全文获取类型
收费全文 | 1556篇 |
免费 | 20篇 |
国内免费 | 4篇 |
专业分类
化学 | 1115篇 |
晶体学 | 16篇 |
力学 | 22篇 |
数学 | 104篇 |
物理学 | 323篇 |
出版年
2021年 | 7篇 |
2020年 | 12篇 |
2019年 | 20篇 |
2018年 | 18篇 |
2017年 | 14篇 |
2016年 | 21篇 |
2015年 | 13篇 |
2014年 | 33篇 |
2013年 | 66篇 |
2012年 | 47篇 |
2011年 | 64篇 |
2010年 | 34篇 |
2009年 | 45篇 |
2008年 | 98篇 |
2007年 | 70篇 |
2006年 | 76篇 |
2005年 | 83篇 |
2004年 | 89篇 |
2003年 | 66篇 |
2002年 | 67篇 |
2001年 | 33篇 |
2000年 | 57篇 |
1999年 | 28篇 |
1998年 | 22篇 |
1997年 | 22篇 |
1996年 | 20篇 |
1995年 | 15篇 |
1994年 | 24篇 |
1993年 | 10篇 |
1992年 | 23篇 |
1991年 | 16篇 |
1990年 | 15篇 |
1989年 | 9篇 |
1987年 | 11篇 |
1986年 | 15篇 |
1985年 | 22篇 |
1984年 | 41篇 |
1983年 | 15篇 |
1982年 | 24篇 |
1981年 | 29篇 |
1980年 | 25篇 |
1979年 | 29篇 |
1978年 | 17篇 |
1977年 | 20篇 |
1976年 | 18篇 |
1975年 | 13篇 |
1974年 | 9篇 |
1973年 | 7篇 |
1972年 | 7篇 |
1968年 | 6篇 |
排序方式: 共有1580条查询结果,搜索用时 0 毫秒
51.
52.
[reaction: see text] We have established an enantioselective synthesis of both C2 symmetric and unsymmetric tetra-ortho-substituted axially chiral biaryls through rhodium-catalyzed double [2 + 2 + 2] cycloaddition (up to >99% ee). This method serves as an attractive new route to enantioenriched tetra-ortho-substituted axially chiral biaryls in view of the one-step access to substrate diynes and tetraynes starting from readily available alkynes. 相似文献
53.
Ryoma Masuda Yuuya Kawasaki Kazunobu Igawa Yoshiyuki Manabe Hiroshi Fujii Nobuo Kato Katsuhiko Tomooka Junko Ohkanda 《化学:亚洲杂志》2020,15(6):742-747
Mid‐sized molecules have emerged as an attractive chemical space and potentially provide a robust basis for the development of synthetic agents to control intracellular protein interactions. However, the limited cell permeability and chemical tractability of such agents remain to be addressed. We envisioned that target‐templated synthesis of such mid‐sized molecules might provide a solution. Here, we exploited a copper‐free Huisgen cycloaddition for template synthesis using a peptide fragment containing a 4,8‐diazacyclononyne (DACN) moiety and an azide‐containing fusicoccin derivative in the presence or absence of recombinant 14‐3‐3ζ protein in vitro. Time‐course changes in the yield of products demonstrated that the reaction was accelerated in the presence of 14‐3‐3 and one of the regioisomers was generated predominantly, supporting the template effect. 相似文献
54.
A novel amphiphilic Tb(3+) complex (TbL(+)) having anionic bis(pyridine) arms and a hydrophobic alkyl chain is developed. It spontaneously self-assembles in water and gives stable vesicles that show sensitized luminescence of Tb(3+) ions at neutral pH. This TbL(+) complex is designed to show coordinative unsaturation, i.e., water molecules occupy some of the first coordination spheres and are replaceable upon binding of phosphate ions. These features render TbL(+) self-assembling receptor molecules which show increase in the luminescence intensity upon binding of nucleotides. Upon addition of adenosine triphosphate (ATP), significant amplification of luminescent intensity was observed. On the other hand, ADP showed moderately increased luminescence and almost no enhancement was observed for AMP. Very interestingly, the increase in luminescence intensity observed for ATP and ADP showed sigmoidal dependence on the concentration of added nucleotides. It indicates positive cooperative binding of these nucleotides to TbL(+) complexes preorganized on the vesicle surface. Self-assembly of amphiphilic Tb(3+) receptor complexes provides nanointerfaces which selectively convert and amplify molecular information of high energy phosphates linked by phosphoanhydride bonds into luminescence intensity changes. 相似文献
55.
