全文获取类型
收费全文 | 1998篇 |
免费 | 58篇 |
国内免费 | 2篇 |
专业分类
化学 | 1414篇 |
晶体学 | 37篇 |
力学 | 63篇 |
数学 | 164篇 |
物理学 | 380篇 |
出版年
2023年 | 18篇 |
2022年 | 32篇 |
2021年 | 47篇 |
2020年 | 40篇 |
2019年 | 70篇 |
2018年 | 49篇 |
2017年 | 46篇 |
2016年 | 62篇 |
2015年 | 39篇 |
2014年 | 62篇 |
2013年 | 154篇 |
2012年 | 127篇 |
2011年 | 143篇 |
2010年 | 76篇 |
2009年 | 55篇 |
2008年 | 102篇 |
2007年 | 98篇 |
2006年 | 91篇 |
2005年 | 84篇 |
2004年 | 100篇 |
2003年 | 49篇 |
2002年 | 35篇 |
2001年 | 21篇 |
2000年 | 17篇 |
1999年 | 24篇 |
1998年 | 9篇 |
1997年 | 9篇 |
1996年 | 12篇 |
1995年 | 16篇 |
1993年 | 14篇 |
1992年 | 14篇 |
1990年 | 10篇 |
1989年 | 17篇 |
1988年 | 10篇 |
1987年 | 14篇 |
1986年 | 11篇 |
1985年 | 11篇 |
1984年 | 14篇 |
1983年 | 15篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1980年 | 26篇 |
1979年 | 12篇 |
1978年 | 14篇 |
1977年 | 23篇 |
1976年 | 13篇 |
1975年 | 12篇 |
1974年 | 8篇 |
1973年 | 9篇 |
1971年 | 8篇 |
排序方式: 共有2058条查询结果,搜索用时 62 毫秒
141.
Vanda Veprek-Bilinski Krishna Narasimhan Andr S. Dreiding 《Helvetica chimica acta》1978,61(8):3018-3027
Reaction of Diphenylcyclopropenone with β-Carbonyl-enolates. I. Use of Acetylacetone, Methyl Acetoacetate, 2-Ethoxycarbonyl-cyclododecanone and Dimethyl Malonate The reaction of the sodium salts of acetylacetone ( 6 ), methyl acetoacetate (7) , 2-ethoxycarbonyl-cyclododecanone (8) , dimethyl malonate (19) and its methyl derivative 20 with diphenylcyclopropenone ( 5 ) in dimethylformamide at room temperature led to the unsaturated γ-lactones 14, 15, 17, 22 and 36 . In the case of dimethyl malonate ( 19 ), the halfester 21 , the acyl-malonic ester 24 and the indenone-malonic ester 23 were also isolated. Several intermediates and the final products were characterised by reactions and spectroscopically. A general mechanism is discussed for the addition of cyclo-propenones ( 1 ) to the enolate salts of β-dicarbonyl compounds 4 involving the bicyclic lactone-enolates 18 and 32 as intermediates. The products formed via 18 and 32 are considered to be the result of an attack of one of the oxygen atoms of the β-carbonyl-enolate anion ( 4 ), the product 24 , on the other hand, of the attack of the α-carbon atom of 4 ; in both cases the attack is on the carbonyl C-atom of 5 . 相似文献
142.
A new stable product from the reaction of nitrosobenzene and N-ethoxycarbonylazepine is shown to be a 1,4-cycloadduct. 相似文献
143.
V. Parthasarathi Paul T. Beurskens H. J. J. Behm S. Garcia-Granda M. Krishna Pillay 《Journal of chemical crystallography》1986,16(6):833-840
The structure of the the title compound, C30H29NO, was determined by X-rays.M
r
=419.57, triclinic, space groupP1,a=5.8922(12),b=8.5855(11),c=12.2216(20) Å,=78.145(12)°,=79.181(15)°, =76.108(14)°,V
c
=581.1(1) Å3,Z=1,D
x
=1.199 Mg m–3. CuK radiation (graphite crystal monochromator, =1.54184 Å),(CuK)=5.17 cm–1,T=290K. Final conventionalR-factor=0.039,R
w
=0.042 for 2795 observed reflections and 377 variables. The structure was solved usingMultan andDirdif. With a view to determining the conformational preference of the piperidine ring when it is highly sustituted with a bulky group like the phenyl group, the present X-ray investigation was undertaken. 相似文献
144.
