全文获取类型
收费全文 | 1161篇 |
免费 | 309篇 |
国内免费 | 77篇 |
专业分类
化学 | 1321篇 |
晶体学 | 4篇 |
力学 | 2篇 |
综合类 | 5篇 |
物理学 | 70篇 |
综合类 | 145篇 |
出版年
2024年 | 3篇 |
2023年 | 18篇 |
2022年 | 45篇 |
2021年 | 44篇 |
2020年 | 63篇 |
2019年 | 61篇 |
2018年 | 47篇 |
2017年 | 30篇 |
2016年 | 129篇 |
2015年 | 87篇 |
2014年 | 94篇 |
2013年 | 119篇 |
2012年 | 83篇 |
2011年 | 62篇 |
2010年 | 40篇 |
2009年 | 42篇 |
2008年 | 53篇 |
2007年 | 44篇 |
2006年 | 54篇 |
2005年 | 36篇 |
2004年 | 40篇 |
2003年 | 66篇 |
2002年 | 49篇 |
2001年 | 17篇 |
2000年 | 28篇 |
1999年 | 14篇 |
1998年 | 18篇 |
1997年 | 27篇 |
1996年 | 12篇 |
1995年 | 23篇 |
1994年 | 14篇 |
1993年 | 10篇 |
1992年 | 15篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1975年 | 8篇 |
1974年 | 6篇 |
1973年 | 3篇 |
排序方式: 共有1547条查询结果,搜索用时 31 毫秒
71.
Dr. Matteo P. Placidi Prof. Dr. Mauro Botta Dr. Ferenc K. Kálmán Dr. Gisela E. Hagberg Dr. Zsolt Baranyai Andreas Krenzer Alexandria K. Rogerson Prof. Dr. Imre Tóth Prof. Dr. Nikos K. Logothetis Priv.‐Doz. Dr. Goran Angelovski 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(35):11644-11660
A series of low molecular weight lanthanide complexes were developed that have high 1H longitudinal relaxivities (r1) and the potential to be used as dual frequency 1H and 19F MR probes. Their behavior was investigated in more detail through relaxometry, pH‐potentiometry, luminescence, and multinuclear NMR spectroscopy. Fitting of the 1H NMRD and 17O NMR profiles demonstrated a very short water residence lifetime (<10 ns) and an appreciable second sphere effect. At lower field strengths (20 MHz), two of the complexes displayed a peak in r1 (21.7 and 16.3 mM ?1 s?1) caused by an agglomeration, that can be disrupted through the addition of phosphate anions. NMR spectroscopy revealed that at least two species are present in solution interconverting through an intramolecular binding process. Two complexes provided a suitable signal in 19F NMR spectroscopy and through the selection of optimized imaging parameters, phantom images were obtained in a MRI scanner at concentrations as low as 1 mM . The developed probes could be visualized through both 1H and 19F MRI, showing their capability to function as dual frequency MRI contrast agents. 相似文献
72.
Prof. Xuefeng Wang Prof. Lester Andrews Dipl.Chem. Felix Brosi Dr. Sebastian Riedel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(4):1397-1409
The reactions of laser‐ablated Au, Ag, and Cu atoms with F2 in excess argon and neon gave new absorptions in the M? F stretching region of their IR spectra, which were assigned to metal‐fluoride species. For gold, a Ng? AuF bond was identified in mixed neon/argon samples. However, this bonding was much weaker with AgF and CuF. Molecules MF2 and MF3 (M=Au, Ag, Cu) were identified from the isotopic distribution of the Cu and Ag atoms, comparison of the frequencies for three metal fluorides, and theoretical frequency calculations. The AuF5 molecule was characterized by its strongest stretching mode and theoretical frequency calculations. Additional evidence was observed for the formation of the Au2F6 molecule. 相似文献
73.
