全文获取类型
收费全文 | 1088篇 |
免费 | 26篇 |
国内免费 | 2篇 |
专业分类
化学 | 876篇 |
晶体学 | 4篇 |
力学 | 9篇 |
数学 | 82篇 |
物理学 | 145篇 |
出版年
2022年 | 4篇 |
2021年 | 7篇 |
2020年 | 9篇 |
2019年 | 11篇 |
2018年 | 11篇 |
2017年 | 12篇 |
2016年 | 31篇 |
2015年 | 17篇 |
2014年 | 22篇 |
2013年 | 41篇 |
2012年 | 63篇 |
2011年 | 71篇 |
2010年 | 37篇 |
2009年 | 36篇 |
2008年 | 60篇 |
2007年 | 77篇 |
2006年 | 68篇 |
2005年 | 52篇 |
2004年 | 60篇 |
2003年 | 60篇 |
2002年 | 54篇 |
2001年 | 12篇 |
2000年 | 19篇 |
1999年 | 13篇 |
1998年 | 13篇 |
1997年 | 7篇 |
1996年 | 20篇 |
1995年 | 6篇 |
1994年 | 15篇 |
1993年 | 11篇 |
1992年 | 11篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1987年 | 11篇 |
1986年 | 15篇 |
1985年 | 21篇 |
1984年 | 17篇 |
1983年 | 6篇 |
1982年 | 8篇 |
1981年 | 14篇 |
1980年 | 11篇 |
1979年 | 11篇 |
1978年 | 12篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1974年 | 3篇 |
1973年 | 4篇 |
1972年 | 5篇 |
排序方式: 共有1116条查询结果,搜索用时 15 毫秒
181.
182.
Optically active o-substituted diphenyl N-substituted sulphilimines are readily synthesised by the reaction of the corresponding sulphides and t-butyl hypochlorite in the presence of l-menthol and amide anions. (−)-N-p-Tolylsulphonylsulphilimines (1, 2) obtained were converted to the corresponding (−)-N-unsubstituted sulphilimines (8, 9) by treating them with concentrated sulphuric acid. When (−)-S-o-anisyl S-phenyl N-(unsubstituted) sulphilimine (8) was treated with acylating agents or acrylonitrile, the corresponding optically active (−)-N-substituted sulphilimines were prepared with complete retention at sulphur. The absolute configuration of (−)-S-o-anisyl S-phenyl N-p-tolylsulphonylsulphilimine (1) was determined by converting it to (+)-S-o-anisyl sulphoxide (17). CD curves of (−)-o-substituted diarylsulphilimines exhibited a negative Cotton effect at around 270–285 nm, which was assigned to (S)-configuration at sulphur by comparing with the analogous sulphoxides. The substituent on the imino group of the sulphilimine gave no appreciable effect on the CD behavior and the lack of substituent effect was considered to be due to the semi-polar character of the S(IV)-N bond. Unusual effect of o-methoxy group on the CD curves was discussed in connection with solvent effect. Mechanism of this asymmetric synthesis has been investigated, and it has become apparent that the diastereomeric menthoxysulphonium chloride was an excess of (RR)-configuration was formed initially and the amide anion attacks the S atom of the salt with net inversion. 相似文献
183.
184.
A novel method for the preparation of N-acylpyrrole is described. The method involves condensation of carboxylic acids with 2,4,4-trimethoxybutan-1-amine, followed by acid-mediated cyclization to form the pyrrole ring. The preparative procedure is highly tolerant of a variety of functional groups. 相似文献
185.
The Hofmann-type rearrangement of aromatic and aliphatic imides using a hypervalent iodine(III) reagent generated in situ from PhI, m-CPBA, and TsOH·H(2)O proceeded in the presence of a base in alcohol to provide anthranilic acid derivatives and amino acid derivatives in high yields, respectively. This reaction proceeds through a tandem reaction via alcoholysis followed by a Hofmann rearrangement promoted by the formation of an imide-λ(3)-iodane intermediate. 相似文献
186.
Tomoyuki Mochida Shinya Suzuki Hiroshi Moriyama Hiroshi Terao Tadashi Sugawara 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(10):1183-1184
The crystal structure of the title compound, (C16H36N)[Ni(C4N2S2)2], shows stacking of the dimerized anions, surrounded by columns of cations. 相似文献
187.
Tanemi Asai Shu Shimamoto Tohru Shibata Tadatomo Kawai Shinya Teramachi 《Macromolecular Symposia》2006,242(1):5-12
The chromatographic method for the determination of the intermolecular acetyl distribution of cellulose triacetate (CTA) was established; the key point of the method is a gradient elution by gradually changing the solvent composition of the ternary mixture of chloroform, methanol, and water on a phenyl-bonded silica gel stationary phase. A positive correlation between the retention time and the degree of acetyl substitution was observed under the established chromatographic conditions. The validity of the elution peak width as the measure of intermolecular acetyl distribution was confirmed by the co-injection and fractionation experiments and the comparison of a peak width with a statistically estimated distribution. 相似文献
188.
An improved synthesis of methyl 4,6-0-benzylidene-2,3-dideoxy-2-nitro-β-D-erythro-hex-2-enopyranoside and its reactions with various nucleophiles are described; all the nucleophiles were found to approach exclusively or predominantly from the equatorial side of the molecule, giving the β-D-glucopyranoside derivatives as the major or exclusive product. The stereochemical course of approach of a nucleophile observed in the present reactions and in the literature are discussed. 相似文献
189.
In the presence of trifluoroacetic acid, N-phenyl-N′-phenoxyurea (1a) rearranges to N-(4′-hydroxy-2-biphenylyl)urea (2a) and N-carbamoyl-2-hydroxy-diphenylamine (3a). The rearrangement is an intramolecular reaction, and the transition state of the breakage of the N-O bond is deduced to be polarized in the form Nδ- --- Oδ+. The reaction is entirely new and constitutes a fundamental aromatic rearrangement. 相似文献
190.
Andreas Galka Tohru Ozaki Jorge Bosch Bayard Okito Yamashita 《Journal of statistical physics》2006,124(5):1275-1315
We address the issue of inferring the connectivity structure of spatially extended dynamical systems by estimation of mutual information between pairs of sites. The well-known problems resulting from correlations within and between the time series are addressed by explicit temporal and spatial modelling steps which aim at approximately removing all spatial and temporal correlations, i.e. at whitening the data, such that it is replaced by spatiotemporal innovations; this approach provides a link to the maximum-likelihood method and, for appropriately chosen models, removes the problem of estimating probability distributions of unknown, possibly complicated shape. A parsimonious multivariate autoregressive model based on nearest-neighbour interactions is employed. Mutual information can be reinterpreted in the framework of dynamical model comparison (i.e. likelihood ratio testing), since it is shown to be equivalent to the difference of the log-likelihoods of coupled and uncoupled models for a pair of sites, and a parametric estimator of mutual information can be derived. We also discuss, within the framework of model comparison, the relationship between the coefficient of linear correlation and mutual information. The practical application of this methodology is demonstrated for simulated multivariate time series generated by a stochastic coupled-map lattice. The parsimonious modelling approach is compared to general multivariate autoregressive modelling and to Independent Component Analysis (ICA). 相似文献