全文获取类型
收费全文 | 2532篇 |
免费 | 87篇 |
国内免费 | 12篇 |
专业分类
化学 | 2007篇 |
晶体学 | 34篇 |
力学 | 32篇 |
数学 | 198篇 |
物理学 | 360篇 |
出版年
2022年 | 19篇 |
2021年 | 18篇 |
2020年 | 34篇 |
2019年 | 42篇 |
2018年 | 19篇 |
2017年 | 25篇 |
2016年 | 46篇 |
2015年 | 54篇 |
2014年 | 58篇 |
2013年 | 123篇 |
2012年 | 142篇 |
2011年 | 161篇 |
2010年 | 77篇 |
2009年 | 92篇 |
2008年 | 152篇 |
2007年 | 149篇 |
2006年 | 141篇 |
2005年 | 130篇 |
2004年 | 124篇 |
2003年 | 102篇 |
2002年 | 92篇 |
2001年 | 31篇 |
2000年 | 54篇 |
1999年 | 16篇 |
1998年 | 35篇 |
1997年 | 28篇 |
1996年 | 28篇 |
1995年 | 26篇 |
1994年 | 20篇 |
1993年 | 25篇 |
1992年 | 38篇 |
1991年 | 29篇 |
1990年 | 23篇 |
1989年 | 31篇 |
1988年 | 17篇 |
1987年 | 25篇 |
1986年 | 21篇 |
1985年 | 29篇 |
1984年 | 31篇 |
1983年 | 28篇 |
1982年 | 38篇 |
1981年 | 29篇 |
1980年 | 36篇 |
1979年 | 23篇 |
1978年 | 23篇 |
1977年 | 13篇 |
1976年 | 19篇 |
1975年 | 18篇 |
1974年 | 15篇 |
1973年 | 15篇 |
排序方式: 共有2631条查询结果,搜索用时 15 毫秒
61.
Yoshihiro Itoh Yotaro Morishima Shun-ichi Nozakura 《Photochemistry and photobiology》1984,39(5):603-608
Abstract— Photosensitized reduction of zwitterionic viologen (SPV) and methyl viologen (MV2+ ) was investigated using an amphiliphilic copolymer having phenanthryl and sulfonate groups (APh) as photosensitizer in aqueous solutions. In the presence of triethanolamine the accumulation of SPV * (photoproduct) was found to be faster than that of MV+ . This attributed to the electrostatic repulsion between SPV. and anionic segments of APh. Such difference between SPV and MV2+ was minimized in the case of the related monomer model. Retardation of the back reaction for the APh-SPV system was also demonstrated by laser photolysis, k b = 8.7 × 107 M -1 s-1 for the polymer system as compared to k b = 2.8 × 109 M -1 s-1 for the monomer model system. Strong salt-effects on the yield of the photoreduction and the rate of back reaction confirm the strong electrostatic interaction between the photoproducts and polyanions. This remarkable electrostatic effect of the polyanions was simulated by electrochemical redox reactions by using a graphite electrode coated with APh. 相似文献
62.
Yoshiyuki Tanaka Makoto Arakawa Yohei Yamaguchi Chieko Hori Masahiro Ueno Takeyuki Tanaka Tatsushi Imahori Yoshinori Kondo 《化学:亚洲杂志》2006,1(4):581-585
A metal‐free acetylide was observed by using NMR spectroscopy. Metal‐free acetylides are closely related to reactive intermediates (carbanions) in solution; therefore, they have been regarded as unobservable species. However, we generated this highly reactive and unstable species through the deprotonation of phenylacetylene by using the strong nonmetallic phosphazene base tBu‐P4. In the presence of tBu‐P4, the J coupling between the ethynyl carbon and hydrogen nuclei (1JC,H) of phenylacetylene disappeared; this indicates the deprotonation of the alkyne terminal. Furthermore, a large low‐field shift (approximately 90 ppm) of the alkyne carbon resonance was observed. We concluded that we have observed a metal‐free carbanion with a formal charge on an sp‐hybridized carbon atom for the first time. 相似文献
63.
