全文获取类型
收费全文 | 3910篇 |
免费 | 145篇 |
国内免费 | 21篇 |
专业分类
化学 | 2633篇 |
晶体学 | 14篇 |
力学 | 70篇 |
数学 | 597篇 |
物理学 | 762篇 |
出版年
2023年 | 27篇 |
2022年 | 71篇 |
2021年 | 93篇 |
2020年 | 94篇 |
2019年 | 80篇 |
2018年 | 70篇 |
2017年 | 64篇 |
2016年 | 128篇 |
2015年 | 107篇 |
2014年 | 119篇 |
2013年 | 275篇 |
2012年 | 289篇 |
2011年 | 308篇 |
2010年 | 189篇 |
2009年 | 155篇 |
2008年 | 218篇 |
2007年 | 231篇 |
2006年 | 219篇 |
2005年 | 182篇 |
2004年 | 162篇 |
2003年 | 124篇 |
2002年 | 117篇 |
2001年 | 69篇 |
2000年 | 56篇 |
1999年 | 60篇 |
1998年 | 32篇 |
1997年 | 25篇 |
1996年 | 34篇 |
1995年 | 38篇 |
1994年 | 35篇 |
1993年 | 38篇 |
1992年 | 35篇 |
1991年 | 26篇 |
1990年 | 23篇 |
1989年 | 27篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 17篇 |
1985年 | 30篇 |
1984年 | 31篇 |
1983年 | 24篇 |
1982年 | 15篇 |
1981年 | 24篇 |
1980年 | 10篇 |
1979年 | 14篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1975年 | 8篇 |
1974年 | 8篇 |
1970年 | 4篇 |
排序方式: 共有4076条查询结果,搜索用时 11 毫秒
261.
262.
Dr. Ramachandran Azhakar Dr. Rajendra S. Ghadwal Prof. Dr. Herbert W. Roesky Prof. Dr. Ricardo A. Mata Hilke Wolf Regine Herbst‐Irmer Prof. Dr. Dietmar Stalke 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(11):3715-3720
Three‐ and five‐membered rings that bear the (Si‐C‐S ) and (Si‐C‐C‐C‐S ) unit have been synthesized by the reactions of L SiCl ( 1 ; L =PhC(NtBu)2) and L′ Si ( 2 ; L′ =CH{(C?CH2)(CMe)(2,6‐iPr2C6H3N)2}) with the thioketone 4,4′‐bis(dimethylamino)thiobenzophenone. Treatment of 4,4′‐bis(dimethylamino)thiobenzophenone with L SiCl at room temperature furnished the [1+2]‐cycloaddition product silathiacyclopropane 3 . However, reaction of 4,4′‐bis(dimethylamino)thiobenzophenone with L′ Si at low temperature afforded a [1+4]‐cycloaddition to yield the five‐membered ring product 4 . Compounds 3 and 4 were characterized by NMR spectroscopy, EIMS, and elemental analysis. The molecular structures of 3 and 4 were unambiguously established by single‐crystal X‐ray structural analysis. The room‐temperature reaction of 4,4′‐bis(dimethylamino)thiobenzophenone with L′ Si resulted in products 4 and 5 , in which 4 is the dearomatized product and 5 is formed under the 1,3‐migration of a hydrogen atom from the aromatic phenyl ring to the carbon atom of the C? S unit. Furthermore, the optimized structures of probable products were investigated by using DFT calculations. 相似文献
263.
Eleonora Freire Sergio Baggio Juan Carlos Muoz Ricardo Baggio 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(8):m299-m301
The structure of the title compound, [Ni(ths)(bbip)(dmf)]·H2O [ths is thiosulfate, S2O3; bbip is 2,6‐bis(1H‐benzimidazol‐2‐yl)pyridine, C21H13N5; and dmf is dimethylformamide, C3H7NO], is monomeric, with the nickel ion octahedrally surrounded by an N,N′,N′′‐tridentate bbip molecule, an S,O‐bidentate ths molecule and an O‐monodentate dmf molecule. The H atoms of the hydration water molecule and the amino groups of bbip are involved in hydrogen bonding and determine a spatial organization of broad layers parallel to (001), which are connected by weak interactions. 相似文献
264.
Toribio L del Nozal MJ Bernal JL Alonso C Jiménez JJ 《Journal of chromatography. A》2007,1144(2):255-261
The chiral resolution of four antifungal compounds, three imidazoles (miconazole, econazole and sulconazole) and one triazole (itraconazole) using supercritical fluid chromatography on the amylose-based chiral stationary phase Chiralpak AD, is presented in this work. The influence of pressure, type and percentage of organic modifier and temperature on retention times and resolution was studied. The enantiomeric separation of the three imidazoles was achieved with resolutions higher than two and analysis times lower than 10 min, obtaining the best results using methanol as modifier. However, the analysis time of the triazole was higher than 80 min due to the existence of a high number of functional groups that were able to interact with the chiral stationary phase. In this case, the resolution of the four stereoisomers was achieved only partially with mixtures of ethanol and 2-propanol as modifier. The isoenantioselective temperatures were obtained from the study of the influence of the temperature, they were above the range of temperatures assayed, except for sulconazole using 2-propanol. 相似文献
265.
