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   3135篇
  免费   109篇
  国内免费   9篇
化学   2389篇
晶体学   57篇
力学   29篇
数学   149篇
物理学   629篇
  2023年   18篇
  2022年   26篇
  2021年   39篇
  2020年   68篇
  2019年   48篇
  2018年   50篇
  2017年   33篇
  2016年   79篇
  2015年   60篇
  2014年   97篇
  2013年   147篇
  2012年   209篇
  2011年   267篇
  2010年   132篇
  2009年   124篇
  2008年   221篇
  2007年   205篇
  2006年   213篇
  2005年   205篇
  2004年   174篇
  2003年   147篇
  2002年   152篇
  2001年   46篇
  2000年   42篇
  1999年   27篇
  1998年   30篇
  1997年   24篇
  1996年   38篇
  1995年   23篇
  1994年   18篇
  1993年   18篇
  1992年   25篇
  1991年   11篇
  1990年   15篇
  1989年   9篇
  1988年   10篇
  1987年   7篇
  1985年   16篇
  1984年   17篇
  1983年   17篇
  1982年   21篇
  1981年   9篇
  1980年   18篇
  1979年   16篇
  1978年   15篇
  1977年   11篇
  1976年   7篇
  1975年   10篇
  1974年   5篇
  1973年   8篇
排序方式: 共有3253条查询结果,搜索用时 15 毫秒
971.
5,6-Di-tert-butyl-2,3,7-trithiabicyclo[2.2.1]hept-5-ene 7-endo-oxide (4) was prepared by addition of S(2)Cl(2) to 3,4-di-tert-butylthiophene 1-oxide (3) in high yield. The oxidation of 4 with dimethyldioxirane gave a 7:1 isomeric mixture of 5,6-di-tert-butyl-2,3,7-trithiabicyclo[2.2.1]hept-5-ene 2-endo-7-endo-dioxide (5a) and 2-exo-7-endo-dioxide (5b) quantitatively. The thermally labile 5 was shown to undergo a retro-Diels-Alder reaction that produces S(2)O and 3 in a reversible way. The resulting S(2)O was trapped by Diels-Alder reactions with dienes to give 3,6-dihydro-1,2-dithiin 1-oxides in good yields. In the absence of the dienes, S(2)O disproportionates to SO(2) and S(3), and the resulting S(3) underwent a 1,3-dipolar cycloaddition with 3 on its syn-pi-face with respect to the S[double bond]O bond to give a trithiolane derivative, whereas in the presence of excess norbornene, it produced the 1,3-dipolar cycloadduct with norbornene in good yield. Thus, the retro-Diels-Alder reaction of 5 functions as an S(2)O and S(3) source. DFT calculations at the B3LYP/6-311+G(3df,2p) level were carried out in order to explain why S(2)O disproportionates to SO(2) and S(3) and why S(2)O acts as a dienophile and not a 1,3-dipole, whereas O(3) and S(3) serve as 1,3-dipoles.  相似文献   
972.
The ruthenium-catalyzed reaction of aryl ethers having a carbonyl group at the ortho position to the ether group with organoboronates (R-B(OCH2CMe2CH2O), R = aryl, alkenyl, and alkyl) resulted in site-selective C-C bond formation. Among the transition metal complexes screened, the RuH2(CO)(PPh3)3 complex showed the highest activity. Several aromatic ketones having methoxy or phenoxy groups at the ortho position can also be used in this coupling reaction. A variety of arylboronates containing electron-donating (NMe2, OMe, methyl, and vinyl) and -withdrawing (F and CF3) groups reacted with methoxy ketones to give the corresponding coupling products in high yields.  相似文献   
973.
A novel and mild method was established to synthesize 6-methylidene penem compounds. This method entails a MgBr(2)/Et(3)N-promoted aldol-type condensation on 6-bromopenem 12 with an appropriately substituted aldehyde to yield the intermediate acetylated bromohydrin, which was smoothly converted to the final product with simultaneous deprotection of C3 carboxylic acid ester using activated zinc dust and phosphate buffer at pH 6.5. This process provides a useful variation of C-C bond formation method for penem derivatives and also serves as a practical synthetic method to prepare 6-exomethylenepenem derivatives without racemization at the C5 position.  相似文献   
974.