Regio- and stereoselective sulfonylation of allenes under Cu catalysis is described. Allenyl sp carbons exclusively react with TsCN to give the corresponding alkenyl sulfones. The reaction is initiated by addition of tosyl radical to form benzyl radical intermediates, which determines the reaction pathway. The structure of the products is highly dependent on the substituents on allenes. 相似文献
56.
Nobuo Ishizawa Reiko Yamashita Shuji Oishi James R. Hester Shunji Kishimoto 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(9):1006-1009
The triclinic superstructure of a small crystal of LiCa2Nb3O10, lithium dicalcium triniobium decaoxide, has been investigated by synchrotron X‐ray diffraction. The unit cell is an almost rectangular parallelepiped, although there is a 0.245° offset from orthogonality for β. The structure essentially belongs to a homologous series of Li[Nan?3Ca2NbnO3n+1] with n = 3, where the moiety in square brackets has a perovskite‐type slab structure. The superstructure has a doubled unit‐cell volume with respect to the tetragonal aristotype. The NbO6 octahedra are rotated about axes parallel to [110] by approximately 10°. Adjacent slabs are connected by Li atoms and are geometrically related by 42 pseudosymmetry lying parallel to c . There are twice as many sites as Li atoms, providing a variation of population at these Li sites. 相似文献
57.
Ono Y Minami A Noike M Higuchi Y Toyomasu T Sassa T Kato N Dairi T 《Journal of the American Chemical Society》2011,133(8):2548-2555
Fusicoccin A and cotylenin A are structurally related diterpene glucosides and show a phytohormone-like activity. However, only cotylenin A induces the differentiation of human myeloid leukemia cells. Since the cotylenin A producer lost its ability to proliferate during preservation, a study on the relationship between structure and activity was carried out and a modified fusicoccin A with hydroxyl group at the 3-position showed a similar biological activity with that of cotylenin A. We then searched for an enzyme source that catalyzes the introduction of a hydroxyl group into the 3-position and found that brassicicene C, which is structurally related to fusicoccin A with hydroxyl group at the 3-position, was produced by Alternaria brassicicola ATCC96836. We recently cloned a brassicicene C biosynthetic gene cluster including the genes encoding fusicocca-2,10(14)-diene synthase and two cytochrome P450s, which were responsible for the formation of fusicocca-2,10(14)-diene-8β,16-diol. In this study, we report that a α-ketoglutarate dependent dioxygenase, the gene coding for which was located in the cluster, catalyzed a hydroxylation at the 3-position of fusicocca-2,10(14)-diene-8β,16-diol. On the other hand, a α-ketoglutarate-dependent dioxygenase, which had been identified in a fusicoccin A biosynthetic gene cluster, catalyzed the 16-oxidation of fusicocca-2,10(14)-diene-8β,16-diol to yield an aldehyde (8β-hydroxyfusicocca-1,10(14)-dien-16-al), although both dioxygenases had 51% amino acid sequence identity. These findings suggested that the dioxygenases played critical roles for the formation of the fusicoccin A-type and cotylenin A-/brassicicene C-type aglycons. Moreover, we showed that short-chain dehydrogenase/reductase located in the fusicoccin A biosynthetic gene cluster catalyzed the reduction of the aldehyde to yield fusicocca-1,10(14)-diene-8β,16-diol. 相似文献
58.