A facile synthesis of a 5,7,5-fused ring system that is present in thapsigargins belonging to a novel family of sesquiterpene lactones, guainanolides, using domino enyne-RCM is reported here. 相似文献
145.
A data-processing technique is proposed for use with conventional frequency-chirped absorption spectroscopy to ensure accurate mapping of spectral features into time-domain signatures with arbitrarily fast readout chirp rates. This technique recovers the spectrum from a signal that is distorted owing to the fast chirp rate and therefore facilitates fast measurement of the spectral features over a broad spectral range with high resolution. Both numerical simulations and experimental results are presented. 相似文献
146.
We report a technique that is capable of making simultaneous two-point time-series measurements of minor-species concentrations in turbulent flames. The experimental setup, which incorporates picosecond time-resolved laser-induced fluorescence, has a spatial resolution of less than 250 microm and a temporal resolution of less than 100 micros, which spatially and temporally resolve microscales in many turbulent flows. Two-point time-series data are given for a standard turbulent nonpremixed flame at Re= 10,000, including a discussion of potential implications. 相似文献
147.
Dubbeldam D Calero S Vlugt TJ Krishna R Maesen TL Beerdsen E Smit B 《Physical review letters》2004,93(8):088302
We present a method to determine potential parameters in molecular simulations of confined systems through fitting on experimental isotherms with inflection points. The procedure uniquely determines the adsorbent-adsorbate interaction parameters and is very sensitive to the size parameter. The inflection points in the isotherms are often related to a subtle interplay between different adsorption sites. If a force field can predict this interplay, it also reproduces the remaining part of the isotherm correctly, i.e., the Henry coefficients and saturation loadings. 相似文献
148.
Detection of hypophosphite, phosphite, and orthophosphate in natural geothermal water by ion chromatography 总被引:3,自引:0,他引:3
McDowell MM Ivey MM Lee ME Firpo VV Salmassi TM Khachikian CS Foster KL 《Journal of chromatography. A》2004,1039(1-2):105-111
Current doctrine states that phosphorus is incorporated into cells in the pentavalent(V) oxidation state as orthophosphate. However, recent studies show that microorganisms contain enzymes used to metabolize reduced forms of phosphorous, including phosphite(III) and hypophosphite(I), which suggests that there is a natural source for these chemical species. This paper will discuss suppressed conductivity ion chromatography methods developed to detect hypophosphite, phosphite, and orthophosphate in a geothermal water matrix containing fluoride, chloride, bromide, nitrate, hydrogen carbonate and sulfate. All peaks were clearly resolved, and calibrations were linear with estimated 3sigma detection limits of 0.83, 0.39, and 0.35 microM for hypophosphite, phosphite, and orthophosphate, respectively. 相似文献
149.
We obtained a long-time-storage mechanism for spectral features in thulium ions doped into YAG by applying a magnetic field that splits the electronic ground state. We show experimentally that the storage time can be more than 30 s, which is 3 orders of magnitude longer than that of the metastable state that normally is used for information storage in this material. Level splitting and storage lifetimes for various magnetic field strengths of as much as 5 T were investigated. This storage mechanism will be relevant in the many coherent transient-based signal-processing schemes in which Tm:YAG is being used, and we demonstrate long-time storage in a basic data storage application. 相似文献
150.
Shultz DA Fico RM Bodnar SH Kumar RK Vostrikova KE Kampf JW Boyle PD 《Journal of the American Chemical Society》2003,125(38):11761-11771
A magnetostructural correlation (conformational electron spin exchange modulation) within an isostructural series of biradical complexes is presented. X-ray crystal structures, variable-temperature electron paramagnetic resonance spectroscopy, zero-field splitting parameters, and variable-temperature magnetic susceptibility measurements were used to evaluate molecular conformation and electron spin exchange coupling in this series of molecules. Our combined results indicate that the ferromagnetic portion of the exchange couplings occurs via the cross-conjugated pi-systems, while the antiferromagnetic portion occurs through space and is equivalent to incipient bond formation. Thus, molecular conformation controls the relative amounts of ferro- and antiferromagnetic contributions to exchange coupling. In fact, the exchange parameter correlates with average semiquinone ring torsion angles via a Karplus-Conroy-type relation. Because of the natural connection between electron spin exchange coupling and electronic coupling related to electron transfer, we also correlate the exchange parameters in the biradical complexes to mixed valency in the corresponding quinone-semiquinone radical anions. Our results suggest that delocalization in the cross-conjugated, mixed-valent radical anions is proportional to the ferromagnetic contribution to the exchange coupling in the biradical oxidation states. 相似文献