全文获取类型
收费全文 | 32560篇 |
免费 | 5847篇 |
国内免费 | 2509篇 |
专业分类
化学 | 20195篇 |
晶体学 | 550篇 |
力学 | 905篇 |
综合类 | 199篇 |
数学 | 972篇 |
物理学 | 10020篇 |
无线电 | 8075篇 |
出版年
2024年 | 50篇 |
2023年 | 254篇 |
2022年 | 444篇 |
2021年 | 590篇 |
2020年 | 911篇 |
2019年 | 968篇 |
2018年 | 899篇 |
2017年 | 1141篇 |
2016年 | 1399篇 |
2015年 | 1740篇 |
2014年 | 1862篇 |
2013年 | 3143篇 |
2012年 | 2227篇 |
2011年 | 2253篇 |
2010年 | 1850篇 |
2009年 | 1901篇 |
2008年 | 2046篇 |
2007年 | 1970篇 |
2006年 | 1824篇 |
2005年 | 1685篇 |
2004年 | 1581篇 |
2003年 | 1430篇 |
2002年 | 1364篇 |
2001年 | 992篇 |
2000年 | 998篇 |
1999年 | 788篇 |
1998年 | 700篇 |
1997年 | 561篇 |
1996年 | 478篇 |
1995年 | 500篇 |
1994年 | 376篇 |
1993年 | 324篇 |
1992年 | 281篇 |
1991年 | 213篇 |
1990年 | 172篇 |
1989年 | 130篇 |
1988年 | 132篇 |
1987年 | 116篇 |
1986年 | 108篇 |
1985年 | 88篇 |
1984年 | 92篇 |
1983年 | 45篇 |
1982年 | 64篇 |
1981年 | 48篇 |
1980年 | 43篇 |
1979年 | 45篇 |
1978年 | 20篇 |
1977年 | 14篇 |
1976年 | 9篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 687 毫秒
991.
L. M. Babkov I. I. Gnatyuk G. A. Puchkovskaya S. V. Trukhachev 《Journal of Structural Chemistry》2004,45(3):374-381
The conformational lability of 4-ethyl-4-cyanobiphenyl molecules in solid crystal (SC) and isotropic liquid (IL) states was investigated by IR spectroscopic techniques (experiment and theory). IR absorption spectra were measured at 28°C–95°C in the frequency range 400 cm–1–4000 cm–1. Spectrum simulation was performed using the fragment method with allowance for the conformational fluctuations of molecules. The experimental and calculated spectra were compared and analyzed, and it was shown that in the IL, the samples are mixtures of conformers. The temperature changes in the spectra in the stated range are caused by the conformational lability of molecules.Original Russian Text Copyright © 2004 by L. M. Babkov, I. I. Gnatyuk, G. A. Puchkovskaya, and S. V. TrukhachevTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 3, pp. 398–405, May–June, 2004. 相似文献
992.
Biomimetic formation of gramicidin S by dimerization-cyclization of pentapeptide precursor on solid support 总被引:1,自引:0,他引:1
The biomimetic formation of gramicidin S, cyclo(-d-Phe-Pro-Val-Orn-Leu-)2, by the dimerization and cyclization of pentapeptide precursor without the protection of δ-amino group of the Orn residue was examined on a solid support. The cyclization of H-d-Phe-Pro-Val-Orn-Leu-oxime on a resin with an oxime group of 0.62 mmol/g in 1,4-dioxane directly gave gramicidin S in a 50% yield. The dimerization-cyclization mode on the solid support was similar to that of the biosynthesis of gramicidin S on an enzyme. 相似文献
993.
Koga T Taguchi K Kobuke Y Kinoshita T Higuchi M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(5):1146-1156
The self-assembly of peptides and proteins into beta-sheet-rich high-order structures has attracted much attention as a result of the characteristic nanostructure of these assemblies and because of their association with neurodegenerative diseases. Here we report the structural and conformational properties of a peptide-conjugated graft copolymer, poly(gamma-methyl-L-glutamate) grafted polyallylamine (1) in a water-2,2,2-trifluoroethanol solution as a simple model for amyloid formation. Atomic force microscopy revealed that the globular peptide 1 self-assembles into nonbranching fibrils that are about 4 nm in height under certain conditions. These fibrils are rich in beta-sheets and, similar to authentic amyloid fibrils, bind the amyloidophilic dye Congo red. The secondary and quaternary structures of the peptide 1 can be controlled by manipulating the pH, solution composition, and salt concentration; this indicates that the three-dimensional packing arrangement of peptide chains is the key factor for such fibril formation. Furthermore, the addition of carboxylic acid-terminated poly(ethylene glycol), which interacts with both of amino groups of 1 and hydrophobic PMLG chains, was found to obviously inhibit the alpha-to-beta structural transition for non-assembled peptide 1 and to partially cause a beta-to-alpha structural transition against the 1-assembly in the beta-sheet form. These findings demonstrate that the amyloid fibril formation is not restricted to specific protein sequences but rather is a generic property of peptides. The ability to control the assembled structure of the peptide should provide useful information not only for understanding the amyloid fibril formation, but also for developing novel peptide-based material with well-defined nanostructures. 相似文献
994.
