全文获取类型
收费全文 | 2550篇 |
免费 | 83篇 |
国内免费 | 9篇 |
专业分类
化学 | 1639篇 |
晶体学 | 5篇 |
力学 | 41篇 |
数学 | 411篇 |
物理学 | 546篇 |
出版年
2023年 | 19篇 |
2022年 | 39篇 |
2021年 | 70篇 |
2020年 | 62篇 |
2019年 | 73篇 |
2018年 | 70篇 |
2017年 | 39篇 |
2016年 | 90篇 |
2015年 | 58篇 |
2014年 | 91篇 |
2013年 | 122篇 |
2012年 | 151篇 |
2011年 | 164篇 |
2010年 | 97篇 |
2009年 | 105篇 |
2008年 | 111篇 |
2007年 | 103篇 |
2006年 | 97篇 |
2005年 | 96篇 |
2004年 | 70篇 |
2003年 | 62篇 |
2002年 | 63篇 |
2001年 | 23篇 |
2000年 | 25篇 |
1999年 | 36篇 |
1998年 | 18篇 |
1997年 | 19篇 |
1996年 | 16篇 |
1995年 | 25篇 |
1994年 | 13篇 |
1993年 | 22篇 |
1992年 | 19篇 |
1991年 | 19篇 |
1989年 | 18篇 |
1984年 | 16篇 |
1982年 | 12篇 |
1979年 | 21篇 |
1978年 | 13篇 |
1977年 | 16篇 |
1976年 | 22篇 |
1975年 | 20篇 |
1974年 | 10篇 |
1957年 | 13篇 |
1956年 | 17篇 |
1955年 | 11篇 |
1954年 | 14篇 |
1929年 | 9篇 |
1927年 | 11篇 |
1924年 | 9篇 |
1907年 | 11篇 |
排序方式: 共有2642条查询结果,搜索用时 265 毫秒
31.
The reaction of trimethylsilyl- and pentafluorophenyl-N-sulfinylamine respectively with 3,5-dihalogeno-1,2,4-trithia-3,5-diborolanes yields the 1,2-dithia-4-aza-3,5-diborolidines1-3.Tert-butyl-N-sulfinylamine and 3,5-dimethyl-1,2,4-trithia-3,5-diborolane react analogous. OtherN-sulfinylamines however split the disulfane bridge in 3,5-dimethyl-1,2,4-trithia-3,5-diborolane and the 1,4-dithia-2-aza-3,5-diborolidines5A-7(A) are formed. Besides of boroxines, cyclo-2,4,6-trimethyl-1,3-dioxa-5-aza-2,4,6-triboranes and cyclo-2,4,6-trimethyl-1-oxa-3,5-diaza-2,4,6-triboranes are formed as byproducts,8–10 have been isolated. In 1,2,4-trithia-3,5-diborolanes and 1,2-dithia-4-aza-3,5-diborolidines the bromo-atoms can be substituted by alkyl (13, 14), by amino (15–20) and by isothiocyanato groups. The compounds were characterised analytically and spectroscopically (MS; NMR:1H,11B,19F,29Si; IR). 相似文献
32.
Zusammenfassung Es werden die Veresterungsgeschwindigkeiten der Monoaminobenzoesäuren und der 1- und 2-Pyridinkarbonsäuren bei 25° mit Chlorwasserstoff als Katalysator sowohl in wasserfreiem als auch in wasserhaltigem Glykol und in wasserfreiem Glyzerin gemessen, die derm- und derp-Aminobenzoesäure und der 2-Pyridinkarbonsäure (Nikotinsäure) auch in wasserhaltigem Glyzerin. Die Abhängigkeit der monomolekularen Geschwindigkeitskonstanten vom Wassergehalt (w) des Mediums und dem Überschuß der Konzentration der Salzsäure über die der organischen Säure, bzw. ihres Esters (C) wird für die drei zuletzt genannten Säuren durch Formeln dargestellt. Diese Geschwindigkeitskonstanten nehmen durchwegs sowohl in wasserfreiem als auch in wasserhaltigem Glykol und Glyzerin nicht nur rascher als die Konzentration der gesamten Salzsäure, sondern auch als die des obigen Überschusses, d. h. der freien Salzsäure, zu.FürC=1/3 sind beiw=0.03 die Geschwindigkeitskonstanten derm- und derp-Aminobenzoesäure und der Nikotinsäure in Äthylalkohol um 20–30 %, die der Anthranilsäure um 70%, die der Pikolinsäure um 130% größer als in Glykol, beiw=0.7 dagegen die der drei ersteren Säuren um 35–50% kleiner.In Glykol sind die Konstanten bei der Anthranilsäure und den Pyridinkarbonsäuren kleiner als in Glyzerin, bei den übrigen wie bei den früher untersuchten Säuren aber größer.Die sterische Wirkung der Aminogruppe in Orthostellung ist in Glykol größer, in Glyzerin kleiner als in Äthylalkohol.Unter gleichen Bedingungen Bind die Konstanten der Veresterungsgeschwindigkeit der Nikotinsäure in Glykol und Glyzerin etwa viermal, in Äthylalkohol nur etwa zweimal größer als die der Pikolinsäure. 相似文献
33.
