全文获取类型
收费全文 | 40487篇 |
免费 | 4102篇 |
国内免费 | 4491篇 |
专业分类
化学 | 33672篇 |
晶体学 | 489篇 |
力学 | 2495篇 |
综合类 | 348篇 |
数学 | 3646篇 |
物理学 | 8430篇 |
出版年
2024年 | 92篇 |
2023年 | 487篇 |
2022年 | 1318篇 |
2021年 | 1206篇 |
2020年 | 1492篇 |
2019年 | 1357篇 |
2018年 | 1245篇 |
2017年 | 1464篇 |
2016年 | 1684篇 |
2015年 | 1538篇 |
2014年 | 1856篇 |
2013年 | 3152篇 |
2012年 | 2270篇 |
2011年 | 2500篇 |
2010年 | 2006篇 |
2009年 | 2467篇 |
2008年 | 2256篇 |
2007年 | 2366篇 |
2006年 | 2128篇 |
2005年 | 1953篇 |
2004年 | 1949篇 |
2003年 | 1519篇 |
2002年 | 1804篇 |
2001年 | 1108篇 |
2000年 | 950篇 |
1999年 | 780篇 |
1998年 | 685篇 |
1997年 | 621篇 |
1996年 | 644篇 |
1995年 | 603篇 |
1994年 | 534篇 |
1993年 | 412篇 |
1992年 | 410篇 |
1991年 | 255篇 |
1990年 | 203篇 |
1989年 | 183篇 |
1988年 | 193篇 |
1987年 | 162篇 |
1986年 | 123篇 |
1985年 | 111篇 |
1984年 | 108篇 |
1983年 | 40篇 |
1982年 | 89篇 |
1981年 | 117篇 |
1980年 | 128篇 |
1979年 | 129篇 |
1978年 | 103篇 |
1977年 | 76篇 |
1976年 | 73篇 |
1973年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Summary Electrostatic and structural properties of a set of -lactam, -lactam and nonlactam compounds have been analyzed and compared with those of a model of the natural substrate d-alanyl-d-alanine for the carboxy- and transpeptidase enzymes. This first comparison of the electrostatic properties has been based on a distributed multipole analysis of high-quality ab initio wave functions of the substrate and potential antibiotics. The electrostatic similarity of the substrate and active compounds is apparent, and contrasts with the electrostatic properties of the noninhibitors. This has been quantified to give a reasonable correlation with the MIC (Minimum Concentration for Inhibition) and with kinetic data (k2/K) in accordance with the model for interaction of the lactam compounds with dd-peptidase. These correlations provide a better prediction of antibacterial activity than purely structural criteria. 相似文献
942.
S. I. Kuzina A. I. Mikhailov G. N. Nesterenko A. A. Shteinman 《Russian Chemical Bulletin》1996,45(4):824-826
-Irradiation of -oxo-bridged binuclear iron complexes FeIII
2OL
n
in a glycerol or dimethylformamide matrix at 77 K affords unstable mixed-valence FeIIFIII forms resulting from the transfer of a mobile electron generated by the ionizing radiation. These nonequilibrium forms retain the ligand environment of the original complexes, and their ESR spectra at 77–200 K are characterized by an asymmetric signal with an axially anisotropicg-factor, which is in agreement with the spectra of the FeIIFe syu forms obtained by chemical reduction.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 869–871, April, 1996. 相似文献
943.
R. Ficarra M. L. Calabrò S. Tommasini D. Costantino M. Carulli S. Melardi M. R. Di Bella F. Casuscelli R. Romeo P. Ficarra 《Chromatographia》1996,43(7-8):365-368
Summary A high-performance liquid chromatographic method is reported for the resolution of the enantiomers of a series of fused -lactams (2,7-diaza-3-oxo[3.3.0]octan-6-ones) with probable anti-HIV and anticancer activity. Resolution was achieved on a Chiralcel® OD column, cellulose tris-(3,5-dimethylphenyl carbamate) adsorbed on macroporous silica gel; mixtures ofn-hexane and isopropyl alcohol in different proportions were used as the mobile phase. The analysis was studied at different temperatures. 相似文献
944.
The morphology and the crystallization behavior of blends of linear low density polyethylene (LLDPE) with an experimental sample of a semiflexible liquid crystalline polymer (SBH 112 by Eniricerche, Italy) have been studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and scanning electron microscopy (SEM). The blends possess a two-phase morphology, due to immiscibility of the two components. SEM observations show that dispersion of the minor SBH phase is favored at low (相似文献
945.
It is shown that, given a lower semicontinuous convex integrandf satisfying a suitable integrability condition, there exists a sequence of Lipschitz simple integrands which Mosco converges tof and such that the sequence of conjugate integrands Mosco converges tof
*. Moreover, this sequence can be chosen so that the sequence of associated integral functionals, respectively defined onL
1(X) andL
(X
*), Mosco converges as well.We wish to thank Professor Erik J. Balder for interesting remarks on the first version of this work. 相似文献
946.
