全文获取类型
收费全文 | 8034篇 |
免费 | 772篇 |
国内免费 | 1214篇 |
专业分类
化学 | 9111篇 |
晶体学 | 32篇 |
力学 | 7篇 |
综合类 | 45篇 |
数学 | 260篇 |
物理学 | 565篇 |
出版年
2024年 | 10篇 |
2023年 | 70篇 |
2022年 | 139篇 |
2021年 | 191篇 |
2020年 | 363篇 |
2019年 | 289篇 |
2018年 | 224篇 |
2017年 | 219篇 |
2016年 | 366篇 |
2015年 | 325篇 |
2014年 | 326篇 |
2013年 | 741篇 |
2012年 | 447篇 |
2011年 | 476篇 |
2010年 | 428篇 |
2009年 | 454篇 |
2008年 | 573篇 |
2007年 | 561篇 |
2006年 | 521篇 |
2005年 | 486篇 |
2004年 | 481篇 |
2003年 | 392篇 |
2002年 | 326篇 |
2001年 | 232篇 |
2000年 | 200篇 |
1999年 | 168篇 |
1998年 | 166篇 |
1997年 | 137篇 |
1996年 | 133篇 |
1995年 | 100篇 |
1994年 | 98篇 |
1993年 | 99篇 |
1992年 | 72篇 |
1991年 | 53篇 |
1990年 | 39篇 |
1989年 | 28篇 |
1988年 | 18篇 |
1987年 | 11篇 |
1986年 | 15篇 |
1985年 | 5篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1979年 | 2篇 |
1978年 | 6篇 |
1977年 | 2篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1968年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 796 毫秒
161.
CCl2自由基与H2O分子反应动力学研究 总被引:2,自引:0,他引:2
用213 nm激光光解CCl4产生CCl2自由基,用LP LIF技术测定了室温下基态CCl2自由基与H2O分子的反应速率常数为(5.45±0.95)×10-14 cm3•molecule-1•s-1.在G2MP2理论水平上计算了CCl2+H2O反应的最低单重态势能面,揭示了插入与加成 消除两种反应机理,得到了三个可能的产物通道:HCl+HClCO、HCl+trans ClCOH以及HCl+cis ClCOH.并用RRKM TST和传统过渡态理论计算了这三个通道的分支比及其温度效应.结果说明在低温下(273 K),插入机理的产物通道的分支比远大于加成 消除机理的产物通道, HCl+HClCO是主要产物,分支比为77.4%,其次是HCl+cis ClCOH,分支比为22.6%.而在高温下(3000 K),加成 消除机理的反应通道大于插入机理, HCl+trans ClCOH分支比为82.3%. 相似文献
162.
Nadine Metz 《European Polymer Journal》2007,43(4):1202-1209
Acetone oxime acrylate has been synthesized as a new active ester monomer. Free radical polymerization yielded a reactive polymer soluble in various organic solvents, such as chloroform, dioxane, DMSO, acetone, methanol, dichloromethane, DMF, and ethanol. Controlled radical polymerization of acetone oxime acrylate was successfully conducted using the RAFT, NMP and Iniferter method. Partly polymer analogous reaction with N-isopropylamine resulted in the reactive copolymer poly(N-isopropylacrylamide-co-acetone oxime acrylate), which featured a lower critical solution temperature (LCST) of 61 °C in water. Further, the reactivity of the copolymer was exemplary proven by complete reaction with ammonia yielding poly(N-isopropylacrylamide-co-acrylamide), which does not possess a LCST. 相似文献
163.
Akikazu Matsumoto Ryuji Kotaki Takayuki Otsu 《Journal of polymer science. Part A, Polymer chemistry》1991,29(12):1707-1715
Bulk polymerization of alkyl N,N-dialkylfumaramates (FAE) and maleamates (MAE) was performed in the presence of a radical initiator. It has been found that FAE is more reactive than MAE when the reactivity of the two geometrical isomers was compared for their homo- and copolymerizations. From investigation on the effect of ester and N-substituents of these monomers, it has been found that the isopropyl ester shows a higher reactivity than the methyl ester and that N-ethyl and n-butyl substitution gives polymers with high molecular weight of more than several thousands. The resulting substituted polymethylenes from FAE and MAE were characterized and compared with each other. The isomerization of MAE to FAE with morpholine as an isomerization catalyst and monomer-isomerization radical polymerization were also investigated. 相似文献
164.
