全文获取类型
收费全文 | 1020篇 |
免费 | 33篇 |
国内免费 | 1篇 |
专业分类
化学 | 671篇 |
晶体学 | 16篇 |
力学 | 24篇 |
数学 | 63篇 |
物理学 | 280篇 |
出版年
2021年 | 9篇 |
2020年 | 11篇 |
2019年 | 10篇 |
2018年 | 8篇 |
2017年 | 7篇 |
2016年 | 20篇 |
2015年 | 18篇 |
2014年 | 18篇 |
2013年 | 45篇 |
2012年 | 45篇 |
2011年 | 53篇 |
2010年 | 35篇 |
2009年 | 22篇 |
2008年 | 41篇 |
2007年 | 48篇 |
2006年 | 32篇 |
2005年 | 57篇 |
2004年 | 48篇 |
2003年 | 26篇 |
2002年 | 39篇 |
2001年 | 18篇 |
2000年 | 51篇 |
1999年 | 10篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 18篇 |
1994年 | 21篇 |
1993年 | 17篇 |
1992年 | 21篇 |
1991年 | 22篇 |
1990年 | 9篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 12篇 |
1986年 | 12篇 |
1985年 | 18篇 |
1984年 | 11篇 |
1983年 | 13篇 |
1982年 | 13篇 |
1981年 | 6篇 |
1980年 | 11篇 |
1979年 | 11篇 |
1978年 | 17篇 |
1977年 | 10篇 |
1976年 | 17篇 |
1975年 | 18篇 |
1974年 | 15篇 |
1973年 | 5篇 |
1923年 | 4篇 |
排序方式: 共有1054条查询结果,搜索用时 0 毫秒
261.
The combination of double-quantum NMR with magic angle spinning is demonstrated for deuterium in solids Under magic angle spinning the single-quantum resonance lines are extremely sensitive to spinner angle adjustment and stability while the double-quantum lines are not. This provides an additional approach to high resolution. 相似文献
262.
Platas-Iglesias C Vander Elst L Zhou W Muller RN Geraldes CF Maschmeyer T Peters JA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(22):5121-5131
In this paper we explore Gd(3+)-doped zeolite NaY nanoparticles for their potential application as a contrast agent in magnetic resonance imaging (MRI). The nanoparticles have an average size of 80-100 nm, as determined by TEM and XRD. A powdered sample loaded with La3+ was characterised by means of multinuclear solid-state NMR spectroscopy. The NMR dispersion (NMRD) profiles obtained from aqueous suspensions of samples with Gd3+ doping ratios of 1.3-5.4 wt% were obtaining at different temperatures. The relaxivity increases drastically as the Gd3+ loading decreases, with values ranging between 11.4 and 37.7 s-1 mM-1 at 60 MHz and 37 degrees C. EPR spectra of aqueous suspensions of the samples suggest that an interaction between neighbouring Gd3+ ions within the same particle produces a significant increase in the transversal electronic relaxation rates in samples with a high Gd3+ content. The experimental NMRD and EPR data are explained with the use of a model that considers the system as a concentrated aqueous solution of Gd3+ in the interior of the zeolite that is in exchange with the bulk water outside the zeolite. The results obtained indicate that the Gd3+ ion is immobilised in the interior of the zeolite and that the relaxivity is mainly limited by the relatively slow diffusion of water protons from the pores of the zeolite channels into the bulk water. 相似文献
263.
Ladame S Fauré R Denier C Lakhdar-Ghazal F Willson M 《Organic & biomolecular chemistry》2005,3(11):2070-2072
A new series of "bi-substrate" analogues have been synthesized as potential inhibitors of the glyceraldehyde-3-phosphate dehydrogenase and one lead compound has been identified that inhibits the enzyme from Trypanosoma cruzi with good affinity and very high (50-fold) specificity. 相似文献
264.
E Payan J Y Jouzeau F Lapicque N Muller J P Payan P Gegout P Bertin P Netter 《Journal of chromatography. A》1991,566(1):9-18
A high-performance liquid chromatographic method for the determination of hyaluronic acid levels in synovial fluids has been developed. The hyaluronidase sample digests, containing an internal standard (benzoic acid), were separated on a reversed-phase octadecylsilyl column eluted with 0.01 M tetrabutylammonium phosphate-acetonitrile (83:17, v/v) at pH 7.35. The determination was made on 1:10 diluted samples, by using a calibration curve from 50 to 500 micrograms/ml of human umbilical cord hyaluronic acid. For validation, the synovial fluids were simultaneously analysed by this method and a radiometric method: a high correlation was found between the two (correlation coefficient 0.94). The proposed method can be used to determine specifically the high hyaluronic acid levels of synovial fluids without interferences from other glycosaminoglycans or non-steroidal anti-inflammatory drug treatment. 相似文献
265.
