全文获取类型
收费全文 | 1493篇 |
免费 | 17篇 |
国内免费 | 6篇 |
专业分类
化学 | 1381篇 |
力学 | 1篇 |
数学 | 68篇 |
物理学 | 66篇 |
出版年
2020年 | 6篇 |
2018年 | 6篇 |
2016年 | 14篇 |
2015年 | 16篇 |
2014年 | 15篇 |
2013年 | 27篇 |
2012年 | 27篇 |
2011年 | 30篇 |
2010年 | 15篇 |
2009年 | 19篇 |
2008年 | 19篇 |
2007年 | 29篇 |
2006年 | 22篇 |
2005年 | 24篇 |
2004年 | 17篇 |
2003年 | 16篇 |
2002年 | 17篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 12篇 |
1998年 | 7篇 |
1997年 | 7篇 |
1996年 | 15篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 5篇 |
1992年 | 16篇 |
1991年 | 8篇 |
1990年 | 20篇 |
1989年 | 5篇 |
1988年 | 10篇 |
1986年 | 6篇 |
1985年 | 13篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 11篇 |
1980年 | 7篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1977年 | 11篇 |
1975年 | 5篇 |
1971年 | 4篇 |
1966年 | 76篇 |
1965年 | 129篇 |
1964年 | 143篇 |
1963年 | 123篇 |
1962年 | 125篇 |
1961年 | 146篇 |
1960年 | 124篇 |
1959年 | 62篇 |
排序方式: 共有1516条查询结果,搜索用时 15 毫秒
11.
Larry I. Amstutz Horst Meyer Samuel M. Myers
Robert L. Mills
《Journal of Physics and Chemistry of Solids》1969,30(12):2693-2697Measurements of T1 in the hep phase of H2, over the temperature range 2°–12°K and the ortho concentration range between 0.5 and 0.97 are presented. At temperatures below 10°K, the thermally activated self-diffusion is negligible and the mechanism for nuclear relaxation is that attributed by Moryia and Motizuki and by Harris to intramolecular dipolar interaction, modulated by intennolecular electric quadrupole-quadrupole (EQQ) interaction. The gaussian approximation for the correlation function was used by these authors to predict T1. From the comparison between experiment and theory, we determine the EQQ parameter Γ/kB to be 0.67°K. Above 10°K the effect of diffusion influences T1, and the experimental results for an 88 per cent ortho H2 sample up to the melting point suggest that the relaxation mechanisms resulting from EQQ interaction and diffusion are not independent of one another. 相似文献
12.
Different methods for representing the upper orbitals in Rydberg transitions are tested by means of a series of ab initio SCF and CI calculations for ethylene and various properties for such diffuse united-atom species are reported. Calculated transition energies indicates the maximum separation between individual components of the (π,3d) Rydberg species to be in the 0.1-0.2 range, with similarly small energy separations being obtained for the corresponding (π, 3p) states 相似文献
13.
14.
15.
Ursula Baumann 《Fresenius' Journal of Analytical Chemistry》1963,193(6):456
Ohne Zusammenfassung
Folin, O.: Hoppe-Seylers Z. physiol. Chem. 41, 223 (1904); vgl. diese Z. 44, 459 (1905). 相似文献
16.
Ursula Schmid Paul Gilgen Heinz Heimgartner Hans-Jürgen Hansen Hans Schmid 《Helvetica chimica acta》1974,57(5):1393-1403
Irradiation of 2, 3-diphenyl-2H-azirine ( 1a ) and 1-azido-1-phenyl-propene, the precursor of 2-methyl-3-phenyl-2H-azirine ( 1b ), in benzene, with a high pressure mercury lamp (pyrex filter) in the presence of acid chlorides yields the oxazoles 5a–d (Scheme 2). Photolysis of 2, 2-dimethyl-3-phenyl-2H-azirine ( 1c ) under the same conditions gives after methanolysis the 5-methoxy-2, 2-dimethyl-4-phenyl-3-oxazolines 7a, b, d , while hydrolysis of the reaction mixture leads to the formation of the 1, 2-diketones 8a, c, d (Scheme 4). The suggested reaction path for all these reactions is a 1, 3-dipolar cycloaddition of the photochemically generated benzonitrilemethylides 2 to the carbonyl double bond of the acid chlorides to give the intermediates 4 , followed by either elimination of hydrogen chloride or solvolysis (Schemes 2 and 4). Irradiation of 1c in the presence of acetic acid anhydride leads via the intermediate 9 to the 5-hydroxy-3-oxazoline 10 and the 5-methylidene-3-oxazoline 11 (Scheme 5). 相似文献
17.
