全文获取类型
收费全文 | 523092篇 |
免费 | 22138篇 |
国内免费 | 12833篇 |
专业分类
化学 | 295739篇 |
晶体学 | 6208篇 |
力学 | 24221篇 |
综合类 | 679篇 |
数学 | 62063篇 |
物理学 | 169153篇 |
出版年
2022年 | 3926篇 |
2021年 | 6632篇 |
2020年 | 8216篇 |
2019年 | 8318篇 |
2018年 | 9352篇 |
2017年 | 8914篇 |
2016年 | 13581篇 |
2015年 | 10161篇 |
2014年 | 14220篇 |
2013年 | 27564篇 |
2012年 | 24472篇 |
2011年 | 28381篇 |
2010年 | 19940篇 |
2009年 | 19527篇 |
2008年 | 24082篇 |
2007年 | 23293篇 |
2006年 | 21601篇 |
2005年 | 19033篇 |
2004年 | 16969篇 |
2003年 | 14407篇 |
2002年 | 13136篇 |
2001年 | 14981篇 |
2000年 | 11208篇 |
1999年 | 9206篇 |
1998年 | 7612篇 |
1997年 | 7349篇 |
1996年 | 7060篇 |
1995年 | 6622篇 |
1994年 | 6474篇 |
1993年 | 6038篇 |
1992年 | 6566篇 |
1991年 | 6542篇 |
1990年 | 6180篇 |
1989年 | 5837篇 |
1988年 | 5732篇 |
1987年 | 5610篇 |
1986年 | 5295篇 |
1985年 | 6728篇 |
1984年 | 6785篇 |
1983年 | 5375篇 |
1982年 | 5436篇 |
1981年 | 5377篇 |
1980年 | 4929篇 |
1979年 | 5326篇 |
1978年 | 5458篇 |
1977年 | 5359篇 |
1976年 | 5309篇 |
1975年 | 4896篇 |
1974年 | 4853篇 |
1973年 | 4862篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
991.
992.
993.
994.
995.
996.
997.
Bio‐Inspired Transition Metal–Organic Hydride Conjugates for Catalysis of Transfer Hydrogenation: Experiment and Theory 下载免费PDF全文
Alex McSkimming Dr. Bun Chan Dr. Mohan M. Bhadbhade Dr. Graham E. Ball Prof. Stephen B. Colbran 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(7):2821-2834
Taking inspiration from yeast alcohol dehydrogenase (yADH), a benzimidazolium (BI+) organic hydride‐acceptor domain has been coupled with a 1,10‐phenanthroline (phen) metal‐binding domain to afford a novel multifunctional ligand ( L BI+) with hydride‐carrier capacity ( L BI++H?? L BIH). Complexes of the type [Cp*M( L BI)Cl][PF6]2 (M=Rh, Ir) have been made and fully characterised by cyclic voltammetry, UV/Vis spectroelectrochemistry, and, for the IrIII congener, X‐ray crystallography. [Cp*Rh( L BI)Cl][PF6]2 catalyses the transfer hydrogenation of imines by formate ion in very goods yield under conditions where the corresponding [Cp*Ir( L BI)Cl][PF6] and [Cp*M(phen)Cl][PF6] (M=Rh, Ir) complexes are almost inert as catalysts. Possible alternatives for the catalysis pathway are canvassed, and the free energies of intermediates and transition states determined by DFT calculations. The DFT study supports a mechanism involving formate‐driven Rh?H formation (90 kJ mol?1 free‐energy barrier), transfer of hydride between the Rh and BI+ centres to generate a tethered benzimidazoline (BIH) hydride donor, binding of imine substrate at Rh, back‐transfer of hydride from the BIH organic hydride donor to the Rh‐activated imine substrate (89 kJ mol?1 barrier), and exergonic protonation of the metal‐bound amide by formic acid with release of amine product to close the catalytic cycle. Parallels with the mechanism of biological hydride transfer in yADH are discussed. 相似文献
998.
Experimental and Theoretical Studies on the Rearrangement of 2‐Oxoazepane α,α‐Amino Acids into 2′‐Oxopiperidine β2,3,3‐Amino Acids: An Example of Intramolecular Catalysis 下载免费PDF全文
Dr. Diego Núñez‐Villanueva Dr. M. Ángeles Bonache Laura Lozano Dr. Lourdes Infantes Prof. José Elguero Prof. Ibon Alkorta Prof. M. Teresa García‐López Dr. Rosario González‐Muñiz Dr. Mercedes Martín‐Martínez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(6):2489-2500
Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis. 相似文献
999.
1000.
DFT mechanistic study of the H2‐assisted chain transfer copolymerization of propylene and p‐methylstyrene catalyzed by zirconocene complex 下载免费PDF全文
Cheng‐Gen Zhang Shu‐Yuan Yu Liaoyun Zhang Huayi Li Zhi‐Xiang Wang 《Journal of polymer science. Part A, Polymer chemistry》2015,53(4):576-585
DFT computations have been performed to investigate the mechanism of H2‐assisted chain transfer strategy to functionalize polypropylene via Zr‐catalyzed copolymerization of propylene and p‐methylstyrene (pMS). The study unveils the following: (i) propylene prefers 1,2‐insertion over 2,1‐insertion both kinetically and thermodynamically, explaining the observed 1,2‐insertion regioselectivity for propylene insertion. (ii) The 2,1‐inserion of pMS is kinetically less favorable but thermodynamically more favorable than 1,2‐insertion. The observation of 2,1‐insertion pMS at the end of polymer chain is due to thermodynamic control and that the barrier difference between the two insertion modes become smaller as the chain length becomes longer. (iii) The pMS insertion results in much higher barriers for subsequent either propylene or pMS insertion, which causes deactivation of the catalytic system. (iv) Small H2 can react with the deactivated [Zr]?pMS?PPn facilely, which displace functionalized pMS?PPn chain and regenerate [Zr]? H active catalyst to continue copolymerization. The effects of counterions are also discussed. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 576–585 相似文献