全文获取类型
收费全文 | 4390篇 |
免费 | 182篇 |
国内免费 | 16篇 |
专业分类
化学 | 2621篇 |
晶体学 | 27篇 |
力学 | 125篇 |
数学 | 742篇 |
物理学 | 671篇 |
无线电 | 402篇 |
出版年
2023年 | 29篇 |
2022年 | 89篇 |
2021年 | 107篇 |
2020年 | 61篇 |
2019年 | 88篇 |
2018年 | 58篇 |
2017年 | 84篇 |
2016年 | 149篇 |
2015年 | 138篇 |
2014年 | 127篇 |
2013年 | 270篇 |
2012年 | 244篇 |
2011年 | 259篇 |
2010年 | 149篇 |
2009年 | 150篇 |
2008年 | 247篇 |
2007年 | 234篇 |
2006年 | 261篇 |
2005年 | 210篇 |
2004年 | 170篇 |
2003年 | 172篇 |
2002年 | 119篇 |
2001年 | 77篇 |
2000年 | 64篇 |
1999年 | 71篇 |
1998年 | 47篇 |
1997年 | 45篇 |
1996年 | 49篇 |
1995年 | 46篇 |
1994年 | 49篇 |
1993年 | 47篇 |
1992年 | 37篇 |
1991年 | 25篇 |
1990年 | 27篇 |
1989年 | 24篇 |
1988年 | 20篇 |
1987年 | 26篇 |
1986年 | 23篇 |
1985年 | 49篇 |
1984年 | 46篇 |
1983年 | 43篇 |
1982年 | 37篇 |
1981年 | 28篇 |
1980年 | 38篇 |
1979年 | 30篇 |
1978年 | 22篇 |
1977年 | 26篇 |
1976年 | 20篇 |
1975年 | 19篇 |
1973年 | 16篇 |
排序方式: 共有4588条查询结果,搜索用时 0 毫秒
1.
2.
3.
Highly Selective Copper‐Catalyzed Asymmetric [3+2] Cycloaddition of Azomethine Ylides with Acyclic 1,3‐Dienes 下载免费PDF全文
María González‐Esguevillas Ana Pascual‐Escudero Dr. Javier Adrio Prof. Dr. Juan C. Carretero 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(12):4561-4565
The first examples of the catalytic asymmetric 1,3‐dipolar cycloaddition of azomethine ylides with acyclic activated 1,3‐dienes (and 1,3‐enynes) are described. Under copper catalysis, a selective cycloaddition at the terminal γ,δ‐C?C bond is observed. In addition, depending on the ligand used, either the exo or the endo adduct can be obtained with high selectivity. Under appropriate reaction conditions, the acyclic 1,6‐addition product is detected, suggesting a stepwise mechanism. The resulting C4‐alkenyl‐substituted pyrrolidines are suitable substrates for further access to polycyclic systems, as highlighted by the preparation of hexahydrochromeno[4,3‐b]pyrrole and the tetracyclic core of the alkaloid gracilamine. 相似文献
4.
Vanessa R. Machado Louis P. Sandjo Giovanni L. Pinheiro Milene H. Moraes Mario Steindel Moacir G. Pizzolatti 《Natural product research》2018,32(3):275-281
The natural product lupeol 1 was isolated from aerial parts of Vernonia scorpioides with satisfactory yield, which made it viable to be used as starting material in semisynthetic approach. Ten lupeol derivatives 2–11 were prepared by classical procedures. Including, five new esters derivatives 7–11, which were obtained by structural modifications in the isopropylidene fragment. All semisynthetic compounds and lupeol 1–11 were confirmed by 1H NMR, 13C NMR and HRMS. Their antiprotozoal activity was evaluated in vitro against L. amazonensis and T. cruzi. Derivative 6 showed the best antitrypanosomal activity (IC50 = 12.48 μg/mL) and the lowest cytotoxic derivative (CC50 = 161.50 μg/mL). The mechanism of action of the most active derivatives (4, 6 and 11) is not dependent from the enzyme trypanothione reductase. 相似文献
5.
Alfons Pascual 《Helvetica chimica acta》1989,72(3):556-569
Preparation of Substituted 2-Aminooxazole-4-carbonitriles During our synthetic programme to convert 4-isoxazolylthioureas 3 into the corresponding carbodiimides 5 , a side reaction leading to the hitherto unknown 2-aminooxazole-4-carbonitriles 6 was observed. By selecting appropriate reaction conditions, it was possible to improve the yields of the carbodiimides 5 as well as of the novel oxazole-carbonitriles 6 at will, thus allowing the synthesis of 6 to be conducted in very good yields. To overcome the difficulty of isolating unstable carbodiimides, the synthesis of 6 is best carried out in a one-pot procedure. A limited mechanistic study showed that the formation of 6 proceeds via 5 as the only intermediate. The stability of the N?C?N bonds against base attack (depending strongly on both sterical hindrance and electronic-density factors) forms the only limitation of this new synthetic pathway to oxazole-4-carbonitriles. 相似文献
6.
In this paper we highlighted the synthesis, characterization, and practical exploitation of different types of polyphosphazenes substituted with fluorinated groups. There are several ways in which fluorine atoms can be inserted into polyphosphazenes, all of which leading to different polymers showing a wide range of characteristics. In general it is true that the insertion of fluorine atoms into phosphazene macromolecules leads to an enhancement of the thermal stability, flame resistance, low-temperature elastomericity, and chemical inertness of the phosphazenes obtained. The contribution of Italian research groups to the preparation and exploitation of organic commercial macromolecules grafted onto fluorinated polyphosphazenes is also reviewed. 相似文献
7.
This is a conceptual overview on a polemical subject: the problem of time asymmetry. It is proved that time asymmetry can
be considered as a global generalized symmetry breaking, produced by a choice of a physically admissible state space, in a
global Gel'fand triplet. The well-known physics of irreversible process can be studied using this mathematical structure and
all the arrows of time can be explained and coordinated. But the deeper problems of time definition and time arrow in quantum
gravity remain outside of this landscape. 相似文献
8.
9.
Onofrio Mario Di Vincenzo Plamen Koshlukov Roberto La Scala 《Advances in Applied Mathematics》2006,37(4):541
In this paper we describe completely the involutions of the first kind of the algebra UTn(F) of n×n upper triangular matrices. Every such involution can be extended uniquely to an involution on the full matrix algebra. We describe the equivalence classes of involutions on the upper triangular matrices. There are two distinct classes for UTn(F) when n is even and a single class in the odd case.Furthermore we consider the algebra UT2(F) of the 2×2 upper triangular matrices over an infinite field F of characteristic different from 2. For every involution *, we describe the *-polynomial identities for this algebra. We exhibit bases of the corresponding ideals of identities with involution, and compute the Hilbert (or Poincaré) series and the codimension sequences of the respective relatively free algebras.Then we consider the *-polynomial identities for the algebra UT3(F) over a field of characteristic zero. We describe a finite generating set of the ideal of *-identities for this algebra. These generators are quite a few, and their degrees are relatively large. It seems to us that the problem of describing the *-identities for the algebra UTn(F) of the n×n upper triangular matrices may be much more complicated than in the case of ordinary polynomial identities. 相似文献
10.
Zero-schemes on smooth complex projective varieties, forcing all elements of ample and free linear systems to be reducible, are studied. Relationships among the minimal length of such zero-schemes, the positivity of the line bundle associated with the linear system, and the dimension of the variety are established. Bad linear spaces are also investigated. 相似文献