全文获取类型
收费全文 | 1644篇 |
免费 | 61篇 |
国内免费 | 54篇 |
专业分类
化学 | 418篇 |
晶体学 | 6篇 |
力学 | 320篇 |
综合类 | 7篇 |
数学 | 510篇 |
物理学 | 498篇 |
出版年
2023年 | 8篇 |
2022年 | 11篇 |
2021年 | 19篇 |
2020年 | 17篇 |
2019年 | 73篇 |
2018年 | 65篇 |
2017年 | 46篇 |
2016年 | 54篇 |
2015年 | 60篇 |
2014年 | 63篇 |
2013年 | 93篇 |
2012年 | 31篇 |
2011年 | 53篇 |
2010年 | 56篇 |
2009年 | 98篇 |
2008年 | 109篇 |
2007年 | 74篇 |
2006年 | 113篇 |
2005年 | 113篇 |
2004年 | 116篇 |
2003年 | 101篇 |
2002年 | 105篇 |
2001年 | 23篇 |
2000年 | 24篇 |
1999年 | 27篇 |
1998年 | 25篇 |
1997年 | 17篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 3篇 |
1993年 | 7篇 |
1992年 | 2篇 |
1990年 | 2篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 8篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 16篇 |
1980年 | 16篇 |
1979年 | 6篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1976年 | 8篇 |
1975年 | 10篇 |
1974年 | 6篇 |
1973年 | 3篇 |
1957年 | 2篇 |
排序方式: 共有1759条查询结果,搜索用时 125 毫秒
31.
Genetic algorithms have properties which make them attractive in de novo drug design. Like other de novo design programs, genetic algorithms require a method to reduce the enormous search space of possible compounds. Most often this is done using information from known ligands. We have developed the ADAPT program, a genetic algorithm which uses molecular interactions evaluated with docking calculations as a fitness function to reduce the search space. ADAPT does not require information about known ligands. The program takes an initial set of compounds and iteratively builds new compounds based on the fitness scores of the previous set of compounds. We describe the particulars of the ADAPT algorithm and its application to three well-studied target systems. We also show that the strategies of enhanced local sampling and re-introducing diversity to the compound population during the design cycle provide better results than conventional genetic algorithm protocols. 相似文献
32.
From the de Broglie matter wave hypothesis and Planck’s energy quantization law, and assuming conservation of energy in the
absorption of a photon and its consequent conversion to kinetic energy of motion by a material particle initially at rest,
one can deduce a simple mathematical relationship between the wavelength λ (or frequency ν), of the photon absorbed by the
particle at rest, and the resulting de Broglie matter wave length, λD, of the particle with kinetic energy of motion of mv2/2. The relationship so deduced, λD∝√λ, suggests that visible wavelengths of light, from about 4000 ?, in the violet, to beyond about 7000 ?, in the red, on
absorption by an electron at rest, lead to material electron wavelengths, λD, of the order of the size of the electron transfer proteins seen in the photosynthetic reaction centers of photosynthesizing
organisms, at about a size of 50–100 ?. In addition to understanding the mechanism of photosynthesis as a material wave mediated
phenomenon, further areas of importance of the relations pointed out in this paper are in the design of experiments to gain
a deeper understanding of the basic tenets of wave mechanics, and in the use of tunable lasers to probe various properties
of material waves, and to precisely control their properties for applications including lithography. 相似文献
33.
Tamás F. Móri 《Journal of Theoretical Probability》1995,8(1):139-164
Considering an infinite string of i.i.d. random letters drawn from a finite alphabet we define the cover timeW
n
as the number of random letters needed until each pattern of lenghtn appears at least once as a substring. Sharp weak and a.s. limit results onW
n
are known in the symmetric case, i.e., when the random letters are uniformly distributed over the alphabet. In this paper we determine the limit distribution ofW
n
in the nonsymmetric case asn. Generalizations in terms of point processes are also proved.Dedicated to Endre Csáki on his 60th birthday. 相似文献
34.
Qing Liu 《Transactions of the American Mathematical Society》1996,348(11):4577-4610
Let be a hyperelliptic curve of genus over a discrete valuation field . In this article we study the models of over the ring of integers of . To each Weierstrass model (that is a projective model arising from a hyperelliptic equation of with integral coefficients), one can associate a (valuation of) discriminant. Then we give a criterion for a Weierstrass model to have minimal discriminant. We show also that in the most cases, the minimal regular model of over dominates every minimal Weierstrass model. Some classical facts concerning Weierstrass models over of elliptic curves are generalized to hyperelliptic curves, and some others are proved in this new setting.
35.
