全文获取类型
收费全文 | 3532篇 |
免费 | 136篇 |
国内免费 | 20篇 |
专业分类
化学 | 2561篇 |
晶体学 | 15篇 |
力学 | 55篇 |
数学 | 664篇 |
物理学 | 393篇 |
出版年
2023年 | 23篇 |
2022年 | 30篇 |
2021年 | 44篇 |
2020年 | 62篇 |
2019年 | 58篇 |
2018年 | 37篇 |
2017年 | 28篇 |
2016年 | 98篇 |
2015年 | 57篇 |
2014年 | 79篇 |
2013年 | 118篇 |
2012年 | 150篇 |
2011年 | 145篇 |
2010年 | 82篇 |
2009年 | 98篇 |
2008年 | 163篇 |
2007年 | 135篇 |
2006年 | 132篇 |
2005年 | 131篇 |
2004年 | 113篇 |
2003年 | 128篇 |
2002年 | 119篇 |
2001年 | 63篇 |
2000年 | 71篇 |
1999年 | 47篇 |
1998年 | 61篇 |
1997年 | 70篇 |
1996年 | 71篇 |
1995年 | 57篇 |
1994年 | 64篇 |
1993年 | 48篇 |
1992年 | 43篇 |
1991年 | 60篇 |
1990年 | 42篇 |
1989年 | 42篇 |
1988年 | 46篇 |
1987年 | 52篇 |
1986年 | 38篇 |
1985年 | 47篇 |
1984年 | 77篇 |
1983年 | 44篇 |
1982年 | 54篇 |
1981年 | 49篇 |
1980年 | 47篇 |
1979年 | 34篇 |
1978年 | 43篇 |
1977年 | 42篇 |
1976年 | 34篇 |
1975年 | 29篇 |
1973年 | 24篇 |
排序方式: 共有3688条查询结果,搜索用时 15 毫秒
71.
Zusammenfassung Es wurde die Kinetik der Bromatbildungsreaktion nach 3 Br3+6 OH=8 Br+BrO3+3 H2O, beziehungsweise 3 Br2+6 OH=5 Br+BrO3+3 H2O untersucht und gefunden, daß die Geschwindigkeit der Bromatbildung mit den Konzentrationen von Brom und Hydroxylion steigt und mit zunehmender Bromionkonzentration abnimmt. Die Werte der Potenzexponenten der Konzentrationen variieren mit der Geschwindigkeit. Neutralsalze verzögern.Es wurde ferner für die rasche Reaktion das Zeitgesetz –d[Br3]/d=k
1[OH]/[Br]3[Br3]2, beziehungsweise –d[Br2]/d=k1[OH]/[Br][Br2]2 und für die langsame Reaktion das Zeitgesetz –d[Br3]/d=k
2[OH]4/[Br]7[Br3]3, beziehungsweise –d[Br2]/d=k2[OH]4/[Br]4[Br2]3 wahrscheinlich gemacht. Der Temperaturkoeffizient der in einer Monophosphat-Biphosphatlösung gemessenen langsamen Reaktion ist von der Größenordnung 17.Aus den Geschwindigkeitskoeffizienten der raschen Reaktion und dem der Reaktion HBrO+OH BrO3 läßt sich das Brom-Hypobromitgleichgewicht und aus den Koeffizienten der langsamen Reaktion und dem der Reaktion BrO3+Br+H. Br2 das Brom-Bromatgleichgewicht berechnen.Vgl. die vorhergehende Arbeit. 相似文献
72.
Maria E. Sulbaran Gerzon E. Delgado Asilo J. Mora Ali Bahsas Hector Novoa de Armas Norbert Blaton 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(9):o543-o545
In the title compound, C6H8N2O2S, also known as N‐acetyl‐2‐thiohydantoin–alanine, the molecules are joined by N—H...O hydrogen bonds, forming centrosymmetric R22(8) dimers; these dimers are linked by C—H...O interactions to form R22(10) rings, thus forming C22(10) chains that run along the [101] direction. 相似文献
73.
