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   1966篇
  免费   57篇
  国内免费   10篇
化学   1046篇
晶体学   48篇
力学   35篇
数学   205篇
物理学   699篇
  2021年   12篇
  2020年   15篇
  2019年   27篇
  2017年   13篇
  2016年   40篇
  2015年   42篇
  2014年   32篇
  2013年   57篇
  2012年   74篇
  2011年   83篇
  2010年   39篇
  2009年   53篇
  2008年   74篇
  2007年   79篇
  2006年   62篇
  2005年   63篇
  2004年   75篇
  2003年   34篇
  2002年   71篇
  2001年   40篇
  2000年   39篇
  1999年   34篇
  1998年   28篇
  1997年   35篇
  1996年   37篇
  1995年   30篇
  1994年   39篇
  1993年   31篇
  1992年   24篇
  1991年   34篇
  1990年   36篇
  1989年   17篇
  1988年   41篇
  1987年   45篇
  1986年   17篇
  1985年   41篇
  1984年   29篇
  1983年   30篇
  1982年   24篇
  1981年   40篇
  1980年   31篇
  1979年   29篇
  1978年   24篇
  1977年   15篇
  1976年   21篇
  1975年   24篇
  1974年   16篇
  1973年   16篇
  1972年   10篇
  1938年   9篇
排序方式: 共有2033条查询结果,搜索用时 31 毫秒
991.
The structure of the active site of human glyoxalase I and the reaction mechanism of the enzyme-catalyzed conversion of the thiohemiacetal, formed from methylglyoxal and glutathione, to S-D-lactoylglutathione has been investigated by ab initio quantum chemical calculations. To realistically represent the environment of the reaction center, the effective fragment potential methodology has been employed, which allows systems of several hundred atoms to be described quantum mechanically. The methodology and the active site model have been validated by optimizing the structure of a known enzyme-inhibitor complex, which yielded structures in good agreement with the experiment. The same crystal structure has been used to obtain the quantum motif for the investigation of the glyoxalase I reaction. The results of our study confirm that the metal center of the active site zinc complex plays a direct catalytic role by binding the substrate and stabilizing the proposed enediolate reaction intermediate. In addition, our calculations yielded detailed information about the interactions of the substrate, the reaction intermediates, and the product with the active site of the enzyme and about the mechanism of the glyoxalase I reaction. The proton transfers of the reaction proceed via the two highly flexible residues Glu172 and Glu99. Information about the structural and energetic effect of the protein on the first-shell complex has been attained by comparison of the structures optimized in the local protein environment and in a vacuum. The environment of the zinc complex disturbs the Cs symmetry found for the complex in a vacuum, which suggests an explanation for the stereochemical behavior of glyoxalase I.  相似文献   
992.
993.
994.
A review about the properties of several types of tracerinput signals is given and in particular the stochastic signals are discussed. An estimation of the necessary measuring time shows, that true stochastic signals are unimportant for tracer experiments. The use of periodic pseudo-random binary signals (PRBS) permits a strong reduction of the measuring time. Therefore their practical application for tracer experiments is possible. The PRBS-method has special advantages in comparison with impulse methods, if fluctuations and disturbances exist within the investigated systems. Correlations for the estimation of parameters, needed for the realization of PRBS-measurements, are given and several measuring arrangements are discussed.

The application of the PRBS-method for measurements of residence time distributions (RDC) of a mixer-settler extractor is described.  相似文献   
995.
This paper examines the computational complexity certification of the fast gradient method for the solution of the dual of a parametric convex program. To this end, a lower iteration bound is derived such that for all parameters from a compact set a solution with a specified level of suboptimality will be obtained. For its practical importance, the derivation of the smallest lower iteration bound is considered. In order to determine it, we investigate both the computation of the worst case minimal Euclidean distance between an initial iterate and a Lagrange multiplier and the issue of finding the largest step size for the fast gradient method. In addition, we argue that optimal preconditioning of the dual problem cannot be proven to decrease the smallest lower iteration bound. The findings of this paper are of importance in embedded optimization, for instance, in model predictive control.  相似文献   
996.
A sequence of a Baeyer–Villiger oxidation and a Lewis acid-promoted reduction of the resulting formate with Et3SiH enabled the metal-free formal decarbonylation of tertiary and secondary aliphatic aldehydes. The new methodology mimics the biosynthetic decarbonylation pathway through oxidative C−C bond cleavage rather than the C(O)−H bond activation known from conventional Tsuji–Wilkinson-type reactions. The substrate scope is complementary to existing transition-metal-catalyzed protocols.  相似文献   
997.
998.
    
Ohne Zusammenfassung  相似文献   
999.
Synthesis, Structures, NMR and EPR Investigations of Binuclear Bis(N,N,N‴,N‴‐tetraisobutyl‐N′,N″‐isophthaloylbis(selenoureato)) Complexes of NiII and CuII The synthesis of binuclear CuII and NiII complexes of the quadridentate ligand N,N,N‴,N‴‐tetraisobutyl‐N′,N″‐isophthaloylbis(selenourea) and their crystal structures are reported. The complexes crystallize monoclinic, P21/c (Z = 2). In the EPR spectra of the binuclear CuII complex exchange‐coupled CuII‐CuII pairs were observed. In addition the signals of a mononuclear CuII species are observed what will be explained with the assumption of an equilibrium between the binuclear CuII‐complex (CuII‐CuII pairs) and oligomeric complexes with “isolated” CuII ions. Detailed 13C and 77Se NMR investigations on the ligand and the NiII complex allow an exact assignment of all signals of the heteroatoms.  相似文献   
1000.
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