Hongren Chen Fengying Hong Dr. Guang Shao Dr. Deyu Hang Lei Zhao Prof. Zhuo Zeng 《化学:亚洲杂志》2014,9(12):3418-3430
By using three synthetic protocols, a series of fluorocarbon and hydrocarbon N‐heterocyclic imidazole‐based liquid crystals (LCs) and related imidazolium‐based ionic liquid crystals (ILCs) have been prepared. The ring size of the N‐heterocycle and the length of the N‐terminal chain (on the imidazolium unit in the ILCs) were modified, and the influence of these structural parameters on liquid‐crystal phases was investigated by means of polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X‐ray diffraction (XRD). These new ILCs exhibit a disordered smectic phase (SmA), good thermal stabilities, a broad smectic phase range, a high dipole moment, relatively low melting points, but high clearing points and strong emission fluorescence relative to imidazole‐based LCs. These encouraging results have led us to believe these fluorocarbon and hydrocarbon N‐heterocyclic imidazole‐based LCs and related imidazolium‐based ILCs could be used as new liquid‐crystalline materials. 相似文献
74.
Constanze Keck Dr. Jennifer Hahn Divanshu Gupta Prof. Dr. Holger F. Bettinger 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(5):e202103614
The boron-nitrogen analogue of ortho-benzyne, 1,2-azaborinine, is a reactive intermediate that features a formal boron-nitrogen triple bond. We here show by combining experimental and computational techniques that the Lewis acidity of the boron center of dibenzo[c,e][1,2]azaborinine allows interaction with the silicon containing single bonds Si−E through the silicon bonding partner E (E=F, Cl, OR, H). The binding to boron activates the Si−E bonds for subsequent insertion reaction. This shows that the BN-aryne is a ferocious species that even can activate and insert into the very strong Si−F bond. 相似文献
75.
Abdurrahman Turksoy Samir Bouayad-Gervais Prof. Franziska Schoenebeck 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(46):e202201435
While the N-trifluoromethylation of cyclic ureas is of interest for the potential to fundamentally change the properties of these biologically relevant moieties, the single synthetic procedure known to date describing their access only gives 4,4-disubstituted or fused aromatic cyclic N-CF3 urea derivatives. We herein report an alternative approach to unleash access to the 4-monosubstituted imidazolidinone motif. The strategy relies on straightforward cyclization of readily accessible acyclic ureas, enabled by Ag-catalysis or light-assisted proton coupled electron transfer. The cyclic core is shown to be highly robust and amenable to various derivatizations, such as tandem Ni-catalysis, C−B, C−N, C−C cross couplings or C−H functionalizations, tolerating basic, nucleophilic and/or oxidizing conditions. 相似文献
76.
Kurt F. Hoffmann Dr. Anja Wiesner Dr. Simon Steinhauer Prof. Dr. Sebastian Riedel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(57):e202201958
Preparation and characterization of the dimeric Lewis superacid [Al(OTeF5)3]2 and various solvent adducts is presented. The latter range from thermally stable adducts to highly reactive, weakly bound species. DFT calculations on the ligand affinity of these Lewis acids were performed in order to rank their remaining Lewis acidity. An experimental proof of the Lewis acidity is provided by the reaction of solvent-adducts of Al(OTeF5)3 with [PPh4][SbF6] and OPEt3, respectively. Furthermore, their reactivity towards chloride and pentafluoroorthotellurate salts as well as (CH3)3SiCl and (CH3)3SiF is shown. This includes the formation of the dianion [Al(OTeF5)5]2−. 相似文献
77.
