全文获取类型
收费全文 | 5907篇 |
免费 | 207篇 |
国内免费 | 33篇 |
专业分类
化学 | 3684篇 |
晶体学 | 23篇 |
力学 | 192篇 |
数学 | 1196篇 |
物理学 | 1052篇 |
出版年
2023年 | 58篇 |
2022年 | 140篇 |
2021年 | 183篇 |
2020年 | 159篇 |
2019年 | 153篇 |
2018年 | 131篇 |
2017年 | 117篇 |
2016年 | 244篇 |
2015年 | 230篇 |
2014年 | 242篇 |
2013年 | 335篇 |
2012年 | 415篇 |
2011年 | 465篇 |
2010年 | 268篇 |
2009年 | 248篇 |
2008年 | 377篇 |
2007年 | 340篇 |
2006年 | 308篇 |
2005年 | 298篇 |
2004年 | 234篇 |
2003年 | 176篇 |
2002年 | 188篇 |
2001年 | 73篇 |
2000年 | 49篇 |
1999年 | 64篇 |
1998年 | 52篇 |
1997年 | 64篇 |
1996年 | 48篇 |
1995年 | 47篇 |
1994年 | 45篇 |
1993年 | 33篇 |
1992年 | 25篇 |
1991年 | 28篇 |
1990年 | 21篇 |
1989年 | 19篇 |
1988年 | 22篇 |
1987年 | 25篇 |
1986年 | 19篇 |
1985年 | 25篇 |
1984年 | 22篇 |
1983年 | 13篇 |
1982年 | 22篇 |
1981年 | 12篇 |
1980年 | 14篇 |
1979年 | 14篇 |
1978年 | 15篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1973年 | 7篇 |
1972年 | 5篇 |
排序方式: 共有6147条查询结果,搜索用时 15 毫秒
31.
Orellana M Arriola P Del Río R Schrebler R Cordova R Scholz F Kahlert H 《The journal of physical chemistry. B》2005,109(32):15483-15488
During the electrochemical oxidation of Prussian blue (PB) to Prussian yellow (PY), an electrocatalytic oxygen production proceeds at the electrode when aqueous electrolyte solutions are used. The formed oxygen is scavenged by the PY, probably by absorption, and it is consumed during the electrochemical reduction of PY to PB by a heterogeneous chemical reaction of PB with oxygen to PY and hydrogen peroxide. Because of this catalytic regeneration of PY, it is impossible to determine the amount of low-spin iron by chronocoulometry using a potential program in which PB is first oxidized to PY and then the charge is measured to reduce PY to PB. The latter charge is biased by the electrocatalytic PY regeneration. 相似文献
32.
The solubility parameter of poly(hexamethylene oxide) has been estimated by measuring solution viscosities and by turbidimetric titrations in a series of solvents. From both experimental methods, a value of () was obtained, whence the cohesive energy density is 274.5 J cm?3 (65.6 cal cm?3). These experimental values are compared with those calculated by empirical methods. 相似文献
33.
Grez P Celedón C Molinari A Oliva A Orellana M Schrebler R del Río R Córdova R 《The journal of physical chemistry. B》2005,109(48):22920-22927
The interaction of the ethyl xanthate (EX) anion with a copper electrode in a borate buffer solution, pH 9.2, has been investigated by cyclic voltammetry (CV), electrochemical quartz crystal microbalance (EQCM), electrochemical impedance spectroscopy (EIS), and measurements of contact angle (CA) under controlled potential. The results obtained allow establishing that, in the potential range from -0.80 and -0.60 V, two parallel reactions were characterized. These reactions were the ethyl xanthate electroadsorption and the hydrogen evolution reaction (HER). This last reaction has not been described by previous authors. Besides, the EIS measurements show that the mechanism of the HER on copper electrodes is not affected by the presence of a ethyl xanthate species. The EQCM study shows that in the electrodesorption process the departure of each ethyl xanthate species from the copper electrode is accompanied with the simultaneous entry of four to five water molecules. This fact is in accordance with the number of copper atoms involved in the adsorption of one ethyl xanthate species. 相似文献
34.
Gianluca Pozzi Marco Cavazzini Orsolya Holczknecht Silvio Quici Ian Shepperson 《Tetrahedron letters》2004,45(22):4249-4251
A fluorous-tagged TEMPO radical has been prepared and its catalytic activity in the chemoselective oxidation of alcohols to carbonyl compounds has been investigated. The new fluorous radical proved to be an efficient, selective and easily recoverable catalyst, which can be conveniently used in standard organic solvents and then isolated and recycled by fluorous liquid-liquid extraction. 相似文献
35.
