全文获取类型
收费全文 | 69376篇 |
免费 | 1975篇 |
国内免费 | 140篇 |
专业分类
化学 | 40722篇 |
晶体学 | 370篇 |
力学 | 1476篇 |
综合类 | 10篇 |
数学 | 12186篇 |
物理学 | 16727篇 |
出版年
2023年 | 414篇 |
2022年 | 443篇 |
2021年 | 854篇 |
2020年 | 1000篇 |
2019年 | 897篇 |
2018年 | 1290篇 |
2017年 | 1180篇 |
2016年 | 2225篇 |
2015年 | 1848篇 |
2014年 | 1880篇 |
2013年 | 3911篇 |
2012年 | 3971篇 |
2011年 | 3965篇 |
2010年 | 2539篇 |
2009年 | 2240篇 |
2008年 | 3437篇 |
2007年 | 3239篇 |
2006年 | 2926篇 |
2005年 | 3373篇 |
2004年 | 3140篇 |
2003年 | 2438篇 |
2002年 | 1928篇 |
2001年 | 1523篇 |
2000年 | 1489篇 |
1999年 | 999篇 |
1998年 | 792篇 |
1997年 | 720篇 |
1996年 | 920篇 |
1995年 | 719篇 |
1994年 | 782篇 |
1993年 | 759篇 |
1992年 | 778篇 |
1991年 | 656篇 |
1990年 | 642篇 |
1989年 | 552篇 |
1988年 | 538篇 |
1987年 | 495篇 |
1986年 | 485篇 |
1985年 | 665篇 |
1984年 | 613篇 |
1983年 | 499篇 |
1982年 | 506篇 |
1981年 | 520篇 |
1980年 | 441篇 |
1979年 | 423篇 |
1978年 | 445篇 |
1977年 | 399篇 |
1976年 | 411篇 |
1974年 | 382篇 |
1973年 | 384篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
991.
Luiz Antonio F. de Godoy 《Tetrahedron letters》2006,47(45):7853-7856
The InCl3-catalyzed addition of carbon nucleophiles to cyclic N-acyliminium and oxocarbenium ions under solvent-free conditions at room temperature is described. The corresponding α-substituted heterocycles were obtained in moderate to excellent yields. 相似文献
992.
Yagüe S Terrón MC González T Zapico E Bocchini P Galletti GC González AE 《Rapid communications in mass spectrometry : RCM》2000,14(10):905-910
Some fractions of beer-factory wastewaters represent an important environmental concern owing to their high content of polyphenols and dark-brown color. The capacity of Coriolopsis gallica to preferentially degrade lignin has been successfully applied in our laboratory to the biotreatment and decolorization of paper-industry effluents. In this work, the ability of this white-rot fungus to degrade high-tannin-containing wastewaters is evaluated. Under all the conditions studied, effluent decolorization and chemical oxygen demand reduction achieved by C. gallica at day 12 of incubation were close to 50 and 65%, respectively. No adhesion of dark color to the fungal mycelium was observed suggesting that decolorization could be ascribed to C. gallica degradation systems. Mycelium dry-weight values showed that C. gallica is tolerant to relatively high tannin content present in the effluent samples. In the sample containing the highest effluent concentration (60% v/v), dry-weight values suggested an inhibition of fungal growth at day 6 of incubation and a further adaptation of the fungus to the stressing tannin effect at day 12 of fungal treatment. Pyrolysis/gas chromatography/mass spectrometry results showed a decrease of polyphenols pyrolysis products, mainly phenol and guaiacol, with the incubation time. All these results indicate the potential use of C. gallica in bioremediation of tannin-containing industrial wastewaters and in other applications where a reduction in polyphenols content is required. 相似文献
993.
Alberti G Brunet E Dionigi C Juanes O de La Mata MJ Rodríguez-Ubis JC Vivani R 《Angewandte Chemie (International ed. in English)》1999,38(22):3351-3353
The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space. 相似文献
994.
An effective multiresidual method for the trace analysis of fifteen compounds from a diverse group of pesticides, polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyl (PCBs) and polybrominated biphenyl (PBBs) in aquaculture feed is described. The analytical procedure is based on the matrix solid-phase dispersion (MSPD) of feed sample and subsequent elution with hexane. The MSPD process was evaluated using an asymmetrical experimental design 2(3)3(2)//9. Factors such as C18 sorbent amount, kind of adsorbents, solvent volume and elution mode were considered. The results suggest that the operational MSPD conditions are elution with pressure, 1 g of C18, basic alumina as adsorbent and 30 mL of hexane. The overall method including MSPD procedure and GC coupled to mass spectrometry (MS/MS) has been applied to several samples of aquaculture feed and marine species. Precision and accuracy of the analytical method were determined using the reference material from the International Atomic Energy Agency (IAEA-406), showing a good agreement to the referenced values. 相似文献
995.