Jernej Wagger David Bevk Anton Meden Jurij Svete Branko Stanovnik 《Helvetica chimica acta》2006,89(2):240-248
A series of racemic dipodazine analogues 9 were prepared in 22–80% yield from (3Z,6RS)‐3‐[(dimethylamino)methylidene]‐6‐methyl‐1‐(phenylmethyl)piperazine‐2,5‐dione ( 7 ) (Scheme 1), which was prepared in four steps from (RS)‐alanine methyl ester hydrochloride. The preparation of nonracemic 7 from (S)‐alanine methyl ester hydrochloride failed, since the introduction of the enamino functionality at position 3 of the precursor 6 was accompanied by almost complete racemization. 相似文献
34.
Ernst Lücker Anton Rosopulo Wilhelm Kreuzer 《Fresenius' Journal of Analytical Chemistry》1991,340(4):234-241
Summary The function of solid sampling graphite furnace atomic absorption spectrometry (SS-GFAAS) in the laboratory-internal preparation, production and standardization of bovine liver reference material is described. Analytical quality assurance was achieved by using a wet digestion decomposition procedure and GFAAS (WD-GFAAS) as well as three different methods of SS-GFAAS. It will be shown, that SS-GFAAS is an adequate and efficacious method of analytical quality control in the production of reference materials. Details are given about the preliminary steps of sample selection and preparation, applying SS-GFAAS both as a screening method and for the detection of contamination during the production of the reference materials. The element contents of the four bovine liver materials measured after decomposition with nitric acid in the open system by GFAAS and also by Flame-AAS, using the slotted tube atom trap, are compared statistically with the results obtained by SS-GFAAS. In solid sampling analyses three different systems were applied: a solid sampling ZAA-spectrometer with platform-boat, a D2AA-spectrometer with solid sampling graphite tube and platform-drawer and an autoprobe-D2AA-spectrometer with solid sampling autoprobe and graphite tube. 相似文献
35.
Anton Meller Walter Maringgele Rainer Oesterle 《Monatshefte für Chemie / Chemical Monthly》1980,111(5):1087-1096
The reaction of galliumtrichloride and galliumtribomide resp. with 2,6-dimethyl- and 2,4,6-trimethyl-N-trimethylsilyl-trifluoroacetanilide leads to the trifluoracetanilidodihalogenogallanes1–3. The 2,6,9-trioxa-4,8-diaza-1,5-digalla-bicyclo-[3.3.1]-nonadiene derivatives1 a–3 a are formed probably as by products.N-Trimethylsilyltrifluoroacetamide reacts with galliumtrichloride to yield the trifluoroacetamido-dichlorogallane4.N-Trimethylsilyl-N,N,N-triorganyl-thioureas react with galliumtrichloride under elimination of chlorotrimethylsilane to give the thio-ureido-gallanes5 and6. IfN,N-diorganylureas andN,N-diorganyl-thio-ureas resp. are reacted with galliumtrichloride and methyl-gallium-dichloride resp., the thio-ureido- and ureido-gallanes7–9 are obtained by elimination of hydrogenchloride. The compounds are characterized by analysis and spectral data (NMR:1H,19F; MS; IR). The substances are monomeric in the gas phase, obviously due to internal coordination.
30. Mitt.:W. Maringgele undA. Meller, J. Organomet. Chem., im Druck. 相似文献
36.
Uwe Klingebiel Gerhild Wendenburg Anton Meller 《Monatshefte für Chemie / Chemical Monthly》1979,110(2):289-296
Lithium salts of hydrazines react with fluorosilanes under formation of fluorosilylhydrazines and LiF. Five membered rings are obtained in the reaction of bis(fluorosilyl)-hydrazines with lithiated amines. The mass,1H-and19F-nmr spectra of the compounds are reported. 相似文献
37.
David Bevk Uroš Grošelj Anton Meden Jurij Svete Branko Stanovnik 《Helvetica chimica acta》2007,90(9):1737-1744
A simple and efficient synthesis of four new substituted pyrimidines, compounds 9a – d , from the title compound 3 is described. Conversion of 3 to methyl (E)‐3‐(dimethylamino)‐2‐(6‐methoxy‐2‐phenylpyrimidin‐4‐yl)prop‐2‐enoate ( 4 ), followed by condensation with various dinucleophiles according to the ‘enaminone methodology’, afforded the target compounds 9 in medium‐to‐good yields. 相似文献
38.
