全文获取类型
收费全文 | 710篇 |
免费 | 32篇 |
国内免费 | 7篇 |
专业分类
化学 | 583篇 |
晶体学 | 4篇 |
力学 | 7篇 |
数学 | 19篇 |
物理学 | 136篇 |
出版年
2023年 | 8篇 |
2022年 | 15篇 |
2021年 | 19篇 |
2020年 | 33篇 |
2019年 | 27篇 |
2018年 | 18篇 |
2017年 | 30篇 |
2016年 | 19篇 |
2015年 | 35篇 |
2014年 | 22篇 |
2013年 | 57篇 |
2012年 | 57篇 |
2011年 | 55篇 |
2010年 | 49篇 |
2009年 | 38篇 |
2008年 | 47篇 |
2007年 | 35篇 |
2006年 | 29篇 |
2005年 | 20篇 |
2004年 | 22篇 |
2003年 | 10篇 |
2002年 | 11篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 8篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1992年 | 3篇 |
1991年 | 5篇 |
1990年 | 8篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 7篇 |
1983年 | 1篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1977年 | 5篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1972年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1947年 | 1篇 |
排序方式: 共有749条查询结果,搜索用时 156 毫秒
61.
Pd-C/ammonium formate efficiently and selectively reduces hetero-aromatic furan ring to the corresponding tetrahydrofuran moiety. Under this reaction condition, carbon-carbon double bond and α,β-unsaturated ketones also reduced to the corresponding alkanes and saturated ketones. 相似文献
62.
63.
R. Nageswara Rao R. Mastan Vali Dhananjay D. Shinde 《Biomedical chromatography : BMC》2009,23(11):1145-1150
A highly sensitive and selective on‐line two‐dimensional reversed‐phase liquid chromatography/electrospray ionization–tandem mass spectrometry (2D‐LC‐ESI/MS/MS) method was developed and validated to determine rifaximin in rat serum by direct injection. The 2D‐LC‐ESI/MS/MS system consisted of a restricted access media column for trapping proteins as the first dimension and a Waters C18 column as second dimension using 0.1% aqueous acetic acid:acetonitrile as mobile phase in a gradient elution mode. Rifampacin was used as an internal standard. The linear dynamic range was 0.5–10 ng/mL (r2 > 0.998). Acceptable precision and accuracy were obtained over the calibration range. The assay was successfully used in analysis of rat serum to support pharmacokinetic studies. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
64.
Jaiprakash N. Sangshetti Nagnnath D. Kokare Sandeep A. Kotharkar Devanand B. Shinde 《Monatshefte für Chemie / Chemical Monthly》2008,139(2):125-127
Summary. 2,4,5-Triaryl-1H-imidazoles were efficiently synthesized from benzoin or benzil, ammonium acetate, and aromatic or heteroaromatic aldehydes
in the presence of sodium bisulfite as catalyst. The present protocol offers significant improvements for the synthesis of
2,4,5-triaryl-1H-imidazoles with regard to yield of products, simplicity in operation, and cheap catalyst. 相似文献
65.
We have developed simple, greener, safer multicomponent synthesis series of 4-arylidene-2-phenyl-5(4H) oxazolones 4(a-r) catalyzed by Bronsted acid ionic liquid as triethylammonium hydrogen sulfate [Et3NH][HSO4] and catalytic amount of acetic anhydride and sodium acetate with excellent yields (90–99%). The protocol offers economical, environmentally benign, solvent-free conditions, and recycle–reuse of the catalyst and easily available starting as benzoyl chloride 1, amino acid 2 and a variety of aldehydes 3. The cyclization followed by condensation of benzoyl chloride, amino acid, and a variety of aldehydes catalyzed by ILs [Et3NH][HSO4] and catalytic amount of acetic anhydride and sodium acetate. The final products were confirmed by their characterization data such as FTIR, 1H-NMR, 13C-NMR, Mass, high-resolution mass spectra and were compared with its reported method. 相似文献
66.
