全文获取类型
收费全文 | 67566篇 |
免费 | 13168篇 |
国内免费 | 20048篇 |
专业分类
化学 | 47561篇 |
晶体学 | 2448篇 |
力学 | 5063篇 |
综合类 | 2166篇 |
数学 | 10664篇 |
物理学 | 32880篇 |
出版年
2024年 | 218篇 |
2023年 | 919篇 |
2022年 | 2472篇 |
2021年 | 2480篇 |
2020年 | 2516篇 |
2019年 | 2400篇 |
2018年 | 2198篇 |
2017年 | 3041篇 |
2016年 | 2586篇 |
2015年 | 3442篇 |
2014年 | 4095篇 |
2013年 | 5460篇 |
2012年 | 5528篇 |
2011年 | 5828篇 |
2010年 | 5518篇 |
2009年 | 5533篇 |
2008年 | 6257篇 |
2007年 | 5510篇 |
2006年 | 5357篇 |
2005年 | 4708篇 |
2004年 | 3608篇 |
2003年 | 2776篇 |
2002年 | 2791篇 |
2001年 | 2721篇 |
2000年 | 2677篇 |
1999年 | 1693篇 |
1998年 | 1003篇 |
1997年 | 842篇 |
1996年 | 823篇 |
1995年 | 728篇 |
1994年 | 750篇 |
1993年 | 640篇 |
1992年 | 572篇 |
1991年 | 443篇 |
1990年 | 410篇 |
1989年 | 413篇 |
1988年 | 311篇 |
1987年 | 264篇 |
1986年 | 230篇 |
1985年 | 171篇 |
1984年 | 152篇 |
1983年 | 164篇 |
1982年 | 128篇 |
1981年 | 109篇 |
1980年 | 75篇 |
1979年 | 67篇 |
1978年 | 26篇 |
1976年 | 22篇 |
1971年 | 19篇 |
1959年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
A three-dimensional global potential energy surface for the ground (X (1)Sigma(+)(g))electronic state of HgH(2) is constructed from more than 13,00 ab initio points. These points are generated using an internally contracted multireference configuration interaction method with the Davidson correction and a large basis set. Low-lying vibrational energy levels of HgH(2), HHgD, and HgD(2) calculated using the Lanczos algorithm are found to be in good agreement with the available experimental band origins. The majority of the vibrational energy levels up to 9000 cm(-1) are assigned with normal mode quantum numbers. Our results indicate a gradual transition for the stretching vibrations from the normal mode regime at low energies to the local mode regime near 9000 and 8000 cm(-1) for HgH(2) and HgD(2), respectively, as evidenced by a decreasing energy gap between the (0,0,n(3)) and (1,0,n(3)-1) vibrational states and bifurcation of the corresponding wave functions. 相似文献
62.
A general and efficient Cu(I)-catalyzed cross-coupling reaction of terminal alkynes and monooxalyl chloride for the synthesis of 2-oxo-3-butynoates and 2-oxo-3-butynoamides was developed. Readily available starting materials, the mild reaction conditions, wide functional group tolerance, and the obviation of stoichiometric organolithium or magnesium reagents combine to highlight this reaction. 相似文献
63.
A new serratane-type triterpene, lycophlegmarin (1), has been isolated from Lycopodium phlegmaria L. Four known related triterpenoids were also found from the title plant. The structure of the new compound was elucidated on the basis of detailed spectroscopic analysis and chemical method. Lycophlegmarin exhibited modest growth-inhibitory activity in vitro against human hepatoma cells BEL 7402. 相似文献
64.
IntroductionCatal}ticrcactionsaffcctcdb}'diffosionaIlimitationincatal}'stporcshavcbccnextensive1}dcaltx`ithinlit.rat.rcll~5I.Scvcraltcchniqucsonthcspccialcase0fFischcr-Tropschs}'nthcsis(FTS)t\crcapplicdtocxplorcthebchaviorofthiscatal}ticrcactionundercxpcrimcntalandopcrationaIconditionsI3~5l.It`"ouldbeofvaluctoindicatchcrcthatthcseinvcstigationsx"ercmainl}'bascdonthesimplcstkineticsofthcfirstordcrinH2proposedbyAndersonl6]attheearIystages.However-thesetboents,striCtlyspeaking,areinvalidfor… 相似文献
65.
氢化物发生-冷原子吸收光谱法测定化妆品中汞 总被引:3,自引:0,他引:3
张宇红 《理化检验(化学分册)》2004,40(9):519-520
介绍用氢化物发生-冷原子吸收光谱法在重铬酸钾-硝酸溶液存在下测定化妆品中汞的含量。方法空白值低、灵敏、准确、精密度好,特别适合于化妆品中汞的测定。 相似文献
66.
