全文获取类型
收费全文 | 2703篇 |
免费 | 68篇 |
国内免费 | 29篇 |
专业分类
化学 | 1728篇 |
晶体学 | 13篇 |
力学 | 57篇 |
数学 | 555篇 |
物理学 | 447篇 |
出版年
2024年 | 7篇 |
2023年 | 13篇 |
2022年 | 100篇 |
2021年 | 101篇 |
2020年 | 43篇 |
2019年 | 56篇 |
2018年 | 55篇 |
2017年 | 48篇 |
2016年 | 98篇 |
2015年 | 75篇 |
2014年 | 111篇 |
2013年 | 190篇 |
2012年 | 181篇 |
2011年 | 187篇 |
2010年 | 127篇 |
2009年 | 123篇 |
2008年 | 147篇 |
2007年 | 161篇 |
2006年 | 148篇 |
2005年 | 155篇 |
2004年 | 121篇 |
2003年 | 84篇 |
2002年 | 60篇 |
2001年 | 25篇 |
2000年 | 26篇 |
1999年 | 30篇 |
1998年 | 23篇 |
1997年 | 29篇 |
1996年 | 24篇 |
1995年 | 30篇 |
1994年 | 22篇 |
1993年 | 22篇 |
1992年 | 14篇 |
1991年 | 18篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 9篇 |
1985年 | 23篇 |
1984年 | 14篇 |
1983年 | 11篇 |
1982年 | 13篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1966年 | 2篇 |
排序方式: 共有2800条查询结果,搜索用时 15 毫秒
91.
92.
Acidolytic cleavage of unsymmetrically substituted dithioacetal monoxides, ArCH(SR)S(O)Me has been used to prepare linear unsymmetrical disulfides RSSMe. Disproportionation was suppressed by the addition of a small amount of benzyl mercaptan. The required starting materials were conveniently obtained from appropriately substituted sulfines ArC(=SO)SR and methyl lithium. The following disulfides were prepared: R =n-C4H9 (73.5%), n-C7H15 (78%), Ph (77.3%), AcO(CH2)10 (81.5%).The unsymmetrical dithioacetal PhCH(SEt)SC7H15-n gave upon oxidation with one equivalent of peracid a mixture of two dithioacetal monoxides which on treatment with HClO4 led to three disulfides. This result is discussed in term of the mechanism of acidolysis. 相似文献
93.
Water-soluble Co (III) and Co (II) complexes with P- or N-coordinated PTA or PTA oxide ligands, respectively, have been prepared and fully characterized, constituting the first examples of cobalt compounds bearing PTA or any ligand with a cage-like PTA core, the latter complex providing also the first PTA oxide coordination to a metal center. 相似文献
94.
95.
Filipiak P Camaioni DM Fessenden RW Carmichael I Hug GL 《The journal of physical chemistry. A》2006,110(38):11046-11052
The reaction of the alpha-hydroxyalkyl radical of 2-propanol (1-hydroxy-1-methylethyl radical) with nitrite ions was characterized. A product of the reaction was assigned as the adduct nitro radical anion, [HO-C(CH(3))(2)NO(2)](*-). This radical was identified using time-resolved electron spin resonance (TRESR). The radical's magnetic parameters, the nitrogen hyperfine coupling constant (a(N) = 26.39 G), and its g-factor (2.0052) were the same as those of the nitro radical anion previously discovered in (*)OH spin-trapping experiments with the aci-anion of (CH(3))(2)CHNO(2). Production of [HO-C(CH(3))(2)NO(2)](*-) was determined to be 38% +/- 4% of the reaction of (CH(3))(2)C(*)-OH with nitrite. The reason why this fraction was less than 100% was rationalized by invoking the competitive addition at oxygen, which forms [HO-C(CH(3))(2)ONO](*-), followed by a rapid loss of (*)NO. Furthermore, by taking this mechanism into account, the bimolecular rate constant for the total reaction of (CH(3))(2)C(*)-OH with nitrite at reaction pH 7 was determined to be 1.6 x 10(6) M(-1) s(-1), using both decay traces of (CH(3))(2)C(*)-OH and growth traces of [HO-C(CH(3))(2)NO(2)](*-). This correspondence further confirms the nature of the reaction. The reaction mechanism is discussed with guidance by computations using density functional theory. 相似文献
96.
