首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2669篇
  免费   120篇
  国内免费   7篇
化学   1699篇
晶体学   73篇
力学   89篇
数学   278篇
物理学   657篇
  2023年   18篇
  2022年   22篇
  2021年   47篇
  2020年   44篇
  2019年   57篇
  2018年   61篇
  2017年   50篇
  2016年   85篇
  2015年   83篇
  2014年   92篇
  2013年   247篇
  2012年   151篇
  2011年   146篇
  2010年   120篇
  2009年   105篇
  2008年   130篇
  2007年   121篇
  2006年   90篇
  2005年   95篇
  2004年   76篇
  2003年   64篇
  2002年   72篇
  2001年   33篇
  2000年   24篇
  1999年   28篇
  1998年   16篇
  1997年   20篇
  1996年   26篇
  1995年   26篇
  1994年   19篇
  1993年   29篇
  1992年   28篇
  1991年   24篇
  1990年   25篇
  1989年   28篇
  1988年   25篇
  1987年   21篇
  1986年   25篇
  1985年   23篇
  1984年   30篇
  1983年   22篇
  1982年   24篇
  1981年   31篇
  1980年   30篇
  1979年   28篇
  1978年   16篇
  1977年   32篇
  1976年   16篇
  1975年   22篇
  1974年   20篇
排序方式: 共有2796条查询结果,搜索用时 199 毫秒
21.
The transient hot-wire technique is widely used for absolute measurements of the thermal conductivity and thermal diffusivity of fluids. It is well established that fluid radiation effects significantly influence these measurements, especially those for the thermal diffusivity. Corrections for radiation effects are based on the models developed and deviations of the measured data from the ideal line source model. In this paper, the effect of fluid radiation on the measurements of the thermal conductivity of n-pentane is presented. For comparison, the influence of thermal radiation effect on measurement of transparent fluids, such as argon is also shown. The difference between the influence of natural convection and thermal radiation is also demonstrated.  相似文献   
22.
Selenocystine ([Sec]2) and aryl-substituted selenocysteine (Sec) derivatives are synthesized, starting from commercially available amino acid l-serine. These compounds are characterized by a number of analytical techniques such as NMR (1H, 13C and 77Se) and TOF mass spectroscopy. This study reveals that the introduction of amino/imino substituents capable of interacting with selenium may stabilize the Sec derivatives. This study further suggests that the oxidation-elimination reactions in Sec derivatives could be used for the generation of biologically active selenols having internally stabilizing substituents.  相似文献   
23.
Summary Finite order expressions are derived for expectation values and transition matrix elements within the framework of the coupled cluster method.  相似文献   
24.
Inter- and intramolecular hetero-Diels-Alder reactions in a series of functionalized 2(1H)-pyrazinones were investigated under controlled microwave irradiation. The cycloaddition reactions were efficiently performed in sealed tubes, utilizing either a combination of 1,2-dichloroethane and a thermally stable ionic liquid, or 1,2-dichlorobenzene as reaction medium. In all cases, a significant rate-enhancement using microwave flash heating as compared to thermal heating was observed.  相似文献   
25.
Signs of the dipole moment derivatives, ?px/?S4 and ?px/?S5 where px is the dipole moment vector along an axis perpendicular to the CO bond in the plane of the molecule and Sj are the symmetry coordinates for the B1 vibrations of Br2CO have been re-evaluated from the reported ?p/?Qi values (p being the dipole moment of the molecule and Qi the normal coordinates) and the L matrix elements. The new set of dipole moment derivatives fits well with similar parameters for Cl2CO and F2CO.  相似文献   
26.
The exchange part of the usual Hartree-Fock potential in the unrestricted Hartree-Fock (UHF) theory is suitablyaveraged to construct an, average one-electron model Hamiltonian which generates a set of spin-restricted one-electron orbitals in a self-consistent manner. These orbitals are then used to calculate the electronic energy of the open-shell system by using the proper functional form for the energy which handles the exchange terms correctly. The eigenvalues ofF av can be used for calculating either the spin-polarised or spin-averaged ionisation potentials of different orbitals at theKoopmans’ theorem level of approximation. Comparison ofE ac with the UHF-energy shows thatE UHF<E ac in each case revealing some kind of an upper bound nature ofE ac. An approximate variational argument is given. Relationship of our model with the hyper-Hartree-Fock theory of slater is explored and the general problem of eliminating ‘self-interaction’ terms in average Fock-operator based theories is discussed.  相似文献   
27.
A microwave-assisted three-component reaction was used to prepare a series of 1,4-disubstituted-1,2,3-triazoles from corresponding alkyl halides, sodium azide, and alkynes. This procedure eliminates the need to handle organic azides, as they are generated in situ, making this already powerful click process even more user-friendly and safe.  相似文献   
28.
A series of 1-(5-phenyl-2H-tetrazol-2-ylacetyl)-4-substituted thiosemicarbazids were prepared as possible antiinflammatory agents. Their antiproteolytic activity was reported.  相似文献   
29.
Korivi RP  Cheng CH 《Organic letters》2005,7(23):5179-5182
[reaction: see text] A wide range of substituted isoquinolines were synthesized via a highly efficient nickel-catalyzed annulation of the tert-butyl imines of 2-iodobenzaldehydes and various alkynes; examination of the regiochemistry of isoquinolines synthesized indicates that there are two different alkyne insertion pathways for the catalytic reactions.  相似文献   
30.
The isolation and the X-ray crystal structure of physiological copper(II)-L-histidine complex are reported. The neutral five-coordinate complex shows distorted square pyramidal geometry with bidentate and tridentate L-histidine ligands. The basic character of the pendent imidazole group and H-bonding interactions of bidentate L-histidine ligand are important for copper transport. The unique structural features help explain the origin of its thermodynamic stability and kinetic reactivity in human blood along with the ternary copper(II)-amino acid complexes. The role of L-histidine in interaction with copper(II)-albumin, in cellular uptake of copper, and in treatment of Menkes disease can be studied using these results.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号