全文获取类型
收费全文 | 3733篇 |
免费 | 174篇 |
国内免费 | 16篇 |
专业分类
化学 | 2669篇 |
晶体学 | 20篇 |
力学 | 54篇 |
数学 | 523篇 |
物理学 | 657篇 |
出版年
2023年 | 24篇 |
2022年 | 58篇 |
2021年 | 77篇 |
2020年 | 57篇 |
2019年 | 64篇 |
2018年 | 58篇 |
2017年 | 50篇 |
2016年 | 129篇 |
2015年 | 112篇 |
2014年 | 121篇 |
2013年 | 225篇 |
2012年 | 221篇 |
2011年 | 251篇 |
2010年 | 173篇 |
2009年 | 161篇 |
2008年 | 218篇 |
2007年 | 250篇 |
2006年 | 232篇 |
2005年 | 215篇 |
2004年 | 176篇 |
2003年 | 125篇 |
2002年 | 111篇 |
2001年 | 122篇 |
2000年 | 83篇 |
1999年 | 85篇 |
1998年 | 44篇 |
1997年 | 44篇 |
1996年 | 65篇 |
1995年 | 36篇 |
1994年 | 36篇 |
1993年 | 30篇 |
1992年 | 26篇 |
1991年 | 22篇 |
1990年 | 18篇 |
1989年 | 20篇 |
1988年 | 17篇 |
1987年 | 15篇 |
1986年 | 24篇 |
1985年 | 20篇 |
1984年 | 17篇 |
1983年 | 13篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1975年 | 7篇 |
1974年 | 4篇 |
排序方式: 共有3923条查询结果,搜索用时 15 毫秒
1.
Experimental and Theoretical Studies on the Rearrangement of 2‐Oxoazepane α,α‐Amino Acids into 2′‐Oxopiperidine β2,3,3‐Amino Acids: An Example of Intramolecular Catalysis 下载免费PDF全文
Dr. Diego Núñez‐Villanueva Dr. M. Ángeles Bonache Laura Lozano Dr. Lourdes Infantes Prof. José Elguero Prof. Ibon Alkorta Prof. M. Teresa García‐López Dr. Rosario González‐Muñiz Dr. Mercedes Martín‐Martínez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(6):2489-2500
Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis. 相似文献
2.
3.
Rafael López 《Annals of Global Analysis and Geometry》1997,15(3):201-210
In this paper we study constant mean curvature compact surfaces with two Jordan curves in parallel planes as boundary and we investigate the point at which the surface inherits the symmetries of its boundary. 相似文献
4.
C. Foces‐Foces M. Lpez‐Rodríguez C. Prez 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(2):o95-o97
The title compound, C12H21NO3, crystallizes in two polymorphic forms, viz. the tetragonal form described here and the monoclinic form described previously [Foces‐Foces, López‐Rodríguez, Pérez, Martín & Pérez‐Hernández (2007). Cryst. Growth Des. 7 , 905–911]. The differences in the conformations of the hydroxymethyl and methylaminocarbonyl substituents have important consequences in the hydrogen‐bond interaction motifs and, therefore, in the packing arrangements. These forms are concomitant polymorphs with melting points differing by 3 K. 相似文献
5.
6.
D. Barrios Rolanía G. Lpez Lagomasino E. B. Saff 《Journal of Approximation Theory》2003,124(2):263-281
Using a convergence theorem for Fourier–Padé approximants constructed from orthogonal polynomials on the unit circle, we prove an analogue of Hadamard's theorem for determining the radius of m-meromorphy of a function analytic on the unit disk and apply this to the location of poles of the reciprocal of Szeg
functions. 相似文献
7.
B. Velázquez-Martí C. Gracia-López 《International Journal of Infrared and Millimeter Waves》2004,25(7):1109-1122
The thermal treatment by millimeter waves for the soil disinfection can be one possible alternative to chemical treatments. This physical method is based on incrementing the soil temperature and its pathogens irradiating with high frequency electromagnetic waves. So the previous knowledge of the temperature distribution in the irradiated soil is essential for achieving an effective bad microorganism and weed seeds elimination. This report analyse the heating kinetic and spatial distribution of the maximum temperatures reached by the soil. It is presented a mathematic model about how are distributed the reached temperatures in the depth of the irradiated soil. This model concludes that when an orchard soil is irradiated superficially by microwaves, the microwaves have a big attenuation due to the soil dielectric properties and the water located in the pores of the most superficial layer. This fact causes a shield effect blocking the waves penetration in few centimetres. The heating by radiation is reduced to the superficial layer. The heating propagation in the depth is occurred by conduction following the Fourier equations. 相似文献
8.
9.
10.
Given a constant of motion for the one-dimensional harmonic oscillator with linear dissipation in the velocity, the problem to get the Hamiltonian for this system is pointed out, and the quantization up to second order in the perturbation approach is used to determine the modification on the eigenvalues when dissipation is taken into consideration. This quantization is realized using the constant of motion instead of the Hamiltonian.
PACS: 03.20.+i, 03.30.+p, 03.65.−w,03.65.Ca 相似文献