全文获取类型
收费全文 | 4105篇 |
免费 | 702篇 |
国内免费 | 670篇 |
专业分类
化学 | 4266篇 |
晶体学 | 17篇 |
力学 | 169篇 |
综合类 | 6篇 |
数学 | 204篇 |
物理学 | 815篇 |
出版年
2024年 | 30篇 |
2023年 | 360篇 |
2022年 | 138篇 |
2021年 | 148篇 |
2020年 | 229篇 |
2019年 | 168篇 |
2018年 | 152篇 |
2017年 | 113篇 |
2016年 | 282篇 |
2015年 | 239篇 |
2014年 | 236篇 |
2013年 | 314篇 |
2012年 | 209篇 |
2011年 | 243篇 |
2010年 | 173篇 |
2009年 | 263篇 |
2008年 | 248篇 |
2007年 | 245篇 |
2006年 | 231篇 |
2005年 | 179篇 |
2004年 | 191篇 |
2003年 | 134篇 |
2002年 | 88篇 |
2001年 | 68篇 |
2000年 | 84篇 |
1999年 | 74篇 |
1998年 | 75篇 |
1997年 | 63篇 |
1996年 | 60篇 |
1995年 | 53篇 |
1994年 | 39篇 |
1993年 | 42篇 |
1992年 | 32篇 |
1991年 | 28篇 |
1990年 | 20篇 |
1989年 | 20篇 |
1988年 | 32篇 |
1987年 | 15篇 |
1986年 | 11篇 |
1985年 | 26篇 |
1984年 | 11篇 |
1982年 | 12篇 |
1981年 | 17篇 |
1980年 | 9篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 12篇 |
1975年 | 8篇 |
1973年 | 9篇 |
1972年 | 5篇 |
排序方式: 共有5477条查询结果,搜索用时 31 毫秒
31.
32.
Kinetics of chemiluminescence reactions of 2-methyl-6-phenylimidazo[1,2-a]pyrazin-3(7H)-one (1c, Cypridina luciferin analogue) and substituent effects of the 6-aryl group of derivatives 1 strongly suggest that the rate-determining step is a single electron transfer from an anion derived from 1 to a triplet molecular oxygen (O2) in the oxygenation process. 相似文献
33.
34.
Sébastien Fiorucci Jer?me Golebiowski Daniel Cabrol-Bass Serge Antonczak 《Chemphyschem》2004,5(11):1726-1733
Flavonoids are naturally occurring phenol derivatives present in substantial amounts in a large variety of plants, fruits and vegetables daily eaten by humans. Most of these compounds exhibit several interesting biological activities, such as antiradical and antioxidant actions. Indeed, by complexation with specific enzymes, flavonoids are notably liable to metabolize molecular dioxygen. On the basis of experimental results describing oxygenolysis of the flavonoid quercetin, activated by the enzyme quercetin 2,3-dioxygenase (2,3-QD),ur attention has focused on the role of metal center in the activation of the substrate quercetin. Thus, in the present study, by means of DFT calculations at the B3LYP/ 6-31(+)G* level on model molecular systems, we describe different mechanisms for dioxygen metabolization by quercetin. Stationary points are described, and energetic and structural analyses along the reaction paths are reported. Our calculations show that the copper cation must act as an oxidant towards the substrate and that the reaction proceeds through a 1,3-cycloaddition. 相似文献
35.
Wolfgang Quapp 《Theoretical chemistry accounts》1989,75(6):447-460
Gradient extremals are curves in configuration space denned by the condition that the gradient of the potential energy is an eigenvector of the Hessian matrix. Solutions of a corresponding equation go along a valley floor or along a crest of a ridge, if the norm of the gradient is a minimum, and along a cirque or a cliff or a flank of one of the two if the gradient norm is a maximum. Properties of gradient extremals are discussed for simple 2D model surfaces including the problem of valley bifurcations. 相似文献
36.
根据反应层的概念对在任一给定形状微电极上进行的准一级和二级均相催化反应机理(EC'')的稳态电化学行为进行了研究。本方法简便易行,不需要解复杂方程的数学技巧。利用推导出的这些方程,可计算准一级和二级均相催化反应的动力学常数。 相似文献
37.
