全文获取类型
收费全文 | 3181篇 |
免费 | 42篇 |
国内免费 | 3篇 |
专业分类
化学 | 2288篇 |
晶体学 | 11篇 |
力学 | 42篇 |
数学 | 475篇 |
物理学 | 410篇 |
出版年
2018年 | 26篇 |
2017年 | 26篇 |
2016年 | 33篇 |
2015年 | 32篇 |
2014年 | 30篇 |
2013年 | 84篇 |
2012年 | 80篇 |
2011年 | 104篇 |
2010年 | 76篇 |
2009年 | 77篇 |
2008年 | 103篇 |
2007年 | 119篇 |
2006年 | 99篇 |
2005年 | 111篇 |
2004年 | 110篇 |
2003年 | 88篇 |
2002年 | 93篇 |
2001年 | 53篇 |
2000年 | 76篇 |
1999年 | 66篇 |
1998年 | 50篇 |
1997年 | 57篇 |
1996年 | 49篇 |
1995年 | 47篇 |
1994年 | 39篇 |
1993年 | 67篇 |
1992年 | 52篇 |
1991年 | 41篇 |
1990年 | 29篇 |
1989年 | 33篇 |
1988年 | 40篇 |
1987年 | 35篇 |
1986年 | 50篇 |
1985年 | 61篇 |
1984年 | 41篇 |
1983年 | 31篇 |
1982年 | 38篇 |
1981年 | 50篇 |
1980年 | 40篇 |
1979年 | 55篇 |
1978年 | 41篇 |
1977年 | 42篇 |
1976年 | 37篇 |
1975年 | 31篇 |
1974年 | 36篇 |
1973年 | 24篇 |
1972年 | 27篇 |
1971年 | 24篇 |
1970年 | 24篇 |
1966年 | 36篇 |
排序方式: 共有3226条查询结果,搜索用时 15 毫秒
901.
The relationship between one-bond 13C-1H coupling constants and internuclear CCC bond angles (n) in hydrocarbons of the type R2CH2 is best approximated by a quadratic expression. 相似文献
902.
Thomas Weis Markus M. Eisl Bernhard M. Reichl Kurt W. Brandl Herbert Störi 《Mikrochimica acta》1997,125(1-4):287-291
The analysis of segregation phenomena in bicrystals is an important step for the understanding of combined effects of the elementary diffusion processes involved in the segregation in polycristalline systems. The segregation of Si and P in a Fe-6at.%Si bicrystal with a (100) and (110) surface has been investigated by means of AES (Auger electron spectroscopy). For these experiments the technique of a linearly increased temperature has been applied. Significant differences between the segregation kinetics at the two surfaces of the sample have been found on the one hand for the maximum coverage of P and on the other hand for the high temperature behaviour of Si. Additionally, model calculations based on the KTBIM (kinetic tight binding Ising model) have been performed to qualitatively describe the experimental results. It is shown, that the striking differences between the segregation behaviour at the two differently oriented surfaces can be explained by different segregation energies of P, whereas Si plays a minor role due to its relatively small segregation energy.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday 相似文献
903.
Preparation, Mössbauer and Vibrational Spectra of the Complexes [SnCl4F]?, [SnCl4(NCS)]?, and [SnCl4(NCS)2]2? N(CH2)4F and N(CH2)4SCN react in liquid SO2 with SnCl4 yielding the adducts [N(CH3)4][SnCl4F] (I), [N(CH3)4][SnCl4(NCS)] (II) and [N(CH3)4]2[SnCl4(NCS)2] (III).respectively. Mössbauer and vibrational spectra indicate for the anion of I a fluoro-bridged species, which is probably tetrameric like the isoelectronic SbCl4F. For II dimeric moieties are proposed with bridging S-atoms, while [SnCl4(NCS)2]2? has an octahedral structure with N-bonded isothiocyanate groups in the trans-positions. 相似文献
904.
In a previous publication a large series of proteins were identified in fetal human brain by the use of two-dimensional electrophoresis (2-DE) with subsequent matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) and MALDI-tandem time-of-flight (TOF/TOF) analysis. Further identification of many more different spots by traditional 2-DE without additional step such as narrow immobilized ph gradient (IPG) strips or prefractionation seems unlikely and we therefore decided to separate extracted brain proteins by ion-exchange chromatography using a TSK gel DEAE-5PW column followed by 2-DE of individual fractions and analysis by MALDI-TOF/TOF with LIFT technology in fetal brain of the early second trimester. About 1880 protein spots corresponding to 543 different gene products were identified. These proteins included housekeeping, signaling, cytoskeletal, metabolic, antioxidant, and neuron/synaptosomal specific proteins. Among these, 314 gene products (314/543, 57.8%), which have never been detected in traditional 2-DE of human fetal brain, were observed by this method. This updated map of fetal brain proteins may serve as data base and reference map for fetal brain proteins, and the methodology applied may be used as a valuable analytical tool for the basis of protein expressional studies in health and disease. 相似文献
905.
Silvia E. Braslavsky † J. Ioan Matthews † Hans J. Herbert † Jacob de Kok† Carel J. P. Spruit Kurt Schaffner† 《Photochemistry and photobiology》1980,31(4):417-420
Abstract— Irradiation of small phytochrome from oat in its Pr form with 15 ns laser pulses of different wavelengths(605–655 nm) gave rise to a difference absorption with maxima at 400 and 685 nm for the first detectable transient. Bleaching of a 660 nm band was observed, non-recuperable up to 1 ms. The transient absorption has a lifetime of 70±15 μs at 273 K. The transient is tentatively identified as lumi-R and the conformation of its chromophore is postulated to be more extended than that of Pr . A deviation from the exponential decay of the lumi-R absorption at 284 and 300 K and the lack of observable enhancement of the far-red absorption within 1 ms are interpreted in terms of the appearance of still other intermediates on this time scale between lumi-R and Pfr phytochrome. 相似文献
906.