Junji Furukawa Eiichi Kobayashi Yoshihiro Arai Toshimitsu Suzuki Yoshinobu Takegami 《Journal of polymer science. Part A, Polymer chemistry》1976,14(10):2553-2563
An investigation by 220-MHz NMR spectroscopy was carried out on the alternating copolymers of acrylic monomer with 2-substituted 1,3-diolefin. The chain structures were determined. The acrylic monomers used were methyl methacrylate (MMA), acrylonitrile (AN), and methacrylonitrile (MAN); isoprene (IP) and chloroprene (CLP) were the 1,3-diolefins. In the MAN–IP alternating copolymer, the 1-position methylene protons of IP showed an AB quartet peak, confirming the α-1 linkage structure. Similarly, in the MMA–CLP and AN–CLP copolymers, the 1-position methylene protons of CLP showed and AB quartet and an ABX pattern, respectively, confirming the α-1 linkage structure in both these cases also. The α-1 linkage structure was also revealed by the decoupling technique in the MAN–CLP alternating copolymer. The AN–IP and MMA–IP alternating copolymers also possess a bond between the α-position of the acrylic monomer and the 1-position of IP. The monomeric units in the alternating copolymers of acrylic monomers with 2-substituted 1,3-diolefins were generally linked at the α-position of acrylic monomer and the 1-position of 1,3-diolefin. On the other hand, in the Diels-Alder adducts of acrylic monomer with 2-substituted 1,3-diolefin, the reaction takes place between the α-position of acrylic monomer and the 4-position of 1,3-diolefin. The regioselectivity of the alternating copolymers and the Diels-Alder adducts is quite compatible with the expectations from molecular orbital theory. 相似文献
64.
Yamamoto Y Terui N Tachiiri N Minakawa K Matsuo H Kameda T Hasegawa J Sambongi Y Uchiyama S Kobayashi Y Igarashi Y 《Journal of the American Chemical Society》2002,124(39):11574-11575
Paramagnetic NMR and optical studies of the oxidized forms of mesophile Pseudomonas aeruginosa cytochrome c(551) and its quintuple mutant (F7A/V13M/F34Y/E43Y/V78I), and thermophile Hydrogenobacter thermophilus cytochrome c(552) demonstrated that the amino acid side chain packings in the protein interior influence the coordination bond between the heme iron and the axial methionine in the proteins. The strength of heme axial coordinations was found to correlate with the overall protein thermostability. 相似文献
65.
A method is described for determination of the steroidal saponin, eruboside B, originating in garlic and garlic products as the p-nitrobenzoyl chloride (PNBC) derivative by reversed-phase liquid chromatography (with ultraviolet detection at 260 nm. Proto-eruboside B was extracted from garlic (Allium sativum L.); subjected to solid-phase extraction (SPE) with a C18 cartridge, Florisil column chromatography, and silica gel column chromatography; and then enzymatically converted to eruboside B, which was applied as an external standard. Steroidal saponins in garlic and commercial garlic products were extracted with methanol and purified by SPE cartridges, followed by enzymatic treatment. A frostanol saponin such as proto-eruboside B is enzymatically transformed to a spirostanol saponin, eruboside B. After the derivatization with PNBC, the saponin derivative was chromatographed on a C8 column with a gradient elution of (A) 80% aqueous acetonitrile and (B) 100% acetonitrile. The detection limit of the developed method was 1 microg/g for the samples. The method was applied to the analysis of garlic and garlic health food products available in Japan. 相似文献
66.
Yoshihiro Osamura Shinichi Yamabe Kichisuke Nishimoto 《International journal of quantum chemistry》1980,18(2):457-462
The photoisomerization of imine compounds is studied in terms of an ab initio MO CI calculation. The potential curves of the syn-anti isomerization via the rotation and the inversion are examined for benzaldimine. It is suggested that the photoisomerization is initiated through the rotation around the C? N bond in both singlet and triplet states. The ease of the photoisomerization is found to be determined by both the conformation of phenyl ring in the ground state and the energy difference of vertically excited states between two isomers. 相似文献
67.
Hidetomo Ashitaka Kazuya Jinda Haruo Ueno 《Journal of polymer science. Part A, Polymer chemistry》1983,21(7):1989-1995
A mechanism is proposed for the polymerization of syndiotactic 1,2-polybutadiene (s-PB) with soluble cobalt-organoaluminum-CS2. The proposed active species have structures which consist of side-on coordination of CS2 to cobalt, anti-π-allyl growing end, cisoid bidentate coordination of butadiene, and activation by complex formation with organoaluminum at the nonbonded sulfur of the coordinated CS2. This proposal is based on findings for the aluminum-free catalyst Co(C4H6)(C8H13)-CS2. It is tentatively interpreted that syndiotactic 1,2 polymerization proceeds under the influence of the side-on coordinated CS2, by which the reactivity between the terminal carbons of butadiene and the C3 of the π-allyl end is enhanced. 相似文献
68.