975.
976.
In this paper we given some basic characterizations of minimal Markov basis for a connected Markov chain, which is used for performing exact tests in discrete exponential families given a sufficient statistic. We also give a necessary and sufficient condition for uniqueness of minimal Markov basis. A general algebraic algorithm for constructing a connected Markov chain was given by Diaconis and Sturmfels (1998,The Annals of Statistics,26, 363–397). Their algorithm is based on computing Gröbner basis for a certain ideal in a polynomial ring, which can be carried out by using available computer algebra packages. However structure and interpretation of Gröbner basis produced by the packages are sometimes not clear, due to the lack of symmetry and minimality in Gröbner basis computation. Our approach clarifies partially ordered structure of minimal Markov basis.  相似文献   
977.
The winner determination problem (WDP) in combinatorial auctions is the problem of, given a finite set of combinatorial bids B, finding a feasible subset B of B with a maximum revenue. WDP is known to be equivalent to the maximum weight set packing problem, and hard to approximate by polynomial time algorithms. This paper proposes three heuristic bid ordering schemes for solving WDP; the first two schemes take into account the number of goods shared by conflicting bids, and the third one is based on a recursive application of such local heuristic functions. We conducted several experiments to evaluate the goodness of the proposed schemes. The result of experiments implies that the first two schemes are particularly effective to improve the performance of the resulting heuristic search procedures. More concretely, they are scalable compared with the conventional linear programming (LP) relaxation based schemes, and could quickly provide an optimum solution under optimization schemes such as the branch-and-bound method. In addition, they exhibit a good anytime performance competitive to the LP-based schemes, although it is sensitive to configurable parameters controlling the strength of contributions of bid conflicts to the resultant bid ordering schemes.  相似文献   
978.
We have reexamined the sequential two-photon absorption of I2 from the X1Σg+ ground state to the lowest D′2g(3P2) ion-pair state using the high vibrational level of the B3Π(0u+) state as an intermediate. The double resonance transition to the D′2g(3P2) state was found to occur through the B3Π(0u+)-b′2u coupled state by hyperfine interaction, which permits to mix the different J levels of different electronic states. We elucidated the B3Π(0u+)-b′2u coupling schemes in the intermediate states. The double resonance spectra recorded for the D′2g(3P2) state in the range v=0-30 were used to determine the absolute energy of the D′2g(3P2) state. The molecular parameters of the D′2g(3P2) state obtained in this study are Y00=40 388.783(2), Y10=103.956 46(57), Y20=−0.207 717(51), Y30=2.199(13)×10−4, Y01=0.020 528 18(62), and Y11=−5.1753(48)×10−5 (all in cm−1 and σ in parentheses). An RKR potential curve constructed using these constants is reported. Combined with the data on the D′2g(3P2)-A3Π(2u) transition from Zheng et al. (J. Chem. Phys.96, 4877 (1992)), we can locate the A3Π(2u) v=0 state at 10 096.444(6) cm−1 above the ground state.  相似文献   
979.
Goto K  Nakagawa T  Nakamura O  Kawata S 《Optics letters》2002,27(20):1797-1799
A compact, photocouplerlike system is presented for wireless transcutaneous transmission of biological signals. The system consists of an optically powered near-infrared light transmitter, a receiver for the signals from it, and a near-infrared light source with which to irradiate it. The transmitter, which is implanted under the skin, is transcutaneously coupled with the receiver and the light source, which are combined and placed on the skin. The transmitter, which is powered transcutaneously by the light source, then amplifies a signal input to it and sends intensity-modulated light transcutaneously to the receiver. With this system, electromyograms and neuronal firing patterns of live rats have been successfully recorded.  相似文献   
980.
The temporal profile and phase of the fifth harmonic of a Ti:sapphire laser were fully characterized by two-photon ionization frequency-resolved optical gating technique for the first time. The fifth harmonic was found to have negative chirp and the pulse compression was demonstrated. The negative chirp is well explained by using a zero-range potential model. This technique is scalable to extreme ultraviolet (XUV) and soft x-ray regions by using currently available light sources, making it possible to measure the pulse duration and phase of vacuum ultraviolet, XUV, and soft x-ray pulses.  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