全文获取类型
收费全文 | 2916篇 |
免费 | 50篇 |
国内免费 | 14篇 |
专业分类
化学 | 2058篇 |
晶体学 | 13篇 |
力学 | 76篇 |
数学 | 372篇 |
物理学 | 461篇 |
出版年
2022年 | 23篇 |
2021年 | 28篇 |
2020年 | 18篇 |
2019年 | 32篇 |
2018年 | 29篇 |
2016年 | 46篇 |
2015年 | 40篇 |
2014年 | 51篇 |
2013年 | 166篇 |
2012年 | 131篇 |
2011年 | 129篇 |
2010年 | 97篇 |
2009年 | 98篇 |
2008年 | 126篇 |
2007年 | 138篇 |
2006年 | 140篇 |
2005年 | 147篇 |
2004年 | 128篇 |
2003年 | 111篇 |
2002年 | 114篇 |
2001年 | 49篇 |
2000年 | 50篇 |
1999年 | 31篇 |
1998年 | 26篇 |
1997年 | 35篇 |
1996年 | 50篇 |
1995年 | 33篇 |
1994年 | 40篇 |
1993年 | 38篇 |
1992年 | 27篇 |
1991年 | 40篇 |
1990年 | 29篇 |
1989年 | 19篇 |
1988年 | 18篇 |
1987年 | 23篇 |
1986年 | 16篇 |
1985年 | 46篇 |
1984年 | 51篇 |
1983年 | 32篇 |
1982年 | 41篇 |
1981年 | 36篇 |
1980年 | 48篇 |
1979年 | 43篇 |
1978年 | 55篇 |
1977年 | 44篇 |
1976年 | 34篇 |
1975年 | 40篇 |
1974年 | 39篇 |
1973年 | 39篇 |
1972年 | 17篇 |
排序方式: 共有2980条查询结果,搜索用时 15 毫秒
991.
Dr. Denis Lesage Prof. Anne Milet Dr. Antony Memboeuf Jérôme Blu Dr. Andrew E. Greene Prof. Jean‐Claude Tabet Dr. Yves Gimbert 《Angewandte Chemie (International ed. in English)》2014,53(7):1939-1942
The mechanism of the Pauson–Khand reaction has been studied by mass spectrometry and it has been found, through ion‐molecule reaction with 13CO, that the carbon monoxide incorporated into the product cyclopentenone is one that has been retained within the complex. Theoretical and kinetic calculations support this finding, which provides a complementary explanation for the effect of Pauson–Khand promoters. 相似文献
992.
Nardjes Mouas Toma Jean‐Claude Daran Hocine Merazig Eric Manoury 《Acta Crystallographica. Section C, Structural Chemistry》2014,70(5):460-464
In our ongoing development of ferrocene ligands, 1‐dimethylamino‐2‐(diphenylphosphinothioyl)ferrocene is being used as a convenient building block to obtain racemic or enantiomerically pure ligands. Using this building block in large excess allowed the formation of several by‐products, two of which have already been reported; the structure of a third by‐product, namely 1‐(diphenylphosphinothioyl)‐2‐{[(diphenylphosphinothioyl)sulfanyl]methyl}ferrocene, [Fe(C5H5)(C30H25P2S3)], is presented here. The crystal structure is built up from a ferrocene unit, with one of the cyclopentadienyl (Cp) rings substituted in the 1‐ and 2‐positions by a protected diphenylphosphinothioyl group and a [(diphenylphosphinothioyl)sulfanyl]methyl fragment, –CH2SP(=S)Ph2. There are C—H...S interactions which result in the formation of chains parallel to the c axis. After desulfurization, the crude material was then reacted with Pd and Pt (M) precursors [MCl2(CH3CN)2] to yield two isostructural dinuclear complexes arranged around twofold axes, namely (R,R/S,S)‐bis{μ‐[2‐(diphenylphosphanyl)ferrocen‐1‐yl]methanethiolato‐κ3P,S:S}bis[chloridopalladium(II)] pentane disolvate, [Pd2{Fe(C5H5)(C18H15PS)}2Cl2]·2C5H12, and the platinum(II) analogue, (R,R/S,S)‐bis{μ‐[2‐(diphenylphosphanyl)ferrocen‐1‐yl]methanethiolato‐κ3P,S:S}bis[chloridoplatinum(II)] toluene monosolvate, [Pt2{Fe(C5H5)(C18H15PS)}2Cl2]·C7H8, in which the two metal atoms present a slightly distorted square‐planar geometry formed by two bridging S atoms and P and Cl atoms. The P,S‐chelating ligand results from the rupture of one of the P—S bonds in the starting ligand. These dinuclear complexes display a butterfly geometry. Surprisingly, only the (R,R/S,S) diastereoisomer has been isolated. 相似文献
993.
