全文获取类型
收费全文 | 8490篇 |
免费 | 654篇 |
国内免费 | 225篇 |
专业分类
化学 | 9013篇 |
晶体学 | 24篇 |
力学 | 3篇 |
综合类 | 23篇 |
数学 | 3篇 |
物理学 | 303篇 |
出版年
2024年 | 6篇 |
2023年 | 88篇 |
2022年 | 240篇 |
2021年 | 222篇 |
2020年 | 275篇 |
2019年 | 215篇 |
2018年 | 210篇 |
2017年 | 186篇 |
2016年 | 310篇 |
2015年 | 300篇 |
2014年 | 357篇 |
2013年 | 680篇 |
2012年 | 449篇 |
2011年 | 423篇 |
2010年 | 381篇 |
2009年 | 427篇 |
2008年 | 432篇 |
2007年 | 504篇 |
2006年 | 440篇 |
2005年 | 440篇 |
2004年 | 446篇 |
2003年 | 350篇 |
2002年 | 492篇 |
2001年 | 180篇 |
2000年 | 156篇 |
1999年 | 124篇 |
1998年 | 128篇 |
1997年 | 118篇 |
1996年 | 103篇 |
1995年 | 103篇 |
1994年 | 64篇 |
1993年 | 68篇 |
1992年 | 66篇 |
1991年 | 53篇 |
1990年 | 45篇 |
1989年 | 34篇 |
1988年 | 42篇 |
1987年 | 29篇 |
1986年 | 14篇 |
1985年 | 20篇 |
1984年 | 35篇 |
1983年 | 13篇 |
1982年 | 21篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 19篇 |
1978年 | 8篇 |
1972年 | 5篇 |
1969年 | 4篇 |
1968年 | 3篇 |
排序方式: 共有9369条查询结果,搜索用时 0 毫秒
101.
Summary An HPLC reversed-phase ion interaction reagent method is presented, which makes use of chiral compounds as the interaction
reagents. By employing the optical isomeric forms of malic, tartaric and mandelic acids as the interaction reagents, a good
separation of D(−) and L(+) ascorbic acid has been achieved. The method has also been applied to the identification of vitamin
C in some medical formulations. The separation of te enantiomeric forms of DL malic acid has been attempted and a shor discussion
is presented about the elution sequence in chromatographic separation of D- and L-enantiomers. 相似文献
102.
CuBr2-catalyzed three-component coupling of N-benzylallylamine, ethyl glyoxalate, and terminal alkynes afforded glycinate-tethered 1,6-enynes, which were further transformed into polycyclic pyrrole-2-carboxylates via iridium-catalyzed cycloisomerization/Diels-Alder cycloaddition/dehydrogenation sequence under conventional and microwave heating conditions. The corresponding β-amino acid analogues were obtained from the Ir-catalyzed reaction of enynes prepared from lithium amides of allylamines with methyl non-2-en-4-ynoate. The Cu-catalyzed Mannich-type condensation was further extended to the synthesis of the glycinate-tethered dienyne and cyclopropylenyne, which were subjected to Rh-catalyzed cycloisomerizations to furnish bicyclic amino acids. 相似文献
103.
Chaikovskaya O. N. Levin P. P. Sul"timova N. B. Sokolova I. V. Kuz"min A. V. 《Russian Chemical Bulletin》2004,53(2):313-317
The spectra and kinetics of short-lived intermediates formed from aqueous (0.1 N NaOH) solutions of the natural mixture of humic and fulvic acids (HFA) were studied by laser flash photolysis using excitation wavelengths of 337, 390, 470, and 520 nm. Laser photolysis of HFA with light of 520 and 470 nm results in the formation of triplet excited states (THFA) characterized by the broad absorption spectrum with a maximum near 630 nm and lifetimes of 0.15 ms in deoxygenated solutions. The formation of two types of THFA with lifetimes of 0.1 and 2 ms and absorption spectra with maxima at 570 nm is observed under photolysis with light of 337 and 390 nm. The estimation of quantum yields of THFA gives 1 and 0.3% under photolysis with excitation wavelengths of 337 and 520 nm, respectively. The rate constants of THFA quenching by molecular oxygen are equal to (7—8)·108 L mol–1 s–1. 相似文献
104.
The reaction of fluorine-containing halides and acetamides with conjugated dienoic acids initiated by sodium dithionite gave halide-free 1,4-adducts in 40-80% yields, with the E configuration as the major products. 相似文献
105.