Yoshinao Nakagawa Nobuo Tajima Kimihiko Hirao 《Journal of computational chemistry》2000,21(14):1292-1304
The hydration reaction of ethylene, C2H4+H2O → C2H5OH, catalyzed by oxoacids (H3PO4, H2SO4, and HClO4) and metal cations (B3+, Al3+, Sc3+, Ga3+, La3+, Be2+, Mg2+, Ca2+, Zn2+, and Sr2+) are studied systematically by density functional theory with a BLYP functional. The reaction profiles of the main reaction and some side reactions, such as ester formation, dimerization of ethylene, and dehydrogenation of ethanol, have been determined with a variety of catalysts. In each case, the intermediate states, the transition states, and their energetics are calculated. Metal cations react more efficiently for the main reaction than oxoacids, but they also make the dehydrogenation reaction active. While the dimerization reaction is strongly affected by the acidity of the catalyst, both the acidity and basicity of the catalyst are important for the dehydrogenation reaction. Efficient formation of ethanol from ethylene over a catalyst is suggested. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1292–1304, 2000 相似文献
59.
Bunsho Ohtani Makoto Nishida Sei-ichi Nishimoto Tsutomu Kagiya 《Photochemistry and photobiology》1986,44(6):725-732
Photooxygenation reaction of an unsaturated fatty acid ester, methyl linoleate (methyl 9- cis. 12- cis -octadecadienoate, ML-H), sensitized by porphyrins and several types of dyes has been studied in aqueous emulsion and acetonitrile solution under air at 40°C. The oxygen (O2 ) uptake proceeded slowly in the absence of sensitizers upon irradiation of an aqueous emulsion and an acetonitrile solution of ML-H (20 m M ) at ℷex > 290 nm (11.4 and 6.1 μmol h-1 , respectively). The rate of O2 uptake was enhanced by a catalytic amount (0.1 m M ) of porphyrins and dyes; hematoporphyrin (HP), zinc tetrakis(N-methyl-4-pyridiniumyi)porphyrin (ZnTMPyP), methylene blue (MB), rose bengal (RB), acridine orange (AO), and acriflavine (AF). In both systems, the sensitized photooxidation of ML-H by O2 proceeded equimolarly to produce isomeric mixture of C9 and C13 hydroperoxides having the trans,cis and trans,trans conjugated diene configurations, independent of the types of the sensitizers used. The yield ratio of trans,trans/ trans,cis products in the MB-sensitized photooxygenation in acetonitrile and aqueous emulsion were almost equal (0.32 and 0.35. respectively). The sensitizing activity of the sensitizers, as measured by the quantum yield of O2 uptake, increased in the order: MB (≃ 0) < ZnTMPyP < RB < HP < AF < AO in the aqueous emulsion and AO < AF < HP < RB = MB in the acetonitrile solution. The order in homogeneous acetonitrile solution was interpreted by the sensitizing ability of the dyes to produce singlet oxygen, while that in heterogeneous aqueous emulsion was correlated to the lipophilicity of dyes as well as the singlet-oxygen-producing ability. 相似文献
60.
Prof. Dr. Toshiaki Mori Takayuki Kodera Hiroshi Yoshimine Prof. Dr. Yoshimitsu Kakuta Prof. Dr. Nobuo Sugiura Prof. Dr. Koji Kimata Prof. Dr. Yoshio Okahata 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(24):7388-7393
Using a highly sensitive flow‐type 27 MHz quartz crystal microbalance, we could detect a small mass change during stepwise and alternating one‐sugar transfer of glucuronic acid (GlcA) and N‐acetylgalactosamine (GalNAc) to an acceptor, catalyzed by chondroitin polymerase from Escherichia coli strain K4 (K4CP), and analyze the elongation mechanism of K4CP. K4CP was found to bind strongly to a chondroitin acceptor (Kd=0.97 μM ). Although the binding affinity and the catalytic rate constant for each monomer were considerably different, the apparent catalytic efficiency (kcat/Km) was similar (6.3×104 M ?1 s?1 for GlcA transfer and 3.4×104 M ?1 s?1 for the GalNAc transfer). This is reasonable for the smooth alternating elongation of GlcA and GalNAc on the acceptor. This is the first study to report the determination of kinetic parameters for enzymatic, alternated, sugar elongation. 相似文献