全文获取类型
收费全文 | 1051篇 |
免费 | 32篇 |
国内免费 | 2篇 |
专业分类
化学 | 896篇 |
晶体学 | 9篇 |
力学 | 8篇 |
数学 | 26篇 |
物理学 | 146篇 |
出版年
2023年 | 5篇 |
2022年 | 5篇 |
2021年 | 4篇 |
2020年 | 7篇 |
2019年 | 15篇 |
2018年 | 5篇 |
2016年 | 17篇 |
2015年 | 14篇 |
2014年 | 25篇 |
2013年 | 53篇 |
2012年 | 78篇 |
2011年 | 66篇 |
2010年 | 33篇 |
2009年 | 28篇 |
2008年 | 66篇 |
2007年 | 69篇 |
2006年 | 72篇 |
2005年 | 79篇 |
2004年 | 60篇 |
2003年 | 66篇 |
2002年 | 66篇 |
2001年 | 17篇 |
2000年 | 18篇 |
1999年 | 7篇 |
1998年 | 16篇 |
1997年 | 12篇 |
1996年 | 13篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1992年 | 8篇 |
1991年 | 8篇 |
1990年 | 5篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 17篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 11篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1973年 | 5篇 |
排序方式: 共有1085条查询结果,搜索用时 78 毫秒
991.
Masato Ikeda Dr. Shuichi Haraguchi Munenori Numata Dr. Seiji Shinkai Prof. 《化学:亚洲杂志》2007,2(10):1290-1298
We synthesized a semiartificial β‐1,3‐glucan, curdlan with dialkylaniline groups (CUR‐DA), that bears chromophoric aromatic groups at its peripheral positions. Spectroscopic studies as well as microscopic observations indicate that CUR‐DA adopts a triple‐stranded helical structure in water‐ or methanol‐rich solutions of dimethyl sulfoxide (DMSO). This triple‐stranded helical structure exhibits high thermal stability and resistance to base, attributes that are similar to those of the triple‐stranded helical structure of native β‐1,3‐glucans such as schizophyllan. Moreover, we found that the stability of the triple‐stranded helical structure can be easily modulated by solvent composition and metal‐ion (Zn2+) binding. As β‐1,3‐glucan polysaccharides are known to serve as “polymeric” hosts, including certain DNA molecules, carbon nanotubes, and conjugated polymers, and complexation occurs only with the single‐stranded structure, this information is very useful for the creation of these attractive polymeric composites, the controlled release of DNA, and so on. 相似文献
992.
Ozawa F Okamoto H Kawagishi S Yamamoto S Minami T Yoshifuji M 《Journal of the American Chemical Society》2002,124(37):10968-10969
The (pi-allyl)palladium complex bearing an sp2-hybridized phosphorus ligand (DPCB-OMe: 1,2-bis(4-methoxyphenyl)-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene) efficiently catalyzes direct conversion of allylic alcohols in the absence of activating agents of alcohols such as Lewis acids. N-Allylation of aniline proceeds at room temperature to afford monoallylated anilines in 90-97% yields. C-Allylation of active methylene compounds is also successful at 50 degrees C using a catalytic amount of pyridine as a base, giving monoallylation products in 85-95% yields. The catalytic mechanism involving hydrido- and (pi-allyl)palladium intermediates has been proposed on the basis of stoichiometric examinations using model compounds of presumed intermediates. 相似文献
993.
Carbocations have been generally considered to be relatively unstable and transient species. But the "cation pool" method enables the easy accumulation of carbocations in conventional reaction media such as dichloromethane. In the "cation pool" method, carbocations are generated by low-temperature electrochemical oxidation and accumulated in a solution. In the next step, the carbocations thus produced are allowed to react with various nucleophiles. Combinatorial parallel synthesis based upon the "cation pool" method has also been developed. The applicability of the "cation pool" method depends upon the stability of the cation that is accumulated. This problem can be overcome by the "cation flow" method. In the "cation flow" method, carbocations are generated in a microflow electrochemical system. Short residence times and efficient temperature control of the microflow system are advantageous. Combinatorial sequential synthesis has been achieved based on the "cation flow" method. 相似文献
994.