Didjour Albert Kambir Ahmont Claude Landry Kablan Thierry Acafou Yapi Sophie Vincenti Jacques Maury Nicolas Baldovini Pierre Tomi Mathieu Paoli Jean Brice Boti Flix Tomi 《Molecules (Basel, Switzerland)》2022,27(12)
The variability of chemical composition of the leaf essential oil (EO) from Neuropeltis acuminata, a climbing liana growing wild in Ivory Coast, was investigated for the first time. The in vitro anti-inflammatory activity was also evaluated. Thirty oil samples were isolated from leaves collected in three forests of the country and analyzed using a combination of Column Chromatography (CC), Gas Chromatography with Retention Indices (GC(FID)), Gas Chromatography-Mass Spectrometry (GC-MS), and 13Carbon-Nuclear Magnetic Resonance (13C-NMR). Fractionation by CC led to the first-time isolation from natural source of δ-cadinen-11-ol, whose structural elucidation by one dimension (1D) and 2D-NMR spectroscopy is reported here. Finally, 103 constituents accounting for 95.7 to 99.6% of the samples’ compositions were identified. As significant variations of the major constituents were observed, the 30 oil compositions were submitted to hierarchical cluster and principal components analyses. Five distinct groups were evidenced: Group I, dominated by (E)-β-caryophyllene, kessane, and δ-cadinene, while the main constituents of Group II were germacrene B, ledol, α-humulene, (E)-γ-bisabolen-12-ol, and γ-elemene. Group III exhibited guaiol, germacrene D, atractylone, (E)-γ-bisabolen-12-ol, δ-cadinene and bulnesol as main compounds. Group IV was dominated by (E)-nerolidol, guaiol, selina-4(15),7(11)-diene and bulnesol, whereas (E)-β-caryophyllene, α-humulene and α-muurolene were the prevalent compounds of Group V. As the harvest took place in the same dry season in the three forests, the observed chemical variability could be related to harvest sites, which includes climatic and pedologic factors, although genetic factors could not be excluded. The leaf oil sample S24 behaved as a high inhibitor of LipOXygenase (LOX) activity (half maximum Inhibitory Concentration, IC50: 0.059 ± 0.001 mg mL−1), suggesting an anti-inflammatory potential. 相似文献
994.
Dr. Lenka Pallova Dr. Etienne S. Gauthier Dr. Laura Abella Marion Jean Dr. Nicolas Vanthuyne Dr. Vincent Dorcet Dr. Laure Vendier Prof. Jochen Autschbach Dr. Jeanne Crassous Dr. Stéphanie Bastin Dr. Vincent César 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(28):7722-7730
The straightforward, multigram-scale synthesis of the partially saturated H6-fluoreno[n]helicenes (n=5 or 7) featuring a central, overcrowded alkene is described. The key cyclization step was based on an intramolecular McMurry reaction from the corresponding 1,5-diketones. Chiral stationary phase HPLC analysis and isomer separation indicate that each helicenic compound is constituted of three diastereoisomers at room temperature, i. e. the configurationally stable (R,R,P)/(S,S,M) pair of enantiomers and an apparently achiral compound resulting from the rapid interconversion between the (R,S,P) and (S,R,M) enantiomers. The partially saturated H6-fluoreno[n]helicenes are oxidatively aromatized to give an efficient access to the corresponding fluoreno[n]helicenes. The chiroptical properties (vibrational and electronic circular dichroism) of the chiral, enantiopure compounds have been measured and analyzed by quantum-chemical calculations, confirming their helicoidal nature. 相似文献
995.
