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   415074篇
  免费   4908篇
  国内免费   1730篇
化学   211714篇
晶体学   5342篇
力学   19971篇
综合类   8篇
数学   71592篇
物理学   113085篇
  2021年   2061篇
  2020年   2461篇
  2019年   2448篇
  2018年   12234篇
  2017年   12023篇
  2016年   10368篇
  2015年   4683篇
  2014年   5502篇
  2013年   15680篇
  2012年   15949篇
  2011年   24932篇
  2010年   14567篇
  2009年   14753篇
  2008年   20143篇
  2007年   22158篇
  2006年   13985篇
  2005年   13818篇
  2004年   12742篇
  2003年   11756篇
  2002年   10652篇
  2001年   10735篇
  2000年   8374篇
  1999年   6595篇
  1998年   5394篇
  1997年   5193篇
  1996年   5324篇
  1995年   4749篇
  1994年   4520篇
  1993年   4333篇
  1992年   4751篇
  1991年   4604篇
  1990年   4277篇
  1989年   4078篇
  1988年   4351篇
  1987年   4021篇
  1986年   3889篇
  1985年   5671篇
  1984年   5712篇
  1983年   4639篇
  1982年   5082篇
  1981年   5061篇
  1980年   4842篇
  1979年   4913篇
  1978年   4901篇
  1977年   4849篇
  1976年   4795篇
  1975年   4687篇
  1974年   4506篇
  1973年   4673篇
  1972年   2661篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Nitrogen position and internal heavy atom effects on the radiative and radiationless transitions from the lowest excited states of the isomeric azaphenanthrenes and some of their methyl, chlorine and bromine derivatives have been studied in E.P.A. solutions at 77 K. The nitrogen position affects the fluorescence and S1-T1 intersystem crossing rates more than the phosphorescence and T1-S0 intersystem crossing rates. Small differences in the behaviour of 9-azaphenanthrene are enhanced in non-hydroxylic solvents and at room temperature, and it is inferred that (n, π*) states play a more important role in the photophysical behaviour of this isomer. Halogen, substitution in all the isomers increases the phosphorescence rate, induces a smaller increase in the T1-S0 intersystem crossing rates and has a negligible effect on the fluorescence rate.  相似文献   
992.
Langlet  J.  Claverie  P.  Pullman  B.  Piazzola  D.  Daudey  J. P. 《Theoretical chemistry accounts》1977,46(2):105-116
The effect of water on the conformational preferences of acetylcholine has been studied within the discrete, the continuum and the combined discrete-continuum models described in parts I and II of this series. All the models lead to the conclusion that the trans-gauche form which is, following refined quantum-mechanical computations, the intrinsically preferred one and the one observed in the crystal of acetylcholine and of a number of analogues should remain also the preferred conformation in water. This result agrees with NMR studies. The results of the empirical discrete model used here compare favorably to those obtained by an ab initio super-molecule treatment. The continuum model utilized here represents a net improvement above such models utilized in other works.  相似文献   
993.
A non-empirical molecular orbital method, particularly suitable for calculations on cage-like molecules, is described. The method uses as basis functions the set of free-electron functions which are the solutions of Schrödinger's equation for an electron confined between two concentric, spherical potential energy barriers. Application of the theory to the SCF calculation of the energies of the delocalized electrons in benzene and tetrasulphur tetranitride shows that the model is capable of interpreting the properties of such systems. However, it does highlight a difficulty in the calculation of excited state energies with one-centre models which appears to be largely unrecognized.Extension of the method to a consideration of all the valence electrons, using P4 as an example, reveals problems the origin of which is an inadequate treatment of the core electrons. It is suggested that these problems may best be dealt with by use of a suitable pseudo potential.  相似文献   
994.
Résumé Nous avons montré par spectrométrie Raman à chaud et mesure des tensions de vapeur que P4S3 se vaporise de façon congruente tandis que P4S7 et P4S10 se dissocient dès le début de leur vaporisation. P4S7 donne réversiblement P4S3 et soufre. P4S10 se dissocie irréversiblement en P4S7 et soufre. A l'état de vapeur non saturante, P4S3 se dissocie au-dessus de 600° avec formation de phosphore, de soufre et d'autres espèces non identifiées.Nous avons mesuré expérimentalement la capacité calorifique de P4S3 liquide, calculé celle de P4S3 gazeux et son entropie standard. Nous avons aussi estimé l'enthalpie standard de vaporisation de P4S3 à l'aide des mesures des tensions de vapeur saturante. Nous en avons déduit l'entropie standard de P4S3 liquide et son point d'ébullition.
It is shown by Raman spectroscopy at high temperature and by vapor tensimetric measurements that the vaporisation of P4S3 is congruent, whereas P4S7 and P4S10 dissociate at the beginning of vaporisation. P4S7 gives P4S3 and sulfur reversibly. The dissociation of P4S10 into P4S7 and sulfur is irreversible. Above 600°, in non-saturated vapour the dissociation of P4S3 gives phosphorus, sulfur and some unidentified gaseous species. The heat capacity of liquid P4S3 has been measured. That of gaseous P4S3 and its standard entropy have been calculated. The vaporisation standard enthalpy of P4S3 has been estimated from the experimental results on the saturated vapour pressures. The standard entropy of liquid P4S3 and its boiling point have been derived from these data.