Susumu Tajima Seiji Tobita Motohiro Mitani Kazuko Akuzawa Hideo Sawada Masaharu Nakayama 《Journal of mass spectrometry : JMS》1991,26(11):1023-1026
Loss of CO from the molecular ions ([CH3OC6H4COF]+˙) of o-, m- and p-anisoyl fluorides has been investigated by mass-analysed ion kinetic energy (MIKE) spectrometry. This reaction involves fluorine atom migration from the carbonyl group to the benzene ring. In the cases of o- and p-anisoyl fluorides, the fluorine atom migrates via a three-membered transition state to form the molecular ions ([CH3OC6H4F]+˙) of o- and p-fluoroanisoles, respectively. On the other hand, in the case of m-anisoyl fluoride, the fluorine atom migrates from the carbonyl group to the benzene ring via a three- or four-membered transition state. 相似文献
995.
Masaru Sato Shinsuke Tanaka Seiji Ebine Kenichi Morinaga Sadatoshi Akabori 《Journal of organometallic chemistry》1985,282(2):247-253
Of the 1,n-dithia[n]ferrocenophanes prepared from disodium 1,1′-ferrocenedithiolate and polymethylene dibromides, the ferrocenophane with n = 7 showed the largest shift in the electronic and 13C NMR spectra and the largest decrease of the redox potential. This can be interpreted by a new type of stereoelectronic interaction between the sulfur atom and the ferrocene nucleus. 相似文献
996.
Osamu Moriya Seiji Arai Takeshi Endo 《Journal of polymer science. Part A, Polymer chemistry》1988,26(9):2573-2579
Tri-n-butyltin hydride (Bu3SnH) was employed for the synthesis of organotin polymers via radical process. The polymers having both organotin and carbonate groups were obtained by the reaction of Bu3SnH with monomers such as diallylcarbonate and diethylene glycol bis(allyl carbonate) via hydrostannation. The copolymerization of diethylene glycol bis(allyl carbonate) and the mono-hydrostannated derivative was also conducted to obtain the corresponding polymers. 相似文献
997.
Yamaguchi S Shirasaka T Akiyama S Tamao K 《Journal of the American Chemical Society》2002,124(30):8816-8817
A series of dibenzoborole derivatives with various groups such as (N,N-diphenylamino)phenyl, thienyl, and bithienyl groups at the 3,7-positions have been synthesized and their photophysical properties studied. These new pi-electron systems show significant solvatochromism in the fluorescence spectra. Thus, about 100-140 nm blue shifts in the emission maxima and 20-30-fold increments in the quantum yields are observed upon changing the solvent from THF to DMF. Similar fluorescence changes are observed upon the addition of n-Bu4NF to their THF solutions, demonstrating their sensing abilities toward a fluoride ion. These fluorescence changes result from the "on/off" control of the ppi-pi* conjugation in their LUMO by the coordination of donor solvents or fluoride ion to the boron atom in the dibenzoborole skeleton. 相似文献
998.