Renata Ga<parová Daniel Zbojek Margita Lácová Katarína Král'ová Anton Gatial Branislav Horváth Alžbeta Krutošíková 《Central European Journal of Chemistry》2005,3(4):622-646
The reactions of substituted furo[3,2-b]pyrrole-5-carboxhydrazides 1 with 5-arylfuran-2-carboxaldehydes 2, 4,5-disubstituted furan-2-carboxaldehydes 3 and thiophene-2-carboxaldehyde 4 has been studied. The advantage of microwave irradiation on some of these reactions was reflected in the reduced reaction
time and increased yields. Reactions of 1 with 4-substituted 1,3-oxazol-5(4H)-ones 11 led to diacylhydrazines 13 or to imidazole derivatives 14 depending on the temperature. 1,2,4-Triazole-3-thione 17 was synthesized by two-step reaction of 1 with phenylisothiocyanate and subsequent base-catalyzed cyclization of thiosemicarbazide 16. The effects of hydrazones 5–10 on inhibition of photosynthetic electron transport in spinach chloroplasts and chlorophyll content in the antialgal suspensions
of Chlorella vulgaris were investigated. 相似文献
39.
5(4H)-Thiazolone derivative 4 , obtained from N-dithiocarbobenzoxyglycine ( 1 ) and N,N-dimethyl-N′-heteroarylformamidines 3 in acetic anhydride, was rearranged with sodium methoxide in methanol followed by acidification with acetic acid into imidazole-4-carboxylic acid derivatives 5, 6 and 7 . These were further converted with methyl iodide into methylthio derivatives 8 , with hydrogen peroxide into the corresponding disulphide 9 , with hydrazine and amines into hydrazide 10 and amides 11 . In the reactions of 4a and 6a with amines in the presence of dichloromethane symetrically disubstituted methanes 14–18 were formed. 相似文献
40.
Myron S. Huzan Manuel Fix Matteo Aramini Peter Bencok J. Frederick W. Mosselmans Shusaku Hayama Franziska A. Breitner Leland B. Gee Charles J. Titus Marie-Anne Arrio Anton Jesche Michael L. Baker 《Chemical science》2020,11(43):11801
Large single-ion magnetic anisotropy is observed in lithium nitride doped with iron. The iron sites are two-coordinate, putting iron doped lithium nitride amongst a growing number of two coordinate transition metal single-ion magnets (SIMs). Uniquely, the relaxation times to magnetisation reversal are over two orders of magnitude longer in iron doped lithium nitride than other 3d-metal SIMs, and comparable with high-performance lanthanide-based SIMs. To understand the origin of these enhanced magnetic properties a detailed characterisation of electronic structure is presented. Access to dopant electronic structure calls for atomic specific techniques, hence a combination of detailed single-crystal X-ray absorption and emission spectroscopies are applied. Together K-edge, L2,3-edge and Kβ X-ray spectroscopies probe local geometry and electronic structure, identifying iron doped lithium nitride to be a prototype, solid-state SIM, clean of stoichiometric vacancies where Fe lattice sites are geometrically equivalent. Extended X-ray absorption fine structure and angular dependent single-crystal X-ray absorption near edge spectroscopy measurements determine FeI dopant ions to be linearly coordinated, occupying a D6h symmetry pocket. The dopant engages in strong 3dπ-bonding, resulting in an exceptionally short Fe–N bond length (1.873(7) Å) and rigorous linearity. It is proposed that this structure protects dopant sites from Renner–Teller vibronic coupling and pseudo Jahn–Teller distortions, enhancing magnetic properties with respect to molecular-based linear complexes. The Fe ligand field is quantified by L2,3-edge XAS from which the energy reduction of 3dz2 due to strong 4s mixing is deduced. Quantification of magnetic anisotropy barriers in low concentration dopant sites is inhibited by many established methods, including far-infrared and neutron scattering. We deduce variable temperature L3-edge XAS can be applied to quantify the J = 7/2 magnetic anisotropy barrier, 34.80 meV (∼280 cm−1), that corresponds with Orbach relaxation via the first excited, MJ = ±5/2 doublet. The results demonstrate that dopant sites within solid-state host lattices could offer a viable alternative to rare-earth bulk magnets and high-performance SIMs, where the host matrix can be tailored to impose high symmetry and control lattice induced relaxation effects.Taking advantage of synchrotron light source methods, we present the geometric and electronic structure of iron doped in lithium nitride. 相似文献