全文获取类型
收费全文 | 699篇 |
免费 | 83篇 |
国内免费 | 99篇 |
专业分类
化学 | 493篇 |
晶体学 | 2篇 |
力学 | 59篇 |
综合类 | 9篇 |
数学 | 108篇 |
物理学 | 210篇 |
出版年
2022年 | 7篇 |
2021年 | 6篇 |
2020年 | 12篇 |
2019年 | 15篇 |
2018年 | 21篇 |
2017年 | 9篇 |
2016年 | 20篇 |
2015年 | 23篇 |
2014年 | 20篇 |
2013年 | 38篇 |
2012年 | 28篇 |
2011年 | 38篇 |
2010年 | 43篇 |
2009年 | 27篇 |
2008年 | 48篇 |
2007年 | 39篇 |
2006年 | 30篇 |
2005年 | 39篇 |
2004年 | 28篇 |
2003年 | 26篇 |
2002年 | 26篇 |
2001年 | 27篇 |
2000年 | 22篇 |
1999年 | 25篇 |
1998年 | 10篇 |
1997年 | 6篇 |
1996年 | 15篇 |
1995年 | 15篇 |
1993年 | 9篇 |
1992年 | 4篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 9篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1976年 | 11篇 |
1975年 | 9篇 |
1973年 | 8篇 |
1972年 | 7篇 |
1971年 | 9篇 |
1970年 | 4篇 |
1909年 | 4篇 |
排序方式: 共有881条查询结果,搜索用时 15 毫秒
21.
Halland N Alstrup Lie M Kjaersgaard A Marigo M Schiøtt B Jørgensen KA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(23):7083-7090
The mechanism for the 2,5-diphenylpyrrolidine-catalyzed enantioselective alpha-chlorination of aldehydes with electrophilic halogenation reagents has been investigated by using experimental and computational methods. These studies have led us to propose a mechanism for the reaction that proceeds through an initial N-chlorination of the chiral catalyst-substrate complex, followed by a 1,3-sigmatropic shift of the chlorine atom to the enamine carbon atom. The suggested reaction course is different from previously proposed mechanisms for organocatalytic enamine reactions, in which the carbon-electrophile bond is formed directly. Furthermore, the rate-determining step in the overall reaction was determined and the presence of nonlinear effects was probed. 相似文献
22.
van der Vlugt JI Rauchfuss TB Wilson SR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,12(1):90-98
The two-step one-pot oxidative decarbonylation of [Fe2(S2C2H4)(CO)4(PMe3)2] (1) with [FeCp2]PF6, followed by addition of phosphane ligands, led to a series of diferrous dithiolato carbonyls 2-6, containing three or four phosphane ligands. In situ measurements indicate efficient formation of 1(2+) as the initial intermediate of the oxidation of 1, even when a deficiency of the oxidant was employed. Subsequent addition of PR3 gave rise to [Fe2(S2C2H4)(mu-CO)(CO)3(PMe3)3]2+ (2) and [Fe2(S2C2H4)(mu-CO)(CO)2(PMe3)2(PR3)2]2+ (R = Me 3, OMe 4) as principal products. One terminal CO ligand in these complexes was readily substituted by MeCN, and [Fe2(S2C2H4)(mu-CO)(CO)2(PMe3)3(MeCN)]2+ (5) and [Fe2(S2C2H4)(mu-CO)(CO)(PMe3)4(MeCN)]2+ (6) were fully characterized. Relevant to the H(red) state of the active site of Fe-only hydrogenases, the unsymmetrical derivatives 5 and 6 feature a semibridging CO ligand trans to a labile coordination site. 相似文献
23.
Simultaneous determination of ondansetron and tropisetron in human plasma using HPLC with UV detection 总被引:4,自引:0,他引:4
Bauer S Störmer E Kaiser R Tremblay PB Brockmöller J Roots I 《Biomedical chromatography : BMC》2002,16(3):187-190
A rapid and sensitive HPLC method for the simultaneous quantitation of ondansetron and tropisetron, two serotonin (5-HT) receptor antagonists frequently used in treatment and prevention of nausea and emesis, is described. The procedure involves liquid-liquid extraction of human plasma with dichloromethane coupled with reversed-phase HPLC and UV detection. The lower limits of quantification (LOQ) were 0.62 ng/mL for ondansetron and 1.25 ng/mL or tropisetron. Intra- and inter-assay coefficients of variation ranged from 1.5 to 7.5% and 5.3 to 13.7%, respectively. The sensitivity and precision were sufficient for determination of plasma concentrations after therapeutic administration of both drugs and the method can be used for the estimation of pharmacokinetic parameters. 相似文献
24.