The multielement trace analytical method ‘total reflection X-ray fluorescence’ (TXRF) has become a successfully established method in the semiconductor industry, particularly, in the ultra trace element analysis of silicon wafer surfaces. TXRF applications can fulfill general industrial requirements on daily routine of monitoring wafer cleanliness up to 300 mm diameter under cleanroom conditions. Nowadays, TXRF and hyphenated TXRF methods such as ‘vapor phase decomposition (VPD)-TXRF’, i.e. TXRF with a preceding surface and acid digestion and preconcentration procedure, are automated routine techniques (‘wafer surface preparation system’, WSPS). A linear range from 108 to 1014 [atoms/cm2] for some elements is regularly controlled. Instrument uptime is higher than 90%. The method is not tedious and can automatically be operated for 24 h/7 days. Elements such as S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Br, Sn, Sb, Ba and Pb are included in the software for standard peak search. The detection limits of recovered elements are between 1×1011 and 1×107 [atoms/cm2] depending upon X-ray excitation energy and the element of interest. For the determination of low Z elements, i.e. Na, Al and Mg, TXRF has also been extended but its implementation for routine analysis needs further research. At present, VPD-TXRF determination of light elements is viable in a range of 109 [atoms/cm2]. Novel detectors such as silicon drift detectors (SDD) with an active area of 5 mm2, 10 mm2 or 20 mm2, respectively, and multi-array detectors forming up to 70 mm2 are commercially available. The first SDD with 100 mm2 (!) area and integrated backside FET is working under laboratory conditions. Applications of and comparison with ICP-MS, HR-ICP-MS and SR-TXRF, an extension of TXRF capabilities with an extremely powerful energy source, are also reported. 相似文献
74.
Osipov SN Golubev AS Sewald N Michel T Kolomiets AF Fokin AV Burger K 《The Journal of organic chemistry》1996,61(21):7521-7528
A new method for the preparation of alpha-chlorodifluoromethyl-, alpha-bromodifluoromethyl-, and alpha-difluoromethyl-substituted alpha-hydroxy and alpha-amino acid esters 11, 19-21 is described. The key step of the synthesis is the regioselective alkylation of ketones 5, 7-9 and imines 16-18 with C-nucleophiles. The ketones 7-9 are readily available from 3,3,3-trifluorolactate 1 by a five-step procedure. Subsequent removal of the protecting groups from 19-21 provides the corresponding free amino acids 25, 26, 28. 相似文献
75.
On singlet excitation (λ = 254 nm, THF, pentane or hexane), the diastereoisomeric methano-epoxydienes (E)- 6 and (E)- 7 undergo interconversion and yield the products 8 – 11 . The main process is the cleavage of the oxirane ring to the vinyl carbene intermediate e which undergoes addition to the adjacent double bond furnishing the cyclopropene 8 . The alternative carbene intermediate f is evidenced by the formation of the cyclobutene 10 . For the fragmentation leading to 11 , the carbene f as well as the dipolar species h are considered as possible intermediates. On triplet sensitization (acetone, λ > 280 nm), (E)- 7 shows behavior typical of epoxydienes, undergoing fission of the C? O bond of the oxirane ring and isomerization to the compounds 13 , 14 and (E/Z)- 15 . 相似文献
76.
Six known metabolites, two new isocoumarins 4 and 8, and one new highly substituted benzoic acid derivative 9 were isolated from the ethyl acetate culture extract of a fungal endophyte, Scytalidium sp. In addition, another new benzoic acid 10 with an unusual 1,2-dicarbonyl side chain was indirectly identified from its methylated derivatives 10a-10d. 相似文献
77.