Zusammenfassung Mittels Raman-Spektrometrie bei erhöhter Temperatur und durch Messung der Dampfdrucke wurde festgestellt, daß sich P4S3 verflüchtigt, während P4S7 und P4S10 mit Beginn der Verflüchtigung dissoziieren. P4S7 ergibt reversibel P4S3 und Schwefel. Im Zustand ungesättigten Dampfes dissoziiert P4S3 oberhalb von 600° unter Bildung von Phosphor, Schwefel und anderer nicht identifizierter Substanzen.Die Wärmekapazität von flüssigem P4S3 wurde gemessen, während die vom gasförmigem P4S3 sowie seine Standard-Entropie berechnet wurden. Die Standard-Enthalpie der Verflüchtigung des P4S3 wurde durch Messungen der Sättigungs-Dampfdrucke ermittelt. Daraus wurden die Standard-Entropie des flüssigen P4S3 sowie sein Siedepunkt berechnet.

- - , P4S3 , P4S7 P4S10 . P4S7 P4S3 , P4S10 P4S7 . 600° P4S3 , . P4S3, P4S3 . P4S3. P4S3 .


Nous remercions Monsieur Letoffe du laboratoire du Professeur J. Bousquet, INSA de Lyon, 20 Avenue Albert Einstein 401, 69621 Villeurbanne, qui a r6alis6 pour nous les déterminations expérimentales des capacités calorifiques de P4S8 liquide.  相似文献   
995.
Oxidative addition of 2 molecules of 3,3-dimethylcyclopropene (I) to α,α′-bipyridyl(1,5-cyclooctadiene)nickel(0) (III) gave the title compound (IV) in over 90% yield. Complex IV was characterized by mass, 1H NMR and 13C NMR spectroscopy. Its structure was determined by X-ray diffraction (a 13.7081(2), b 14.638(2), c 9.5139(1) Å, β 110.82(1)°, C2/c, R = 0.05 for 1614 reflections).  相似文献   
996.
The 34S(p, n)34mCl reaction induced by 13-MeV protons is used for the determination of sulphur in copper, nickel and aluminium alloys. The 34mCl is separated by repeated precipitation as silver chloride. The results obtained were 3.08 ± 0.47, 1.47 ± 0.17 and <1μg g-1 for copper, nickel and aluminium alloys, respectively.  相似文献   
997.
The goals and limitations of some clustering methods are briefly reviewed. In order to avoid the usual methods which are very demanding on computer time and space, a new scheme is proposed for updating (generation) of, and retrieval from, large data bases organized as binary trees. The method is based on calculation of three distances at any given vertex l on the level n, A(l,n) instead of two. The respective distances d1(X,Al), d2(X,Ar), d3(AlAr) are calculated between the input vector X and the left and right descendant of the vertex A(l,n), Al and Ar, respectively. The advantage of the method compared to the standard clustering methods or formation of hierarchal trees is that the required memory or computational time is reduced from N2 to approximately N log N (N is the number of clustering objects).  相似文献   
998.
An efficient method for the preparation of crystalline α-hydroxy previtamin D3 is described. Also the stereoselective reduction of 1-keto previtamin 9 is discussed; it was observed that with aluminum hydride 7 is the most abundant product. This stands in contrast with previously reported LiAlH4 and NaBH4 mediated reductions.  相似文献   
999.
A simple method for the separation of hemoglobin chains from hemolysate or globin, by sodium dodecyl sulfate polyacrylamide gel electrophoresis, is described. The alpha, beta, and gamma chains can be clearly separated from each other. The alpha chain has the highest mobility, the beta chain has a slower mobility than the gamma chain, while the delta chain has about the same mobility as the beta chain. Hemoglobins with elongated chains can easily be detected by this method. Tak-beta, elongated by 11 residues, moves much more slowly than betaA but is much faster than alpha Constant Spring which is elongated by 31 residues. Screening of several individuals with slow-moving hemoglobins using this method led to the finding of a case with Hb Tak-beta thalassemia and other carriers of Hb Tak.  相似文献   
1000.
Polarographic and cyclovoltammetric investigation of the thiooxalates 16 shows that they are reversibly reduced to the corresponding radical anions. Accordingly, EPR spectra are obtained after in-situ electroreduction. Taking also the electronic spectra into consideration, conclusions about the spin density distribution can be drawn. It is shown thatpπ is higher at the oxygen atoms of oxoesters than at the sulfur atoms of the corresponding thiono esters.  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