全文获取类型
收费全文 | 3267篇 |
免费 | 51篇 |
国内免费 | 4篇 |
专业分类
化学 | 2286篇 |
晶体学 | 11篇 |
力学 | 50篇 |
数学 | 505篇 |
物理学 | 470篇 |
出版年
2018年 | 29篇 |
2017年 | 29篇 |
2016年 | 38篇 |
2015年 | 35篇 |
2014年 | 33篇 |
2013年 | 89篇 |
2012年 | 80篇 |
2011年 | 111篇 |
2010年 | 80篇 |
2009年 | 81篇 |
2008年 | 104篇 |
2007年 | 121篇 |
2006年 | 102篇 |
2005年 | 113篇 |
2004年 | 110篇 |
2003年 | 91篇 |
2002年 | 96篇 |
2001年 | 58篇 |
2000年 | 78篇 |
1999年 | 70篇 |
1998年 | 50篇 |
1997年 | 60篇 |
1996年 | 53篇 |
1995年 | 54篇 |
1994年 | 44篇 |
1993年 | 65篇 |
1992年 | 55篇 |
1991年 | 44篇 |
1990年 | 31篇 |
1989年 | 35篇 |
1988年 | 44篇 |
1987年 | 38篇 |
1986年 | 54篇 |
1985年 | 65篇 |
1984年 | 43篇 |
1983年 | 34篇 |
1982年 | 41篇 |
1981年 | 53篇 |
1980年 | 40篇 |
1979年 | 56篇 |
1978年 | 44篇 |
1977年 | 45篇 |
1976年 | 40篇 |
1975年 | 35篇 |
1974年 | 37篇 |
1973年 | 23篇 |
1972年 | 25篇 |
1971年 | 26篇 |
1970年 | 30篇 |
1934年 | 21篇 |
排序方式: 共有3322条查询结果,搜索用时 31 毫秒
51.
Nonracemic sec-alcohols of opposite absolute configuration were obtained either by asymmetric reduction of the corresponding ketone using 2-propanol as hydrogen donor or by enantioselective oxidation through kinetic resolution of the rac-alcohol using acetone as hydrogen acceptor employing whole lyophilized cells of Rhodococcus ruber DSM 44541. The microbial oxidation/reduction system exhibits not only excellent stereo- and enantioselectivity but also a broad substrate spectrum. Due to the exceptional tolerance of the biocatalyst toward elevated concentrations of organic materials (solvents, substrates and cosubstrates), the process is highly efficient. The simple preparation of the biocatalyst and its ease of handling turns this system into a versatile tool for organic synthesis. 相似文献
52.
We have used the oscillating peroxidase-oxidase (PO) reaction as a model system to study how oscillatory dynamics may affect the influence of toxic reaction intermediates on enzyme stability. In the peroxidase-oxidase reaction reactive intermediates, such as hydrogen peroxide, superoxide, and hydroxyl radical are formed. Such intermediates inactivate many cellular macromolecules such as proteins and nucleic acids. These reaction intermediates also react with peroxidase itself to form an inactive enzyme. The fact that the PO reaction shows bistability between an oscillatory and a steady state gives us a unique possibility to compare such inactivation when the system is in one of these two states. We show that inactivation of peroxidase is slower when the system is in an oscillatory state, and using numerical simulations we provide evidence that oscillatory dynamics lower the average concentration of the reactive intermediates. 相似文献
53.
Mathematische Semesterberichte - Nicolaus Cusanus’ Beiträge zur Mathematik des 15. Jahrhunderts betreffen hauptsächlich Kreisquadraturen. Wir besprechen seinen... 相似文献
54.
55.
The $t\bar t$ system allows a truly perturbative treatment of the potential. Completing previous computations, we calculate the contributions of QCD box graph corrections, which make the “relativistic”O(α 4) corrections in the non-Abelian case differ from the well known corrections of the same order in QED. 相似文献
56.