Karappulli P. Balakrishnan Thomas A. Kaden Liselotte Siegfried Andreas D. Zuberbühler 《Helvetica chimica acta》1984,67(4):1060-1069
The complexation of Cu(I) and Cu(II) by a series of 12-, 14- and 16-membered macrocyclic ligands 1–6 containing the N2S2 donor set has been studied potentiometrically, spectrophotometrically and voltammetrically. In the case of Cu(II), mononuclear complexes CuL2+ with stability constants of 1010–1015 are formed. In addition, partially hydrolyzed species Cu(L)OH+ are observed at pH > 10 for the 12-membered ligands. For Cu(I), beside the specis CuL+ with stabilities of 1012–1014, the unexpected formation of protonated species CuLH2+ was detected. In contrast to the well-known general trends in coordination chemistry, the stability of these protonated species increases relative to that of the complexes with the neutral ligand when the ring size and concomitantly the distance between neighbouring donor atoms is decreased. From the stability constants of the Cu(I)- and Cu(II)-complexes the redox potentials have been calculated and are compared to the values of E1/2 obtained by cyclic voltammetry. Despite the identical donor set the Cu(II)/Cu(I) redox potentials of the complexes are spanning a range of 340 mV or six orders of magnitude in relative stability, reflecting the importance of subtle differences in steric requirements. 相似文献
78.
The kinetics of the on/off reaction of the amino group in the side chain of tetraazamacrocyclic Cu2+, Ni2+ and Co2+ complexes has been measured. The rate law k(obs)=k(0)+k(H)[H+]+k(OH)/[H+], the sum of the forward and reverse reaction, gives rise to u-shaped pH dependences from which the three rate constants can be determined. k(H) describes the proton assisted dissociation of the amino group bound to the metal ion and is roughly correlated to the equilibrium constant of the reaction. k(OH) is determined by the protonation constant of the free amino group and the rate constant describing the binding of the amino group to the metal ion. k(0) is composed of the rate constant for the opening of the chelate ring without proton assistance and the rate for the reactivity of the ammonium group in the formation of the chelate ring. Our results show that the rates of the opening and closing of the chelate ring are very little dependent on the nature of the metal ion. 相似文献
79.
Densities of aqueous solutions of a series of polymethonium chloride and bromide salts (CH3)3–N–(CH2)n–N–(CH3)3X2 have been measured at 25°C. Apparent molal volumes have been calculated, and methylene-group contributions to the limiting apparent molal volumes °v have been estimated. Constant values of the methylene-group contribution of 16.5 and 17.0 cm3-mole–1 were obtained for the bromide and chloride salts, respectively. These values are consistent with methylene-group contributions reported for other series of organic electrolytes. 相似文献
80.
Hans Günther Wahl Silke-Yvonne Habel Norbert Schmieder Hartmut M. Liebich 《Journal of separation science》1994,17(7):543-548
Identification of cis/trans isomers of unsaturated fatty acids cannot usually be achieved by GC-MS (gas chromatography-mass spectrometry) without reference substances. In this study a GC-FTIR-MS system (gas chromatography-Fourier transform-mass spectrometry) was used to identify fatty acid methyl esters (FAMEs) and differentiate between the cis/trans isomers. Besides methyl esters, 2-alkenyl-4,4-dimethyloxazoline derivatives (DMOX), which have been used to locate double bond positions of unsaturated fatty acids, were examined with respect to their suitability for cis/trans differentiation. A combined GC-FTIR-MS system with a wide band (4000–550 cm?1) mercury cadmium telluride (MCT) detector was used in series and parallel to identify 31 reference unsaturated fatty acids, including 7 pairs of cis/trans isomers. Serum samples of healthy persons and commercially available fish oil were analyzed as examples of complex mixtures. Using splitless injection the detection limit for the less sensitive IR detector was 25 ng/μl in case of the weak cis and trans bands. In the FTIR spectra cis/trans isomers were identified by analysis of bands arising from C? H out-of-plane (oop) bending: for both the FAME and DMOX derivatives cis-1,2-disubstituted double bonds give a strong band near 720 cm?1 and the corresponding trans isomers near 967 cm?1. cis Isomers could be identified further by a band at 3012 cm?1. With the combined data of the GC-FTIR-MS system it is now possible to identify polyunsaturated fatty acids with regard to the discrimination of cis/trans isomers. 相似文献