Aleksandra JankowiakEmilia Obijalska Piotr Kaszynski Adam Pieczonka Victor G. Young Jr. 《Tetrahedron》2011,67(19):3317-3327
Four heterocyclic salts 1a-d were prepared by Ca2+-assisted cyclization of fluoro derivatives 3, and investigated by spectroscopic (NMR and UV), electrochemical, and computational (DFT and MP2) methods. The mechanism for the formation of the cations was investigated at the DFT level of theory. 2-D NMR spectroscopy for 1[ClO4] in DMSOd6 aided with DFT results permitted the assignment of 1H and 13C NMR signals in cations 1. The molecular and crystal structures for 1a[ClO4] [C13H10ClNO4 triclinic, P−1, a=9.6517(12) Å, b=11.0470(13) Å, c=12.2373(15) Å, α=67.615(1)°, β=78.845(2)°, γ=87.559(2)°; V=1183.0(2) Å3, Z=4] and 1d[ClO4] [C12H9ClN2O4 triclinic, P−1, a=5.9525(6) Å, b=8.3141(9) Å, c=12.2591(13) Å, α=73.487(1)°, β=83.814(1)°, γ=83.456(1)°; V=576.07(10) Å3, Z=2] were determined by X-ray crystallography and compared with results of DFT and MP2 calculations. Electrochemical analysis gave the reduction potential order (1b>1c∼1d>1a), which is consistent with computational results. 相似文献
97.
Piotr Hanczyc Katarzyna Matczyszyn Krzysztof Pawlik Joanna Olesiak Herve Leh Malcolm Buckle 《Liquid crystals》2013,40(4):461-468
The liquid crystalline (LC) properties of two supercoiled plasmid DNA samples, pBSK (2958 bp) and pGEM (3000 bp), have been studied using polarised light microscopy (PLM), circular dichroism (CD) and UV–Vis spectroscopy. The influence of methods of isolation on plasmid LC behaviour is described, and using PLM we have demonstrated the spontaneous formation of cholesteric fingerprint-like textures. Preliminary studies of LC phase transitions in pGEM show the irreversibility of LC phase formation, as a consequence of changes in the tertiary structure of supercoiled plasmids. Using UV–Vis spectroscopy a hyperchromic effect was observed with increasing temperature. The CD spectra clearly showed structural changes, and probably mismatching of DNA bases, during cooling. Finally, we have observed an irreversible phase transition in plasmid DNA which is very different from that previously reported in linear DNA. 相似文献
98.
Józef Drabowicz Piotr Łyżwa Jan Omelańczuk Marian Mikołajczyk 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1)
Abstract The title compound optically active t-butylphenylphosphine oxide 1 is widely used as a key substrat for the synthesis of other optically active derivatives such as α-hydroxyphosphine oxides, vinylphosphine oxides Till now, it has been obtained by a few rather laborious procedures1 相似文献
99.
Roza Trzcinska Piotr Suder Anna Bodzon-Kulakowska Magdalena Skalska Andrzej Marcinkowski Jerzy Kubacki Roman Pedrys Jerzy Silberring Andrzej Dworak Barbara Trzebicka 《Analytical and bioanalytical chemistry》2013,405(28):9049-9059
Peptide surfaces were obtained by the covalent immobilisation of fluorescently labelled pentapeptides carboxyfluorescein–glycine–arginine–methionine–leucine–glycine, either directly or through a poly(ethylene glycol) (PEG) linker on modified silicon wafers. Each step during the preparation of the peptide surfaces was confirmed by several surface characterisation techniques. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy were used to determine the surface composition, the wafers philicity was measured by contact angle and atomic force microscopy was used to investigate the surface morphology. Exposure of the peptide surfaces to trypsin resulted in the release of a fluorescently labelled peptide product, which allowed the kinetics of the enzymatic reaction to be followed with the aid of fluorescence spectroscopy. The electrospray ionisation mass spectrometry analysis of the post-digestion solution confirmed that the pentapeptides attached to the solid support undergo specific trypsin hydrolysis at the C-terminus of the arginine residues. Detailed surface analyses before and after the enzyme action was performed using ToF-SIMS. Because of the limited accessibility of the short peptide directly attached to the surface, a quantitative yield of enzymatic hydrolysis was observed only in case when the peptide was bound through the PEG linker. The insertion of the PEG linker increased the number of immobilised peptides and the rate of enzymatic digestion which consequently improved the quality of the enzyme assays. The described approach may be used for different peptide sequences designed for other proteases. Figure
Monitoring of trypsin hydrolysis on PEG-peptide surface 相似文献
100.
Grzegorz Zi?ba Marcin Rojkiewicz Violetta Kozik Krystyna Jarzembek Anna Jarczyk Aleksander Sochanik Piotr Ku? 《Monatshefte für Chemie / Chemical Monthly》2012,58(4):153-159