全文获取类型
收费全文 | 2292篇 |
免费 | 110篇 |
国内免费 | 17篇 |
专业分类
化学 | 1657篇 |
晶体学 | 31篇 |
力学 | 31篇 |
数学 | 134篇 |
物理学 | 566篇 |
出版年
2023年 | 22篇 |
2021年 | 21篇 |
2020年 | 37篇 |
2019年 | 57篇 |
2018年 | 50篇 |
2017年 | 23篇 |
2016年 | 46篇 |
2015年 | 73篇 |
2014年 | 67篇 |
2013年 | 141篇 |
2012年 | 136篇 |
2011年 | 136篇 |
2010年 | 102篇 |
2009年 | 94篇 |
2008年 | 150篇 |
2007年 | 155篇 |
2006年 | 150篇 |
2005年 | 133篇 |
2004年 | 145篇 |
2003年 | 104篇 |
2002年 | 65篇 |
2001年 | 49篇 |
2000年 | 24篇 |
1999年 | 25篇 |
1998年 | 11篇 |
1997年 | 13篇 |
1996年 | 12篇 |
1995年 | 17篇 |
1994年 | 20篇 |
1993年 | 40篇 |
1992年 | 29篇 |
1991年 | 19篇 |
1990年 | 15篇 |
1989年 | 16篇 |
1988年 | 14篇 |
1987年 | 10篇 |
1986年 | 14篇 |
1985年 | 17篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 11篇 |
1981年 | 18篇 |
1980年 | 12篇 |
1979年 | 13篇 |
1978年 | 12篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1975年 | 15篇 |
1974年 | 13篇 |
1973年 | 9篇 |
排序方式: 共有2419条查询结果,搜索用时 46 毫秒
11.
12.
Naoki Tsuchiya Tetsuhiro Yamamoto Hiroki Akagawa Prof. Dr. Takashi Nishikata 《Angewandte Chemie (International ed. in English)》2023,62(21):e202301343
Herein, we report a copper-catalyzed stereospecific fluorination involving CsF and α-bromocarboxamides as tertiary alkyl sources that, unlike traditional stereospecific routes involving stereoinversive SN2 reactions, proceeds with retention of stereochemistry. The developed stereospecific Cu-catalyzed reaction is among the most efficient methods for synthesizing fluorinated molecules that possess highly congested stereogenic carbon centers. Mechanistic studies revealed that the combined reactivity of CuF2 and Cs salt is essential for completing the catalytic cycle. Our catalytic system underwent fluorination exclusively with tertiary alkyl bromides and did not react with primary alkyl bromides, indicating that this stereospecific fluorination methodology is suitable for synthesizing fluorinated building blocks possessing stereo-defined F-containing tertiary carbon stereogenic center. 相似文献
13.
Hiroki Hasegawa Daisuke Sakamaki Hideki Fujiwara 《Angewandte Chemie (International ed. in English)》2023,62(26):e202302498
In this work, we aimed to develop a dicyanomethyl radical that undergoes both reversible C−C bond formation/dissociation and metal-ligand coordination reactions to combine dynamic covalent chemistry (DCC) based on organic radicals with coordination chemistry. We have previously reported a dicyanomethyl radical conjugated with a triphenylamine ( 1 ⋅) that exhibits a monomer/dimer equilibrium between the σ-bonded dimer ( 12 ). We designed and synthesized a novel dicyanomethyl radical with a pyridyl group as a coordination point ( 2 ⋅) by replacing the phenyl group of 1 ⋅ with a 3-pyridyl group. We showed that 2 ⋅ is also in an equilibrium with the σ-bonded dimer ( 22 ) in solution and has suitable thermodynamic parameters for application in DCC. 22 coordinates to PdCl2 in a 2 : 2 ratio to selectively form a metallamacrocycle ( 22 )2(PdCl2)2, and its structure was clarified by single crystal X-ray analysis. Variable-temperature NMR, ESR, and electronic absorption measurements revealed that ( 22 )2(PdCl2)2 also undergoes the reversible C−C bond formation/dissociation reaction. Ligand-exchange experiment showed that 22 was liberated from ( 22 )2(PdCl2)2 by the addition of another ligand with a higher affinity for PdII. This work demonstrated that DCC based on dicyanomethyl radicals works orthogonally to metal-ligand coordination reactions. 相似文献
14.