Cascella M Micheletti C Rothlisberger U Carloni P 《Journal of the American Chemical Society》2005,127(11):3734-3742
The biological function of the aspartic protease from HIV-1 has recently been related to the conformational flexibility of its structural scaffold. Here, we use a multistep strategy to investigate whether the same mechanism affects the functionality in the pepsin-like fold. (i) We identify the set of conserved residues by using sequence-alignment techniques. These residues cluster in three distinct regions: near the cleavage-site cavity, in the four beta-sheets cross-linking the two lobes, and in a solvent-exposed region below the long beta-hairpin in the N-terminal lobe. (ii) We elucidate the role played by the conserved residues for the enzymatic functionality of one representative member of the fold family, the human beta-secretase, by means of classical molecular dynamics (MD). The conserved regions exhibit little overall mobility and yet are involved into the most important modes of structural fluctuations. These modes influence the substrate-catalytic aspartates distance through a relative rotation of the N- and C-terminal lobes. (iii) We investigate the effects of this modulation by estimating the reaction free energy at different representative substrate/enzyme conformations. The activation free energy is strongly affected by large-scale protein motions, similarly to what has been observed in the HIV-1 enzyme. (iv) We extend our findings to all other members of the two eukaryotic and retroviral fold families by recurring to a simple, topology-based, energy functional. This analysis reveals a sophisticated mechanism of enzymatic activity modulation common to all aspartic proteases. We suggest that aspartic proteases have been evolutionarily selected to possess similar functional motions despite the observed fold variations. 相似文献
18.
On the biogenesis of betacyanins. Expaiments with [2-14C]-dopaxanthine Labelled dopaxanthine is prepared from betanin by a double exchange procedure replacing first its cyclodopa part by diethylamine and effecting the second exchange with 2-[14C]-dopa on the purified intermediate ‘DEA-betalain’; the specific activity delivered with the dopa allowed accurate determination of ? in the electronic spectrum of dopaxanthine (λmax 488 nm, ? = 41.800). The C(2)-labelled dopaxanthine is incubated into the fruits of Opuntia bergeriana. The incorporation rates into betanin are found to correspond approximately to those obtained in earlier experiments with labelled dopa alone: Only 1--5% of the label showed up in the cyclodopa moiety of the betanin (which is isolated as cyclodopa-glucoside) whereas 99--95% of the radioactivity was associated with the betalamic acid part (recovered in form of indicaxanthine). 相似文献
19.
EPR, UV/Vis and FTIR spectroscopy as well as thermal analysis (TA/MS) were applied to study the influence of sulfate species present in the anatase support on the specific nature of VOx species in supported VOx/TiO2 catalysts. Those sulfate species modify the local structure of the supported vanadyl species and lead to the formation of two types of VO2+ sites instead of only one type being formed on sulfate‐free anatase. EPR and FTIR spectroscopic measurements revealed that a part of the VO2+ species are directly bound to the surface sulfate species. By TA/MS it was found that SO2 is released at lower temperature from VOx/TiO2 in comparison to the vanadium‐free support. The direct bonding between sulfate and VOx species stabilizes the latter on the surface of VOx/TiO2 resulting in three effects: 1) a higher V site dispersion in comparison to sulfate‐free TiO2, 2) a better resistance of surface vanadyls against diffusion into the bulk of the support and 3) a much faster reoxidation of reduced V sites than observed on sulfate‐free TiO2. 相似文献
20.
An evaluation of the non-boiling evaporation technique for the preconcentration of Pb, Cd, Cu, Zn at the pg g?1 level in water samples is presented. Various improvements were made to allow efficient control of contamination problems at these extremely low concentrations. They include the choice of FEP Teflon for the evaporation containers and the use of sophisticated cleaning, ageing and pre-conditionning procedures. Detailed calibration graphs were obtained down to the sub-pg g?1 level by processing ultra-low concentration standards. This technique was then applied to the determination of these four metals in snow samples collected in Greenland and Antarctica. 相似文献