Sergio Caucao Gabriel N. Gatica Ricardo Oyarzúa 《Numerical Methods for Partial Differential Equations》2019,35(1):295-324
In this article, we consider an augmented fully mixed variational formulation that has been recently proposed for the nonisothermal Oldroyd–Stokes problem, and develop an a posteriori error analysis for the 2‐D and 3‐D versions of the associated mixed finite element scheme. More precisely, we derive two reliable and efficient residual‐based a posteriori error estimators for this problem on arbitrary (convex or nonconvex) polygonal and polyhedral regions. The reliability of the proposed estimators draws mainly upon the uniform ellipticity of the bilinear forms of the continuous formulation, suitable assumptions on the domain and the data, stable Helmholtz decompositions, and the local approximation properties of the Clément and Raviart–Thomas operators. On the other hand, inverse inequalities, the localization technique based on bubble functions, and known results from previous works are the main tools yielding the efficiency estimate. Finally, several numerical results confirming the properties of the a posteriori error estimators and illustrating the performance of the associated adaptive algorithms are reported. 相似文献
266.
Amovilli and March (2006) [8] used diffusion quantum Monte Carlo techniques to calculate the non-relativistic ionization potential I(Z) in He-like atomic ions for the range of (fractional) nuclear charges Z lying between the known critical value Zc=0.911 at which I(Z) tends to zero and Z=2. They showed that it is possible to fit I(Z) to a simple quadratic expression. Following that idea, we present here a semiempirical fine-tuning of Hartree–Fock ionization potentials for the isoelectronic series of He, Be, Ne, Mg and Ar-like atomic ions that leads to excellent estimations of Zc for these series. The empirical information involved is experimental ionization and electron affinity data. It is clearly demonstrated that Hartree–Fock theory provides an excellent starting point for determining I(Z) for these series. 相似文献
267.
Aline Lima de Oliveira Carlos Henrique Alves de Oliveira Laura Maia Mairink Francine Pazini Ricardo Menegatti Luciano Morais Lião 《Magnetic resonance in chemistry : MRC》2011,49(8):537-542
Complete assignment of 1H and 13C NMR chemical shifts and J(1H/1H and 1H/19F) coupling constants for 22 1‐phenyl‐1H‐pyrazoles' derivates were performed using the concerted application of 1H 1D and 1H, 13C 2D gs‐HSQC and gs‐HMBC experiments. All 1‐phenyl‐1H‐pyrazoles' derivatives were synthesized as described by Finar and co‐workers. The formylated 1‐phenyl‐1H‐pyrazoles' derivatives were performed under Duff's conditions. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
268.
269.
Rosa Maria Alonso‐Salces Claude Guillou Luis A. Berrueta 《Rapid communications in mass spectrometry : RCM》2009,23(3):363-383
Liquid chromatography coupled with a photodiode array detector, electrospray ionization, collision‐induced dissociation and tandem mass spectrometry (LC‐DAD/ESI‐CID‐MS/MS) on a triple quadrupole (QqQ) has been used to detect and characterize polyphenols and methylxanthines in green coffee beans: three phenolic acids (caffeic acid, ferulic acid and dimethoxycinnamic acid), three isomeric caffeoylquinic acids (Mr 354), three feruloylquinic acids (Mr 368), one p‐coumaroylquinic acid (Mr 338), three dicaffeoylquinic acids (Mr 516), three feruloyl‐caffeoylquinic acids (Mr 530), four p‐coumaroyl‐caffeoylquinic acids (Mr 500), three diferuloylquinic acids (Mr 544), six dimethoxycinnamoyl‐caffeoylquinic acids (Mr 544), three dimethoxycinnamoyl‐feruloylquinic acids (Mr 558), six cinnamoyl‐amino acid conjugates, three cinnamoyl glycosides, and three methylxanthines (caffeine, theobromine and theophylline). Dimethoxycinnamic acid, three isomers of dimethoxycinnamoyl‐caffeoylquinic acids and another three of dimethoxycinnamoyl‐feruloylquinic acids, as well as the three cinnamoyl glycosides, had not previously been reported in coffee beans. Structures have been assigned on the basis of the complementary information obtained from UV‐visible spectra, relative hydrophobicity, scan mode MS spectra, and fragmentation patterns in MS2 spectra (both in the positive and negative ion modes) obtained using a QqQ at different collision energies. A structure diagnosis scheme is provided for the identification of different isomers of polyphenols and methylxanthines. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
270.