Anna B. DenisovaVyacheslav Ya. Sosnovskikh Wim Dehaen Suzanne ToppetLuc Van Meervelt Vasiliy A. Bakulev 《Journal of fluorine chemistry》2002,115(2):183-192
Reaction of 2-hydrazinothiazoles 1 with 1-thienyl- and 1-furyl-1,3-butanediones 2a,b in methanol in the presence of hydrochloric acid mainly leads to a mixture of pyrazoles 3 and pyrazolines 4 or pyrazoles 3 and 5 in strong acidic conditions. Isomeric hydrazones 6 and pyrazolines 4 were formed and isolated in these reactions in the absence of hydrochloric acid. It has been shown that the regioselectivity in the reaction of diketones 2 with hydrazine 1 is governed by both the concentration of acid and the nature of substituents in the 1,3-diketones 2. Cyclization of hydrazones 6 is shown to occur under milder conditions than dehydration for pyrazolines 4. The new heterocyclic compounds were prepared and fully characterized by NMR spectra and by X-ray analysis for 3c. 相似文献
36.
钴配合物的合成晶体结构及活性研究 总被引:13,自引:0,他引:13
The complex [Co2Cl2(EGTB)]·(ClO4)2·5H2O was synthesized in methanol-aqueous solution and its crystal structure was determined by X-ray diffraction method. The crystal belongs to monoclinic with space group of P21/c. The cell parameters are a=1.42231(2)nm,b=2.53181(3)nm,c=1.43392(2)nm,β=102.0518(5)°, Z=4, F(000)=2360,Dc=1.508g·cm-3, The final R=0.0788,wR2=0.232, The catalytic rate constant of the dismutation of superoxide radicals has been obtained by means of photo reduction of nitroblue tetrazolium(NBT),and it is 1.875×106mol-1·L·s-1. 相似文献
37.
38.
Kiyokawa T Kanaori K Tajima K Kawaguchi M Mizuno T Oku J Tanaka T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(14):3548-3554
The alpha-helical coiled coils have a representative amino acid sequence of (abcdefg)(n) heptad repeats. We previously reported that two peptides named IZ-2A and IZ-2W formed an (IZ-2A)(2)/IZ-2W heterotrimer with an Ala-Ala-Trp interaction in the hydrophobic core. In this paper, we describe the selective formation of AAB- and ABC-type heterotrimers. To increase the selectivity of the AAB-type heterotrimeric formation, Lys residues at the f position were mutated to either an Ala or a Gln residue to form IZ-2A(fA) or IZ-2W(fQ). Separately, both IZ-2A(fA) and IZ-2W(fQ) have a random structure at pH 7 and 20 degrees C. However, together IZ-2A(fA) and IZ-2W(fQ) form a 2:1 complex with a thermal transition midpoint (Tm) of 48 degrees C. This procedure was applied to prepare the ABC-type heterotrimer, in which two sets of Ala-Ala-Trp interactions were designed in the hydrophobic core. Interhelical interaction between the e and g positions and the alpha-helical propensity of the amino acid at the f position were also considered in the design. The resultant three peptides selectively formed the ABC-type heterotrimer with a Tm of 51 degrees C. Other peptide combinations had random coil properties. 相似文献
39.
Alphonse Ekouya Raymond Calas Jacques Dunogues Claude Biran Norbert Duffaut 《Journal of organometallic chemistry》1979,177(1):137-144
Aliphatic saturated amides treated with Me3SiCl/Li/THF were found to react in two ways. Either alkoxysilanes, mono- or di-silylated at the functional carbon, or C-silylated amines were obtained dependent of the original amide structure and the experimental conditions. (Me3Si)2CHN(SiMe3)2 exhibited Physicochemical properties that are particular for hindered rotation about the CN bond. A general mechanism is proposed to explain these results. 相似文献
40.
In an effort to develop an efficient synthetic method of highly diastereoselective (2′R)- and (2′S)-2′-deoxy[2′-2H]guanosines, chemoenzymatic conversion was investigated. The synthesis of (2′R > 98% de)-2′-deoxy[2′-2H]guanosine was achieved by biological transdeoxyribosylation using (2′R > 98% de)-2′-deoxy[2′-2H]uridine, 2,6-diaminopurine, and Enterobacter aerogenes AJ-11125, followed by treatment with adenosine deaminase. (2′S > 98% de)-2′-Deoxy[2′-2H]guanosine was synthesized from (2′S > 98% de)-2′-deoxy[2′-2H]uridine and 2,6-diaminopurine using thymidine phosphorylase and purine nucleoside phosphorylase instead of E. aerogenes AJ-11125. 相似文献