165.
166.
Lycium species growing in Turkey have not so far been studied sufficiently. For this reason, non-polar and polar extracts obtained from the fruits of Lycium barbarum L. and L. Ruthenicum Murray (Solanaceae) were assessed both in vitro for their potential as free radical scavenger crude extracts and their phenolic composition. Fruits of Lycium species were sequentially extracted with petroleum ether, ethyl acetate, methanol, n-butanol, and water in a Soxhlet extractor. All the extracts were assessed for the scavenging of the nitrogen-centered free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH.) by in vitro method. Furthermore, the composition of each extract was investigated both in terms of its Folin-Ciocalteau reactive components and its qualitative content. The phenolic compounds within the extracts were determined as benzoic acid and hydroxycinnamic acid derivatives, flavonoids, and anthocyanins according to their retention time and UV spectral data by HPLC-DAD system. 相似文献
167.
Boris A TrofimovSvetlana F Malysheva Boris G SukhovNatal'ya A Belogorlova Elena Yu SchmidtLyubov N Sobenina Vladimir A KuimovNina K Gusarova 《Tetrahedron letters》2003,44(13):2629-2632
Secondary phosphines 1-3 react readily with N-vinylpyrroles 4 and 5 under radical initiation to give regiospecifically anti-Markovnikov adducts, diorganyl-2-(1-pyrrolyl)ethylphosphines 6a-d, highly reactive building blocks for organic synthesis, in 88-91% yields. 相似文献
168.
The oxidative decomposition of various ketones (including acetone, 2-butanone, 4-heptanone, cyclopentanone and cyclohexanone) over dehydrated TiO(2) (P25) powder is investigated by electron paramagnetic resonance (EPR) spectroscopy. For the first time, a series of thermally unstable radical intermediates are observed both on the activated and reduced TiO(2) surface, depending on the adopted experimental conditions. These radical intermediates are identified as organoperoxy-based species of general formula ROO(.-) and RCO(3) (.-). They are formed by reaction of photogenerated charge carriers (either trapped electrons or trapped holes) with the adsorbed ketones in the presence of molecular oxygen. The organoperoxy intermediates are thermally unstable and decompose at temperatures in the region of 180-250 K. This work demonstrates that free-radical pathways involving both organoperoxy and superoxide radicals can be responsible for the thermal- and photodecomposition of ketones over polycrystalline TiO(2) (P25). 相似文献
169.
Effects of double substitution on the thermodynamic stabilities of nitrogen radicals 总被引:1,自引:0,他引:1
Studiesonkineticsofradicalreactions[1]andonCHbonddissociationenergies(BDE)[2]haveshownthatcarbonradicalsarestabilizedbyeitheranelectrondonatinggroup(EDG)oranelectronwithdrawinggroup(EWG),andthatifacarbonradicalissubstitutedwithbothanEDGandanEWGatthesamecen… 相似文献
170.
The products of high-vacuum pyrolysis of 1,1,1-trifluoro-2-bromo-2-chloroethane were studied by matrix IR spectroscopy. The
decomposition of 1,1,1-trifluoro-2-bromo-2-chloroethane was shown to occur predominantlyvia two directions: to form the 1,1,1-trifluoro-2-chloroethyl radical and trifluoromethylcarbene isomerizing to trifluoroethylene.
The CF3CHCl radical has been detected in the matrix for the first time. The bands observed in the IR spectrum were calculated by
the quantum-chemical B3LYP/6-311 G(d,p) method and assigned to normal vibrations of the radical.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2085–2088, October, 1998. 相似文献