Homoleptic cyclometalated iridium complexes with highly efficient red phosphorescence and application to organic light-emitting diode 总被引:7,自引:0,他引:7
Tsuboyama A Iwawaki H Furugori M Mukaide T Kamatani J Igawa S Moriyama T Miura S Takiguchi T Okada S Hoshino M Ueno K 《Journal of the American Chemical Society》2003,125(42):12971-12979
Phosphorescence studies of a series of facial homoleptic cyclometalated iridium(III) complexes have been carried out. The complexes studied have the general structure Ir(III)(C-N)(3), where (C-N) is a monoanionic cyclometalating ligand: 2-(5-methylthiophen-2-yl)pyridinato, 2-(thiophen-2-yl)-5-trifluoromethylpyridinato, 2,5-di(thiophen-2-yl)pyridinato, 2,5-di(5-methylthiophen-2-yl)pyridinato, 2-(benzo[b]thiophen-2-yl)pyridinato, 2-(9,9-dimethyl-9H-fluoren-2-yl)pyridinato, 1-phenylisoquinolinato, 1-(thiophen-2-yl)isoquinolinato, or 1-(9,9-dimethyl-9H-fluoren-2-yl)isoquinolinato. Luminescence properties of all the complexes at 298 K in toluene are as follows: quantum yields of phosphorescence Phi(p) = 0.08-0.29, emission peaks lambda(max) = 558-652 nm, and emission lifetimes tau = 0.74-4.7 micros. Bathochromic shifts of the Ir(thpy)(3) family [the complexes with 2-(thiophen-2-yl)pyridine derivatives] are observed by introducing appropriate substituents, e.g., methyl, trifluoromethyl, or thiophen-2-yl. However, Phi(p) of the red emissive complexes (lambda(max) > 600 nm) becomes small, caused by a significant decrease of the radiative rate constant, k(r). In contrast, the complexes with the 1-arylisoquinoline ligands are found to have marked red shifts of lambda(max) and very high Phi(p) (0.19-0.26). These complexes are found to possess dominantly (3)MLCT (metal-to-ligand charge transfer) excited states and have k(r) values approximately 1 order of magnitude larger than those of the Ir(thpy)(3) family. An organic light-emitting diode (OLED) device that uses Ir(1-phenylisoquinolinato)(3) as a phosphorescent dopant produces very high efficiency (external quantum efficiency eta(ex) = 10.3% and power efficiency 8.0 lm/W at 100 cd/m(2)) and pure-red emission with 1931 CIE (Commission Internationale de L'Eclairage) chromaticity coordinates (x = 0.68, y = 0.32). 相似文献
69.
M. Itoh Y. Yamada N. Kiriyama K. Komura K. Ueno M. Sakanoue 《Journal of Radioanalytical and Nuclear Chemistry》1993,172(2):289-298
For determining low level lithium concentrations in water, a neutron activation method based on the measurement of tritium radioactivity produced by6Li(n,)3H reaction has been developed. This method is specific and free from interference by other chemical elements. Using a low background liquid scintillation counter for tritium measurement, the detection limit is approximately 0.3 ppm during irradiation at a thermal neutron flux density of 1.1·107n·cm–2·s–1 for 6 hours by a small nuclear reactor and liquid scintillation counting for 2000 minutes 相似文献
70.
Naofumi Terada Minoru Morimoto Hiroyuki Saimoto Yoshiharu Okamoto Saburo Minami Yoshihiro Shigemasa 《先进技术聚合物》2003,14(1):40-51
Oxidized chitosan derivatives with various degrees of oxidation (DS, 0.1–1.0) were prepared by the treatment of chitosan with CrO3/aq HClO4 or by the oxidation of 3‐O‐ and N‐protected chitosan with 30% aq H2O2/Na2WO4 followed by 3‐O‐ and N‐deprotection. The oxidized products were then N‐acetylated with Ac2O in order to improve their water‐solubility. Although the oxidized chitosan derivative of DS 0.28 and the degree of N‐acetylation of chitosan (DA) 38% was insoluble in the pH 3–8 region, that of DS 0.26 and DA 76% was soluble in the neutral pH range. The newly‐prepared acetylated and oxidized chitosan derivatives were found to suppress the chemiluminescence response of inflammatory cells such as canine polymorphonuclear cells (PMNs). Analysis by the surface plasmon resonance method revealed that the bind and release behavior of PMNs to acetylated oxidized chitosan derivatives was similar to that against carboxymethylated chitosan derivatives. The amount of water‐soluble chitosan derivative bound to cytokine IL‐8 was found to be affected by the structural and electronic features of the chitosan substituents in the chitosan chain. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献