全文获取类型
收费全文 | 512篇 |
免费 | 16篇 |
专业分类
化学 | 324篇 |
晶体学 | 16篇 |
力学 | 12篇 |
数学 | 28篇 |
物理学 | 148篇 |
出版年
2023年 | 7篇 |
2022年 | 10篇 |
2021年 | 12篇 |
2020年 | 12篇 |
2019年 | 12篇 |
2018年 | 12篇 |
2017年 | 9篇 |
2016年 | 20篇 |
2015年 | 9篇 |
2014年 | 17篇 |
2013年 | 34篇 |
2012年 | 38篇 |
2011年 | 45篇 |
2010年 | 21篇 |
2009年 | 19篇 |
2008年 | 21篇 |
2007年 | 26篇 |
2006年 | 15篇 |
2005年 | 22篇 |
2004年 | 27篇 |
2003年 | 17篇 |
2002年 | 29篇 |
2001年 | 6篇 |
2000年 | 7篇 |
1999年 | 9篇 |
1998年 | 2篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1991年 | 4篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有528条查询结果,搜索用时 15 毫秒
11.
Alessandro Gambaro Valerio Peruzzo Gualtiero Plazzogna Guiseppe Tagliavini 《Journal of organometallic chemistry》1980,197(1):45-50
Mixed allylbutyltin halides (CH2CHCH2)SnBu3-nCln (n = 0–3) have been prepared, and characterized by carbon-13 NMR spectroscopy. Their ability to bring about allylstannylation of ketones and aldehydes, to form organostannoxy compounds, Bu3-nSnClnOC(R′)(R″)CH2CHCH2, has been shown to increase on increasing the value of n, that is on increasing the acceptor ability of the tin centre. 相似文献
12.
Truemper configurations (thetas, pyramids, prisms, and wheels) have played an important role in the study of complex hereditary graph classes (eg, the class of perfect graphs and the class of even-hole-free graphs), appearing both as excluded configurations, and as configurations around which graphs can be decomposed. In this paper, we study the structure of graphs that contain (as induced subgraphs) no Truemper configurations other than (possibly) universal wheels and twin wheels. We also study several subclasses of this class. We use our structural results to analyze the complexity of the recognition, maximum weight clique, maximum weight stable set, and optimal vertex coloring problems for these classes. Furthermore, we obtain polynomial -bounding functions for these classes. 相似文献
13.
Dr. Amy C. Topley Valerio Isoni Dr. Thomas A. Logothetis Dr. Duncan Wynn Dr. Harry Wadsworth Dr. Alex M. R. Gibson Imtiaz Khan Dr. Neil J. Wells Prof. Cécile Perrio Prof. Richard C. D. Brown 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(5):1720-1725
A Resin‐linker‐vector (RLV) strategy is described for the radiosynthesis of tracer molecules containing the radionuclide 18F, which releases the labelled vector into solution upon nucleophilic substitution of a polystyrene‐bound arylsulfonate linker with [18F]‐fluoride ion. Three model linker‐vector molecules 7 a – c containing different alkyl spacer groups were assembled in solution from (4‐chlorosulfonylphenyl)alkanoate esters, exploiting a lipase‐catalysed chemoselective carboxylic ester hydrolysis in the presence of the sulfonate ester as a key step. The linker‐vector systems were attached to aminomethyl polystyrene resin through amide bond formation to give RLVs 8 a – c with acetate, butyrate and hexanoate spacers, which were characterised by using magic‐angle spinning (MAS) NMR spectroscopy. On fluoridolysis, the RLVs 8 a , b containing the longer spacers were shown to be more effective in the release of the fluorinated model vector (4‐fluorobutyl)phenylcarbamic acid tert‐butyl ester ( 9 ) in NMR kinetic studies and gave superior radiochemical yields (RCY≈60 %) of the 18F‐labelled vector. The approach was applied to the synthesis of the radiopharmaceutical O‐(2‐[18F]‐fluoroethyl)‐L ‐tyrosine ([18F]‐FET), delivering protected [18F]‐FET in >90 % RCY. Acid deprotection gave [18F]‐FET in an overall RCY of 41 % from the RLV. 相似文献
14.