Sophie Laurent Franois Botteman LuceVander Elst RobertN. Muller 《Helvetica chimica acta》2004,87(5):1077-1089
In our efforts of finding new specific contrast agents of higher relaxivity and selectivity, we have prepared the two new benzyl‐functionalized DTPA (‘diethylenetriamine pentaacetate’) gadolinium complexes (S)‐ 3 and (R,S)‐ 4 , and compared their properties with those of the known regioisomers (S)‐ 2 and (S)‐ 1 . The theoretical fitting of the reduced transverse relaxation rates of the 17O‐nucleus of H2O gave values for the water‐residence time (τM) of 86–143 ns at 310 K, values that are not limiting the proton relaxivity at body temperature. 1H‐NMRD (nuclear magnetic‐relaxation dispersion) Profiles showed that the relaxivity of 1 – 4 (r1=4.3–5.1 s?1 mM ?1 at 20 MHz and 310 K) is higher than for the Gd? DTPA parent compound 5 . Transmetallation assessment demonstrated that all substituted compounds, except for (S)‐ 2 , are more stable than 5 . The highest stability towards Zn2+‐induced transmetallation was achieved with complexes 3, 1 , and 4 (in decreasing order). Apparently, the steric hindrance of the benzyl substituents in positions 5, 4, and 2, respectively, favorably reduces the accessibility of Zn ions. From a synthetic point of view, 4‐substituted DTPA complexes of type 1 are more readily accessible than 5‐substituted compounds of type 3 . Therefore, the former seem to be superior for linking substituted DTPA complexes to macromolecules or specific vectors. 相似文献
266.
We use the density functional theory and x-ray and neutron diffraction to investigate the crystal structures and reaction mechanisms of intermediate phases likely to be involved in decomposition of the potential hydrogen storage material LiAlH(4). First, we explore the decomposition mechanism of monoclinic LiAlH(4) into monoclinic Li(3)AlH(6) plus face-centered cubic (fcc) Al and hydrogen. We find that this reaction proceeds through a five-step mechanism with an overall activation barrier of 36.9 kcal/mol. The simulated x ray and neutron diffraction patterns from LiAlH(4) and Li(3)AlH(6) agree well with experimental data. On the other hand, the alternative decomposition of LiAlH(4) into LiAlH(2) plus H(2) is predicted to be unstable with respect to that through Li(3)AlH(6). Next, we investigate thermal decomposition of Li(3)AlH(6) into fcc LiH plus Al and hydrogen, occurring through a four-step mechanism with an activation barrier of 17.4 kcal/mol for the rate-limiting step. In the first and second steps, two Li atoms accept two H atoms from AlH(6) to form the stable Li-H-Li-H complex. Then, two sequential H(2) desorption steps are followed, which eventually result in fcc LiH plus fcc Al and hydrogen: Li(3)AlH(6)(monoclinic)-->3 LiH(fcc)+Al(fcc)+3/2 H(2) is endothermic by 15.8 kcal/mol. The dissociation energy of 15.8 kcal/mol per formula unit compares to experimental enthalpies in the range of 9.8-23.9 kcal/mol. Finally, we explore thermal decomposition of LiH, LiH(s)+Al(s)-->LiAl(s)+12H(2)(g) is endothermic by 4.6 kcal/mol. The B32 phase, which we predict as the lowest energy structure for LiAl, shows covalent bond characters in the Al-Al direction. Additionally, we determine that transformation of LiH plus Al into LiAlH is unstable with respect to transformation of LiH through LiAl. 相似文献
267.
A gel retardation assay system for studying protein binding to simple repetitive DNA sequences. 总被引:2,自引:0,他引:2
Simple repetitive DNA sequences have been regarded as mere "junk" present in all eukaryotic genomes. In fact, mixed simple repeat (gt)n(ga)m sequences are present in major histocompatibility complex MHC-DRB genes for long evolutionary times, including such distant animals as artiodactyla and man. We describe herein an unsophisticated method which reveals that at least certain simple repetitive (gt)n(ga)m sequences bind nuclear proteins and show characteristics of a specific DNA-protein interaction via gel retardation. 相似文献
268.
269.
Thermal cyclization of the pyrimidine-N-oxide dicarbamate 2 gives a 95:5 mixture of the 2-oxo-2H-[1,2,4]oxadiazolo[2,3-a]- and [2,3-c]pyrimidinecarbamates 3 and 4. An efficient procedure for the conversion of 3 to 4 , and vice versa, is described. 相似文献
270.
Gautier-Luneau I Merle C Phanon D Lebrun C Biaso F Serratrice G Pierre JL 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(7):2207-2219
Despite the crucial role of "iron(III) citrate systems" in the iron metabolism of living organisms (bacteria as well as plants or mammals), the coordination chemistry of ferric citrate remains poorly defined. Variations in the experimental conditions used for the preparation of so-called ferric citrates (iron salt, Fe:cit molar ratio, base, pH, temperature, solvent) lead to several different species, which are in equilibrium in solution. To date, six different anionic complexes have been structurally characterized in the solid state, by ourselves or others. In the work described herein, we have established the experimental conditions leading to each of them. Five were obtained from aqueous solution. With the exception of a nonanuclear species (of which fragments have been detected), all were identified in aqueous solution on the basis of electrospray ionization mass spectrometry. In addition, the spectra revealed a new trinuclear species, which could not be crystallized. Kinetic studies of iron uptake from citrate species by iron chelators confirmed the results indicated by the ESI-MS studies. These studies also allowed the relative molar fraction of mononuclear versus polynuclear complexes to be determined, which depends on the Fe:cit molar ratio. 相似文献