Hirokazu Shimooka Ken-Ichi Yamada Seiji Takahashi Makoto Kuwabara 《Journal of Sol-Gel Science and Technology》1998,13(1-3):873-876
Transparent, partially-crystallized nanostructured barium titanate (BaTiO3) monolithic xerogels (dried at 90°C) have been successfully synthesized via hydrolysis of Ba, Ti alkoxide precursor solutions in a concentration range of 1.0 mol/l with addition of water with a molar ratio of H2O/Ba 6.3. Transparent monolithic xerogels obtained from a precursor solution of 1.0 mol/l remained transparent even after firing at 500°C in oxygen, although the degree of their transparency was considerably decreased. Firing at temperatures above 500°C yielded translucent ceramics of BaTiO3, and ultimately, firing above 600°C resulted in normal opaque ceramic bodies. Those obtained from a more concentrated precursor solution of 1.2 mol/l were, on the other hand, still transparent after firing at 600°C in oxygen, and turned opaque at 700°C. The results demonstrate that the materials retained their transparency even after pyrolysis of organic compounds involving exothermic oxidation at temperatures in the range of 200 to 400°C. The densification behavior of transparent BaTiO3 monolithic xerogels obtained was found to be excellent; for example, those derived from a 1.0 mol/l precursor solution could be sintered to form monolithic BaTiO3 ceramics with a relative sintered density of about 94% and an average grain size of 1 m by firing at 1100°C for 2 h in oxygen. 相似文献
999.
Here, we describe the synthesis and incorporation of the nucleoside base analogue C-deoxyribonucleoside 3 carrying thiophenol into DNA. The 1′-β compound 3 was synthesized by Friedel-Crafts alkylation, followed by deprotection. The coupling reaction with 3,5-ditoluoyl-1-α/β-methoxy-2-deoxy-d-ribose and diphenyldisulfide in the presence of SnCl4 afforded the α/β mixture 2 (β/α=2.8), and the β-form was separated by silica gel chromatography. After formation of the phosphoramidite derivative, the C-nucleoside 3 was incorporated into DNA. When the mercapto-bases were incorporated into complementary singled-stranded (ss) DNAs, the resulting duplex displayed high thermal stabilization on treatment with bubbling O2 (Tm 73 °C), but was destabilized in the presence of mercaptoethanol (Tm 33 °C). CD spectra showed that the duplex had a right-handed double-stranded structure. Imino proton NMR studies of temperature stability suggested that the strength of hydrogen bonding around the mercapto C-nucleoside was larger when treated with bubbling O2 than when in treated with reducing agent. Thus, formation of the base-to-base disulfide bond increased the stability of the duplex; correspondingly, reduction of the disulfide to two thiol bases destabilized the DNA reversibly. The duplex-forming disulfide base pair showed resistance to exonulease III. The present strategy could be used to introduce new functionalities into cells and novel biomaterials. 相似文献
1000.
Okamoto H Yamaji M Satake K Tobita S Kimura M 《The Journal of organic chemistry》2004,69(23):7860-7868
The triplet-sensitized photoreactions of the title biplanophane system 6, the photoisomer of a 2,11-diaza[3,3](9,10)anthracenoparacyclophane derivative 5, were investigated by stationary and laser-flash photolyses using xanthone (XT) and benzophenone (BP) as triplet sensitizers. When photoisomer 6 underwent XT-sensitized irradiation, a triplet cyclophane 5 and a novel polycyclic product 7 were obtained via an adiabatic cycloreversion and a formal [2pia + 2pia + 2sigmas] rearrangement, respectively. The maximum quantum yield for the formation of cyclophane 5 (0.69) and the upper-limit efficiency for the formation of polycycle 7 (0.31) were determined by laser photolysis techniques. For BP-sensitized photolysis of photoisomer 6, oxetane 8, in addition to triplet cyclophane 5 and polycycle 7, was formed by a Paterno-Buchi reaction. The quenching rate constant (k(q)) of triplet BP by photoisomer 6 (3.4 x 10(8) dm(3) mol(-)(1) s(-)(1)) was found to be 1 order of magnitude smaller than that for XT (5.0 x 10(9) dm(3) mol(-)(1) s(-)(1)). On the basis of the relationship between k(q) and the triplet donor-acceptor energy gap, the triplet energy level of photoisomer 6 was estimated to be approximately 71 kcal mol(-)(1). The photochemical and the photophysical processes involved in the sensitized photolyses are summarized in an energetic reaction diagram and discussed in detail. 相似文献