全文获取类型
收费全文 | 39950篇 |
免费 | 3052篇 |
国内免费 | 4345篇 |
专业分类
化学 | 28213篇 |
晶体学 | 293篇 |
力学 | 1129篇 |
综合类 | 724篇 |
数学 | 6862篇 |
物理学 | 10126篇 |
出版年
2023年 | 363篇 |
2022年 | 616篇 |
2021年 | 1655篇 |
2020年 | 1031篇 |
2019年 | 1046篇 |
2018年 | 751篇 |
2017年 | 831篇 |
2016年 | 1124篇 |
2015年 | 1149篇 |
2014年 | 1387篇 |
2013年 | 2514篇 |
2012年 | 1667篇 |
2011年 | 1867篇 |
2010年 | 1749篇 |
2009年 | 2282篇 |
2008年 | 2402篇 |
2007年 | 2512篇 |
2006年 | 2024篇 |
2005年 | 1398篇 |
2004年 | 1355篇 |
2003年 | 1350篇 |
2002年 | 3759篇 |
2001年 | 1465篇 |
2000年 | 1038篇 |
1999年 | 817篇 |
1998年 | 802篇 |
1997年 | 517篇 |
1996年 | 577篇 |
1995年 | 527篇 |
1994年 | 526篇 |
1993年 | 533篇 |
1992年 | 527篇 |
1991年 | 324篇 |
1990年 | 277篇 |
1989年 | 224篇 |
1988年 | 259篇 |
1987年 | 208篇 |
1986年 | 209篇 |
1985年 | 325篇 |
1984年 | 240篇 |
1983年 | 151篇 |
1982年 | 296篇 |
1981年 | 466篇 |
1980年 | 424篇 |
1979年 | 466篇 |
1978年 | 369篇 |
1977年 | 279篇 |
1976年 | 237篇 |
1974年 | 75篇 |
1973年 | 150篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
Isolation of lipase producer and its performance in enantioselective hydrolysis of glycidyl butyrate
A racemic glycidyl butyrate resolving strain, preliminarily identified as a Rhizopus sp., had been isolated from soil. Its extracellular lipase was found to enantioselectively hydrolyze the (S)-enantiomer of
the chiral ester, with optimal activities at pH 5.3 and 42°C. Higher en antioselectivity of theenzyme was observed at lower
temperatures, while the best anantioselectivity was obtained at pH 5.5–6.0, with an, E value (enantiomeric ratio) of 57. 相似文献
962.
The essential features (geometries of the minima and of the saddle points, energy barriers) of the potential energy surface of the four hydrides YXH4 mentioned in the title have been determined with two basis sets, of minimal and DZ quality respectively. The importance of the different extent of the deformation of the XH4 group in the different structures of the four hydrides is brought out and discussed. The aspects of charge distribution and bonding are examined drawing on population analysis, comparison of the electrostatic molecular potentials and decomposition of the interaction energy (this last referred to the Y+ + XH
4
–
YXH4 process). The capability of XH3 in effecting the etherolytic disruption of the Y-H bond is finally brought out. 相似文献
963.
H. Reul 《Fresenius' Journal of Analytical Chemistry》1977,285(2):126
964.
The selectivity of a column system, S, defined by equation 28, includes a variation coefficient (β) of plate number with capacity ratio which has a large influence on the peak capacity, as shown in Fig. 18. Some typical chromatograms are given. In order to predict S for a column system from Kovat's Index, equations 40 and 41 are given to calculate the constant of the carbon number rule for squalane at different temperatures or for different stationary phases. The specific retention value of heptane on squalane at different temperatures can be calculated from equation 42. The nonpolarity index, defined by equation 43, was used to calculate the retention value of heptane on various stationary phases. In liquid chromatography, the order of elution may be reversed by changing the composition of the eluent on the same chemically bonded silica (manufactured in China). The linear relations between the log retention values of different kinds of solutes or of a single solute on the silicas of different surface areas when using the same eluent are given. 相似文献
965.
Biflavonoids, widely distributed in natural plants, had strong biological activities including spasmolysis, peripheral vasodilatation, antibradykinin activity and antispasmogenic action against prostaglandin PGE1, inhibition of cyclic GMP and cyclic AMP phosphodiesterase and inhibition of hepatoma cells. Recently, some biflavonoids were demonstrated to enhance suppersion of lymphocyte proliferation, inhibition of phospholipaseCr1, anti-inflammatory activity, anti-HIV activity, anticompleme… 相似文献
966.