Wojciechowski G Huang L Ortiz de Montellano PR 《Journal of the American Chemical Society》2005,127(45):15871-15879
The mammalian peroxidases eosinophil peroxidase, lactoperoxidase (LPO), and myeloperoxidase oxidize thiocyanate to the antimicrobial agents hypothiocyanous acid (HOSCN) and (SCN)2 and are part of a defense system that protects the host from infections. Horseradish peroxidase (HRP), a plant enzyme, also oxidizes thiocyanate. We report here that the prosthetic heme vinyl groups of HRP react with the catalytically generated HOSCN and (SCN)2 to form at least nine vinyl-modified heme adducts. Mass spectrometry combined with analysis of the equivalent reactions of HRP reconstituted with 2- or 4-cyclopropylheme, or mesoheme-d4, shows that all of the prosthetic heme modifications result from addition of oxidized thiocyanate to the heme vinyl groups. No delta-meso-substitution of the heme was observed, in contrast to what is observed with radical agents. Model studies show that incubation of either HRP with preformed HOSCN or a solution of heme with preformed (SCN)2 gives rise to the same products obtained in the HRP-catalyzed reaction. Model studies also demonstrate that the SCN* radical, if formed, should add to a meso-carbon. These findings implicate an electrophilic addition mechanism. In contrast, oxidation by LPO of thiocyanate, the normal substrate of this enzyme, does not result in heme modification. In view of the demonstrated intrinsic reactivity of the heme group, LPO must actively suppress heme modification. As the key difference between LPO (and other mammalian peroxidases) and HRP is the presence of two covalent ester links between the heme and the protein, we propose that these links contribute to steric protection of the adjacent heme vinyl groups. 相似文献
996.
The method of capillary isotachophoresis with conductivity detection was applied for the determination of the physico-chemical characteristics (conditional stability constants log beta') of holmium and yttrium complexes with DTPA (diethylenetriaminepentaacetic acid) and DOTA (1,4,7,10-tetraazadodecane-N,N',N',N'-tetraacetic acid). The log beta' determination is based on the linear relation between the stability constants of lanthanide-DTPA (lanthanide-DOTA) complexes and the reduction of the zone of the complex owing to the bleeding phenomena (liberating free metal ion). The stability constants calculated using this relationship are comparable with the literary data obtained by other methods for both holmium (log beta'(Ho-DTPA)=21.9, log beta'(Ho-DOTA)=24.5) and yttrium complexes (log beta'(Y-DTPA)=21.2, log beta'(Y-DOTA)=24.4). Capillary isotachophoresis was applied for the determination of the optimum composition of the reaction mixture (metal:ligand ratio) as well. 相似文献
997.
Conejo-García A Campos JM Entrena A Sánchez-Martín RM Gallo MA Espinosa A 《The Journal of organic chemistry》2003,68(22):8697-8699
A complete study of the conformational behavior of 4,8-diaza-3(1,4),9(4,1)-dipyridina-1,6(1,4)-dibenzenacyclodecaphan-3(1),9(1)-bis(ilium) bishexafluorophosphate is described. This study allows us to conclude that the process observed by which the different chemical shifts of the pyridinium protons show coalescence at a high-temperature 1H NMR is the rotation around the C-N bond, whereas the conformational equilibrium between the four conformers is produced at low temperature. 相似文献
998.
A simple and sensitive electroanalytical method was developed for microvolume trace elements determination. Commercially available
interdigitated microelectrodes (IDA) from ALS Japan Inc. based on a thin carbon film technology were used. Anodic stripping
voltammetry with collection at IDA was applied. Carbon IDA microbands were coated with a pre-deposited mercury film for better
performance. The method with this sensor enables analysis of some heavy metals in total sample volumes as small as 0.05 cm3. The method was applied to Pb in blood samples determination by a standard addition method. 相似文献
999.
ágnes Zsigmond Ramóna Madácsi Ferenc Notheisz Erzsébet Mernyák and Ferenc Joó 《Reaction Kinetics and Catalysis Letters》2006,87(2):297-304
Summary [RuCl2(PPh3)3], [{RuCl2(TPPMS)2}2] and their heterogenized analogs were applied in the selective hydrogenation of 17-keto- and α,β-unsaturated ketosteroids.
In basic conditions these complexes selectively hydrogenated the C=O bonds, similarly to the results obtained in the case
of α,β-unsaturated aldehydes. A new method was developed for the synthesis of an expensive steroid alcohol, which can be prepared
traditionally in a more complicated way. 相似文献
1000.
Determination of trans-galactooligosaccharides in selected food products by ion-exchange chromatography: collaborative study 总被引:1,自引:0,他引:1
de Slegte J 《Journal of AOAC International》2002,85(2):417-423
Eleven collaborating laboratories assayed 7 blind duplicate pairs of food and feed products for tans-galactooligosaccharides. The 7 laboratory sample pairs ranged from low (2%) to high levels (15%). Following the proposed method, the test samples were treated with beta-galactosidase and the released galactose was determined by ion-exchange chromatography. Repeatability standard deviation ranged from 2.9 to 11.6%; reproducibility standard deviation ranged from 4.6 to 11.6%. 相似文献