全文获取类型
收费全文 | 785篇 |
免费 | 8篇 |
国内免费 | 2篇 |
专业分类
化学 | 403篇 |
晶体学 | 1篇 |
力学 | 5篇 |
数学 | 61篇 |
物理学 | 325篇 |
出版年
2020年 | 4篇 |
2019年 | 6篇 |
2018年 | 3篇 |
2015年 | 7篇 |
2014年 | 9篇 |
2013年 | 20篇 |
2012年 | 22篇 |
2011年 | 37篇 |
2010年 | 15篇 |
2009年 | 17篇 |
2008年 | 38篇 |
2007年 | 19篇 |
2006年 | 27篇 |
2005年 | 29篇 |
2004年 | 23篇 |
2003年 | 17篇 |
2002年 | 10篇 |
2001年 | 22篇 |
2000年 | 24篇 |
1999年 | 10篇 |
1998年 | 10篇 |
1997年 | 9篇 |
1996年 | 19篇 |
1995年 | 18篇 |
1994年 | 20篇 |
1993年 | 34篇 |
1992年 | 29篇 |
1991年 | 13篇 |
1990年 | 16篇 |
1989年 | 12篇 |
1988年 | 12篇 |
1987年 | 14篇 |
1986年 | 15篇 |
1985年 | 21篇 |
1984年 | 9篇 |
1983年 | 11篇 |
1982年 | 15篇 |
1981年 | 8篇 |
1980年 | 13篇 |
1979年 | 19篇 |
1978年 | 6篇 |
1977年 | 19篇 |
1976年 | 16篇 |
1975年 | 12篇 |
1974年 | 7篇 |
1973年 | 13篇 |
1972年 | 4篇 |
1971年 | 4篇 |
1969年 | 7篇 |
1968年 | 4篇 |
排序方式: 共有795条查询结果,搜索用时 171 毫秒
31.
John J. D'Amico Frederic G. Bellinger John J. Freeman 《Journal of heterocyclic chemistry》1988,25(5):1503-1509
The reaction of the appropriate 2-benzothiazolinone with 2-chloroacetamide under basic conditions afforded the 2-oxo-3(2H)-benzothiazolineacetamides 6–9. The 2-thioxo-3(2H)-benzothiazolineacetamide ( 10 ) was prepared by the reaction of 3-(carbethoxymethyl)benzothiazoline-2-thione with ammonium hydroxide. The reaction of acetamides 6–10 with the appropriate anhydride containing a catalytic amount of the sodium salt of the acid corresponding to the anhydride afforded the titled compounds 11–18 in excellent yields. The omission of the catalyst in the same reaction furnished a mixture containing 57% of the titled compound, 37% of the nitrile and 6% of an unknown. Possible mechanism and supporting nmr, ir and mass spectral data are discussed. 相似文献
32.
We present studies of the potential energy landscape of selected binary Lennard-Jones 13 atom clusters. The effect of adding selected impurity atoms to a homogeneous cluster is explored. We analyze the energy landscapes of the studied systems using disconnectivity graphs. The required inherent structures and transition states for the construction of disconnectivity graphs are found by combination of conjugate gradient and eigenvector-following methods. We show that it is possible to controllably induce new structures as well as reorder and stabilize existing structures that are characteristic of higher-lying minima. Moreover, it is shown that the selected structures can have experimentally relevant lifetimes. 相似文献
33.
Wenzel TJ Freeman BE Sek DC Zopf JJ Nakamura T Yongzhu J Hirose K Tobe Y 《Analytical and bioanalytical chemistry》2004,378(6):1536-1547
Chiral crown ethers 1 and 5 are useful enantiomeric discriminating agents in 1H NMR spectroscopy for neutral and protonated primary amines, amino acids, and amino alcohols. The presence of the carboxylic acid groups in 1 and 5 provide sites at which ytterbium(III) can bind. Adding ytterbium(III) nitrate to crown–substrate mixtures in methanol-d4 causes shifts in the spectra of substrates and often enhances the chiral discrimination in the 1H NMR spectrum. The enhancement in enantiomeric discrimination that occurs in the presence of ytterbium(III) allows lower concentrations of the crown ether to be used in chiral recognition studies. Several amide derivatives of 1 were prepared and evaluated as chiral NMR discriminating agents, although except for 1e, these were less effective than 1. 相似文献
34.
