全文获取类型
收费全文 | 3367篇 |
免费 | 187篇 |
国内免费 | 18篇 |
专业分类
化学 | 2593篇 |
晶体学 | 28篇 |
力学 | 82篇 |
数学 | 466篇 |
物理学 | 403篇 |
出版年
2023年 | 26篇 |
2022年 | 35篇 |
2021年 | 66篇 |
2020年 | 89篇 |
2019年 | 90篇 |
2018年 | 57篇 |
2017年 | 56篇 |
2016年 | 124篇 |
2015年 | 82篇 |
2014年 | 121篇 |
2013年 | 149篇 |
2012年 | 249篇 |
2011年 | 302篇 |
2010年 | 111篇 |
2009年 | 87篇 |
2008年 | 234篇 |
2007年 | 201篇 |
2006年 | 211篇 |
2005年 | 192篇 |
2004年 | 151篇 |
2003年 | 124篇 |
2002年 | 118篇 |
2001年 | 30篇 |
2000年 | 32篇 |
1999年 | 21篇 |
1998年 | 29篇 |
1997年 | 26篇 |
1996年 | 28篇 |
1995年 | 33篇 |
1994年 | 21篇 |
1993年 | 28篇 |
1992年 | 28篇 |
1991年 | 27篇 |
1990年 | 15篇 |
1989年 | 19篇 |
1988年 | 18篇 |
1987年 | 21篇 |
1985年 | 31篇 |
1984年 | 30篇 |
1983年 | 18篇 |
1982年 | 37篇 |
1981年 | 24篇 |
1980年 | 24篇 |
1979年 | 20篇 |
1978年 | 23篇 |
1977年 | 23篇 |
1976年 | 19篇 |
1975年 | 12篇 |
1974年 | 10篇 |
1973年 | 10篇 |
排序方式: 共有3572条查询结果,搜索用时 15 毫秒
221.
Baer AJ Koivisto BD Côté AP Taylor NJ Hanan GS Nierengarten H Van Dorsselaer A 《Inorganic chemistry》2002,41(20):4987-4989
Diplatinum metalloreceptors anti-4a and anti-4b exhibit dynamic behavior in solution that is modified by anion binding. An X-ray crystal structure determination of anti-4a supports its proposed solution structure. 相似文献
222.
A pulsed jet cavity Fourier transform microwave spectrometer has been used to measure the rotational spectra of OCAgX (X = F, Cl, Br) in the frequency range 5-22 GHz. Metal atoms were generated via laser ablation and were allowed to react with CO and a halide precursor, prior to stabilization of the products within a supersonic jet of argon. These are the first experimental observations of OCAgF and OCAgBr, and the first high resolution spectroscopic study of OCAgCl. All three molecules are linear. Accurately determined rotational constants have been used to evaluate the various internuclear distances, which are found to be consistent with trends established for OCAuX and OCCuX species. The C-O distances are short, and the M-C distances are significantly longer than those in other molecules containing a metal-carbonyl bond. Precise values of centrifugal distortion constants and halogen nuclear quadrupole coupling constants have also been determined. The coupling constants are compared with the results of previous studies of OCCuX and OCAuX and are used to infer trends in the electron distributions of the molecules. Ab initio calculations have been performed and employed to predict the geometries, vibrational frequencies, and Mulliken valence orbital populations of the various species. 相似文献
223.
Zhang Z Yang Z Wong H Zhu J Meanwell NA Kadow JF Wang T 《The Journal of organic chemistry》2002,67(17):6226-6227
Conditions for attachment of acetyl chloride, benzoyl chloride, and chloromethyl oxalate to the 3-position of 4-, 5-, 6-, or 7-azaindoles were explored. Best results were achieved with an excess of AlCl(3) in CH(2)Cl(2) followed by the addition of an acyl chloride at room temperature. 相似文献
224.
Johnson BA Kleinman MH Turro NJ Garcia-Garibay MA 《The Journal of organic chemistry》2002,67(20):6944-6953
The rates of phosphorescence decay of 4,7-dimethylindanone (2), 6,9-dimethylbenzosuberone (3), and several related compounds have been analyzed between 4 and 100 K to determine the contributions of intramolecular hydrogen atom tunneling from the o-methyl group to the excited state carbonyl oxygen. Changes in the benzocycloalkanone ring size from five to seven not only affect the geometry at the reaction center, but they also affect the electronic configuration of the triplet excited state in a significant manner. While the triplet state of 5,8-dimethyltetralone (1) in nonpolar glasses can be clearly described as having a predominant n,pi configuration, compounds 2 and 3 have a significantly larger contribution of the less reactive pi,pi state. 4,7-Dimethylindanone (2) is stable under cryogenic conditions and in solution at ambient temperature. In contrast, triplet lifetimes and product analysis indicate that 6,9-dimethylbenzosuberone (3) reacts by quantum mechanical tunneling at temperatures as low as 4 K. A surprisingly small isotope effect k(H)/k(D) approximately 1.1 between 4 and 50 K increases steadily up to k(H)/k(D) approximately 5.1 at 100 K. This unusual observation is interpreted in terms of a vibrationally activated quantum mechanical tunneling process with hydrogen atom transfer at the lowest temperatures being mediated by zero-point-energy reaction-promoting skeletal motions. Results presented here indicate that the combined effects of increasing pi,pi character and unfavorable reaction geometry contribute to the diminished reactivity of o-methyl ketones 2 and 3 as compared to those of tetralone 1. 相似文献
225.
The deracemisation and stereoinversion of both cyclic and acyclic DL-alpha-amino acids, using porcine kidney D-amino acid oxidase (DAAO) and a hydride reducing agent (NaCNBH3-NaBH4), has been investigated. 相似文献
226.
227.
228.
Mycothiol is a low-molecular weight thiol produced by actinomycetes that serves to protect these organisms from oxidative stress and alkylating agents. We report the total synthesis of mycothiol bimane (1) which is a commonly isolated derivative of mycothiol. The synthesis confirms the original structure assignment and unambiguously establishes the absolute stereochemistry of mycothiol to be 1-d-myo-inosityl 2-deoxy-2-(N-acetamido-l-cysteinamido)-alpha-d-glucopyranoside. 相似文献
229.
The new anhydrous borate Na2Co2B12O21 has been synthesized by flux methods and studied by single-crystal X-ray diffraction (space group I2/a with a = 17.1447(15) A, b = 4.5530(5) A, c = 19.4408(15) A, beta = 103.212(5) degrees , V = 1477.4(2) A3, Z = 4). Refinement of its structure reveals it is the first metaborate exhibiting a tunnel network, with internal dimensions of 4.5 x 8.8 A2. Further single-crystal diffraction studies show that the Na+ ions within the tunnels are exchangeable with Li+ along with the absorption of water molecules to form Li2(H2O)2Co2B12O21, making this compound a unique non-siliceous zeotype. 相似文献
230.
Experiments with fast folding proteins are beginning to address the relationship between collapse and folding. We investigate how different scenarios for folding can arise depending on whether the folding and collapse transitions are concurrent or whether a nonspecific collapse precedes folding. Many earlier studies have focused on the limit in which collapse is fast compared to the folding time; in this work we focus on the opposite limit where, at the folding temperature, collapse and folding occur simultaneously. Real proteins exist in both of these limits. The folding mechanism varies substantially in these two regimes. In the regime of concurrent folding and collapse, nonspecific collapse now occurs at a temperature below the folding temperature (but slightly above the glass transition temperature). 相似文献