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   2851篇
  免费   38篇
  国内免费   6篇
化学   2052篇
晶体学   12篇
力学   44篇
综合类   1篇
数学   460篇
物理学   326篇
  2016年   32篇
  2015年   30篇
  2014年   34篇
  2013年   96篇
  2012年   76篇
  2011年   71篇
  2010年   49篇
  2009年   46篇
  2008年   76篇
  2007年   93篇
  2006年   71篇
  2005年   71篇
  2004年   58篇
  2003年   62篇
  2002年   49篇
  2001年   28篇
  2000年   26篇
  1999年   28篇
  1998年   25篇
  1997年   36篇
  1996年   32篇
  1995年   38篇
  1994年   49篇
  1993年   31篇
  1992年   28篇
  1991年   35篇
  1990年   31篇
  1989年   40篇
  1988年   33篇
  1987年   37篇
  1986年   37篇
  1985年   50篇
  1984年   58篇
  1983年   30篇
  1982年   53篇
  1981年   50篇
  1980年   43篇
  1979年   37篇
  1978年   64篇
  1977年   60篇
  1976年   45篇
  1975年   38篇
  1974年   34篇
  1973年   47篇
  1972年   31篇
  1971年   46篇
  1970年   30篇
  1969年   20篇
  1968年   19篇
  1966年   24篇
排序方式: 共有2895条查询结果,搜索用时 295 毫秒
51.
Decision theory models of group decision processes usually assume a given set of alternatives, from which the group has to choose. In realistic group decision situations, however, alternatives are often not specified a priori, but are created during the group process from different components introduced by the group members. This paper develops methods for systematically creating such composite alternatives, also taking into account the necessity to keep both the computational effort and the cognitive load to group members within reasonable limits.Paper presented at the International Conference on Support Systems for Decision and Negotiation Processes DNS-92, Warsaw, 1992.  相似文献   
52.
53.
Electroactive conducting copolymers of aniline (ANI) and o-aminophenol (OAP) and two-layered poly(o-aminophenol) (POAP)/polyaniline (PANI) composites were prepared in aqueous acidic solution by electrode potential cycling. Copolymerization was carried out at different feed concentrations of OAP and ANI on a gold electrode. A strong inhibition of electropolymerization was found at a high molar fraction of OAP in the feed. The copolymers showed good adherence on the electrode surface and gave a redox response up to pH=10.0. Two transitions were observed in the in situ conductivities of the copolymers (as with PANI), but the conductivities were lower by 2.5–3 orders of magnitude as compared to PANI. Electrosynthesis of PANI on POAP modified electrodes showed copolymer formation after reaction initiation and finally formation of a PANI layer at the copolymer/solution interface. The ‘memory effect’ of the bilayer structures of both polymers was discussed in terms of protonation/deprotonation and anion consumption taking place during redox processes of both polymers.  相似文献   
54.
Rigler R 《Electrophoresis》2002,23(5):805-808
The behavior of negatively charged single rhodamine-labeled molecules excited to fluorescence in confocal volume elements has been investigated in oscillating voltage gradients between 1 and 15 kV/cm. The effective charge of rhodamin labeled deoxy-uridine triphosphate (Rh-dUTP) is dependent on the field strength applied and likely is due to differences in the screening by counter ions. It can be shown that the attractive forces generated by an oscillating dipole can increase the concentration of Rh-dUTP by a factor 2. Likewise a substantial increase of concentration can be obtained for Rh-labeled DNA molecules when linear voltage gradients are applied. The importance for diagnostic analysis at the single molecule level is discussed.  相似文献   
55.
56.
The ZnCl2 catalyzed reaction of p-methoxybenzyl chloride with alkenes yields the 1:1 addition products 3, which are converted into the γ-lactones 4 via Ru(VIII) catalyzed oxidative degradation of the aromatic ring.  相似文献   
57.
58.
The model of two interacting ethylene molecules having D 2h symmetry was studied using generalized Hückel method. The validity of - separation was tested on this model. The general character of the ground state and lower lying excited states of the model was discussed and the implications drawn concerning transannular interaction and excimer formation. The values obtained for the dissociation energy of the first excited state of our model and corresponding equilibrium intermolecular distance are of right order of magnitude.
Zusammenfassung Mittels der verallgemeinerten Hückel-Methode wurde das Modell von zwei Äthylenmolekülen mit der Symmetrie D 2h studiert. An diesem Modell wurde die Gültigkeit der - -Separation geprüft. Der allgemeine Charakter des Grundzustandes und der niedriger liegenden angeregten Zustände des Modells wird erörtert und die Folgerungen in Bezug auf transannulare Wechselwirkung und Excimer-Entstehung diskutiert. Die erhaltenen Werte für die Dissoziationsenergie des ersten angeregten Zustandes von unserem Modell und den entsprechenden intermolekularen Gleichgewichtsabstand haben die richtige Größenordnung.

Résumé Le modèle à symétrie D 2h de deux molécules d'éthylène en interaction a été étudié en utilisant la méthode généralisée de Hückel. On a examiné la validité de la séparation - sur ce modèle. Le caractère général de l'état fondamental et celui des états excités inférieurs du modèle furent discutés et on a fait des déductions sur l'interaction transannulaire et sur la formation d'«excimères». Les valeurs obtenues pour l'énergie de dissociation du premier état excité de notre modèle et la distance intermoléculaire d'équilibre corréspondante ont l'ordre de magnitude correct.
  相似文献   
59.
Synthesis of 15-Hydroxy[9]metacyclophane 3-(1-Nitro-2-oxocyclododecyl)propanal ( 1 ) was converted to 15-hydroxy[9]metacyclophane ( 3 ) on two different routes. In the first case the internal aldol reaction product of 1 was treated with K2CO3/THF to give 3 in 29 % yield with regard to cyclododecanone. Alternatively, the aldehyde 1 reacted with a primary amine to form e.g. 4 which gave 3 in the presence of CH3I/K2CO3 in 48 % yield.  相似文献   
60.
Alkylation of bicyclo[3.3.0]octane-2,8-dione ( 1 ), which is prepared by a modification of the procedure described in the literature, gives the methyl- and propynyl-derivatives 6 and 7 (Scheme 1). In addition to the method described previously (Scheme 2), 9-methyl-cis-decalin-1,8-dione 9 is obtainable stereoselectively either by cyclization of keto-acid 16 , or by aldol cyclization of keto-aldehyde 26 and oxydation of the resulting alcohols 24 and 25 (Scheme 4). The β-keto-alcohols 24 and 25 undergo a base-catalyzed isomerization; the trans-decalin isomers 27 and 28 are not detected in this equilibrium mixture (Schemes 4 and 5)l. Monoreduction of cis-dione 9 gives the endo-alcohol 25 , while 27 is the favored product of the reductin of trans-dione 10 (Scheme 4). Optically pure (+)- 25 can be prepared from (9S,10R)-monoacetal 29 (Scheme 5).  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