全文获取类型
收费全文 | 4469篇 |
免费 | 77篇 |
国内免费 | 10篇 |
专业分类
化学 | 3118篇 |
晶体学 | 19篇 |
力学 | 81篇 |
数学 | 979篇 |
物理学 | 359篇 |
出版年
2019年 | 34篇 |
2016年 | 74篇 |
2015年 | 54篇 |
2014年 | 35篇 |
2013年 | 126篇 |
2012年 | 91篇 |
2011年 | 143篇 |
2010年 | 103篇 |
2009年 | 117篇 |
2008年 | 147篇 |
2007年 | 147篇 |
2006年 | 150篇 |
2005年 | 137篇 |
2004年 | 107篇 |
2003年 | 108篇 |
2002年 | 107篇 |
2001年 | 57篇 |
2000年 | 75篇 |
1999年 | 64篇 |
1998年 | 61篇 |
1997年 | 57篇 |
1996年 | 73篇 |
1995年 | 81篇 |
1994年 | 62篇 |
1993年 | 68篇 |
1992年 | 98篇 |
1991年 | 95篇 |
1990年 | 78篇 |
1989年 | 74篇 |
1988年 | 59篇 |
1987年 | 66篇 |
1986年 | 63篇 |
1985年 | 79篇 |
1984年 | 88篇 |
1983年 | 79篇 |
1982年 | 77篇 |
1981年 | 103篇 |
1980年 | 59篇 |
1979年 | 81篇 |
1978年 | 71篇 |
1977年 | 96篇 |
1976年 | 65篇 |
1975年 | 86篇 |
1974年 | 88篇 |
1973年 | 67篇 |
1972年 | 54篇 |
1971年 | 50篇 |
1970年 | 38篇 |
1967年 | 31篇 |
1966年 | 30篇 |
排序方式: 共有4556条查询结果,搜索用时 15 毫秒
91.
92.
Adenosine 5'-triphosphate (ATP(4-)) and related nucleoside 5'-triphosphates (NTP(4-)) serve as substrates in the form of metal ion complexes in enzymic reactions taking part thus in central metabolic processes. With this in mind, the coordination chemistry of NTPs is critically reviewed and the conditions are defined for studies aiming to describe the properties of monomeric complexes because at higher concentrations (>1 mM) self-stacking may take place. The metal ion (M(2+)) complexes of purine-NTPs are more stable than those of pyrimidine-NTPs; this stability enhancement is attributed, in accord with NMR studies, to macrochelate formation of the phosphate-coordinated M(2+) with N7 of the purine residue and the formation degrees of the resulting isomeric complexes are listed. Furthermore, the formation of mixed-ligand complexes (including also those with buffer molecules), the effect of a reduced solvent polarity on complex stability and structure (giving rise to selectivity), the use of nucleotide analogues as antiviral agents, and the effect of metal ions on group transfer reactions are summarized. 相似文献
93.
Sonnenschein H Kreher T Gründemann E Krüger RP Kunath A Zabel V 《The Journal of organic chemistry》1996,61(2):710-714
The first compounds of a new series of redox-active cyclophanes were prepared by dehydrocyclization of bridged indolizines of type 1. The bridged dipyridino-compounds 2a and 2b obtained by reaction of 2 mol of lithiated alpha-picolines with dihalides were used as starting materials. Subsequent treatment of 2a,b with 2 mol of alpha-bromo ketones gave quaternary pyridinium halides. Ring closure in an alkaline medium (Chichibabin reaction) yielded the starting material for the synthesis of the macrocycles. Oxidative C-C coupling gave the diastereomeric cyclophanes of type 3. In all cases one pair of the enantiomers was obtained in excess. CV-investigations have shown that the main products are reversible redox systems. To clarify their conformations, compounds 3c, 3d/1, and3d/2 were subjected to X-ray analysis. 相似文献
94.