Vikas Kumar Mehak Gupta Sumit G. Gandhi Sonali S. Bharate Ajay Kumar Ram A. Vishwakarma Sandip B. Bharate 《Tetrahedron letters》2017,58(42):3974-3978
Herein we report isolation of a new chromone alkaloid chrotacumine K (12) from fruits and a chromone glycoside schumaniofioside A (13) from leaves of Dysoxylum binectariferum Hook f. Schumaniofioside A is reported for the first time from Meliaceae family. Other known alkaloids isolated include rohitukine (1) and chrotacumine E (6). The structure of new alkaloid 12 was elucidated on the basis of extensive 1D and 2D NMR analysis, synthesis and chemical hydrolysis. Chemically, chrotacumine K (12) is a 3′-O-acetyl rohitukine which on chemical or enzymatic hydrolysis produces rohitukine. The new alkaloid 12 is also present in seeds and stem-barks of this plant. The glycoside schumaniofioside A (13) is present only in leaves, and in abundance (~1% w/w of dried leaves). The isolated compounds and extracts were evaluated for in vitro effect on the proinflammatory cytokines (TNF-α and IL-6) in human monocytic THP-1 cells. The alkaloid 12 displayed potent inhibition (57%) of TNF-α at 0.3 µM, and was non-toxic to THP-1 cells up to 40 µM, indicating its excellent therapeutic window. Furthermore, a nitrobenzoyl ester analog 15e showed better inhibition of IL-6 than parent natural product chrotacumine K. 相似文献
67.
Chemical oscillation is an interesting nonlinear dynamical phenomenon which arises due to complex stability condition of the steady state of a reaction far away from equilibrium which is usually characterised by a periodic attractor or a limit cycle around an interior stationary point. In this context Lienard equation is specifically used in the study of nonlinear dynamical properties of an open system which can be utilized to obtain the condition of limit cycle. In conjunction with the property of limit cycle oscillation, here we have shown the condition for isochronicity for different chemical oscillators with the help of renormalisation group method with multiple time scale analysis from a Lienard system. When two variable open system of equations are transformed into a Lienard system of equation the condition for limit cycle and isochronicity can be stated in a unified way. For any such nonlinear oscillator we have shown the route of a dynamical transformation of a limit cycle oscillation to a periodic orbit of centre type depending on the parameters of the system. 相似文献
68.
Four new neutral copper-azido polymers [Cu(6)(N(3))(12)(aem)(2)](n)(1), [Cu(6)(N(3))(12)(dmeen)(2)(H(2)O)(2)](n) (2), [Cu(6)(N(3))(12)(N,N'-dmen)(2)](n) (3), and [Cu(6)(N(3))(12)(hmpz)(2)](n) (4) [aem = 4-(2-aminoethyl)morpholine; dmeen = N,N-dimethyl-N'-ethylethylenediamine; N,N'-dmen = N,N'-dimethylethylenediamine and hmpz = homopiperazine] have been synthesized by using 0.33 mol equiv of the chelating diamine ligands with Cu(NO(3))(2)·3H(2)O/CuCl(2)·2H(2)O and an excess of NaN(3). Single crystal X-ray structures show that the basic unit of these complexes, especially 1-3, contains very similar Cu(II)(6) building blocks. But the overall structures of these complexes vary widely in dimensionality. While 1 is three-dimensional (3D) in nature, 2 and 3 have a two-dimensional (2D) arrangement (with different connectivity) and 4 has a one-dimensional (1D) structure. Cryomagnetic susceptibility measurements over a wide range of temperature exhibit dominant ferromagnetic behavior in all the four complexes. The experimental susceptibility data have been analyzed by some theoretical model equations. 相似文献
69.
Pravin V. Shinde 《Tetrahedron letters》2010,51(9):1309-6782
A one-pot three-component condensation of an aldehyde, malononitrile, and thiophenol has been achieved by conventional and ultrasound method. The reaction has been catalyzed by boric acid in aqueous medium. This protocol afforded corresponding 2-amino-3,5-dicarbonitrile-6-thio-pyridines in shorter reaction times and high yields with the green aspects by avoiding toxic catalysts and solvents. 相似文献
70.
Sandip R. Sabale D. V. Jadhav B. S. Mohite 《Journal of Radioanalytical and Nuclear Chemistry》2010,283(2):273-278
A simple and efficient column chromatographic method has been developed for the sequential separation of U(VI), Th(IV) and
Ce(III) using poly[dibenzo-18-crown-6] as stationary phase and l-arginine as a counter ion. The different elution patterns with various eluting agents were observed for individual element.
The capacity of poly[dibenzo-18-crown-6] for U(VI), Th(IV) and for Ce(III) was found to be 0.96, 0.86 and 1.49 (±0.01) mmol/g
of crown polymer, respectively. The method is efficient to separate the elements in multicomponent mixtures and has good recovery.
The method is extended to determine the U(VI), Th(IV) and Ce(III) from monazite sand. The method is simple, rapid and selective
having good reproducibility (~±2%). 相似文献