全文获取类型
收费全文 | 10032篇 |
免费 | 860篇 |
国内免费 | 835篇 |
专业分类
化学 | 7659篇 |
晶体学 | 108篇 |
力学 | 284篇 |
综合类 | 66篇 |
数学 | 1681篇 |
物理学 | 1929篇 |
出版年
2024年 | 9篇 |
2023年 | 105篇 |
2022年 | 191篇 |
2021年 | 304篇 |
2020年 | 329篇 |
2019年 | 296篇 |
2018年 | 253篇 |
2017年 | 249篇 |
2016年 | 469篇 |
2015年 | 428篇 |
2014年 | 492篇 |
2013年 | 704篇 |
2012年 | 895篇 |
2011年 | 974篇 |
2010年 | 692篇 |
2009年 | 596篇 |
2008年 | 752篇 |
2007年 | 735篇 |
2006年 | 651篇 |
2005年 | 541篇 |
2004年 | 476篇 |
2003年 | 302篇 |
2002年 | 252篇 |
2001年 | 128篇 |
2000年 | 107篇 |
1999年 | 116篇 |
1998年 | 67篇 |
1997年 | 67篇 |
1996年 | 66篇 |
1995年 | 50篇 |
1994年 | 41篇 |
1993年 | 41篇 |
1992年 | 37篇 |
1991年 | 29篇 |
1990年 | 34篇 |
1989年 | 22篇 |
1988年 | 18篇 |
1987年 | 24篇 |
1986年 | 15篇 |
1985年 | 27篇 |
1984年 | 20篇 |
1983年 | 13篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1975年 | 6篇 |
1974年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
An efficient and reusable heterogeneous catalytic assembly of PdCl2 held in ionic liquid brushes has been synthesized and an environmentally‐friendly procedure was developed for coupling aryl iodides with acrylic acid. These reactions were conducted in water under aerobic conditions with water‐insoluble or even solid aryl iodides and they proceeded smoothly and cleanly without any organic co‐solvent or other additives. A 0.5 mol% (based on Pd atom) dose of the catalyst was found to be sufficient for Mizoroki–Heck reaction. The catalyst is easily recovered post reaction, via simple filtration, and reused at least eight times without a noticeable loss of activity. The protocol has the advantages of excellent yield, environmental friendliness, and catalyst recyclability. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
972.
Remedios Fernández-Fernández Juan Carlos López-Martínez Roberto Romero-González José Luis Martínez-Vidal María Isabel Alarcón Flores Antonia Garrido Frenich 《Chromatographia》2010,72(1-2):55-62
A rapid, reliable and sensitive method has been developed to determine malic and citric acid in fruits and vegetables. The methodology is based on simple extraction with an aqueous solution of ethanol (80% v/v) and subsequent chromatographic analysis by liquid chromatography coupled to mass spectrometry. Electrospray ionization in negative mode was used. The best response for citric and malic acid was provided by molecular ions [M?H]? at m/z 191 and 133 respectively. These ions were used for quantification, whereas other fragments were used as confirmation ions. Different variables involved in the separation and detection process, such as mobile phase, gradient profile and flow rate have been optimised. Linearity, repeatability, recovery and limits of quantification were evaluated. Good linearity was obtained up to 5,000 mg kg?1. Recovery ranged from 90.0 to 104.6%, repeatability (expressed as RSD) was <8% for tested matrices, and limits of quantification were equal or lower than 65 mg kg?1. Finally, the method was applied to the analysis of samples of orange, tomato and pepper. 相似文献
973.
Du‐Qiang Luo Wen‐Liang Zhu Xiao‐Long Yang Shu‐Juan Liang Zhi‐Ran Cao 《Helvetica chimica acta》2010,93(6):1209-1215
Two new daphniphyllum alkaloids named 2‐hydroxyyunnandaphnine D ( 1 ) and methyl 7‐hydroxyhomodaphniphyllate ( 2 ), together with eight known alkaloids, daphnioldhanin D, calyciphylline F, calyciphylline B, deoxycalyciphylline B, daphnicyclidin H, macropodumine C, 9,10‐epoxycalycine A, and yunnandaphnine A, were isolated from the stems and leaves of Daphniphyllum calycinum. Their structures and relative configurations were established on the basis of spectral evidence (including 2D‐NMR) and subsequently confirmed by a single‐crystal X‐ray crystallographic diffraction analysis. 相似文献
974.
975.
