全文获取类型
收费全文 | 125829篇 |
免费 | 31816篇 |
国内免费 | 35000篇 |
专业分类
化学 | 82397篇 |
晶体学 | 3397篇 |
力学 | 8743篇 |
综合类 | 2148篇 |
数学 | 18141篇 |
物理学 | 77819篇 |
出版年
2024年 | 554篇 |
2023年 | 1150篇 |
2022年 | 2609篇 |
2021年 | 2545篇 |
2020年 | 2639篇 |
2019年 | 3016篇 |
2018年 | 2894篇 |
2017年 | 4510篇 |
2016年 | 3647篇 |
2015年 | 4763篇 |
2014年 | 5292篇 |
2013年 | 7431篇 |
2012年 | 8187篇 |
2011年 | 10092篇 |
2010年 | 13805篇 |
2009年 | 14307篇 |
2008年 | 8804篇 |
2007年 | 7735篇 |
2006年 | 7214篇 |
2005年 | 6876篇 |
2004年 | 6784篇 |
2003年 | 5051篇 |
2002年 | 4996篇 |
2001年 | 5289篇 |
2000年 | 4814篇 |
1999年 | 3804篇 |
1998年 | 2781篇 |
1997年 | 2395篇 |
1996年 | 2641篇 |
1995年 | 3147篇 |
1994年 | 3151篇 |
1993年 | 3213篇 |
1992年 | 2769篇 |
1991年 | 2325篇 |
1990年 | 2017篇 |
1989年 | 2113篇 |
1988年 | 2023篇 |
1987年 | 1313篇 |
1986年 | 1317篇 |
1985年 | 937篇 |
1984年 | 1083篇 |
1982年 | 972篇 |
1981年 | 805篇 |
1980年 | 825篇 |
1979年 | 609篇 |
1978年 | 559篇 |
1977年 | 643篇 |
1976年 | 1052篇 |
1972年 | 532篇 |
1971年 | 445篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
941.
The synthesis has been achieved of the hexadecyl -glycosides of N-acetylmur-amoyl-L-alanyl-D-glutamic acid and N-acetyl-nor-muramoyl-L-alanyl-D-glutamic acid and of the -octadecylamide of N-acetylmuramoyl-L-alanyl-D-glutamic acid — lipophilic glycopeptides of differing hydrophilic-lipophilic balance.M. V. Frunze Simferopol' State University. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 553–557, July–August, 1991. 相似文献
942.
In order to confirm the structures of the alkaloids delcorine and delcoridine (O-demethyldelcorine) suggested previously, a direct passage has been performed from delcoridine to delcorine and from delcorine to delphatine. It has been shown that the configuration of C1 - OCH3 previously suggested for delcorine and delcoridine is incorrect — it should be .Institute of the Chemistry of Plant Substances of the Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 621–623, September–October, 1981. 相似文献
943.
V. E. Agabekov N. L. Budeiko E. T. Denisov N. I. Mitskevich 《Reaction Kinetics and Catalysis Letters》1977,7(4):437-443
A competition method (R.+O2RO
2
.
, R.+XHRH+X.) is proposed to measure the relative rate constants for the interaction of alkyl radicals with different organic compounds. Partial (per C–H bond) rate constants have been determined for the reaction of undecyl radicals with cumene, ethylbenzene and toluene.
(R.+O2RO 2 . , R.+XHRH+X.), . ( C–H ) , .相似文献
944.
N4-Benzoylcytosine 1-(methyl 2,3,4-tri-O-acetyl--D-glucopyranosiduronate) has been obtained with a yield of 70% by the glycosylation of the trimethylsilyl derivative of N4-benzoylcytosine with methyl 1,2,3,4-tetra-O-acetyl--D-glucopyranuronate in the presence of three equivalents of SnCl4 as condensing agent. Cytosine 1-(-D-glucopyranosiduronamide) (IV) — the amide of pentopyranic acid — has been synthesized in practically quantitative yield by the ammonolysis of the nucleoside (I).Peat Institute, Academy of Sciences of the Belorussian SSR, Minsk. Institute of Bioorganic Chemistry, Academy of Sciences of the Belorussian SSR, Minsk. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 617–619, September–October, 1983. 相似文献
945.
Two quaternary alkaloids were isolated from Ochrosia sandwicensis A. Gray. One was shown to be hunterburnine -methochloride (I), while the other has been named ochrosandwine and its probable structure is 10-hydroxydihydrocorynantheol methochloride (II). The stereochemistry of the related alkaloid huntrabrine methochloride (IV) is elaborated. Two previously isolated yellow Ochrosia bases were shown to be ellipticine hydrochloride and methoxyellipticine for which the structure-8-methoxyellipticine (V) is suggested. A rapid separation of alkaloids from plant material is described, which involves extraction with hot dilute acetic acid, precipitation with Mayer's reagent and conversion of the complex to the chlorides by anion exchange. 相似文献
946.
