... 相似文献
124.
Doklady Physics - In this paper, we study the statistical characteristics of the Riemann wave and the integral of it. Such problems arise in the physics of fronts’ propagation, in particular,... 相似文献
125.
126.
127.
Hejia Hu Linlin Wu Mei Li Cun Xue Guangcheng Wang Siying Chen Yong Huang Lin Zheng Aimin Wang Yueting Li Zipeng Gong 《Biomedical chromatography : BMC》2020,34(1):e4714
Eucommia ulmoides Oliv. (E. ulmoides) is a valuable and nourishing medicinal herb in China that has been used in the treatment of hypertension. Given the fact that most traditional Chinese medicine is mainly used to treat disease, investigating the pharmacokinetics of traditional Chinese medicines in the pathological state is more useful than that in the normal state. However, the differences in the absorption kinetics of active ingredients of E. ulmoides extract between pathological and physiological conditions have not been reported. Therefore, in this study, the rat intestinal in situ circulatory perfusion model was used to investigate the differences in absorption kinetics of seven active ingredients of E. ulmoides extract in normal and spontaneously hypertensive rats, namely, genipinic acid, protocatechuic acid, neochlorogenic acid, chlorogenic acid, cryptochlorogenic acid, (+)-pinoresinol di-O-β-D -glucopyranoside and (+)-pinoresinol 4′-O-β-D -glucopyranoside. Our results indicate that the pathological state of spontaneous hypertension may change the absorption of active components of E. ulmoides extracts, and these findings may provide a reference for improving the rational use of E. ulmoides in the clinic. 相似文献
128.
129.
Ain Uddin Kyle N. Plunkett 《Journal of polymer science. Part A, Polymer chemistry》2020,58(22):3165-3169
A series of donor-acceptor copolymers with dicyclopenta[cd,jk]pyrene and dicyclopenta[cd,lm]perylene acceptor units was prepared via palladium catalyzed cyclopenta-annulation reactions. The acceptor units were paired with diethynyl containing donor groups based on benzo[1,2-b:4,5-b’]dithiophene, thieno[3,2-b]thiophene, and 4-octyl-4H-dithieno[3,2-b:2′,3′-d]pyrrole to create six polymer variants. The cyclopentannulation polymerization resulted in copolymers with molecular weights (Mn) of 6–14 kDa and broad light absorption in the visible region with band gaps of 1.38–1.85 eV. The synthetic methodology, as well as optoelectronic properties, including thin-film absorption and cyclic voltammetry, of the donor-acceptor copolymers are presented. 相似文献
130.
Doklady Mathematics - Integro-differential equations with unbounded operator coefficients in a Hilbert space are studied. The equations under consideration are abstract hyperbolic equations... 相似文献