全文获取类型
收费全文 | 27948篇 |
免费 | 796篇 |
国内免费 | 158篇 |
专业分类
化学 | 18467篇 |
晶体学 | 213篇 |
力学 | 487篇 |
综合类 | 4篇 |
数学 | 4674篇 |
物理学 | 5057篇 |
出版年
2023年 | 176篇 |
2022年 | 412篇 |
2021年 | 501篇 |
2020年 | 425篇 |
2019年 | 493篇 |
2018年 | 474篇 |
2017年 | 451篇 |
2016年 | 901篇 |
2015年 | 778篇 |
2014年 | 835篇 |
2013年 | 1601篇 |
2012年 | 1695篇 |
2011年 | 2427篇 |
2010年 | 1205篇 |
2009年 | 1139篇 |
2008年 | 1660篇 |
2007年 | 1591篇 |
2006年 | 1446篇 |
2005年 | 1355篇 |
2004年 | 1100篇 |
2003年 | 926篇 |
2002年 | 783篇 |
2001年 | 582篇 |
2000年 | 518篇 |
1999年 | 413篇 |
1998年 | 314篇 |
1997年 | 291篇 |
1996年 | 365篇 |
1995年 | 261篇 |
1994年 | 281篇 |
1993年 | 265篇 |
1992年 | 251篇 |
1991年 | 217篇 |
1990年 | 196篇 |
1989年 | 149篇 |
1988年 | 142篇 |
1987年 | 158篇 |
1986年 | 150篇 |
1985年 | 180篇 |
1984年 | 189篇 |
1983年 | 122篇 |
1982年 | 143篇 |
1981年 | 119篇 |
1980年 | 116篇 |
1979年 | 89篇 |
1978年 | 78篇 |
1977年 | 82篇 |
1976年 | 68篇 |
1975年 | 55篇 |
1973年 | 65篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Chmielewski MK Marchán V Cieślak J Grajkowski A Livengood V Münch U Wilk A Beaucage SL 《The Journal of organic chemistry》2003,68(26):10003-10012
Thermolytic groups structurally related to well-studied heat-sensitive phosphate/thiophosphate protecting groups have been evaluated for 5'-hydroxyl protection of deoxyribonucleosides as carbonates and for potential use in solid-phase oligonucleotide synthesis. The spatial arrangement of selected functional groups forming an asymmetric nucleosidic 5'-O-carbonic acid ester has been designed to enable heat-induced cyclodecarbonation reactions, which would result in the release of carbon dioxide and the generation of a nucleosidic 5'-hydroxyl group. The nucleosidic 5'-O-carbonates 3-8, 10-15, and 19-21 were prepared and were isolated in yields ranging from 45 to 83%. Thermolytic deprotection of these carbonates is preferably performed in aqueous organic solvent at 90 degrees C under near neutral conditions. The rates of carbonate deprotection are dependent on the nucleophilicity of the functional group involved in the postulated cyclodecarbonation reaction and on solvent polarity. Deprotection kinetics increase according to the following order: 4 < 5 < 10 < 6 < 12 < 7 < 13 < 8 < 14 congruent with 19-21 and CCl4 < dioxane < MeCN < t-BuOH < MeCN:phosphate buffer (3:1 v/v, pH 7.0) < EtOH:phosphate buffer (1:1 v/v, pH 7.0). Complete thermolytic deprotection of carbonates 7, 8, 13, and 14 is achieved within 20 min to 2 h under optimal conditions in phosphate buffer-MeCN. The 2-(2-pyridyl)amino-1-phenylethyl and 2-[N-methyl-N-(2-pyridyl)]aminoethyl groups are particularly promising for 5'-hydroxyl protection of deoxyribonucleosides as thermolytic carbonates. 相似文献
992.
A. B. Garcia E. R. Galán J. A. S. Blázquez C. V. Calahorro 《Journal of Thermal Analysis and Calorimetry》1986,31(2):247-251
The thermal behaviour of some 2-benzylamino-2-deoxyheptonic acids obtained from 2-benzylamino-2-deoxy-D-glycero-L-gluco, 2-benzylamino-2-deoxy-D-glycero-D-ido and 2-benzylamino-2-deoxy-D-glycero-D-taloheptononitriles (reported) previously has been studied in air (static atmosphere) and N2 (dynamic atmosphere, flow rate 200 ml/min).
Zusammenfassung Das thermische Verhalten einiger aus 2-Benzylamino-2-desoxy-D-glycero-L-gluco-, 2-Benzylamino-2-desoxy-D-glycero-D-ido und 2-Benzylamino-2-desoxy-D-glycero-D-talo-heplononitril (kürzlich beschrieben) erhaltenen 2-Benzylamino-2-desoxy-heptonsäuren wurde ín Luft (statische Atmosphäre) und N2 (dynamische Atmosphäre, Strömungsgeschwindigkeit 200 ml/min) untersucht.