Zi-Zhong Li Qing-Xiang Guo Tan Ren Xiao-Qing Zhu You-Cheng Liu 《Journal of inclusion phenomena and macrocyclic chemistry》1993,15(1):37-42
Tetramethylsilane (TMS) can be included by -cyclodextrin (-CD), and sodium 2,2-dimethylsilapentane-5-sulphonate (DSS) can form inclusion complexes with - and -CD. The NMR chemical shifts are changed considerably as a result of the strong interaction between CD and the guest compound in the inclusion complexes. A downfield shift of as much as 0.63 ppm shift downfield has been observed for the protons of external TMS in CD aqueous solution. In view of this, the question arises of whether TMS and DSS can be used as internal references. DSS in D2O is suggested as an external reference for aqueous cyclodextrin solution in NMR measurements. 相似文献
67.
Adsorption of NH3 onto activated carbon prepared from palm shells impregnated with H2SO4 总被引:3,自引:0,他引:3
Adsorption of ammonia (NH3) onto activated carbons prepared from palm shells impregnated with sulfuric acid (H2SO4) was investigated. The effects of activation temperature and acid concentration on pore surface area development were studied. The relatively large micropore surface areas of the palm-shell activated carbons prepared by H2SO4 activation suggest their potential applications in gas adsorption. Adsorption experiments at a fixed temperature showed that the amounts of NH3 adsorbed onto the chemically activated carbons, unlike those prepared by CO2 thermal activation, were not solely dependent on the specific pore surface areas of the adsorbents. Further adsorption tests for a wide range of temperatures suggested combined physisorption and chemisorption of NH3. Desorption tests at the same temperature as adsorption and at an elevated temperature were carried out to confirm the occurrence of chemisorption due to the interaction between NH3 and some oxygen functional groups via hydrogen bonding. The surface functional groups on the adsorbent surface were detected by Fourier transform infrared spectroscopy. The amounts of NH3 adsorbed by chemisorption were correlated with the contents of elemental oxygen present in the adsorbents. Mechanisms for chemical activation and adsorption processes are proposed based on the observed phenomena. 相似文献
68.
High-level computations at G3, CBS-Q, and G3B3 levels were conducted, and good-quality C-H and N-H bond dissociation energies (BDEs) were obtained for a variety of saturated and unsaturated strained hydrocarbons and amines for the first time. From detailed NBO analyses, we found that the C-H BDEs of hydrocarbons are determined mainly by the hybridization of the parent compound, the hybridization of the radical, and the extent of spin delocalization of the radical. The ring strain has a significant effect on the C-H BDE because it forces the parent compound and radical to adopt certain undesirable hybridization. A structure-activity relationship equation (i.e., BDE (C-H) = 61.1-227.8 (p(parent)% - 0.75)(2) + 152.9 (p(radical)% - 1.00)(2) + 40.4 spin) was established, and it can predict the C-H BDEs of a variety of saturated and unsaturated strained hydrocarbons fairly well. For the C-H BDEs associated with the bridgehead carbons of the highly rigid strained compounds, we found that the strength of the C-H bond can also be predicted from the H-C-C bond angles of the bridgehead carbon. Finally, we found that N-H BDEs show less dependence on the ring strain than C-H BDEs. 相似文献
69.
An HPLC method has been developed for the determination of pseudoephedrine in plasma using column switching. Preparation of the sample was simple in that only 1000 microL of water was added to 200 microL of plasma before injection. A 900 microL aliquot was injected onto the precolumn. Double distilled water was used to elute and remove proteins and polar components in the sample. The components retained on the precolumn were flushed forward onto the analytical column by the mobile phase (acetonitrile-0.2 mol/L ammonium sulphate, 10:90 v/v) with automated column switching. The limit of determination of pseudoephedrine in plasma was 12 ng/mL. The relative standard deviations of intra- and inter-assay for the determination of pseudoephedrine in plasma were 1.2-9.8% over the concentration range 1020-21.8 ng/mL. The mean recovery by on-line solid phase extraction was 94.76% (RSD = 1.1%). 相似文献
70.
Shenghai Guo 《Tetrahedron》2007,63(9):2009-2018
An improved zirconium-mediated alkyne-aldehyde cross-coupling reaction has been achieved to afford the stereodefined (Z)-allylic alcohols or 3-iodinated allylic alcohols selectively via protonolysis or iodinolysis of the corresponding five-membered oxazirconacycles. This method has also been successfully applied to the synthesis of (Z)-enynols through cross-coupling reactions of three different components involving alkyne, aldehyde, and alkynyl bromide in a one-pot procedure. 相似文献