Dr. Yu Harabuchi Dr. Hiroki Hayashi Dr. Hideaki Takano Prof. Tsuyoshi Mita Prof. Satoshi Maeda 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2023,135(1):e202211936
Systematic reaction path exploration revealed the entire mechanism of Knowles's light-promoted catalytic intramolecular hydroamination. Bond formation/cleavage competes with single electron transfer (SET) between the catalyst and substrate. These processes are described by adiabatic processes through transition states in an electronic state and non-radiative transitions through the seam of crossings (SX) between different electronic states. This study determined the energetically favorable SET path by introducing a practical computational model representing SET as non-adiabatic transitions via SXs between substrate's potential energy surfaces for different charge states adjusted based on the catalyst's redox potential. Calculations showed that the reduction and proton shuttle process proceeded concertedly. Also, the relative importance of SET paths (giving the product and leading back to the reactant) varies depending on the catalyst's redox potential, affecting the yield. 相似文献
15.
Hiroki Yamamoto Takuya Fujiwara Takashi Funatsu Makoto Tsunoda 《Molecules (Basel, Switzerland)》2021,26(8)
Biothiols, such as cysteine and glutathione, play important roles in various intracellular reactions represented by the redox equilibrium against oxidative stress. In this study, a method for intracellular thiol quantification using HPLC-fluorescence detection was developed. Thiols were derivatized with a thiol-specific fluorescence derivatization reagent, viz. ammonium 7-fluoro-2,1,3-benzoxadiazole-4-sulfonate (SBD-F), followed by reversed-phase separation on an InertSustain AQ-C18 column. Six different SBD-thiols (homocysteine, cysteine, cysteinylglycine, γ-glutamylcysteine, glutathione, and N-acetylcysteine as an internal standard) were separated within 30 min using a citric buffer (pH 3.0)/MeOH mobile phase. The calibration curves of all the SBD-thiols had strong linearity (R2 > 0.999). Using this developed method, the thiol concentrations of human chronic myelogenous leukemia K562 cell samples were found to be 5.5–153 pmol/1 × 106 cells. The time-dependent effect of a thiol scavenger, viz. N-ethyl maleimide, on intracellular thiol concentrations was also quantified. This method is useful for elucidating the role of intracellular sulfur metabolism. 相似文献
16.
Florian Venel Dr. Hiroki Nagashima Dr. Andrew G. M. Rankin Christelle Anquetil Dr. Vytautas Klimavicius Dr. Torsten Gutmann Prof. Gerd Buntkowsky Dr. Sylvie Derenne Prof. Olivier Lafon Dr. Arnaud Huguet Dr. Frédérique Pourpoint 《Chemphyschem》2021,22(18):1907-1913
Estuaries are key ecosystems with unique biodiversity and are of high economic importance. Along the estuaries, variations in environmental parameters, such as salinity and light penetration, can modify the characteristics of dissolved organic matter (DOM). Nevertheless, there is still limited information about the atomic-level transformations of DOM in this ecosystem. Solid-state NMR spectroscopy provides unique insights into the nature of functional groups in DOM. A major limitation of this technique is its lack of sensivity, which results in experimental time of tens of hours for the acquisition of 13C NMR spectra and generally precludes the observation of 15N nuclei for DOM. We show here how the sensitivity of solid-state NMR experiments on DOM of Seine estuary can be enhanced using dynamic nuclear polarization (DNP) under magic-angle spinning. This technique allows the acquisition of 13C NMR spectra of these samples in few minutes, instead of hours for conventional solid-state NMR. Both conventional and DNP-enhanced 13C NMR spectra indicate that the 13C local environments in DOM are not strongly modified along the Seine estuary. Furthermore, the sensitivity gain provided by the DNP allows the detection of 15N NMR signal of DOM, in spite of the low nitrogen content. These spectra reveal that the majority of nitrogen is in the amide form in these DOM samples and show an increased disorder around these amide groups near the mouth of the Seine. 相似文献
17.
