全文获取类型
收费全文 | 315082篇 |
免费 | 4844篇 |
国内免费 | 2660篇 |
专业分类
化学 | 155545篇 |
晶体学 | 5280篇 |
力学 | 15696篇 |
综合类 | 198篇 |
数学 | 44972篇 |
物理学 | 100895篇 |
出版年
2021年 | 1957篇 |
2020年 | 2321篇 |
2019年 | 2528篇 |
2018年 | 11294篇 |
2017年 | 12184篇 |
2016年 | 8229篇 |
2015年 | 3787篇 |
2014年 | 4579篇 |
2013年 | 10441篇 |
2012年 | 12264篇 |
2011年 | 21854篇 |
2010年 | 13808篇 |
2009年 | 13744篇 |
2008年 | 18649篇 |
2007年 | 21973篇 |
2006年 | 7832篇 |
2005年 | 13678篇 |
2004年 | 9617篇 |
2003年 | 9070篇 |
2002年 | 6973篇 |
2001年 | 6764篇 |
2000年 | 5350篇 |
1999年 | 3937篇 |
1998年 | 3074篇 |
1997年 | 2973篇 |
1996年 | 3006篇 |
1995年 | 2764篇 |
1994年 | 2601篇 |
1993年 | 2409篇 |
1992年 | 3053篇 |
1991年 | 2934篇 |
1990年 | 2862篇 |
1989年 | 2867篇 |
1988年 | 2882篇 |
1987年 | 2856篇 |
1986年 | 2726篇 |
1985年 | 3446篇 |
1984年 | 3445篇 |
1983年 | 2721篇 |
1982年 | 2814篇 |
1981年 | 2871篇 |
1980年 | 2628篇 |
1979年 | 2983篇 |
1978年 | 2977篇 |
1977年 | 3009篇 |
1976年 | 2878篇 |
1975年 | 2606篇 |
1974年 | 2561篇 |
1973年 | 2553篇 |
1968年 | 1800篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
S. Venkatesan S. Suresh P. Ramu M. Kandasamy J. Arumugam S. Thambidurai K.M. Prabu N. Pugazhenthiran 《印度化学会志》2022,99(5):100436
A facile biosynthesis route was followed to prepare zinc oxide nanoparticles (ZnO NPs) using Euphorbia milii (E. milii) leaf constituents. The SEM images exhibited presence of spherical ZnO NPs and the corresponding TEM images disclosed monodisperse nature of the ZnO NPs with diameter ranges between 12 and 20 nm. The Brunauer–Emmett–Teller (BET) analysis revealed that the ZnO NPs have specific surface area of 20.46 m2/g with pore diameter of 2 nm–10 nm and pore volume of 0.908 cm3/g. The EDAX spectrum exemplified the existence of Zn and O elements and non-appearance of impurities that confirmed pristine nature of the ZnO NPs. The XRD pattern indicated crystalline peaks corresponding to hexagonal wurtzite structured ZnO with an average crystallite size of 16.11 nm. The FTIR spectrum displayed strong absorption bands at 512 and 534 cm?1 related to ZnO. The photocatalytic action of ZnO NPs exhibited noteworthy degradation of methylene blue dye under natural sunlight illumination. The maximum degradation efficiency achieved was 98.17% at an illumination period of 50 min. The reusability study proved considerable photostability of the ZnO NPs during photocatalytic experiments. These findings suggest that the E. milii leaf constituents can be utilized as suitable biological source to synthesis ZnO NPs for photocatalytic applications. 相似文献
22.
Satyanarayana A. V. S. Rao M. Jagannadha Mouli K. Chandra 《Journal of Radioanalytical and Nuclear Chemistry》2022,331(3):1509-1513
Journal of Radioanalytical and Nuclear Chemistry - 相似文献
23.
Pengjin Qin Li-An Wang Prof. Joseph M. O'Connor Prof. Kim K. Baldridge Yifan Li Burak Tufekci Jiyue Chen Prof. Arnold L. Rheingold 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(41):18114-18121
Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally harsh reaction conditions, chemists have long sought to develop catalytic variants based upon the activating power of metal–alkene coordination. Herein, we demonstrate the first successful implementation of such a strategy by utilizing [(C5H5)Ru(NCMe)3]PF6 as a precatalyst for the disrotatory 6π electrocyclization of highly substituted trienes that are resistant to thermal cyclization. Mechanistic and computational studies implicate hexahapto transition-metal coordination as responsible for lowering the energetic barrier to ring closure. This work establishes a foundation for the development of new catalysts for stereoselective electrocyclizations. 相似文献
24.
Lei Chen Hui-Qing Yang Cheng-Yu Jin Zhao-Xu Chen 《International journal of quantum chemistry》2020,120(22):e26366
We studied the ring opening of propylene oxide (PO) by salen-M coordinated OH− group [M = Al(III), Sc(III), Cr(III), Mn(III), Fe(III), Co(II), Co(III), Ni(II), Cu(II), Zn(II), Ru(III) and Rh(III)]. The results show that the ring-opening energy barriers for M(II) complexes are much lower than those with M(III) complexes in the gas phase, and the barriers correlate linearly with the negative charges on the OH− group and the Fukui function condensed on the OH− group. The nucleophilicity ordering in the gas phase can be rationalized by the ratio of formal positive charges/radius of M cations. Solvent effect greatly increases the barriers of M(II) complexes but slightly changes the results of M(III) ones, making the barriers similar. Analysis indicates that the reaction heats are linearly proportional to the reverse reaction barriers. The relationships established here can be used to estimate the ring-opening barriers and to screen epoxide ring-opening catalysts. 相似文献
25.
26.
27.
28.
29.
Synthesis of Highly Substituted γ‐Butyrolactones by a Gold‐Catalyzed Cascade Reaction of Benzyl Esters 下载免费PDF全文
Maria Camila Blanco Jaimes Alexander Ahrens Daniel Pflästerer Dr. Matthias Rudolph Prof. Dr. A. Stephen K. Hashmi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(1):427-433
Easily accessible benzylic esters of 3‐butynoic acids in a gold‐catalyzed cyclization/rearrangement cascade reaction provided 3‐propargyl γ‐butyrolactones with the alkene and the carbonyl group not being conjugated. Crossover experiments showed that the formation of the new C?C bond is an intermolecular process. Initially propargylic–benzylic esters were used, but alkyl‐substituted benzylic esters worked equally well. In the case of the propargylic–benzylic products, a simple treatment of the products with aluminum oxide initiated a twofold tautomerization to the allenyl‐substituted γ‐butyrolactones with conjugation of the carbonyl group, the olefin, and the allene. The synthetic sequence can be conducted stepwise or as a one‐pot cascade reaction with similar yields. Even in the presence of the gold catalyst the new allene remains intact. 相似文献
30.