( 200 /) 2--2- , 2--2---uepo-L, 2-2- ---- 2-2--- -- 2--2----ma .相似文献
993.
The paper shows a computer-assisted procedure for the optimization of selectivity of two columns coupled in series by tuning the working temperature (using the isothermal mode) and columns coupling-point pressure at constant inlet and outlet carrier gas pressures. The optimization procedure validation was tested by the separation of 32 hydrocarbons in a column series with the aim to resolve the maximum number of components in the shortest possible analysis time.Dedicated to Professor J. F. K. Huber on the occasion of his 65th birthday 相似文献
994.
995.
Christer Albano William Dunn Ulf Edlund Erik Johansson Bo Nordén Michael Sjöström Svante Wold 《Analytica chimica acta》1978,103(4):429-443
Problems of pattern recognition in chemistry and other subjects can be divided conveniently into four different types depending on the level of scope of the problem.(1) Classification into one of a number of defined classes. As an example blood samples taken from persons known to be either controls or welders are considered. The problem is whether trace element concentrations in these samples contain information on whether or not a person is a welder.(2) Level 1 plus the possibility that an object is an outlier, i.e. does not belong to any of the defined classes. As an example, the üse of 13C-n.m.r. data to decide whether 2-substituted norbornanes have the exo or endo structure is discussed. (2A) Level 2, asymmetric. This situation occurs when one class does not have a systematic structure, but another class is homogeneous and can be described by a level 2 model. This occurs in the classification of materials or compounds as good or bad, active or inactive, and in binary classifications. As an example the use of trace element data to classify steel samples as having good or poor properties of strength is discussed.(3) Level 2 plus the ability to relate the variables measured to external properties of continuous character. As an example, the classification of a series of chemical compounds as β -receptor blockers, β -receptor stimulants, or neither, on the basis of their structural variables is discussed. In addition, relations between these structural variables and the measured biological activity are sought within each of the two classes.(4) Level 3 with the difference that several external property variables in the objects are measured. It may be desirable to use variables of the objects both for classification and for relations to several property variables: such examples are numerous in analytical chemistry. 相似文献
996.
The reactions of palladium(II) compounds of the type Cl2PdL2 with bromobis(pentafluorophenyl)thallium(III) has been reexamined. The reported preparation of the organo palladium(IV) complex Cl2Pd(C6F5)2(PPh3)2 could not be repeated, and instead mixtures of binuclear palladium(II) compounds, Cl2Pd(C6F5)2L2, and mononuclear palladium(II) compounds were obtained. The binuclear are transformed into the mononuclear complexes on addition of an excess of ligand L.The chlorine bridging atoms of the binuclear complexes can be replaced by other halogens or pseudohalogens by treatment with salts of the MX type (X = Br, I, SCN). 相似文献
997.
A new, fast, and simple method based on size exclusion chromatography permits determination of the volumes of open and closed pores of rigid solids such as silica or porous glasses, together with the specific surface and the pore-size distribution for pore diameters ranging from 10 to 4000Å. The results are in good agreement with those obtained by classical methods (capillary condensation, mercury porosimetry). 相似文献
998.
The reduction of titanocene dithiocyanate in various solvents has been examined using the techniques of polarography, voltammetry, controlled potential electrolysis and cyclic voltammetry. In THF and CH2Cl2, Cp2Ti(NCS)2 undergoes successive one-electron transfer reactions, and by using a combination of electrochemical and EPR techniques it has been possible to confirm the stability of [Cp2Ti(NCS)2]? in these solvents. In DMF, however, an irreversible dimerization follows the first electron transfer. 相似文献
999.
-erythro- and
-threo-β-methylphenylalanines were synthesized and incorporated into the delta opioid receptor-selective deltorphin C peptide by solid-phase peptide synthesis. The enantiomeric composition of β-methylphenylalanine and the diastereomeric composition of the peptide were determined. To distinguish exactly between the two enantiomeric pairs of β-methylphenylalanine, different liquid, gas and thin-layer chromatographic methods are suggested, while the four diastereomers Of [β-Mephe3]deltorphin C Can be identified and separated conveniently by high-performance liquid chromatography. 相似文献
1000.
We studied the reactions of some N-methylpyrrole derivatives with isopropenyl acetate and different acetic anhydrides. We have found that the reaction with isopropenyl acetate afforded 5-acetylpyrrole derivatives while with acetic anhydrides acetylation occurred on the α-carbon atom of the side-chain of pyrrole-2-acetic acid derivatives. 相似文献