One‐Pot Synthesis of 3‐Acetyl‐2‐aryl‐3,4‐dihydroquinazolines from N‐[2‐(Azidomethyl)phenyl]benzamides Utilizing Intramolecular Aza‐Wittig Reaction 下载免费PDF全文
Kazuhiro Kobayashi Naoki Matsumoto Mika Nagashima Hiroki Inouchi 《Helvetica chimica acta》2015,98(2):184-189
A new and convenient method for the preparation of 3,4‐dihydroquinazolines 5 with aryl and Ac groups at C(2) and N(3), respectively, has been developed. The key sequence is the formation of aza‐phosphorane intermediates by the reaction of N‐[2‐(azidomethyl)phenyl]benzamides 1 with Ph3P, followed by intramolecular aza‐Wittig reaction and 3‐acetylation, which can be conducted in one‐pot. 相似文献
18.
Hiroki Hirai Kiichi Nakajima Soichiro Nakatsuka Kazushi Shiren Jingping Ni Shintaro Nomura Toshiaki Ikuta Takuji Hatakeyama 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(46):13785-13789
The development of a one‐step borylation of 1,3‐diaryloxybenzenes, yielding novel boron‐containing polycyclic aromatic compounds, is reported. The resulting boron‐containing compounds possess high singlet‐triplet excitation energies as a result of localized frontier molecular orbitals induced by boron and oxygen. Using these compounds as a host material, we successfully prepared phosphorescent organic light‐emitting diodes exhibiting high efficiency and adequate lifetimes. Moreover, using the present one‐step borylation, we succeeded in the synthesis of an efficient, thermally activated delayed fluorescence emitter and boron‐fused benzo[6]helicene. 相似文献
19.
meso‐Dibenzoporphycene has a Large Bathochromic Shift and a Porphycene Framework with an Unusual cis Tautomeric Form 下载免费PDF全文
Dr. Koji Oohora Ayumu Ogawa Tamaki Fukuda Dr. Akira Onoda Prof. Dr. Jun‐ya Hasegawa Prof. Dr. Takashi Hayashi 《Angewandte Chemie (International ed. in English)》2015,54(21):6227-6230
meso‐Monobenzoporphycene (mMBPc) and meso‐dibenzoporphycene (mDBPc), in which one or two benzene moieties are fused at ethylene‐bridged positions (meso‐positions) of porphycene, were prepared in an effort to further delocalize the π‐electrons within the porphycene molecule. mMBPc and mDBPc were fully characterized by mass spectrometry, 1H and 13C NMR spectroscopy, and X‐ray crystallography. The longest‐wavelength Q‐bands of mMBPc and mDBPc are red‐shifted by 92 nm and 418 nm, respectively, compared to that of the unsubstituted porphycene (Pc). Electrochemical measurements indicate that the HOMO is destabilized and the LUMO is stabilized by the fused benzene moieties at the meso positions. Furthermore, both XPS and theoretical studies support the presence of a cis tautomeric form in the ground state of mDBPc, despite the fact that essentially all known porphycene derivatives adopt the trans tautomeric form. 相似文献
20.
The transient heat transfer behavior in the case of heat removal from a cylindrical heat storage vessel packed with spherical particles was investigated experimentally for various factors (flow rate, diameter of spherical particles packed, temperature difference between flowing cold air and spherical particles accumulating heat, and physical properties of spherical particles). The experiments were covered in ranges of Reynolds number based on the mean diameter of spherical particles packed Red = 10.3–2200, porosity?=0.310 to 0.475, ratio of spherical particle diameter to cylinder diameterd/D = 0.0075–0.177 and ratio of length of the cylinder to cylinder diameterL/D=2.5–10. It was found that especially the flow rate and the dimension of spherical particles played an important role in estimating the transient local heat transfer characteristics near the wall of the cylindrical vessel in the present heat storage system. As flow rate and diameter of spherical particles were increased under a given diameter of the cylinder heat storage vessel, the mean heat transfer coefficient between the flow cold air and the hot spherical particles increased and the time period to finish removing heat from the vessel reduced. In addition, the useful experimental correlation equations of mean heat transfer coefficient between both phases and the time period to finish removing heat from the vessel were derived with the functional relationship of Nusselt numberNu d=f [modified Prandtl numberPr * (d/D), Red) and Fourier numberFo = f(d/D, L/D, Pr*, Red). 相似文献