The reaction mechanism of the l-proline-catalyzed α-aminoxylation reaction between aldehyde and nitrosobenzene has been investigated using density functional theory (DFT) calculation. Our calculation results reveal following conclusions [1]. The first step that corresponds to the formation of C–O bond, is the stereocontrolling and rate-determining step [2]. Among four reaction channels, the syn-attack reaction channel is more favorable than that of the anti one, and the TS-ss channel dominates among the four channels for this reaction in the step of C–O bond formation [3]. The intermolecular hydrogen bond between the acidic hydrogen of l-proline and the N atom of the nitrosobenzene in an early stage of the process catalyzes very effectively the C–O bond formation by a large stabilization of the negative charge that is developing at the O atom along the electrophilic attack [4]. The effect of solvent decreases the activation energy, and also, the calculated energy barriers are decrease with the enhancement of dielectric constants for C–O bond formation step. These results are in good agreement with experiment, and allow us to explain the origin of the catalysis and stereoselectivity for l-proline-catalyzed α-aminoxylation of aldehyde reaction. The addition of H2O to substituted imine proline, intermolecular proton-transfer steps, and the l-proline elimination process were also studied in this paper. 相似文献
38.
J Robert Huber 《Chemphyschem》2004,5(11):1663-1669
This Minireview gives an account of the photochemical decay of nitric acid HNO3 in the gas phase, which has been well investigated under bulk and molecular-beam conditions. Due to the importance of this molecule in atmospheric chemistry, attention was paid to the irradiation regions around 300 and 200 nm, where solar photolysis of HNO3 is expected to be particularly efficient. While the low-energy region is characterized by the products OH and NO2, the high-energy region gives rise to a variety of photochemical decay pathways, dominated by channels which lead to the products HONO + O in different electronic states. 相似文献
39.
Svetlana V. Amosova Vladimir A. Shagun Natalia A. Makhaeva Konstantin A. Lysenko 《Journal of organometallic chemistry》2007,692(16):3307-3315
The structure of 2,4-dibromo-2-dibromomethyl-3,3-dimethyl-1-selena-3-silacyclopentene-4, formed by regioselective electrophilic addition of SeBr4 to dimethyl diethynyl silane, has been determined using X-ray analysis technique. Quantum chemistry methods were used to study elementary stages of the reaction. It was found that the first stage consisted of SeBr4 conversion into bimolecular complex Br2?SeBr2, initiated by dimethyl diethynyl silane. Possible formation of five-membered and six-membered heterocycles involves different cyclization mechanisms. The formation of only five-membered heterocycle is explained by kinetically preferable ring closure through four-center transition state. The conclusions obtained by calculations were confirmed by monitoring of the reaction using 1H NMR method. 相似文献
40.
A comparative investigation has been made of the nonisothermal, solid-state thermal decompositions of the oxalates of six divalent transition metals (cations: manganese, iron, cobalt, nickel, copper and zinc) in alternative flowing atmospheres, inert (N2, CO2), reducing (H2) and oxidizing (air). Derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) response peak maxima, providing a measure of reaction temperatures, have been used to determine salt reactivities and thus to characterize the factors that control the relative stabilities of this set of chemically related reactants. Two trends were identified. Trend (1): in the inert and reducing atmospheres, the decomposition temperature (salt stability) increased with rise in enthalpy of formation of the divalent transition metal oxide, MO. It is concluded that the rupture of the cation-oxygen (oxalate) bond is the parameter that determines the stability of salts within this set. Trend (2): the diminution of decomposition temperatures from values for reactions in inert/reducing atmosphere to those for reactions in an oxidizing atmosphere increased with the difference in formation enthalpy between MO and the other participating oxide (MO3/2 or MO1/2). The change of cation valence tended to promote reaction, most decompositions in O2 occurred at lower temperatures, but the magnitude of the effect varied considerably within this set of reactants. Observed variations in stoichiometric and kinetic characteristics with reaction conditions are discussed, together with the mechanisms of thermal decompositions of these solid oxalates.This approach to the elucidation of crystolysis reaction mechanisms emphasizes the value of comparative investigations within the group of chemically related reactants. Previous isothermal kinetic studies had been made for each of the reactants selected here. From these, much has been learned about the form of the (isothermal) solid-state yield-time curves, often interpreted to provide information about the geometry of interface development for the individual rate processes. However, identification of the controls of reactivity, reaction initiation (nucleation) and advance (nucleus growth), is much more difficult to characterize and less progress has been made towards elucidation of the interface chemistry. The trends of reactivity changes with salt compositions, identified here, offer a complementary approach to that provided by the study of single salts. Much of the recent literature on thermal decompositions of solids has been concerned with individual reactants, but many results and conclusions are not presented in the widest possible perspective. Comparisons between systematically related reactants are identified here as providing a chemical context for the elucidation of the chemical steps that participate in interface reactions. The article advocates the use of a more chemical approach in investigations of crystolysis (solid-state chemical) reactions. 相似文献