全文获取类型
收费全文 | 5190篇 |
免费 | 175篇 |
国内免费 | 32篇 |
专业分类
化学 | 2939篇 |
晶体学 | 56篇 |
力学 | 198篇 |
数学 | 1254篇 |
物理学 | 950篇 |
出版年
2023年 | 51篇 |
2022年 | 147篇 |
2021年 | 160篇 |
2020年 | 143篇 |
2019年 | 174篇 |
2018年 | 174篇 |
2017年 | 167篇 |
2016年 | 229篇 |
2015年 | 172篇 |
2014年 | 240篇 |
2013年 | 373篇 |
2012年 | 351篇 |
2011年 | 376篇 |
2010年 | 201篇 |
2009年 | 216篇 |
2008年 | 263篇 |
2007年 | 227篇 |
2006年 | 177篇 |
2005年 | 144篇 |
2004年 | 142篇 |
2003年 | 105篇 |
2002年 | 93篇 |
2001年 | 81篇 |
2000年 | 59篇 |
1999年 | 41篇 |
1998年 | 32篇 |
1997年 | 39篇 |
1996年 | 54篇 |
1995年 | 45篇 |
1994年 | 39篇 |
1993年 | 44篇 |
1992年 | 37篇 |
1991年 | 40篇 |
1990年 | 35篇 |
1989年 | 26篇 |
1988年 | 33篇 |
1987年 | 40篇 |
1986年 | 41篇 |
1985年 | 39篇 |
1984年 | 31篇 |
1983年 | 26篇 |
1982年 | 35篇 |
1981年 | 33篇 |
1980年 | 27篇 |
1979年 | 39篇 |
1978年 | 29篇 |
1977年 | 22篇 |
1976年 | 25篇 |
1975年 | 21篇 |
1974年 | 15篇 |
排序方式: 共有5397条查询结果,搜索用时 15 毫秒
61.
A.Lahmam Bennani H.F. Wellenstein A. Duguet B. Nguyen A.D. Barlas 《Chemical physics letters》1976,41(3):470-473
We present a new experimental determination of the Compton profiles of CH4 and C2H4 molecules using high energy electron impact. The observable q range has been extended up to 10 au (6 au for C2H4). Good agreement is found with an earlier X-ray scattering experiment and with new theoretical calculations. 相似文献
62.
Van T.H. Nguyen Joachim T. Anders Qingjun Ma Regine Herbst-Irmer Peter Langer 《Tetrahedron》2007,63(52):12975-12985
Acid-mediated ring-transformations of 5-alkylidene-2,5-dihydropyrrol-2-ones, available by cyclization of 1,3-diketone dianions with bis(imidoyl) dichlorides of oxalic acid, resulted in formation of functionalized pyran-4-ones, such as 5,6-dihydrobenzo[h]chromones and 5,6,7,8-tetrahydrochromones. 相似文献
63.
Mint Tho Nguyen Georges Leroy Michel Sana Jos Elguero 《Journal of heterocyclic chemistry》1982,19(4):943-944
An ab initio study of the Dimorth rerarrangement of the -amino-1,2,3,4-tetrazole leads to the conclusion teht in vapour phase the rate determining step is not the ring-chain isomerism, but is either the Z-E iomerism around the C?N double bond or the 1,3-sigmatropic shift of the proton. 相似文献
64.
In spite of its industrial importance, the detailed reaction mechanism of cyclohexane autoxidation by O2 is still insufficiently known. Based on quantum chemical potential energy surfaces, rate coefficients of the primary and secondary chain propagation steps involving the cyclohexylperoxyl (CyOO) radical were evaluated using multiconformer transition-state theory. Including tunneling and hindered-internal-rotation effects, the rate coefficient for hydrogen-atom abstraction from cyclohexane (CyH) by CyOO was calculated to be k(T)= 1.46 x 10(-11) x exp(-17.8 kcal mol(-1)/ RT) cm3s(-1) (300-600K), close to the experimental data. A "Franck-Rabinowitch cage" reaction between the nascent cyclohexylhydroperoxide (CyOOH) and cyclohexyl radical, products from CyOO + CyH, is put forward as an initially important cyclohexanol (CyOH) formation channel. alphaH abstraction by CyOO. from cyclohexanone was calculated to be only about five times faster than that from CyH, too slow to explain all the observed side products. The a-hydrogen (alphaH) abstractions from CyOH and CyOOH by CyOO. are predicted to be about 10 and 40 times faster, respectively, than the CyOO. +CyH reaction. The very fast CyOO.+CyOOH reaction proceeds through the unstable Cy-alphaH .OOH radical that decomposes spontaneously into the ketone (Q=O) plus the OH radical; the "hot" .OH is found to produce the bulk of the alcohol via a second, "activated cage" reaction analogous to that above. It is thus shown how the very reactive CyOOH intermediate is the predominant source of ketone and alcohol, while it also leads to some side products. The alpha-hydroxycyclohexylperoxyl radical formed during the moderately fast oxidation of CyOH is shown to decompose fast into HO2 + cyclohexanone in a rapidly equilibrated reaction, which constitutes a smaller, second ketone source. These two fast cyclohexanone forming routes avoid the need for unfavorable molecular routes hitherto invoked as ketone sources. The theoretical predictions are supported and complemented by experimental findings. The newly proposed scheme is also largely applicable to the oxidation of other hydrocarbons, such as toluene, xylene, and ethylbenzene. 相似文献
65.