Dr. Hashem Amini Dr. Nancy Weisbach Dr. Sébastien Gauthier Dr. Helene Kuhn Dr. Nattamai Bhuvanesh Dr. Frank Hampel Dr. Joseph H. Reibenspies Prof. Dr. John A. Gladysz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(49):12619-12634
The silylated hexatriynyl complex trans-(C6F5)(p-tol3P)2Pt(C≡C)3SiEt3 ( PtC6TES ) is converted in situ to PtC6H (wet n-Bu4N+ F−, THF) and cross coupled with the diyne H(C≡C)2SiEt3 ( HC4TES ; CuCl/TMEDA, O2) to give PtC10TES (71 %). This sequence is repeated twice to afford PtC14TES (65 %) and then PtC18TES (27 %). An analogous series of reactions starting with PtC8TES gives PtC12TES (60 %), then PtC16TES (43 %), and then PtC20TES (17 %). Similar cross couplings with H(C≡C)2Si(i-Pr)3 ( HC4TIPS ) give PtC12TIPS (68 %), PtC14TIPS (68 %), and PtC16TIPS (34 %). The trialkylsilyl species (up to PtC18TES ) are converted to 3+2 “click” cycloadducts or 1,4-disubstituted 1,2,3-triazoles trans-(C6F5)(p-tol3P)2Pt(C≡C)n-1C=CHN(CH2C6H5)N=N (29–92 % after workups). The most general procedure involves generating the terminal polyynes PtC x H (wet n-Bu4N+ F−, THF) in the presence of benzyl azide in DMF and aqueous CuSO4/ascorbic acid. All of the preceding complexes are crystallographically characterized and the structural and spectroscopic properties analyzed as a function of chain length. Two pseudopolymorphs of PtC20TES are obtained, both of which feature molecules with parallel sp carbon chains in a pairwise head/tail packing motif with extensive sp/sp van der Waals contacts. 相似文献
996.
Nicole Fernandez-Tejero Quentin Gauthier Sohee Cho Bruce R. McCord 《Electrophoresis》2023,44(3-4):371-377
The determination of tissue type is important when reconstructing a crime scene as skin cells may indicate innocent contact, whereas other types of cells, such as blood and semen, may indicate foul play. Up to now, there has been no specific DNA methylation-based marker to distinguish skin cell DNA from other body fluids. The goal of this study is to develop a DNA methylation-based assay to detect and identify skin cells collected at forensic crime scenes for use in DNA typing. For this reason, we have utilized a DNA methylation chip array-based genome-wide association study to identify skin-specific DNA methylation markers. DNA obtained from skin along with other body fluids, such as semen, saliva, blood, and vaginal epithelia, were tested using five genes that were identified as sites for potential new epigenetic skin markers. Samples were collected, bisulfite converted, and subjected to real-time polymerase chain reaction (PCR) with high-resolution melt analysis. In our studies, when using WDR11, PON2, and NHSL1 assays with bisulfite-modified PCR, skin/sweat amplicons melted at lower temperatures compared to blood, saliva, semen, and vaginal epithelia. One-way analysis of variance demonstrates that these three skin/sweat markers are significantly different when compared with other body fluids (p < 0.05). These results demonstrate that high-resolution melt analysis is a promising technology to detect and identify skin/sweat DNA from other body fluids. 相似文献
997.