Jun Li 《Journal of Cluster Science》2002,13(1):137-163
The electronic structures and bonding of isopoly oxometalates M6O19
2– (M=Cr, Mo, W) have been investigated by using ab initio and relativistic density functional methods. We have discussed the role of the central oxygen atom and the (d-p) conjugation interactions between the metal and bridging oxygen atoms. It is found that there exist 12 three-centered two-electron (d-p-d) bonds for the three M4(-O)4 planar rings in M6O19
2– ions and these hexametalates are considered to have quasi-aromaticity. The (d-p) conjugation effects play essential role in stabilizing these cluster compounds, and the reduced (d-p) conjugation effects account for the instability of the isopoly oxochromate ion, Cr6O19
2–. The vibrational spectra and electronic spectra of M6O19
2– ions are evaluated and assigned theoretically and the calculated spectra are in fairly good agreement with the measured experimental results. 相似文献
995.
Block copolymer self-assembly and supramolecular chemistry can be combined most naturally to prepare smart polymer nanomaterials. An attractive route is based on comb-shaped supramolecules, obtained by attaching side chains to (co)polymers by physical (non-covalent) interactions. Hydrogen bonding is a key element of our approach. It combines an ease of synthesis with other important approach-specific elements, such as hierarchical self-assembly, strongly enhanced processability, swelling, and cleaving. Functional properties discussed include anisotropic proton conductivity, switching proton conductivity, electronically conducting nanowires, polarized luminance, dielectric stacks (optical reflectivity), functional membranes, and nano objects. 相似文献
996.
It is difficult to quantify NH4+ by ion chromatography in the presence of high concentrations of Na+ due to peak overlap. The Dionex IonPac CS15 column, which contains phosphonate, carboxylate, and 18-crown-6 functional groups, was originally developed to overcome this problem. We have found that the addition of 18-crown-6 to the eluent promotes improved peak resolution between Na+ and NH4+ even at concentrations as high as 60,000 to 1 using this column. Its use also improves the separation of alkali and alkaline earth metal and amine cations. Mobile phase 18-crown-6 increased the retention times of CH3NH3+, NH4+, and K+, and decreased the retention time of Sr2+. The retention times of Li+, Na+, Mg2+, Ca2+, (CH3)2NH2+, and (CH3)3NH+ were not affected. This method makes possible the direct analysis of ammonia from nitrogenase, the enzyme responsible for biological nitrogen fixation. The resolution of the NH4+ peak from the Na+ and Mg2+ peaks improved from zero resolution to values of 6.19 and 5.65, respectively. This technique considerably reduces the analysis time of NH4+ in the presence of high concentrations of Mg2+ and Na+ over traditional indophenol measurements. 相似文献
997.
《Journal of separation science》2003,26(11):1028-1034
Reversed phase liquid chromatography with diode array detection (DAD) and electrospray mass spectrometric (ESI MSD) methods were developed for the identification of anthraquinone color components of cochineal, lac dye, and madder – red natural dyestuffs. Electrospray mass spectrometry was found to be more suitable than diode array detection for such analysis because of its higher sensitivity (detection limits in the range 30–90 ng mL–1) and selectivity. The developed method permitted unequivocal identification of carminic acid and laccaic acid A as coloring matters in examined preparations of cochineal and lac dye, respectively. In madder more chemical color species were found: alizarin, purpurin, lucidin, ruberythric acid, and also aluminum and calcium alizarin lake. Among the methods recommended so far, the present one allows fast, direct, and unequivocal identification of components of very complicated natural products used in art. 相似文献
998.
Summary A simple and sensitive method has been developed for the liquid chromatographic determination of short-chain aliphatic amines
in water. Analytes are retained in solid-phase extraction (SPE) cartridges, and then derivatized by drawing an aliquot of
the fluorogeneic reagent 9-fluorenylmethyl chloroformate (FMOC) through the cartridges. After a certain reaction time the
derivatives formed are desorbed with acetonitrile. The collected extracts are then chromatographed on a LiChrospher 100 RP18 125 mm×4 mm i.d., 5 μm, column using an acetonitrile-water gradient. The influence of experimental conditions (SPE material,
volume of sample, concentration of FMOC, time of reaction and pH) has been investigated. Optimal results have been obtained
with C18 SPE cartridges using a sample volume of 5.0 mL. For derivatization, 0.25 mL aliquots of 25 mM FMOC have been used, the reaction
time being only 2 min. The method has been applied to the quantification of several aliphatic amines: methylamine, ethylamine,
dimethylamine,n-butylamine,n-pentylamine andn-hexylamine. Under the proposed conditions the percentages of analytes retained plus derivatized were of about 54–107% compared
to those obtained with direct solution derivatization. The method provided good reproducibility, linearity and accuracy within
the 0.050–1.0 mg L−1 concentration range. The limits of detection were in the 0.25–5.0 μg L−1 range. The utility of the described approach has been tested by analysing tap water, river water and industrial waste water. 相似文献
999.
Michael Sax 《Tetrahedron》2005,61(1):205-211
A novel solid phase synthesis of 2-substituted 2,3-dihydropyridin-4(1H)-ones using Rinkamide-polystyrene-resin is described. The key step involves a hetero-Diels-Alder reaction of Danishefsky's diene with solid phase bound imines, which was carefully optimized. Using this method even ketones are transformed into 2,2-disubstituted dihydropyridones. 相似文献
1000.
Hydrothermal phase equilibria studies have been carried out in the Ln2O3-H2O systems (Ln = La, Pr, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) and the stability fields of the phases Ln(OH)3 LnOOH and Ln2O3-C have been established in the pressure-temperature range of 25000 psi and 900° C. The sequioxides Ln2O3-C are stable only in the last four systems of Er to Lu along with the Ln(OH)3 and LnOOH. The systems from Nd to Ho have only Ln(OH)3 and LnOOH as stable phases and those from La to Pr have only Ln(OH)3 as the stable phase. The unit cell parameters of trihydroxides deviate from the values reported in the literature and this
is attributed to the contamination of CO2 in the starting material. 相似文献