Yue‐Hu Wang Qian‐Yun Sun Fu‐Mei Yang Chun‐Lin Long Fu‐Wei Zhao Gui‐Hua Tang Hong‐Mei Niu Huan Wang Qiao‐Qin Huang Jin‐Jin Xu Li‐Juan Ma 《Helvetica chimica acta》2010,93(12):2467-2477
Ten new phenolic compounds including the six neolignans 1 – 3 and 6 – 8 and four caffeoyl derivatives, i.e., myo‐inositol 1‐caffeate ( 9 ), myo‐inositol 6‐caffeate ( 10 ), myo‐inositol 5‐caffeate ( 11 ), and paucine 3′‐β‐D ‐glucopyranoside ( 12 ) were isolated from the whole plants of Selaginella moellendorffii (caffeic acid=3‐(3,4‐dihydroxyphenyl)prop‐2‐enoic acid). Their structures were established by spectroscopic and chemical methods. 相似文献
976.
Hydroformylation of oct‐1‐ene leading to nonanal (denoted by n) and 2‐methyloctanal (denoted by iso), in a novel series of caprolactam‐based and common imidazolium‐based ionic liquid crystals (ILCs; see Fig. 1) carried out for the first time (caprolactam=hexahydro‐2H‐azepin‐2‐one) (Scheme). Variation of the chain length (n) of the alkyl substituent (Cn) at the caprolactam cation (CP+) from n=12 to 18 caused the n/iso ratios to vary from 1.7 to 2.9. Meanwhile, the TOF (turnover frequency) decreased from 148 to 122 mol mol−1 h−1. Hydroformylation in the imidazolium‐based ILCs revealed that [C16MIm]⋅BF4 (n/iso 5.2, TOF 969 mol mol−1 h−1) was more favorable than [C16MIm]⋅MsO (n/iso 3.7, TOF 969 mol mol−1 h−1) for the formation of the unbranched aldehyde. Although the n/iso ratio in caprolactam‐based ILCs was lower than that in imidazolium‐based ILCs, the conversions are higher in the former ILCs on the whole. It should be noted that the lamellar mesophase has a strong effect on the regioselectivity and TOF of the hydroformylation. Also, it is evident that the influences of different ILCs on the hydroformylation under the various reaction conditions are greatly different. The identification of the reaction products was established by GC and GC/MS analyses. 相似文献
977.
978.
Rodrigo Casasnovas David Fernández Joaquín Ortega-Castro Juan Frau Josefa Donoso Francisco Muñoz 《Theoretical chemistry accounts》2011,130(1):1-13
CBS-QB3, two simplified and less computationally demanding versions of CBS-QB3, DFT-B3LYP, and HF quantum chemistry methods
have been used in conjunction with the CPCM continuum solvent model to calculate the free energies of proton exchange reactions
in water solution following an isodesmic reaction approach. According to our results, the precision of the predicted pK
a values when compared to experiment is equivalent to that of the thermodynamic cycles that combine gas-phase and solution-phase
calculations. However, in the aqueous isodesmic reaction schema, the accuracy of the results is less sensitive to the presence
of explicit water molecules and to the global charges of the involved species since the free energies of solvation are not
required. In addition, this procedure makes easier the prediction of pK
a values for molecules that undergo large conformational changes in solvation process and makes possible the pK
a prediction of unstable species in gas-phase such as some zwitterionic tautomers. The successive pK
a values of few amino acids corresponding to the ionization of the α-carboxylic acid and α-amine groups, which is one of the
problematic cases for thermodynamic cycles, were successfully calculated by employing the aqueous isodesmic reaction yielding
mean absolute deviations of 0.22 and 0.19 pK
a units for the first and second ionization processes, respectively. 相似文献
979.
Maria Rambla-Alegre Juan Peris-Vicente Sergio Marco-Peiró Beatriz Beltrán-Martinavarro Josep Esteve-Romero 《Talanta》2010,81(3):894-274
Melamine is a toxic triazine, illegally used as an additive in milk to apparently increase the amount of protein. A chromatographic procedure using a C18 column and a micellar mobile phase of sodium dodecyl sulphate (0.05 M) and propanol (7.5%), buffered at pH 3, and a detection set by absorbance at 210 nm, was reported for the resolution and quantification of melamine in liquid and powdered milk samples. In this work, samples were diluted with a SDS solution and were directly injected, thus avoiding long extraction and experimental procedures. Melamine was eluted in nearly 9.3 min without overlapping the protein band or other endogeneous compounds. The optimal mobile phase composition was taken using a chemometrical approach that considers the retention factor, efficiency and peak shape. Validation was performed following the European Commission's indications (European Decision 2002/657/EC), and the main analytical parameters studied were: linearity (0.02-100 ppm; r2 = 0.999), limit of detection (5 ppb), intra- and inter-day precision (R.S.D. <7.6% and <9.7%, respectively) and robustness (R.S.D. <7.4% for retention time and <5.0% for area). Sensitivity was adequate to detect melamine under the safety limits proposed by the US FDA. Finally, recoveries for several milk samples were found in the 85-109% range. 相似文献
980.
Mathematical Programming - In this paper we consider an aggregation technique introduced by Yıldıran (J Math Control Inf 26:417–450, 2009) to study the convex hull of regions... 相似文献