Summary The spectrophotometric determination of thallium with Rhodamine B as Rhodamine B-thallium(III) tetrabromide using extraction of the complex into benzene was investigated. For the oxidation of thallium(I) ceric sulphate was used. The optimum conditions and interferences of some ions for the determination of microgram amounts of Tl are described. The sensitivity is 0.0028 g of Tl per 1 ml of benzene. 5 g of Tl can be determined with a precision of ±2.4%. The method described may be applied in trace analyses of thallium, where sulphuric-nitric acid is used for the decomposition of the sample.
Zusammenfassung Ein spektrophotometrisches Verfahren zur Thalliumbestimmung mit Rhodamin B wird beschrieben, bei dem der gebildete Thallium(III)-komplex mit Benzol extrahiert wird. Zur Oxydation von einwertigem Thallium wird Cer(IV)-sulfat verwendet. Die optimalen Bedingungen sowie Störungen durch Fremdionen bei der Bestimmung von g-Mengen Thallium werden beschrieben. Die Empfindlichkeit betrÄgt 0,0028 g Tl/1 ml Benzol. 5 g Tl können mit einer Genauigkeit von±2,4% bestimmt werden. Das beschriebene Verfahren kann bei der Spurenanalyse von Thallium in solchen Proben benutzt werden, die mit Schwefel- und SalpetersÄure aufgeschlossen worden sind.相似文献
947.
R. N. Tursunova V. A. Maslennikova N. K. Abubakirov 《Chemistry of Natural Compounds》1976,11(2):183-187
Summary The structures of two pregnane glycosides fromCynanchum sibiricum Willd. have been established. Sibiricoside D (II) is sibirigenin (cynanchogenin) 3-D-glucosido-D-cymarosido-D-cymarosido-D-cymarosido-D-oleandroside. Sibiricoside E (I) is sibirigenin (or cynanchogenin) 3-D-glucosido-D-glucosido-D-cymarosido-D-cymarosido-D-cymarosido-D-oleandroside.The hypothesis has been put forward that glycosides of isolinoleone are present in the native state which are isomerized into linoleone derivatives in the process of isolation and treatment with acid and alkaline agents.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 171–177, March–April, 1975. 相似文献
948.
S. Pandey K. A. Fletcher W. E. Acree Jr C. Fetzer 《Fresenius' Journal of Analytical Chemistry》1998,360(6):669-674
Applicability of the nitromethane selective quenching rule for discriminating between alternant versus nonalternant polycyclic
aromatic hydrocarbons (PAHs) is examined for 20 representative PAH solutes dissolved in micellar sodium dodecylsulfate (SDS)
+ cetyltrimethylammonium bromide (CTAB), SDS + dodecyltrimethylammonium bromide (DTAB), SDS + Brij-35, and SDS + sodium octanoate
(SO) mixed surfactant solvent media. Experimental results show that nitromethane quenched fluorescence of all 8 alternant
PAHs studied in the four different solvent systems. Unexpected quenching behavior was observed, however, in the case of nonalternant
PAHs. Nitromethane quenched fluorescence emission of nonalternant PAHs dissolved in the SDS + SO solvent media, which is contrary
to the selective quenching rule. In the case of the mixed anionic + cationic surfactant solvent media, nitromethane quenching
selectivity was restored at concentration ratios of approximately 4 : 1 (anionic:cationic) or less.
Received: 22 May 1997 / Revised: 29 September 1997 / Accepted: 3 October 1997 相似文献
949.
Previous animal experiments suggested that the Magos cold vapor atomic absorption spectroscopic (CVAAS) method might overestimate the concentrations of inorganic mercury (I-Hg) in the presence of methylmercury (MeHg). In the present study it is shown that this error is due to a fast degradation of MeHg during the formation of the analytical signal. For brain samples, about 5% of the total amount of MeHg in the reaction vessel is degraded to I-Hg. Speciation of Hg in aqueous solution of MeHg chloride, after purification with ionexchange chromatography using the Magos method, showed that about 9% was I-Hg. Analysis by NMR of MeHg chloride and MeHg hydroxide showed that less than 1% was in the form of I-Hg. The absolute magnitude of the error in the CVAAS method is dependent on the amounts of SnCl2 and MeHg in the reaction vessel; however, the ratio of I-Hg to total (T-Hg) is shown to be independent of the amount of MeHg (25.5–255 ng as Hg) in the reaction vessel. A procedure for corrections is proposed, based on the results from these studies and empirical data from speciation analyses of brain tissue from MeHg-exposed rats and rabbits. 相似文献
950.
V. A. Raldugin M. M. Shakirov T. V. Leibyuk S. A. Shevtsov 《Chemistry of Natural Compounds》1992,27(4):444-449
From an acid fraction of an ethereal extract of Siberian fir needles two new stereoisomeric acids, the molecules of which have a modified lanostane carbon skeleton, have been isolated in the form of methyl esters. The structures of the molecules of these substances have been established by the use of13C and1H NMR,1H-1H and13C-1H two-dimensional NMR spectra, circular dichroism, and chemical transformations as (24Z)- and (24E)-8(14 13)-abeo-17,13-friedo-lanosta-8,14(30),24-triene-3,23-dion-26-oic acids.Institute of Organic Chemistry, Siberian Branch, Academy of Sciences of the USSR, Novosibirsk. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 511–517, July–August, 1991. 相似文献