The racemic spirosesquiterpenes β-acorenol ( 1 ), β-acoradiene ( 2 ), acorenone-B ( 3 ) and acorenone ( 4 ) (Scheme 2) have been synthesized in a simple, flexible and highly stereoselective manner from the ester 5 . The key step (Schemes 3 and 4), an intramolecular thermal ene reaction of the 1,6-diene 6 , proceeded with 100% endo-selectivity to give the separable and interconvertible epimers 7a and 7b . Transformation of the ‘trans’-ester 7a to (±)- 1 and (±)- 2 via the enone 9 (Scheme 5) involved either a thermal retro-ene reaction 10 → 12 or, alternatively, an acid-catalysed elimination 11 → 13 + 14 followed by conversion to the 2-propanols 16 and 17 and their reduction with sodium in ammonia into 1 which was then dehydrated to 2 . The conversion of the ‘cis’-ester 7b to either 3 (Scheme 6) or 4 (Scheme 7) was accomplished by transforming firstly the carbethoxy group to an isopropyl group via 7b → 18 → 19 → 20 , oxidation of 20 to 21 , then alkylative 1,2-enone transposition 21 → 22 → 23 → 3 . By regioselective hydroboration and oxidation, the same precursor 20 gave a single ketone 25 which was subjected to the regioselective sulfenylation-alkylation-desulfenylation sequence 25 → 26 → 27 → 4 . 相似文献
907.
Dieter Kovar Kurt Utvary Edwin Hengge 《Monatshefte für Chemie / Chemical Monthly》1979,110(6):1295-1300
29Si-NMR spectras of some new cyclosilane derivatives were measured. First aspects of the dependence of chemical shift on ring size and kind of substituents are given. 相似文献
908.
Hella Folkerts Bernhard Neumüller Kurt Dehnicke Wolfgang Hiller Martin Herker 《无机化学与普通化学杂志》1995,621(4):531-536
Phosphorane Iminato Complexes of Sulfur. Syntheses and Crystal Structures of [O3SS(NPPh3)2] · CH3CN, [SO(NPPh3)2], and [SCl(NPMe3)2]Cl The title compounds have been prepared by the reaction of Me3SiNPPh3 with SO2 and SOCl2, respectively, and by the reaction of Me3SiNPMe3 with S2Cl2. They form colourless, moisture sensitive crystals, which were characterized by IR spectroscopy and by crystal structure determinations. [O3SS((NPPh3)2)] · CH3CN : Space group Pca21, Z = 4, structure solution with 4016 observed unique reflections, R = 0.050. Lattice dimensions at ?60°C: a = 1865.1, b = 1168.4, c = 1569.0 pm. The compound has a zwitterionic structure with a S? S bond length of 218.2 pm and bond lengths S? N of 161.2 and P? N of 160.1 pm. [SO(NPPh3)2] : Space group P21/c, Z = 4, structure solution with 2854 observed unique reflections, R = 0.113. Lattice dimensions at ?50°C: a = 1173.1, b = 1585.6, c = 1619.2 pm, b? = 98.13°. The compound forms monomeric molecules, in which the positions of S and N atoms are disordered in two positions. The bond lengths are S? N 166 pm and P? N 163 pm in average. [SCl(NPMe3)2]Cl : Space group P1 , Z = 2, structure solution with 2416 observed unique reflections, R = 0.038. Lattice dimensions at 20°C: a = 613.2, b = 1030.3, c = 1111.4 pm, α = 88.48°, b? = 88.01°, γ = 83.10°. The compound forms ions [SCl(NPMe3)2]+ and Cl?. In the cation the sulfur atom is ?-tetrahedrally coordinated with a long S? Cl distance of 246.9 pm and bond lengths S? N of 155.3 pm and P? N of 164.3 pm in average. 相似文献
909.
Phytochemistry of genus Gentiana, XIX: Identification of new C-glycosylflavones in Gentiana pyrenaica L . Five flavone C-glucosides [6-C-β-D -glucopyranosyl-tricine ( 1 ) or isopyrenine; 7-O-β-D -glucopyranosyl-6-C-β-D -glucopyranosyltricine ( 2 ) or isopyrenine-7-O-glucoside; isoscoparine-7-O-glucoside ( 3 ); 7-O-β-D -glucopyranosyl-6-C-β-D -glucopyranosylacacetine ( 4 ) and isovitexine-7-O-glucoside ( 5 ) or saponarine] as well as the flavonol glucoside isoquercitrine ( 9 ) have been isolated chromatographically, using polyamide and cellulose columns, from the aerial parts of Gentiana pyrenaica L . 1 and 2 are new compounds and the first glycoflavones trisubstituted on cycle B. Compound 4 is also encountered for the first time in nature. 相似文献
910.
Time-dependent photoconductivity and photo-absorption measurements, the latter using probe beams of 0.1 eV and 1.4 eV, were analyzed on the basis of a saturated band tail model. By a best fit procedure of theory to experiment band structure and recombination parameters for a-As2Se3 were obtained. 相似文献