全文获取类型
收费全文 | 38090篇 |
免费 | 688篇 |
国内免费 | 170篇 |
专业分类
化学 | 21365篇 |
晶体学 | 253篇 |
力学 | 1201篇 |
综合类 | 9篇 |
数学 | 5353篇 |
物理学 | 10767篇 |
出版年
2022年 | 407篇 |
2021年 | 489篇 |
2020年 | 502篇 |
2019年 | 418篇 |
2018年 | 514篇 |
2017年 | 444篇 |
2016年 | 799篇 |
2015年 | 700篇 |
2014年 | 828篇 |
2013年 | 1455篇 |
2012年 | 1787篇 |
2011年 | 2086篇 |
2010年 | 1165篇 |
2009年 | 1089篇 |
2008年 | 1774篇 |
2007年 | 1805篇 |
2006年 | 1697篇 |
2005年 | 2141篇 |
2004年 | 2054篇 |
2003年 | 1492篇 |
2002年 | 1086篇 |
2001年 | 961篇 |
2000年 | 892篇 |
1999年 | 533篇 |
1998年 | 458篇 |
1997年 | 457篇 |
1996年 | 581篇 |
1995年 | 518篇 |
1994年 | 489篇 |
1993年 | 554篇 |
1992年 | 516篇 |
1991年 | 449篇 |
1990年 | 366篇 |
1989年 | 360篇 |
1988年 | 352篇 |
1987年 | 343篇 |
1986年 | 295篇 |
1985年 | 393篇 |
1984年 | 375篇 |
1983年 | 309篇 |
1982年 | 335篇 |
1981年 | 319篇 |
1980年 | 300篇 |
1979年 | 285篇 |
1978年 | 314篇 |
1977年 | 240篇 |
1976年 | 255篇 |
1975年 | 203篇 |
1974年 | 234篇 |
1973年 | 210篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
Using CD data (solution, solid state, various temperatures), X-ray data and MO calculations for a number of substituted mandelic acids (phenylglycolic acids) an attempt was made to explain the different results for the relation between sign of the 1Lb CD band and substitution pattern as described in the literature for various types of aromatic compounds. Moreover the reported sector rule has been reconsidered taking into account the sign and magnitude of the spectroscopic moments. It has been found that in any explanation of the signs of the 1Lb CD bands of substituted aromatic compounds both the conformational behaviour and the spectroscopic moments should be taken into account. 相似文献
202.
M. Domínguez-Pérez J. Jiménez de Llano L. Segade C. Franjo O. Cabeza 《Journal of Thermal Analysis and Calorimetry》2005,79(2):289-293
Summary This paper reports excess molar enthalpies of the binary systems hexane+ethyl benzene, hexane+o-xylene, hexane+m-xylene and hexane+p-xylene at 298.15 K and atmospheric pressure, over the whole composition range. The data was measured directly using a Calvet microcalorimeter. The excess magnitude was correlated to a Redlich-Kister type equation for each mixture. Also, we will discuss the results for the four mixtures studied here and by comparison with the same binary systems but containing propyl propanoate as first component. Finally, we will correlate our results with the Nitta-Chao and the three UNIFAC theoretical approximations. 相似文献
203.
Arthur de Sousa 《Microchemical Journal》1973,18(2):137-141
An indirect microdetermination of phosphate via EDTA titration is described, which can be applied to minerals, soils, fertilizers, biological samples, drugs and organo-phosphorus compounds. The method is based on the precipitation of phosphate as the very insoluble silver orthophosphate (Ksp = 1.3 × 10?20), dissolution of this precipitate in a solution of potassium cyanonickelate and titration of the nickel displaced by silver. The phosphate content is obtained indirectly by multiplying the number of ml of the titrant by a factor. The method takes about an hour after the phosphate is brought into solution. The accuracy is about 1% for samples containing 5–50 mg PO43? and about 3–5% for samples with 100 to 5000 μg PO43?. 相似文献
204.
[reaction: see text]. A very simple methodology to stereoselectively achieve tricyclic isonucleosides (nucleobase = thymine, uracil, and 5-fluoruracil) and 3'-C-branched nucleosides (nucleobase = theophylline) was performed by means of a DBU-mediated addition process using a readily available 2-bromo sugar. The mechanism for these transformations implies the loss of both substituents at C-2 and C-3 on the sugar moiety, and although it seems that DBU is probably involved, its involvement has not yet been ascertained. Cytosine did not react under these conditions. 相似文献
205.