Fractionation of a steam distilled residue of Commiphora confusa resin has yielded four novel dammarane triterpenes characterised as (20S)-3beta-acetoxy-12beta,16beta-trihydroxydammar-24-ene, (20S)-12beta,16beta-trihydroxydammar-24-ene-3beta-O-beta-glucopyranoside, (20S)-3beta-acetoxy-12beta,16beta,25-tetrahydroxydammar-23-ene, and (20S)-3beta,12beta,16beta,25-pentahydroxydammar-23-ene. The known compounds beta-amyrin, 3beta-amyrinacetate, 2-methoxyfuranodienone, 2-acetoxyfuranodienone, (20R)-3beta-acetoxy-16beta-dihydroxydammar-24-ene, (20R)-3beta,16beta-trihydroxydammar-24-ene, 3beta-acetoxy-16beta-hydroxydammar-24-ene, 3beta-hydroxydammar-24-ene, 3beta-acetoxydammar-24-ene, and beta-sistosterol were also isolated from the same extract. The structures of the compounds were determined using spectroscopic, physical, and chemical methods. 相似文献
25.
E. Clementi W. Koos G. C. Lie G. Ranghino 《International journal of quantum chemistry》1980,17(3):377-398
The energy of (H2O)3 has been calculated for 29 geometrical configurations of the trimer using the SCF LCAO MO method and extended as well as minimal basis sets of Gaussian functions. For two configurations two intermediate basis sets have also been tested. The results show the nonadditive component of the interaction energy to be small. They also indicate that fairly reliable results for the trimer can be obtained using minimal basis sets and the counterpoise method to eliminate the basis set superposition error. The nonadditive contribution to the interaction energy is shown to be mainly due to the long-range induction interaction. 相似文献
26.
HPLC determination of four active saponins from Panax notoginseng in rat serum and its application to pharmacokinetic studies 总被引:1,自引:0,他引:1
Four main active saponins (ginsenosides Rg1, Rb1, Rd and notoginsenoside R1) in Panax notoginseng in rat serum after oral and intravenous administration of total saponins of P. notoginseng (PNS) to rats were determined using a simple and sensitive high-performance chromatographic method. The serum samples were pretreated with solid-phase extraction before analysis. The calibration curves for the four saponins were linear in the given concentration ranges. The intra-day and inter-day assay coefficients in serum were less than 10.0% and the recoveries of the method were higher than 80.0% in the high, middle and low concentrations. This method was applied to study the pharmacokinetics following oral and intravenous administration of PNS. 相似文献
27.
28.
M S Lie Ken Jie 《Journal of chromatography. A》1975,109(1):81-87
Five trimethylene-interrupted methyl octadecadiynoates, C18 delta-2a,-7a; delta-3a,-8a; delta-4a,-9a; delta-5a,-10a and delta-6a,-11a, and the corresponding cis,cis-octadecadienoates were synthesized, and their gas-liquid chromatographic properties were studied on Apiezon L, diethylene glycol succinate polyester and Silac 10C stationary phases. The equivalent chain lengths of these esters have been determined, and the separation of mixtures and the prediction of gas chromatographic behaviour of these isomers are discussed. 相似文献
29.
应用原子吸收光谱法测定了内蒙古地产的笃斯茎、叶中的Cu,Fe,Zn,Mn,Ca,Mg等六种人体必需微量元素及宏量元素的含量。从而为评价笃斯的品质及其药理、药效提供了具有一定价值的数据。 相似文献
30.
Kätlin Kaare Ivar Kruusenberg Maido Merisalu Leonard Matisen Väino Sammelselg Kaido Tammeveski 《Journal of Solid State Electrochemistry》2016,20(4):921-929
Manganese phthalocyanine (MnPc) and copper phthalocyanine (CuPc)-modified electrodes were prepared using multi-walled carbon nanotubes (MWCNTs) as a support material. The catalyst materials were heat treated at four different temperatures to investigate the effect of pyrolysis on the oxygen reduction reaction (ORR) activity of these electrocatalysts. The MWCNT to metal phthalocyanine ratio was varied. Scanning electron microscopy (SEM) was employed to visualise the surface morphology of the electrodes and the x-ray photoelectron spectroscopic (XPS) study was carried out to analyse the surface composition of the most active catalyst materials. The ORR was studied in 0.1 M KOH solution employing the rotating disk electrode (RDE) method. Glassy carbon (GC) electrodes were modified with carbon nanotube-supported metal phthalocyanine catalysts using Tokuyama AS-4 ionomer. The RDE results revealed that the highest electrocatalytic activity for ORR was achieved upon heat treatment at 800 °C. CuPc-derived catalyst demonstrated lower catalytic activity as compared to the MnPc-derived counterpart, which is in good agreement with previous literature, whereas the activity of MnPc-based catalyst was higher than that reported earlier. 相似文献