Dr. Aaron R. Finney Dr. Sébastien Lectez Dr. Colin L. Freeman Prof. John H. Harding Dr. Stephen Stackhouse 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(37):8725-8740
A better understanding of the solution chemistry of the lanthanide (Ln) salts in water would have wide ranging implications in materials processing, waste management, element tracing, medicine and many more fields. This is particularly true for minerals processing, given governmental concerns about lanthanide security of supply and the drive to identify environmentally sustainable processing routes. Despite much effort, even in simple systems, the mechanisms and thermodynamics of LnIII association with small anions remain unclear. In the present study, molecular dynamics (MD), using a newly developed force field, provide new insights into LnCl3(aq) solutions. The force field accurately reproduces the structure and dynamics of Nd3+, Gd3+ and Er3+ in water when compared to calculations using density functional theory (DFT). Adaptive-bias MD simulations show that the mechanisms for ion pairing change from dissociative to associative exchange depending upon cation size. Thermodynamics of association reveal that whereas ion pairing is favourable, the equilibrium distribution of species at low concentration is dominated by weakly bound solvent-shared and solvent-separated ion pairs, rather than contact ion pairs, reconciling a number of contrasting observations of LnIII–Cl association in the literature. In addition, we show that the thermodynamic stabilities of a range of inner sphere and outer sphere coordination complexes are comparable and that the kinetics of anion binding to cations may control solution speciation distributions beyond ion pairs. The techniques adopted in this work provide a framework with which to investigate more complex solution chemistries of cations in water. 相似文献
35.
High-performance liquid chromatography has become an important analytical tool for the quantitation of opioid drugs. Using solid-phase extraction and coulometric electrochemical detection, we have developed a chromatographic method for the simultaneous measurement of morphine and hydromorphone which is both sensitive and specific. Using 1 ml of plasma, intra-assay and inter-assay data show that the detection limit for accurate quantitation of these compounds is about 1.2 ng/ml (coefficient of variation 11.6%) for morphine and 2.5 ng/ml (coefficient of variation 10.5%) for hydromorphone. The method is simple and readily adaptable to most pharmacokinetic studies and toxic screens involving these drugs. 相似文献
36.
An europium-sensitized time-resolved luminescence (TRL) method was developed to determine oxytetracycline (OTC) in cultivated catfish muscle. Extraction of OTC from fish muscle was performed with pH 4.0 ethylenediaminetetraacetic acid (EDTA)-McIlvaine buffer and clean up with hydrophilic-lipophilic balanced copolymer solid phase extraction (SPE) cartridges. The eluate was used without further concentration for TRL measurement in pH 9.0 micellar tris(hydroxylmethyl)aminomethane (TRIS) buffer. Cetyltrimethylammonium chloride (CTACl) was used as surfactant and EDTA as a co-ligand. The excitation and emission wavelengths were set at 388 and 615 nm, respectively. The linear dynamic range was 0-1000 ng g−1 (R2=0.9995). The recovery was 92-112% in the fortification range of 50-200 ng g−1 and the limits of detection (LOD) ranged from 3 to 7 ng g−1. Incurred catfish samples were used to demonstrate the performance of the method around 100 ng g−1, the European Union maximum residue level. 相似文献
37.
Milligan DB Freeman CG Maclagan RG McEwan MJ Wilson PF Anicich VG 《Journal of the American Society for Mass Spectrometry》2001,12(5):557-564
The ion-molecule reactivity of the products formed in the association reactions of HCNH+ with C2H2 (C3H4N+) and C2H4 (C3H6N+) has been investigated to provide information on the structures of the adducts thus formed. The C3H4N+ and C3H6N+ adducts were formed in the reaction flow tube of a flowing afterglow sourced-selected ion flow tube (FA-SIFT) and their reactivity with a neutral molecular "probe" examined. The reactivity of possible known structural isomers for the C3H4N+ and C3H6N+ ions was investigated in both the FA-SIFT and an ion cyclotron resonance spectrometer (ICR). Ab initio investigations of the potential energy surfaces for both structures at the G2(MP2) level have also been performed and structures corresponding to local minima on both surfaces have been identified and evaluated. The results of these experimental and theoretical studies show that at room temperature, the C3H4N+ adduct ion contains two isomers; a less reactive one that is likely to be a four-membered cyclic covalent isomer (approximately 70%) and a faster reacting component that is probably an electrostatic complex (approximately 30%). The C3H6N+ adduct ion formed from HCNH+ + C2H4 at room temperature is a single isomer that is likely to be the four-membered covalently bound cyclic CH2CH2CHNH+ species. 相似文献
38.
39.
40.