Ricardo Aguilar-Villalva Gustavo A. Molina Beatriz Liliana España-Sánchez Luis Fernando Díaz-Peña Alberto Elizalde-Mata Edgar Valerio Cristy Azanza-Ricardo Miriam Estevez 《Arabian Journal of Chemistry》2021,14(7):103239
In recent years, the food, pharmacy, and cosmetic industries have focused on the search of natural compounds with antimicrobial and antioxidant properties; commonly, these compounds are obtained from Kingdom plantae. The aim of the present work is comparing antibacterial and antioxidant capacity of Annona cherimola Mill leaves, using different extraction methods. The ultrasound assisted extraction technique (UAE) was compared with conventional techniques: Soxhlet and maceration. Water and ethanol were used as solvents for leaves extractions performed with these three methods. The main acetogenins reported in Annona cherimola Mill and Annona muricata L. species were simulated using the functional hybrid B3LYP and to confirm its presence, analysis of the compound composition was performed using FT-IR, UV–Vis and HPLC. Total phenolics (TP) and flavonoids (TF) were determined by spectroscopy techniques and novel Differential Pulse Voltammetry (DPV) electrochemical technique. Total Antioxidant Capacity (TAC) of the extracts was measured, using the DPPH, FRAP and CUPRAC techniques. The highest antioxidant content was found in the Soxhlet water extracts; even so, the UAE technique presented an attractive alternative due to considerable reduction in extraction time, which was greater than 99%, and possible selectivity in compounds extraction. Finally, antibacterial activity of the extracts was evaluated, obtaining the best results against gram-positive bacteria using UAE water extract. In this way, the UAE technique presents an excellent extraction option due to the considerable reduction in time and energy, as well as the increase in antibacterial activity. 相似文献
15.
Measurable spectra are often derived from contractions of many-body Green's functions. One calculates hence more information than needed. Here we present and illustrate an in principle exact approach to construct effective potentials and kernels for the direct calculation of electronic spectra. In particular, a dynamical but local and real potential yields the spectral function needed to describe photoemission. We discuss for model solids the frequency dependence of this "photoemission potential" stemming from the nonlocality of the corresponding self-energy. 相似文献
16.
Dr. Guylhaine Clavel Valerio Molinari Dr. Alexander Kraupner Dr. Cristina Giordano 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(29):9018-9023
In the search for alternative materials to current expensive catalysts, Ni has been addressed as one of the most promising and, on this trail, its corresponding nitride. However, nickel nitride is a thermally unstable compound, and therefore not easy to prepare especially as nanoparticles. In the present work, a sol–gel‐based process (the urea glass route) is applied to prepare well‐defined and homogeneous Ni3N and Ni nanoparticles. In both cases, the prepared crystalline nanoparticles (~25 nm) are dispersed in a carbon matrix forming interesting Ni3N‐ and Ni‐based composites. These nanocomposites were characterised by means of several techniques, such as XRD, HR‐TEM, EELS, and the reaction mechanism was investigated by TGA and IR and herein discussed. The catalytic activity of Ni3N is investigated for the first time, to the best of our knowledge, for hydrogenation reactions involving H2, and here compared to the one of Ni. Both materials show good catalytic activities but, interestingly, give a different selectivity between different functional groups (namely, nitro, alkene and nitrile groups). 相似文献
17.
We study the thermodynamics of the spin-S two-dimensional quantum Heisenberg antiferromagnet on the square lattice with nearest (J1) and next-nearest (J2) neighbor couplings in its collinear phase (J(2)/J(1)>0.5), using the pure-quantum self-consistent harmonic approximation. Our results show the persistence of a finite-temperature Ising phase transition for every value of the spin, provided that the ratio J(2)/J(1) is greater than a critical value corresponding to the onset of collinear long-range order at zero temperature. We also calculate the spin and temperature dependence of the collinear susceptibility and correlation length, and we discuss our results in light of the experiments on Li2VOSiO4 and related compounds. 相似文献
18.