A spectrophotometric method has been developed for the simultaneous determination ofo-cresol andm-cresol in water by reaction withp-aminophenol (PAP). Three different methodologies have been assayed; (i) batch analysis, after reaction in an alkaline medium in the presence of dissolved molecular oxygen as oxidizing agent, (ii) a stopped-flow procedure, carried out in the presence of KIO4 and (iii) a flow injection method based on the same approach. The batch procedure requires 22 min for the full development of colour witho-cresol and 12 min form-cresol. In the stopped-flow mode, using KIO4 and a reaction time of 12 min, better sensitivity can be obtained for both compounds and limits of detection of 10 g 1–1 foro-cresol and 30 g 1–1 form-cresol were found. The flow injection method has a lower sensitivity but permits more than 80 injections per hour. Based on the different maximum absorbance wavelengths obtained for the reaction products of PAP witho-cresol (614 nm) andm-cresol (632 nm), both compounds can be simultaneously determined in water samples and recoveries of 90 to 115% were found in spiked water samples of different types. 相似文献
206.
In fluprednisolone and chloroprednisone acetate, the polarographic reduction of the carbon—halogen bond in position 6 occurs first. The carbanion—enolate formed is reduced at the dropping mercury electrode at more negative potentials than the conjugate acid. Controlled potential electrolysis at a mercury pool electrode where the carbanion—enolate can be protonated, yields the unsaturated ketone. Polarographic reduction of clobetasol-17-propionate and of clobetasone-17-butyrate results in cleavage of the C—Cl bond in the side-chain. This process is followed by reduction of the α,β-unsaturated ketone in the A-ring. Analytical methods for the determination of these compounds in ointments, creams and eye/ear drops gave results with standard deviations of 1–2%. 相似文献
207.
A new synthesis of 5-chloro- and 5-bromo-1,7-naphthyridine, using 8-amino-1,7-naphthyridine as starting material is described. On amination with potassium amide in liquid ammonia, the 5-bromo compound undergoes a tele-amination into 8-amino- and 2-amino-1,7-naphthyridine and a Chichibabin reaction yielding 8-amino-5-bromo-1,7-naphthyridine. The reaction with the 5-chloro compound occurs at a much lower rate than the 5-bromo compound and only gives 8-amino-5-chloro-1,7-naphthyridine in a small yield. Convincing 1H-nmr evidence is presented, showing that the 5-bromo- and 5-chloro-1,7-naphthyridine give addition of the amide ion at position 8 and that the 5-chloro compound also gives addition at position 2. 相似文献
208.
A new reagentless system for sulphite (or sulphur dioxide) determination is reported based on the use of an organic conducting polymer, polyaniline, and its absorbance variation at 550 nm, depending on the sulphite concentration. After chemical polymerisation of aniline a very thin film of polyaniline is obtained. Although the response is not fully reversible, each film can be used for at least 10 measurements for low analyte concentrations (up to 0.5 mg l−1) and five measurements for higher sulphite concentrations. Moreover, the reproducibility, ease of preparation and low cost of the films, permit the use of a new disposable system for each measurement. When the change in absorbance at 550 nm was measured for 210 s (stabilisation time), the system showed a linear response, which ranged from 0.025 to 1.50 mg l−1 of sulphite. A theory with regard to the reaction mechanism between the polyaniline films and sulphite is also proposed. The system was applied to sulphite determination in wine samples and the results were in agreement with those obtained by the Official Method of Analysis (iodometric titration). 相似文献
209.
Irradiation of in methanoi with λ 254 nm yields and as the main primary products which result from the excited singlet state hy initial cyclopropane bond homolysis, but no primary photosolvolysis products. 相似文献
210.
Moser J Sarabia Z Minter H Lovell WW van Henegouwen GM 《Journal of photochemistry and photobiology. B, Biology》2000,58(1):37-45
Ketoprofen (KP), a non-steroidal anti-inflammatory drug of the 2-aryl propionic class, has been shown to produce photoallergic side effects as well as cutaneous photosensitizing properties that induce other phototoxic effects. In the present study we investigated photobinding of ketoprofen to both human serum albumin (HSA), a model protein, and to ex vivo pig skin and its photodegradation. Results demonstrate that photoadduct formation and photodegradation progressively increased with irradiation time where they reach a maximum. Maximum photobinding to the viable layer of the epidermis was about 7-8% of the initial radiolabelled KP added, in the region of 15-30 min UV irradiation. These results were comparable to in vitro results that were seen with photobinding of KP to HSA; in this case, the quantity of covalently bound material was approximately 10% of the initial, after a maximum of 18 min irradiation. It was found by HPLC analysis that the KP decrease is accompanied by an increase of the corresponding photoproduct, decarboxylated ketoprofen (DKP). The yield of DKP reaches a maximum at around 15 min. DKP appears to play an important role in vitro and ex vivo, being the major photoproduct and responsible for the photobinding process. Using micro-autoradiographical techniques we investigated the penetration and distribution of ketoprofen in ex vivo pig skin in greater detail. It was apparent that percutaneous absorption was taking place and that most of the ketoprofen was predominately localised in fibroblasts in the papillary dermis. No other specific localisation within the skin architecture was identified. Although there were differences in the quantities of bound ketoprofen within the different layers of the skin, these levels did not appear to correlate with irradiation time. 相似文献