The title pyridazine 1 was found to react with both 2,3-dihydrofuran (2) and 3,4-dihydro-2H-pyran (9) to give the tetracyclic skeletons 5-8 and the phthalonitrile 12 through the intermediates 4 and 10, respectively. A more complex mechanism was ascertained for the reaction of 1 with the pyrroline 14 which, under suitable conditions, afforded the bicyclic derivative 19 as the predominant product; selective elaborations of this species into the 5,6-dicyanoindoles 22 and 23 are reported. 相似文献
36.
Nicolas?Ferré Alessandro?Cembran Marco?Garavelli Massimo?OlivucciEmail author 《Theoretical chemistry accounts》2004,112(4):335-341
A special hybrid quantum mechanics/molecular mechanics forcefield is defined, parameterized and validated for studying the photoisomerization path of the retinal chromophore in the rhodopsin protein. It couples a multireference ab initio Hamiltonian (CASSCF and second-order multireference many-body perturbation theory using a CASSCF reference) to describe the chromophore while the rest of the protein is approximated with the Amber forcefield. The frontier has been carefully parameterized in order to reproduce full quantum mechanics torsional energy profiles, for both the ground state and the first excited state. It is also shown that replacing the chromophore counterion with point charges is a valid approximation. This result is interpreted in terms of a cancellation effect for which a possible explanation is given. 相似文献
37.
Massimo Lucarini Fabio Sciubba Donatella Capitani Maria Enrica Di Cocco Laura D’Evoli Alessandra Durazzo 《Natural product research》2020,34(1):53-62
AbstractThe study focuses on the understanding, at molecular level, the mechanism of interaction between protein and flavonoids. Collagen and catechin interactions were investigated by NMR in solution and solid state. The effect of catechin on the stability of collagen to oxidation was also explored. Collagen was treated with two concentrations of catechin solutions. Oxidation was carried out by incubation of collagen solution with three oxidation systems: Fe(II)/H2O2, Cu(II)/H2O2, and NaOCl/H2O2. The effects of oxidation systems were evaluated by high resolution 1?D and 2?D proton spectroscopy and solid state NMR (13C CP MAS) experiments. Interactions between collagen and catechin preferentially occur between catechin B ring and the amino acids Pro and Hyp of collagen. Results showed that both iron and copper oxidation systems were able to interact with collagen by site specific attack. Moreover, catechin protects collagen proline from oxidation by metal/H2O2 systems, preventing copper and iron approach to collagene molecule;this behaviour was more evident for the copper/H2O2 system. 相似文献
38.
Marco Häser 《Theoretical chemistry accounts》1993,87(1-2):147-173
Summary A novel formulation of MP2 theory is presented which starts from the Laplace transform MP2 ansatz, and subsequently moves from a molecular orbital (MO) representation to an atomic orbital (AO) representation. Consequently, the new formulation is denoted AO-MP2. As in traditional MP2 approaches electron repulsion integrals still need to be transformed. Strict bounds on the individual MP2 energy contribution of each intermediate four-index quantity allow to screen off numerically insignificant integrals with a single threshold parameter. Implicit in our formulation is a bound to two-particle density matrix elements. For small molecules the computational cost for AO-MP2 calculations is about a factor of 100 higher than for traditional MO-based approaches, but due to screening the computational effort in larger systems will only grow with the fourth power of the size of the system (or less) as is demonstrated both in theory and in application. MP2 calculations on (non-metallic) crystalline systems seem to be a feasible extension of the Laplace transform approach. In large molecules the AO-MP2 ansatz allows massively parallel MP2 calculations without input/output of four-index quantities provided that each processor has in-core memory for a limited number of two-index quantities. Energy gradient formulas for the AO-MP2 approach are derived.Dedicated to Prof. W. Kutzelnigg whose books on theoretical chemistry aroused my interest in this field 相似文献
39.
Acetylcholine and choline sensors are prepared by immbilizing enzymes on nylon net attached to a hydrogen peroxide snsor. Choline oxidase is used for the choline sensor; acetylcholinesterase choline oxidase are used for acetylcholine. The platinum/silver electrode pair is polarized at +0.6 V. The assembly is protected with an acetate cellulose membrane to enhance selectivity. The ranges measured are 1–10 μmol l?1 in 0.1–1 ml of sample. The response times are 1–2 min. 相似文献
40.
An experimental design procedure was applied to optimize the operating conditions of an axially-viewed inductively coupled plasma emission spectrometer instruments equipped with echelle optics with cross dispersion and charge transfer device. The multivariate effect of carrier gas flow rate and r.f. power on several analytical figures was investigated and discussed. Both ultrasonic and pneumatic nebulization were used. For the final choice of the optimum, different criteria were taken into account, mainly plasma robustness, instrumental precision, analyte and background net emission, detection limits and signal-to-background ratios. It was found that the use of moderate power (1100W) and mean carrier gas flow rate (0.75 L/min) allows to obtain sufficient plasma robustness, satisfactory precision, and excellent signal-to-background ratios and limits of detection, favorable for ultratrace element determinations in environmental matrices. 相似文献