Maria Concetta Bruzzoniti Edoardo Mentasti Corrado Sarzanini Peter Hajs 《Journal of chromatography. A》1997,770(1-2):13-22
The retention behavior of biologically relevant monovalent (formic, acetic, propionic, lactic and pyruvic) and divalent (oxalic, malonic, succinic, fumaric, maleic and tartaric acids) car☐ylic acids together with inorganic analytes (chloride and sulphate) has been studied. The separation was performed on a latex-based strong anion-exchange resin using carbonate buffer systems in suppressed IC. The retention behaviour of analytes was investigated at different pH values and [HCO3−]+[CO32−] concentrations. A theoretical model, involving ion-exchange equilibria of sample and eluent anions, was derived and applied to the chromatographic data obtained. Chromatographic ion exchange selectivity values were determined and retention data were calculated for the anions using different carbonate eluent conditions. The average of errors between the predicted and the measured retention volumes of the analytes studied does not exceed 4.0%. The study effectively characteristics the behaviour of different analytes under elution conditions of practical importance. 相似文献
106.
Significant improvement in enantioselectivity and catalyst activity was achieved for the catalytic enantioselective Strecker reaction. Using a catalyst (1-2.5 mol %) prepared from Gd(OiPr)3 and d-glucose derived ligand 1, and in the presence of 2,6-dimethylphenol as an additive, high enantioselectivity was obtained from a wide range of ketoimines, including heteroaromatic and cyclic ketoimines. The new method was applied to an efficient catalytic asymmetric synthesis of sorbinil, a therapeutic agent for diabetic complications. 相似文献
107.
In this paper, we describe the design and synthesis of a novel hybrid foldamer, derived from a conformationally constrained aliphatic-aromatic amino acid conjugate that adopts a well-defined, compact, three-dimensional structure, governed by a combined conformational restriction imposed by the individual amino acids from which the foldamer is composed. Conformational investigations confirmed the prevalence of a unique doubly bent conformation for the foldamer, in both solid and solution states, as evidenced from single crystal X-ray and 2D NOESY studies, respectively. The findings suggest that constrained aliphatic-aromatic amino acid conjugates offer new avenues for the de novo design of hybrid foldamers with distinctive structural architectures. Furthermore, the de novo design strategy disclosed herein has the potential for significantly augmenting the ‘tool-box’ of the modern day peptidomimetic chemist, as well as providing a novel approach to the field of rational design. 相似文献
108.
Ayhan S. Demir Ömer Reis Ilker Esiringü Barbaros Reis Sehriban Baris 《Tetrahedron》2007,63(1):160-165
Acylphosphonates, which are easily available from carboxylic acids, are potent acyl anion precursors and undergo cyanide ion promoted phosphonate-phosphate rearrangement to provide the corresponding acyl anion equivalents as reactive intermediates. The protonation of these acyl anion equivalents furnished cyanohydrin O-phosphates in good yields. For the high yield formation of cyanohydrin O-phosphates from arylphosphonates THF should be used and from alkylphosphonates DME was used. 相似文献
109.
O. A Rozentsvet T. Rezanka E. S. Bosenko E. A. Uzhametskaya V. M. Dembitskii 《Chemistry of Natural Compounds》2005,41(5):487-490
Fatty acids, total lipids, phospholipids, and the betaine lipid DGTS in aerial and underwater leaves and sori of the aquatic
fern Salvinia natans were investigated. Lipid compositions from various parts (organs) of the fern were different. The distributions of polar
lipids and fatty acids in various organs were discussed. Special attention was paid to the DGTS-forming capability of this
fern.
__________
Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 399–402, September–October, 2005. 相似文献
110.
Rhodococcus equi A4, Rhodococcus erythropolis NCIMB 11540 and Rhodococcus sp. R312 were investigated towards their ability to produce β-amino amides and acids from β-amino nitriles. The microorganisms show comparable trends: five-membered alicyclic 2-amino nitriles were transformed significantly faster than the six-membered compounds and the products of trans-2-amino nitriles (amides and acids) were formed considerably faster than the cis-counterparts (amides). The trans-five membered nitriles gave the amides (1b, 5b) in excellent enantiomeric excess (94-99%), the biotransformation of trans-six membered substrates resulted in the formation of the acid (3c, 7c) in excellent ee (87-99%). The ee's of the cis-compounds were throughout lower. Fifteen new substances were synthesized and characterized in the course of this work. 相似文献