Aboshyan-Sorgho L Nozary H Aebischer A Bünzli JC Morgantini PY Kittilstved KR Hauser A Eliseeva SV Petoud S Piguet C 《Journal of the American Chemical Society》2012,134(30):12675-12684
This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using trivalent chromium chromophores as sensitizers. Reactions of the segmental ligand L2 with stoichiometric amounts of M(CF(3)SO(3))(2) (M = Cr, Zn) and Ln(CF(3)SO(3))(3) (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield the D(3)-symmetrical trinuclear [MLnM(L2)(3)](CF(3)SO(3))(n) complexes (M = Zn, n = 7; M = Cr, n = 9), in which the central lanthanide activator is sandwiched between the two transition metal cations. Visible or NIR irradiation of the peripheral Cr(III) chromophores in [CrLnCr(L2)(3)](9+) induces rate-limiting intramolecular intermetallic Cr→Ln energy transfer processes (Ln = Nd, Er, Yb), which eventually produces lanthanide-centered near-infrared (NIR) or IR emission with apparent lifetimes within the millisecond range. As compared to the parent dinuclear complexes [CrLn(L1)(3)](6+), the connection of a second strong-field [CrN(6)] sensitizer in [CrLnCr(L2)(3)](9+) significantly enhances the emission intensity without perturbing the kinetic regime. This work opens novel exciting photophysical perspectives via the buildup of non-negligible population densities for the long-lived doubly excited state [Cr*LnCr*(L2)(3)](9+) under reasonable pumping powers. 相似文献
998.
Zhong YL Gauthier DR Shi YJ McLaughlin M Chung JY Dagneau P Marcune B Krska SW Ball RG Reamer RA Yasuda N 《The Journal of organic chemistry》2012,77(7):3297-3310
An efficient, new, and scalable semisynthesis of glucan synthase inhibitors 1 and 2 from the fermentation product enfumafungin 3 is described. The highlights of the synthesis include a high-yielding ether bond-forming reaction between a bulky sulfamidate 17 and alcohol 4 and a remarkably chemoselective, improved palladium(II)-mediated Corey-Yu allylic oxidation at the highly congested C-12 position of the enfumafungin core. Multi-hundred gram quantities of the target drug candidates 1 and 2 were prepared, in 12 linear steps with 25% isolated yield and 13 linear steps with 22% isolated yield, respectively. 相似文献
999.
Cencek W Szalewicz K Leforestier C van Harrevelt R van der Avoird A 《Physical chemistry chemical physics : PCCP》2008,10(32):4716-4731
The ab initio water dimer interaction energies obtained from coupled cluster calculations and used in the CC-pol water pair potential (Bukowski et al., Science, 2007, 315, 1249) have been refitted to a site-site form containing eight symmetry-independent sites in each monomer and denoted as CC-pol-8s. Initially, the site-site functions were assumed in a B-spline form, which allowed a precise optimization of the positions of the sites. Next, these functions were assumed in the standard exponential plus inverse powers form. The root mean square error of the CC-pol-8s fit with respect to the 2510 ab initio points is 0.10 kcal mol(-1), compared to 0.42 kcal mol(-1) of the CC-pol fit (0.010 kcal mol(-1) compared to 0.089 kcal mol(-1) for points with negative interaction energies). The energies of the stationary points in the CC-pol-8s potential are considerably more accurate than in the case of CC-pol. The water dimer vibration-rotation-tunneling spectrum predicted by the CC-pol-8s potential agrees substantially and systematically better with experiment than the already very accurate spectrum predicted by CC-pol, while specific features that could not be accurately predicted previously now agree very well with experiment. This shows that the uncertainties of the fit were the largest source of error in the previous predictions and that the present potential sets a new standard of accuracy in investigations of the water dimer. 相似文献
1000.
Piperazine-2,5-diones are formed by Dieckmann cyclization (NaH, THF) of substructures of the type CH(2)-N(R)C(O)CH(2)N(R')CO(2)Ph in which the terminal methylene (CH(2)) that is adjacent to nitrogen closes onto the carbonyl group of the phenyl carbamate unit at the other end of the chain. R and R' are alkyl groups, and the terminal methylene is activated by a ketone carbonyl, a nitrile, an ester, or a phosphoryl group. The starting materials are assembled by standard acylation and oxidation processes, starting from a β-(alkylamino)alcohol, an (alkylamino)acetonitrile, an (alkylamino) ester, or an (alkylamino)methyl phosphonate. 相似文献