全文获取类型
收费全文 | 672篇 |
免费 | 8篇 |
国内免费 | 1篇 |
专业分类
化学 | 398篇 |
晶体学 | 1篇 |
力学 | 10篇 |
数学 | 52篇 |
物理学 | 220篇 |
出版年
2020年 | 5篇 |
2019年 | 5篇 |
2017年 | 8篇 |
2016年 | 7篇 |
2014年 | 6篇 |
2013年 | 16篇 |
2012年 | 22篇 |
2011年 | 25篇 |
2010年 | 13篇 |
2009年 | 5篇 |
2008年 | 13篇 |
2007年 | 16篇 |
2006年 | 25篇 |
2005年 | 19篇 |
2004年 | 21篇 |
2003年 | 12篇 |
2002年 | 21篇 |
2001年 | 21篇 |
2000年 | 17篇 |
1999年 | 17篇 |
1998年 | 9篇 |
1997年 | 8篇 |
1996年 | 20篇 |
1995年 | 10篇 |
1994年 | 10篇 |
1993年 | 15篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 9篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 13篇 |
1985年 | 15篇 |
1984年 | 17篇 |
1983年 | 8篇 |
1982年 | 13篇 |
1981年 | 14篇 |
1980年 | 13篇 |
1979年 | 15篇 |
1978年 | 20篇 |
1977年 | 19篇 |
1976年 | 17篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1973年 | 15篇 |
1972年 | 6篇 |
1969年 | 5篇 |
1968年 | 5篇 |
1894年 | 5篇 |
排序方式: 共有681条查询结果,搜索用时 15 毫秒
41.
Alan R. Katritzky Kenneth A. Heck Jianqing Li Adam Wells Catherine Garot 《合成通讯》2013,43(14):2657-2670
α-Lithiated 1-(1-alkenyl)benzotriazoles, generated from the reactions of 1-(1-alkenyl)benzotriazoles with n-BuLi, react with a variety of electrophiles to afford α-substituted 1-(1-alkenyl)benzotriazoles which undergo epoxidation with m-CPBA followed by hydrolysis to give α-hydroxy ketones in good yields. Thus, 1-(1-alkenyl)benzotriazoles behave as α-hydroxyacyl anion equivalents. 相似文献
42.
Dr. Amy C. Topley Valerio Isoni Dr. Thomas A. Logothetis Dr. Duncan Wynn Dr. Harry Wadsworth Dr. Alex M. R. Gibson Imtiaz Khan Dr. Neil J. Wells Prof. Cécile Perrio Prof. Richard C. D. Brown 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(5):1720-1725
A Resin‐linker‐vector (RLV) strategy is described for the radiosynthesis of tracer molecules containing the radionuclide 18F, which releases the labelled vector into solution upon nucleophilic substitution of a polystyrene‐bound arylsulfonate linker with [18F]‐fluoride ion. Three model linker‐vector molecules 7 a – c containing different alkyl spacer groups were assembled in solution from (4‐chlorosulfonylphenyl)alkanoate esters, exploiting a lipase‐catalysed chemoselective carboxylic ester hydrolysis in the presence of the sulfonate ester as a key step. The linker‐vector systems were attached to aminomethyl polystyrene resin through amide bond formation to give RLVs 8 a – c with acetate, butyrate and hexanoate spacers, which were characterised by using magic‐angle spinning (MAS) NMR spectroscopy. On fluoridolysis, the RLVs 8 a , b containing the longer spacers were shown to be more effective in the release of the fluorinated model vector (4‐fluorobutyl)phenylcarbamic acid tert‐butyl ester ( 9 ) in NMR kinetic studies and gave superior radiochemical yields (RCY≈60 %) of the 18F‐labelled vector. The approach was applied to the synthesis of the radiopharmaceutical O‐(2‐[18F]‐fluoroethyl)‐L ‐tyrosine ([18F]‐FET), delivering protected [18F]‐FET in >90 % RCY. Acid deprotection gave [18F]‐FET in an overall RCY of 41 % from the RLV. 相似文献
43.