Eric Berson R. Mane Trupti V. Kurt Svihla C. Hanley Thomas R. 《Applied biochemistry and biotechnology》1998,(1):615-627
Variations to the original aeration system in a continuous roller bottle reactor of novel design have been tested and compared
for optimal oxygen (O) delivery. Reactor operating parameters that affect O transfer are rotation rate, liquid-volume level,
fresh-feed rate, and supplementary-aeration rate. Design modifications to enhance gas-liquid O transfer include the addition
of wall baffles and center baffles. The number and location of each of these baffles are compared for their effect on kLa values in the reaction chamber. The liquid feed into the system has been modified to improve the axial liquid mixing and
O transfer. 相似文献
57.
The crystallochemistry of and the bonding in the orthorhombic four-connected nets of BaIn(2) (CeCu(2) structure) and of CaPtSn (TiNiSi structure, a derivative of the CeCu(2) structure) are analyzed with approximate molecular orbital calculations. Following the Zintl concept, in BaIn(2) the In(-) ions are isoelectronic with group IV tin and should adopt a four-connected structure. In contrast to alpha-tin, which has a cubic diamond structure, the indium ions in BaIn(2) build up an orthorhombic three-dimensional four-connected net containing distorted tetrahedra and ladder polymers of four-membered rings. In the CeCu(2) structure (space group Imma) two bond angles in these distorted tetrahedra are fixed at 90 degrees. The four-connected net in the CeCu(2) structure is topologically related to the layers in black phosphorus (space group Cmca). In CaPtSn (TiNiSi structure) the orthorhombic four-connected net is formed by (PtSn)(2)(-) ions in an ordered arrangement. Calculations on BaIn(2) and CaPtSn show that the four-connected nets are increasingly stabilized as the valence electron count is increased from 16 to 30 valence electrons per 4 formula units. For more than 30e, the nets are destabilized due to filling of M-E antibonding states. Structural data obtained by precise single crystal investigations for the TiNiSi series CaPdIn (20e), CaPdSn (24e), CaPdSb (28e), and CaAgSb (32e), confirm the results of the extended Hückel calculations. We find an interesting and understandable angular asymmetry of the tetrahedral sites in these ternary compounds. 相似文献
58.
Kurt Günther Reinhold Carle Ilona Fleischhauer Stefan Merget 《Fresenius' Journal of Analytical Chemistry》1993,345(12):787-790
Summary A semi-preparative low-pressure liquid-chromatographic procedure using tribenzoylcellulose has been developed for the separation and isolation of the four -bisabolol stereoisomers. Stereochemical assignments were made by HPLC-polarimeter coupling and by using the authentical isomers. This method can be applied to the quality control of chamomile preparations, as it allows the detection of adulterations resulting from admixtures of synthetic -bisabolol. In addition, the isolation of all four -bisabolol stereoisomers from commercially available synthetic -bisabolol has been made possible. 相似文献
59.
C(alpha)-Formylglycine, the catalytic amino acid residue in the active site of sulfatases, is generated by post-translational modification of a cysteine or serine residue. We describe a highly sensitive procedure for the detection of C(alpha)-formylglycine-containing peptides in tryptic digests of sulfatase proteins. The protocol is based on the formation of hydrazone derivatives of C(alpha)-formylglycine-containing peptides when using dinitrophenylhydrazine as a matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The hydrazone derivatives desorb and ionize with high efficiency and can be detected in the sub-femtomole range. The presence of C(alpha)-formylglycine is indicated by a mass increment of 180.13 u, corresponding to the hydrazone moiety, and also by a unique C-terminal fragment ion, characteristic of sulfatases, that becomes prominent in MALDI post-source decay mass spectra of the hydrazone derivatives. 相似文献
60.
The literature data on heteropolar and homopolar 2-center bond dissociation energies in organic molecules in the gas phase and the corresponding heats of formation of radicals and ions have been critically evaluated. Data for more than 500 bonds are represented in tabular form together with the pertinent literature references. Selected electron affinities and π-bond dissociation energies have also been incorporated. The follow-up paper will discuss some empirical general aspects of these data particularly regarding the effect of structure on the bond dissociation energies. 相似文献