A rapid, selective, and sensitive method is described for the purification and analysis of fluoxetine and norfluoxetine using a solid-phase extraction column and gas chromatography-electron-capture detection. Linear quantitative response curves for fluoxetine and norfluoxetine are generated over a concentration range of 20-200 ng/ml. Overall extraction efficiency of the extraction procedure is found to be greater than 90% and greater than 75% with correlation coefficients of 0.997 and 0.993 for fluoxetine and norfluoxetine, respectively. 相似文献
66.
Zeta-potentials of self-assembled surface micelles of ionic surfactants adsorbed at hydrophobic graphite surfaces 总被引:1,自引:0,他引:1
Vamsi K. Paruchuri Anh V. Nguyen Jan D. Miller 《Colloids and surfaces. A, Physicochemical and engineering aspects》2004,250(1-3):519-526
The zeta-potentials of the self-assembled surface ionic surfactants (sodium dodecyl sulfate—SDS and hexadecyltrimethyl ammonium bromide—CTAB) on graphite surfaces were determined both from streaming potential and electrophoretic mobility measurements. The adsorption of the surfactants at graphite–liquid interfaces has been studied using atomic force microscopy (AFM) soft-contact imaging which shows the formation of linear, parallel hemicylinders with headgroups oriented towards the solution. The magnitude of the zeta-potential increased with an increase in surfactant concentration, reaching a constant value at a concentration corresponding to the point of surface micelle formation as confirmed from AFM imaging. The streaming potential and electrophoretic mobility measurements showed that the zeta-potentials of SDS and CTAB surface micelles adsorbed at the graphite surface were about −75 and +70 mV, respectively, well in agreement with the values reported for bulk phase micelles in the literature. 相似文献
67.
Nguyen Huyen Rao A. Madhusudhan Phillips J. B. John Vijay T. Reed Wayne F. 《Applied biochemistry and biotechnology》1991,(1):843-853
We describe a technique to modify protein solubility and optimize enzyme activity in reversed micellar solutions. The technique
is based on the ability of hydrates of natural gas to form in the micro-aqueous phase. Clathrate hydrates are crystalline
inclusions of water and gas, and their formation in bulk water has traditionally been studied with relevance to natural gas
recovery. We have found that hydrates can form in the environment of the microaqueous pools of reversed micelles, and that
their extent of formation can be well controlled through the thermodynamic variables of temperature and pressure. Additionally,
formation of hydrates affects the size and aggregation number of the micelles, and thus influences the solubility and conformation
of encapsulated proteins. We demonstrate how the concept can be used in two applications: (i) protein extraction into reversed
micelles and subsequent recovery, and (ii) optimization of enzyme activity in reversed micelles. 相似文献
68.
C. Hinnen C. Nguyen Van Huong A. Rousseau J.P. Dalbera 《Journal of Electroanalytical Chemistry》1979,95(2):131-146
The admittance and differential reflectivity of an electrode of polycrystalline gold in contact with aqueous 0.02 M NaF, KClO4, 0.01 M Na2SO4 and KCl have been measured in the range of potential where the electrode is ideally polarized (?0.7 V to 0.8 V NHE). In NaF and KClO4 evidence was obtained for the compression of the adsorbed layer of water. The increase of the number of water molecules, with respect to the number in the least original state (at?4μC cm?2), is 3% at +8 μC cm?2 and at?17 μC cm?2. At the potential of zero charge the water dipoles are oriented preferentially with the oxygen towards the metal. In KCl the optical measurements confirm the existence of two types of Au?Cl? interaction and the covalent character of the bond at the most positive charges. 相似文献
69.
Chong Han Sean M. Kelly Theresa Cravillion Scott J. Savage Tina Nguyen Francis Gosselin 《Tetrahedron》2019,75(32):4351-4357
An efficient synthesis of PI3K inhibitor GDC-0077, featuring two consecutive Cu-catalyzed CN coupling reactions, is reported. The described synthetic route involves a chemoselective Ullmann-type coupling of a chiral difluoromethyl-substituted oxazolidinone, a Cu-catalyzed N-arylation of l-alanine with high stereochemical integrity, and a high-yielding final amide bond formation step to produce GDC-0077 in >99.5 area % HPLC purity. 相似文献
70.
T. Hiep Nguyen Peter S. Clezy Gary D. Willett George L. Paul John Tann Peter J. Derrick 《Journal of mass spectrometry : JMS》1991,26(4):215-226
Laser desorption Fourier transform ion cyclotron resonance positive- and negative-ion mass spectra are presented for dimethyl 8-acetyl-3,7,12,17-tetramethylporphyrin-2,18-dipropanoate. The 248-nm laser ionization thresholds for both positive and negative ions are observed to be about 2.5 MW cm?2. The M+˙ molecular ion is assigned to the base peak in the low-power spectra whereas it is the M?˙ ion for the corresponding anion spectra. Increased intensities of [M + H]+ and [M ? H]? are observed with increased laser fluences of up to 38 MW cm?2. At high laser powers the negative-ion results reveal that a series of carbon-nitrogen cumulene and polyacetylene cluster ions are formed. Laser evaporation/multiphoton ionization/ and thermal evaporation/electron impact ionization/collision-induced dissociation experiments carried out on the porphyrin M+˙ and [M + H]+ ions over a range of translational kinetic energies and delay times after acceleration are compared and used to obtain mechanistic and structural information. In contrast to the electron impact experiments, which show only side-chain cleavage, the laser-based collision-induced dissociation experiments reveal that, in addition to side-chain cleavage, it is possible to cleave the porphyrin ring to various extents depending on the ion translational energy selected. 相似文献