J.B Goodenough J.-S Zhou F Rivadulla E Winkler 《Journal of solid state chemistry》2003,175(1):116-123
Bond-length fluctuations in transition-metal oxoperovskites may give rise to two-phase fluctuations in what appears to be a single phase to a diffraction experiment. Orbital disorder at Jahn-Teller ions results in bond-length fluctuations that give 3D-ferromagnetic, vibronic Mn(III)-O-Mn(III) superexchange interactions and allow disproportionation into Mn(IV) and Mn(II) in LaMnO3; where orbitally ordered and disordered phases coexist, an external magnetic field stabilizes the orbitally disordered, ferromagnetic phase relative to the orbitally ordered, antiferromagnetic phase. Spin-lattice interactions in the paramagnetic phase of charge-transfer compounds give bond-length fluctuations arising from the semicovalent component of the superexchange interactions. At the crossover from localized to itinerant electronic behavior, the coexistence of two-phase fluctuations has been demonstrated in both the single-valent RNiO3 family (R=rare-earth, A=alkaline-earth) and the mixed-valent R0.5A0.5MnO3 perovskites. “Bad-metal” behavior is found to be associated with bond-length fluctuations. 相似文献
967.
Yuri N. Belokon’Devayani Bhave Daniela D'AddarioElizabetta Groaz Viktor MaleevMichael North Armine Pertrosyan 《Tetrahedron letters》2003,44(10):2045-2048
Copper(salen) complex 1 has been found to catalyse the asymmetric alkylation of enolates derived from a variety of amino acids. There is a clear relationship between the size of the side chain in the substrate and the enantioselectivity of the process, so that the enantioselectivity decreases in the order alanine>aminobutyric acid>allylglycine>leucine>phenylalanine>valine. A transition state model which accounts for the influence of the size of the side chain on the enantioselectivity of the reactions is presented. 相似文献
968.
A. M. Demchenko V. A. Yanchenko V. V. Kisly M. S. Lozinskii 《Chemistry of Heterocyclic Compounds》2005,41(5):668-672
We have developed a method for synthesis of aryl isothiocyanates by means of thiocarbamoylation of aromatic amines by tetramethylthiuram disulfide followed by degradation of the intermediate N(1)-aryl-N,N-dimethylthiourea by concentrated HCl. We have shown that thiocarbamoylation of 4-amino-5-ethyl-4H-1,2,4-triazole-3-thiol occurs at the 2 position of the triazole ring, while thiocarbamoylation of 4-amino-3-methyl-6-phenyl-4,5-dihydro-1,2,4-triazin-5-one leads to the dihetaryl-substituted thiourea. We consider the possibility of using N(1)-aryl-N,N-dimethylthioureas as analogs of isothiocyanates in reactions with N-nucleophiles.__________Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 775–780, May, 2005. 相似文献
969.
Fandong Meng Genhui Xu Baowei Wang Xinbin Ma. School of Chemical Engineering Technology Tianjin University Tianjin China . Luoyang Petrochemical Engineering Corporation Luoyang ChinaManuscript received March revised Apr 《天然气化学杂志》2002,(Z1)
This article describes a process for the synthesis of diethyl oxalate by a coupling reaction of carbon monoxide, catalyzed by palladium in the presence of ethyl nitrite. The kinetics and mechanism of the coupling and regeneration reaction are also discussed. This paper presents the results of a scale-up test of the catalyst and the process based on an a priori computer simulation. 相似文献
970.
The present paper describes constructional details and evaluations of an at-column injector for capillary GC. Injections were made via a sample loop on a 0.32 mm i.d. capillary column. Two rotary valves were employed to allow a wash of the sample loop and a backflush of the transfer line. Repeatability, calculated from absolute area counts for n-alkanes was between 0.3–1% RSD, for injected sample volumes between 5 and 100 μl. Promising results were also obtained with syringe-based injections on narrow bore (100 μ i.d.) columns. Repeatability on the basis of normalized area counts was in the order of 0.1–0.2% RSD, while solvent tailing was practically absent. 相似文献