Helmut Meyn 《Designs, Codes and Cryptography》1995,6(2):107-116
For an odd prime powerq the infinite field GF(q
2 )=
n0 GF (q
2n
) is explicitly presented by a sequence (f
n)1 ofN-polynomials. This means that, for a suitably chosen initial polynomialf
1, the defining polynomialsf
nGF(q)[x] of degrees2
n are constructed by iteration of the transformation of variablexx+1/x and have linearly independent roots over GF(q). In addition, the sequences are trace-compatible in the sense that the relative traces map the corresponding roots onto each other. In this first paper the caseq1 (mod 4) is considered and the caseq3 (mod 4) will be dealt with in a second paper. This specific construction solves a problem raised by A. Scheerhorn in [11]. 相似文献
95.
We give a necessary and sufficient condition for a Bell-type inequality to hold in a horizontal sum of finitely many finite Boolean algebras. 相似文献
96.
Helmut Röhrl 《Geometriae Dedicata》1993,45(3):323-340
In this paper we classify all real convexity theories that contain the standard convexity theory c. For this purpose we consider three subcases: finitary; infinitary and (sc\c)Ø; infinitary and sc=c. In each of these subcases one encounters a phenomenon resembling bifurcation.This research was supported by the Deutsche Forschungsgemeinschaft. 相似文献
97.
Motivated by the concept of maximum entropy methods in signal and image processing, we introduce and discuss a class of ‘directed diffusion equations’ with suitable boundary conditions. The paradigmatic ‘directed diffusion equation’ is The relative entropy $ Sb[f](t): = - \int_\Omega {f(t,x)} \;\ln \;(f(t,x)/b(x))dx $ is rapidly increasing along solution trajectories of (i). This suggests that solving (i) will yield efficient procedures for entropy maximization. We also discuss the asymptotic behavior of solutions of (i)—this is readily done because (i) has a large family of Ljapunov functionals. 相似文献
98.
99.
Sánchez-Moreno MJ Gómez-Coca RB Fernández-Botello A Ochocki J Kotynski A Griesser R Sigel H 《Organic & biomolecular chemistry》2003,1(10):1819-1826
The synthesis of (1H-benzimidazol-2-yl-methyl)phosphonic acid, H2(Bimp)+/-, is described: 2-chloromethylbenzimidazole was reacted with ethylchloroformate to give 1-carboethoxy-2-chloromethylbenzimidazole which was treated with trimethyl phosphite and after hydrolysis with aqueous HBr H2(Bimp)+/- was obtained. In H2(Bimp)+/- one proton is at the N-3 site and the other at the phosphonate group; both acidity constants were determined in aqueous solution by potentiometric pH titrations (25 degrees C; I = 0.1 M, NaNO3) and this furnished the pKa values of 5.37 +/- 0.02 and 7.41 +/- 0.02, respectively. The acidity constant for the release of the primary proton from the P(O)(OH)2 group of H3(Bimp)+ was estimated: pKa = 1.5 +/- 0.2. Moreover, Bimp2- can be further deprotonated at its neutral (N-1/N-3)H site to give the benzimidazolate residue, but this reaction occurs only in strongly alkaline solution (KOH); application of the H_ scale developed by G. Yagil (J. Phys. Chem., 1967, 71, 1034) together with UV spectrophotometric measurements gave pKa = 14.65 +/- 0.12. Comparisons with acidity constants taken from the literature show that this latter pKa value is far too large and this allows the conclusion that an intramolecular hydrogen bond is formed between the (N-1/N-3)H site and the phosphonate group of Bimp2-; the formation degree of this hydrogen-bonded isomer is estimated to be 98 +/- 2%. The general relevance of this and the other results are shortly discussed and the species distribution for the Bimp system in dependence on pH is provided. 相似文献
100.
Under O2, the Aspidosperma alkaloid tabersonine ( 1 ) was converted by a crude enzyme preparation from leaves of mature plants of Catharanthus roseus G DON into the trimeric 3-hydroxy-14′-(3α″-tabersonyl)voafrine B ( 4 ) which was easily reduced by NaBH4 to 14′-(3α″-tabersonyl)voafrine B ( = tertabersonine; 5 ). Compounds such as 4 and 5 are the first trimeric alkaloids in the series of monoterpenoid indole alkaloids. 相似文献