Evan Xuguang Chen Michael Gehm Ryan Danell Mitch Wells Jeffrey T. Glass David Brady 《Journal of the American Society for Mass Spectrometry》2014,25(7):1295-1304
Conventionally, quadrupole ion trap mass spectrometers eject ions of different mass-to-charge ratio (m/z) in a sequential fashion by performing a scan of the rf trapping voltage amplitude. Due to the inherent sparsity of most mass spectra, the detector measures no signal for much of the scan time. By exploiting this sparsity property, we propose a new compressive and multiplexed mass analysis approach—multi Resonant Frequency Excitation (mRFE) ejection. This new approach divides the mass spectrum into several mass subranges and detects all the subrange spectra in parallel for increased mass analysis speed. Mathematical estimation of standard mass spectrum is demonstrated while statistical classification on the parallel measurements remains viable because of the sparse nature of the mass spectra. This method can reduce mass analysis time by a factor of 3–6 and increase system duty cycle by 2×. The combination of reduced analysis time and accurate compound classification is demonstrated in a commercial quadrupole ion trap (QIT) system. Figure
? 相似文献
44.
This paper presents a time-domain technique for identifying modal parameters of test specimens based on the log-decrement method. For lightly damped multidegree-of-freedom or continuous systems, the conventional method is usually restricted to identification of fundamental-mode parameters only. Implementation of band-pass filters makes it possible for the proposed technique to extract modal information of higher modes. The method has been applied to a polymethyl methacrylate (PMMA) beam for complex modulus identification in the frequency range 10-1100 Hz. Results compare well with those obtained using the Least Squares method, and with those previously published in literature. Then the accuracy of the proposed method has been further verified by experiments performed on a QuietSteel specimen with very low damping. The method is simple and fast. It can be used for a quick estimation of the modal parameters, or as a complementary approach for validation purposes. 相似文献
45.
Christian Fabiansen Charles W. Yaméogo Sarita Devi Henrik Friis Anura Kurpad Jonathan C. Wells 《Isotopes in environmental and health studies》2017,53(4):344-355
Childhood malnutrition is highly prevalent and associated with high mortality risk. In observational and interventional studies among malnourished children, body composition is increasingly recognised as a key outcome. The deuterium dilution technique has generated high-quality data on body composition in studies of infants and young children in several settings, but its feasibility and accuracy in children suffering from moderate acute malnutrition requires further study. Prior to a large nutritional intervention trial among children with moderate acute malnutrition, we conducted pilot work to develop and adapt the deuterium dilution technique. We refined procedures for administration of isotope doses and collection of saliva. Furthermore, we established that equilibration time in local context is 3?h. These findings and the resulting standard operating procedures are important to improve data quality when using the deuterium dilution technique in malnutrition studies in field conditions, and may encourage a wider use of isotope techniques. 相似文献
46.
The 1:1 mole ratio reaction of t-Bu3In with Sb(SiMe3)3 results in the formation of the Lewis acid-base adduct t-Bu3In Sb(SiMe3)3 (1), while the dimeric compound [t-Bu2InSb(SiMe3)2]2 (2) is isolated from the 1:1 reaction of t-Bu2InCl with Sb(SiMe3)3. Both the 2:1 reactions of t-Bu2InCl with Sb(SiMe3)3 and t-Bu2InCl with 2 result in the formation of the mixed-bridge compound
(3), however, 1H NMR studies suggest that this compound is unstable in solution. Thermolysis of 1, 2, and t-Bu3Ga Sb(SiMe3)3 (4) results in the formation of nanocrystalline InSb or GaSb through a -hydride elimination pathway. 相似文献
47.
We enumerate, up to isomorphism, several classes of labeled vertex-transitive digraphs with a prime number of vertices. 相似文献
48.
Ph. Gavillet C. Dionisi A. Gurtu R.J. Hemingway M.J. Losty J.C. Marin M. Mazzucato L. Montanet E. Pagiola R. Blokzijl B. Jongejans J.C. Kluyver G.G.G. Massaro J.J. Engelen J.S.M. Vergeest M. Zralek B. Foster P. Grossmann J. Wells 《Physics letters. [Part B]》1978,78(1):158-161
The backward production of the B(1235) meson is studied in the reaction K?p→Σ?π+ω. This reaction is observed in the final state Σ?π+π+π?π0. A π+ω mass enhancement is visible in the region of the B meson for events with small |u|K? →Σ?) squared four-momentum transfer. The properties of the enhancement agree with those of the B meson. The cross section for K?p→Σ?B+ at incident K? momentum is (3.2 ± 0.5) μb. The backward production of the B meson is compared with similar baryon exchange productions of the A1 and C(Q1) axial vector mesons observed in the same experiment. 相似文献
49.
A facile method for the preparation of the aromatic pyrrolo[1,5-b:2,3-c']dipyridazine ring system is reported. A number of compounds have been prepared and the biological screening data reported. 相似文献
50.