Changjun Gao Youjun Lu You-Gen Shen Valerio Faraoni 《General Relativity and Gravitation》2018,50(1):15
The Hawking–Penrose singularity theorem states that a singularity forms inside a black hole in general relativity. To remove this singularity one must resort to a more fundamental theory. Using a corrected dynamical equation arising in loop quantum cosmology and braneworld models, we study the gravitational collapse of a perfect fluid sphere with a rather general equation of state. In the frame of an observer comoving with this fluid, the sphere pulsates between a maximum and a minimum size, avoiding the singularity. The exterior geometry is also constructed. There are usually an outer and an inner apparent horizon, resembling the Reissner–Nordström situation. For a distant observer the horizon crossing occurs in an infinite time and the pulsations of the black hole quantum “beating heart” are completely unobservable. However, it may be observable if the black hole is not spherical symmetric and radiates gravitational wave due to the quadrupole moment, if any. 相似文献
19.
4,4′-bipyridine bridged two Cu (II) complexes, [Cu2L12(4,4′-bipy)(H2O)2](ClO4)2 ( 1 ) and [Cu2L22(4,4′-bipy)]n·(2H2O)n ( 2 ) (where, HL1 = 2-[(3-methylamino-propylimino)-methyl]-phenol, H2L2 = 3-[(2-hydroxy-3-methoxy-benzylidene)-amino]-propionic acid, and 4,4′-bipy = 4,4′-bipyridine) have been synthesized and characterized by single crystal structure determination, mass spectrometry, FT-IR, electronic absorption, and emission spectroscopy. Complex 1 is dinuclear cationic compound and counter balanced by perchlorate anion, whereas complex 2 possesses 1D poly-nuclear structure. Both the complexes crystallize in monoclinic system with P21/c space group and the copper centers possess square pyramidal geometry. H-bonding, C-H···π, π···π interactions results the formation of two dimentional supramolecular structure for both the complexes. Interactions of complexes with bovine serum albumins (BSA) and human serum albumins (HSA) have been studied by using electronic absorption and emission spectroscopic technique. The calculated values of binding constants (Kb) are (9.22 ± 0.26) × 105 L mol−1 ( 1 -BSA), (7.19 ± 0.16) × 105 L mol−1 ( 1 -HSA), (5.05 ± 0.20) × 105 L mol−1 ( 2 -BSA) and (3.56 ± 0.25) × 105 L mol−1 ( 2 -HSA). The mechanism of serum albumins-complex interactions have been investigated by fluorescence lifetime measurement. Fluorescence spectroscopic studies indicate that both the complexes interact with calf thymas-DNA. Catecholase activity of the complexes has been studied in methanol using 3,5-di-tert-butylcatechol (3,5-DTBC) as substrate and the result show that both the complexes are active for catalytic oxidation of 3,5-DTBC to 3,5-di-tert-butylquinone (3,5-DTBQ) in presence of molecular oxygen. Calculated values of turnover numbers are 71.81 ± 1.04 h−1 and 69.45 ± 0.74 h−1 for 1 and 2 , respectively. 相似文献
20.
The rotational analysis of the infrared absorption spectrum of CH3 79Br and CH3 81Br between 2150 and 2510 cm-1 was performed on a Fourier transform spectrum with a resolution of 0·007 cm-1. The bands v 2 + v 6(E) and v 5 + v 6(A 1 + A 2 + E) occur in this region, giving rise to several perturbations as in the corresponding system of methyl chloride [3]. Forbidden transitions, observed in correspondence of the level crossing of the x-y Coriolis coupling between v 2 + v 6 and v 5 + v 6(E), enabled us to estimate the value of A″ - 225D″K at 5·16186 cm-1 for CH3 79Br and 5·16173 cm-1 for CH3 81Br. The parallel system of v 5 + v 6 exhibits a perpendicular structure, and an l-type resonance couples those levels of the parallel and perpendicular components of v 5 + v 6 involved in transitions from the K″ = 0 levels of the ground state. The QQ 0 branches of the A 2 component of v 5 + v 6, made active by this resonance, are observed for both isotopic species. 相似文献