全文获取类型
收费全文 | 21828篇 |
免费 | 1521篇 |
国内免费 | 2133篇 |
专业分类
化学 | 17053篇 |
晶体学 | 280篇 |
力学 | 280篇 |
综合类 | 101篇 |
数学 | 4928篇 |
物理学 | 2840篇 |
出版年
2024年 | 20篇 |
2023年 | 222篇 |
2022年 | 306篇 |
2021年 | 394篇 |
2020年 | 515篇 |
2019年 | 950篇 |
2018年 | 607篇 |
2017年 | 857篇 |
2016年 | 647篇 |
2015年 | 763篇 |
2014年 | 899篇 |
2013年 | 1694篇 |
2012年 | 1304篇 |
2011年 | 1250篇 |
2010年 | 986篇 |
2009年 | 1261篇 |
2008年 | 1443篇 |
2007年 | 1270篇 |
2006年 | 1111篇 |
2005年 | 1026篇 |
2004年 | 1015篇 |
2003年 | 879篇 |
2002年 | 1028篇 |
2001年 | 631篇 |
2000年 | 582篇 |
1999年 | 468篇 |
1998年 | 440篇 |
1997年 | 384篇 |
1996年 | 327篇 |
1995年 | 312篇 |
1994年 | 270篇 |
1993年 | 216篇 |
1992年 | 213篇 |
1991年 | 141篇 |
1990年 | 137篇 |
1989年 | 120篇 |
1988年 | 105篇 |
1987年 | 83篇 |
1986年 | 65篇 |
1985年 | 74篇 |
1984年 | 75篇 |
1983年 | 40篇 |
1982年 | 49篇 |
1981年 | 57篇 |
1980年 | 50篇 |
1979年 | 46篇 |
1978年 | 31篇 |
1976年 | 18篇 |
1974年 | 20篇 |
1973年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
Steven A. Barker Jimo Borjigin Izabela Lomnicka Rick Strassman 《Biomedical chromatography : BMC》2013,27(12):1690-1700
We report a qualitative liquid chromatography–tandem mass spectrometry (LC/MS/MS) method for the simultaneous analysis of the three known N,N‐dimethyltryptamine endogenous hallucinogens, their precursors and metabolites, as well as melatonin and its metabolic precursors. The method was characterized using artificial cerebrospinal fluid (aCSF) as the matrix and was subsequently applied to the analysis of rat brain pineal gland‐aCSF microdialysate. The method describes the simultaneous analysis of 23 chemically diverse compounds plus a deuterated internal standard by direct injection, requiring no dilution or extraction of the samples. The results demonstrate that this is a simple, sensitive, specific and direct approach to the qualitative analysis of these compounds in this matrix. The protocol also employs stringent MS confirmatory criteria for the detection and confirmation of the compounds examined, including exact mass measurements. The excellent limits of detection and broad scope make it a valuable research tool for examining the endogenous hallucinogen pathways in the central nervous system. We report here, for the first time, the presence of N,N‐dimethyltryptamine in pineal gland microdialysate obtained from the rat. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
922.
N. Iranpoor H. Firouzabadi R. Heydari 《Phosphorus, sulfur, and silicon and the related elements》2013,188(5):1027-1035
Silica-chloride was reacted with different quantities of H(OCH 2 CH 2 ) n OH (n = 2-4) to furnish silica-based linear polyethylene glycols and cyclic polyethylene glycolic ethers. The N 2 O 4 complex of silica-tetraethylene glycolic ether ( III ) was selected and used as a stable, cheap, and heterogeneous silica-based reagent for the selective mono- and dinitration of phenols and nitrosation of thiols. 相似文献
923.
Batool Akhlaghinia Mozafar Asadi Elham Safaee Mozhgan Heydarpoor 《Phosphorus, sulfur, and silicon and the related elements》2013,188(10):2099-2104
Alcohols and phenols are protected with hexamethyldisilazane in the presence of N,N′,N′,N?-tetramethyletra-2,3-pyridinoporphyrazinato copper (II) in good-to-excellent yields at room temperature. 相似文献
924.
Issa Yavari Vahideh Hadigheh-Rezvan 《Phosphorus, sulfur, and silicon and the related elements》2013,188(5):1127-1136
N -isopropenylbenzimidazolone undergoes a smooth reaction with triphenylphosphine and dialkyl acetylenedicarboxylates to produce highly functionalized salt-free phosphorus ylides in good yields. These stabilized phosphorus ylides exist as a mixture of two geometrical isomers as a result of restricted rotation around the carbon-carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group. These ylides are converted to dialkyl 2-(1-isopropenyl-1H-benzimidazol-2-yl)-but-2-enedioates in boiling toluene. 相似文献
925.
《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(42):13138-13141
We have gained new insight into the so‐called hydrophobic gap, a molecularly thin region of decreased electron density at the interface between water and a solid hydrophobic surface, by X‐ray reflectivity experiments and molecular dynamics simulations at different hydrostatic pressures. Pressure variations show that the hydrophobic gap persists up to a pressure of 5 kbar. The electron depletion in the interfacial region strongly decreases with an increase in pressure, indicating that the interfacial region is compressed more strongly than bulk water. The decrease is most significant up to 2 kbar; beyond that, the pressure response of the depletion is less pronounced. 相似文献
926.
Two new guaianolide-type sesquiterpenoids,4β,9β-dihydroxy-1α,5α-H-guaia-6,10(14)-dien(1) and 4β,9β,10α-trihydroxy-1α,5α-H-guaia-6-en(2),along with four known sesquiterpenoids(3a,3b,4 and 5),were isolated from Kadsura interior.Their structures and configurations were elucidated by spectroscopic methods including 2D-NMR and HR-MS techniques.Compounds 3a and 3b were obtained as a pair of enantiomers,and their structure and absolute configuration were established from their extensive NMR spectra and by single-crystal X-ray analysis. 相似文献
927.
928.
Abstract The chemical modified β-cyclodextrin(CD), which bears a fluorophore group — N,N-dimethylamino-chalcone (DMAC), was synthesized. The self-inclusion behavior of this compound in aqueous solution and in alcohol/water mixed solvents was investigated in detail, and compared with the inclusion system of non-modified CD with free DMAC molecule. The results showed that a deep self-inclusion complex was formed for this compound in aqueous solution and had the ability to recognize different organic molecules by two fully different modes: “in-out” and “co-inclusion” mechanisms. The inclusion behavior of these systems in various ratio of alcohol/water mixture as solvents was investigated. The results indicated that the self-inclusion complex has a higher stability in alcohol/water mixed solvents than that in the case of non-modified CD. The chalcone group appended at β-CD enabled the host as a sensitive probe to study the inclusion behavior of CD. 相似文献
929.
930.
We classify moduli spaces of arrangements of 10 lines with quadruple points. We show that moduli spaces of arrangements of 10 lines with quadruple points may consist of more than 2 disconnected components, namely 3 or 4 distinct points. We also present defining equations to those arrangements whose moduli spaces are still reducible after taking quotients of complex conjugations. 相似文献