Polyimide nanocomposites having low-k and UV shielding properties have been developed using fluorine functionalized graphene oxide and bis(quinoline amine) based polyimide. The polyimide was synthesized using bis(quinoline amine) and pyromellitic dianhydride at appropriate experimental conditions, and its molecular structure was confirmed through various spectral analysis such as FTIR and NMR. The polyimide (PI) composites were prepared using bis(quinoline amine), pyromellitic dianhydride, and separately filled with 1, 5, 10 wt% of fluorinated graphene oxide (FGO) through in situ polymerization. The polymer composites were characterized using thermo gravimetric analysis (TGA), X-ray powder diffraction (XRD), and scanning electron microscopy (SEM). In addition, the water contact angle, dielectric behavior, and UV–Vis shielding behavior of FGO/PI composites were evaluated. The value of the water contact angle of the polyimide was increased with increment of FGO in the polyimide matrix. The highest water contact angle of polyimide composites observed 108° was obtained for 15 wt% FGO reinforced polyimide composite. The value of the dielectric constant for neat, 1, 5, and 15 wt% FGO reinforced polyimide composites was obtained as 4.5, 3.7, 2.6, and 2.0, respectively. It is also observed from by UV–Vis spectroscopy analysis that the FGO reinforced polyimide composites have good UV shielding behavior. 相似文献
78.
Stefan Dix Paul Golz Jonas R. Schmid Prof. Dr. Sebastian Riedel Prof. Matthew N. Hopkinson 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(45):11554-11558
Trifluoromethoxylated (hetero)arenes are of great interest for several disciplines, especially in agro- and medicinal chemistry. Radical C−H trifluoromethoxylation of (hetero)arenes represents an attractive approach to prepare such compounds, but the high cost and low atom economy of existing . OCF3 radical sources make them unsuitable for the large-scale synthesis of trifluoromethoxylated building blocks. Herein, we introduce bis(trifluoromethyl)peroxide (BTMP, CF3OOCF3) as a practical and efficient trifluoromethoxylating reagent that is easily accessible from inexpensive bulk chemicals. Using either visible light photoredox or TEMPO catalysis, trifluoromethoxylated arenes could be prepared in good yields under mild conditions directly from unactivated aromatics. Moreover, TEMPO catalysis allowed for the one-step synthesis of valuable pyridine derivatives, which have been previously prepared via multi-step approaches. 相似文献
79.
Prof. Juan Carlos López Dr. Alberto Macario Prof. Assimo Maris Prof. Ibon Alkorta Prof. Susana Blanco 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(55):13870-13878
The rotational spectrum of the weakly bound complex pentafluoropyridine⋅⋅⋅formaldehyde has been investigated using Fourier transform microwave spectroscopy. From the analysis of the rotational parameters of the parent species and of the 13C and 15N isotopologues, the structural arrangement of the adduct has been unambiguously established. The full ring fluorination of pyridine has a dramatic effect on its binding properties: It alters the electron density distribution at the π-cloud of pyridine creating a π-hole and changing its electron donor-acceptor capabilities. In the complex, formaldehyde lies above the aromatic ring with one of the oxygen lone pairs, as conventionally envisaged, pointing toward its centre. This lone pair⋅⋅⋅π-hole interaction, reinforced by a weak C−H⋅⋅⋅N interaction, indicates an exchange of the electron-acceptor roles of both molecules when compared to the pyridine⋅⋅⋅formaldehyde adduct. Tunnelling doublets due to the internal rotation of formaldehyde have also been observed and analysed leading to a discussion on the competition between lone pair⋅⋅⋅π-hole and π⋅⋅⋅π stacking interactions. 相似文献
80.
Copper‐Catalyzed Intermolecular Trifluoromethylazidation of Alkenes: Convenient Access to CF3‐Containing Alkyl Azides
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Fei Wang Xiaoxu Qi Zhaoli Liang Dr. Pinhong Chen Prof. Dr. Guosheng Liu 《Angewandte Chemie (International ed. in English)》2014,53(7):1881-1886
A novel copper‐catalyzed intermolecular trifluoromethylazidation of alkenes has been developed under mild reaction conditions. A variety of CF3‐containing organoazides were directly synthesized from a wide range of olefins, including activated and unactivated alkenes, and the resulting products can be easily transformed into the corresponding CF3‐containing amine derivatives. 相似文献