K. Bogdanowicz-Szwed M. Krasodomska M. Lipowska B. Rys A. Skonecka 《Monatshefte für Chemie / Chemical Monthly》1993,124(6-7):721-731
Summary The tandem Michael addition-cyclization of 2-oxo-cycloalkane carbothioic acid anilides1–3 to benzylidenemalononitrile4 yielded spiroannulated pyridines5–7. Reaction of acrylonitrile with2 and3 gave 2,2-disubstituted Michael adducts14,15, whereas with1 led to 2,2,5-tri(2-cyanoethyl)-cyclopentanone11.
Synthese von Pyridinderivaten durch Reaktionen von ,-ungesättigten Nitrilen mit 2-Oxo-cycloalkano-thiokohlensäure-aniliden
Zusammenfassung Die Michael Tandem-Addition-Cyclisierung von 2-Oxo-cycloalkano-thiokohlensäure-aniliden1–3 mit Benzylidenmalononitril4 ergab die spiroannelierten Pyridine5–7. Reaktion von Acrylnitril mit2 und3 ergab die 2,2-disubstituierten Michael-Addukte14,15, wohingegen mit1 2,2,5-Tri(2-cyanethyl)-cyclopentanon11 erhalten wurde.相似文献
947.
L. F. Capitán-Vallvey R. Avidad Castañeda M. del Olmo Iruela J. L. Vilchez Quero 《Mikrochimica acta》1993,112(1-4):55-62
Benzo(a)pyrene (BaP) and pyrene (Pyr) are two polycyclic aromatic hydrocarbons (PAHs) showing native fluorescence in solution. Both compounds have been determined in water at trace levels by solid-phase spectrofluorimetry, in which BaP and Pyr are fixed on Sephadex G-25 gel and the relative fluorescence intensity is measured after the system is packed in a 1-mm silica cell. First-derivative synchronous spectra obtained at =38 nm were used to determine BaP and Pyr in the presence of other potentially interferent PAHs. The spectral characteristics of the PAHs-gel system are described, the applicable concentration ranges being 0.4–2.5 ng/ml for BaP and 0.7–4.5 ng/ml for Pyr. The relative standard deviations were 1.1% and 1.4% for BaP and Pyr respectively. The detection limits were 0.04 ng/ml for BaP and 0.1 ng/ml for Pyr. The method was applied to the analysis of both compounds in water at trace levels and a recovery study on tap, natural and sea waters was carried out. 相似文献
948.
Otto Funk Lothar Schwabe Karl-Heinz Frömming 《Journal of inclusion phenomena and macrocyclic chemistry》1993,16(4):299-314
The purpose of the study was to examine the formation of inclusion compounds in the freeze-dried products obtained from aqueous solutions of nicotinic acid and -cyclodextrin (-CD), or heptakis (2,6-0-dimethyl)--cyclodextrin (DIMEB). The molar ratios used were 1:1 and 2:1. In addition two freezing temperatures (–40 and –196°C) and different secondary drying temperatures (+50 and +80°C) were used. Freeze-dried products with -CD obtained after low temperature freezing are of the same crystallographic structure as seen in a pure inclusion compound prepared by coprecipitation. Amorphous products were formed after fast freezing. The molar ratios of included nicotinic acid in the freeze-dried products vary — dependent on the preparation conditions — between 0.75:1 and 1:1. A factorial design proves that the included drug amount can be increased by enhancement of the amount of nicotinic acid used, by faster freezing, and by the combination of these two factors. The proof of inclusion formation was given by a combination of X-ray diffractography, differential scanning calorimetry, thermogravimetry and thermofractography.The freeze-dried preparations obtained with DIMEB were amorphous mixtures of the two components. No proof for inclusion of the nicotinic acid in the cyclodextrin cavity could be given. Higher (–40°C) or lower (–196°C) freezing temperatures and the running of the secondary drying process could not influence these results. The very low stability constant of the complex and steric reasons are responsible for this behavior. 相似文献
949.
Yoshitsugu Hasegawa Suguru Kontani Isao Tomita 《Journal of inclusion phenomena and macrocyclic chemistry》1993,16(4):329-337
Intercalation of alkylalcohols into -zirconium phosphate was investigated at 25°C and/or under reflux.n-Alcohols having two to five carbons and 2-propanol were taken up at 25°C. These alcohols, andn-hexanol and heptanol, also intercalated under reflux.n-Alcohols having eight to eighteen carbon atoms intercalated when a stepwise method was employed. 2-Butanol and tertiary amylalcohols intercalated under reflux using a butanol intercalate as a starting material. Ethanol-to-butanol intercalates were unstable at room temperature, losing alcohols and changing to -zirconium phosphate. The particle size of the -zirconium phosphate did not affect the intercalation of alcohols. 相似文献
950.
A. Szafranek J. Szafranek 《Journal of inclusion phenomena and macrocyclic chemistry》1993,15(4):351-358
This paper reports TG analyses of inclusion complexes of-cyclodextrin with benzene, acetylsalicylic acid and methyl salicylate. The data were used for calculation of the compositions of the three body complexes and the apparent kinetic parameters of the thermal decompositions. Water exclusion proceeds as a reaction with ordern=1 and an activation energy about 20 kJ/mol. The expulsion of aromatic guest molecules follows ann=1 order process with the activation energy above 155 kJ/mol, except that for the methyl salicylate complex, which was found to be ca. 64 kJ/mol. 相似文献