全文获取类型
收费全文 | 4292篇 |
免费 | 111篇 |
国内免费 | 12篇 |
专业分类
化学 | 3052篇 |
晶体学 | 52篇 |
力学 | 66篇 |
综合类 | 1篇 |
数学 | 224篇 |
物理学 | 1020篇 |
出版年
2023年 | 23篇 |
2022年 | 20篇 |
2021年 | 39篇 |
2020年 | 58篇 |
2019年 | 66篇 |
2018年 | 51篇 |
2017年 | 31篇 |
2016年 | 67篇 |
2015年 | 76篇 |
2014年 | 92篇 |
2013年 | 193篇 |
2012年 | 186篇 |
2011年 | 230篇 |
2010年 | 146篇 |
2009年 | 142篇 |
2008年 | 232篇 |
2007年 | 261篇 |
2006年 | 259篇 |
2005年 | 191篇 |
2004年 | 166篇 |
2003年 | 171篇 |
2002年 | 159篇 |
2001年 | 118篇 |
2000年 | 134篇 |
1999年 | 84篇 |
1998年 | 60篇 |
1997年 | 57篇 |
1996年 | 47篇 |
1995年 | 43篇 |
1994年 | 44篇 |
1993年 | 59篇 |
1992年 | 50篇 |
1991年 | 49篇 |
1990年 | 38篇 |
1989年 | 49篇 |
1988年 | 49篇 |
1987年 | 42篇 |
1986年 | 35篇 |
1985年 | 54篇 |
1984年 | 58篇 |
1983年 | 48篇 |
1982年 | 48篇 |
1981年 | 31篇 |
1980年 | 34篇 |
1979年 | 49篇 |
1978年 | 43篇 |
1977年 | 48篇 |
1975年 | 26篇 |
1974年 | 32篇 |
1967年 | 16篇 |
排序方式: 共有4415条查询结果,搜索用时 15 毫秒
61.
Shin-ichiro Shoda Yoshinori Misawa Yousuke Nishijima Yukiko Tawata Tomohiko Kotake Masato Noguchi Atsushi Kobayashi Takeshi Watanabe 《Cellulose (London, England)》2006,13(4):477-484
Chitinase-catalyzed hydrolytic and transglycosylating behavior of 1,2-oxazoline derivative of N-acetyllactosamine (LacNAc-oxa) 1 has been investigated. An extremely rapid hydrolysis (ring-opening of the oxazoline moiety) could be observed, suggesting that 1 behaves as a transition state analogue substrate for chitinase A1 (Bacillus circulans WL-12). This disaccharide monomer 1 was found to polymerize under basic conditions, giving rise to novel oligosaccharides having a β(1-4)–β(1-6) repeating unit in the main chain. The degree of polymerization of the resulting oligosaccharides was up to 5. This is the first example of enzymatic glycosylation reaction forming a β(1-6) bond catalyzed by chitinase. 相似文献
62.
The dielectric solvent effect on HCCH, HCNH and HCNC spin coupling constants in ethane, tetrachloroethane and trans N-methylformamide has been calculated by finite perturbation theory based on the INDO and CNDO/2 approximations incorporating solvaton theory. The available experimental data are interpreted using the calculated variations of spin coupling constants. The effect of dielectric constant on the general form of the Karplus relation is included in the finite perturbation calculations. 相似文献
63.
Shin-ichi Hirano Toshinobu Yogo Wataru Sakamoto Seiji Yamada Tomoyuki Nakamura Tomoe Yamammto Hiroyuki Ukai Kouichi Banno Tomoko Nakafuku Yukari Ando 《Journal of Sol-Gel Science and Technology》2003,26(1-3):35-41
Nano sized crystalline particles/polymer hybrids were synthesized form designed metal-organic precursors. The newly developed method is composed of the synthesis of organic matrix by polymerization and the in situ nucleation and growth of crystalline oxide particles in the organic matrix below 100°C. The design of metal-organic precursor modified with polymerizable ligand and the selection of reaction conditions does influence the size and crystallinity of ceramic particles in organic matrix. The nano-sized magnetic particle/polymer hybrid exhibits the interesting feature of superparamagnetism and quantum size effect. The crystalline particles of BaTiO3/, PbTiO3/, and KNbO3/polymer hybrids behave to be dielectric and show the typical electro-rheological behavior. 相似文献
64.
Otaka A Watanabe J Yukimasa A Sasaki Y Watanabe H Kinoshita T Oishi S Tamamura H Fujii N 《The Journal of organic chemistry》2004,69(5):1634-1645
A samarium diiodide (SmI(2))-mediated reduction of gamma,gamma-difluoro-alpha,beta-enoates (15, 29, and 34) was successfully applied to the synthesis of (Z)-fluoroalkene dipeptide isosteres (23, 30, and 35), which have served as potential dipeptide mimetics. Reduction of the gamma,gamma-difluoro-alpha,beta-enoates by SmI(2) proceeded via successive two-electron transfers to form dienolate species which upon kinetically controlled trapping with t-BuOH yielded Xaa-Gly-type fluoroalkene isosteres exemplified by 23, 30, and 35. Replacement of the t-BuOH kinetic trapping agent with aldehydes or ketones provided access to alpha-substituted fluoroalkene isosteres (43 and 45) through aldol reactions of Sm-dienolates with the carbonyl compounds. Of particular note, the use of the SmI(2)-HCHO reagent system with chiral enoate 34 provided D-Phe-psi[(Z)-CF[double bond]CH]-D/L-Ser isosteres (45), which could be converted to enantiomerically pure isosteres (49-52) that bore a variety of side chain functionalities at the alpha-position. This was achieved by a sequence of manipulations consisting of beta-lactone formation followed by chromatographic separation and ring-opening with soft nucleophiles. Included in the present work is the first utilization of a Rh-catalyzed Reformatsky reaction of chiral imines for the stereoselective preparation of alpha,alpha-difluoro-beta-amino acid derivatives (28 and 33). The appropriate choice of reagents (carbonyl compounds for kinetic trapping or ring-opening nucleophiles and imines for Reformatsky reactions) allows the presented methodology to yield various fluoroalkene isosteres possessing a wide range of side chain functionalities. 相似文献
65.
Kawasaki T Nonaka Y Watanabe K Ogawa A Higuchi K Terashima R Masuda K Sakamoto M 《The Journal of organic chemistry》2001,66(4):1200-1204
The reverse aromatic Cope rearrangement of 2-allyl-3-alkylideneindolines obtained by Horner-Wadsworth-Emmons olefination of 2-allylindolin-3-ones was performed. When 2-allylindolin-3-ones were treated with phosphonium ylides in refluxing toluene, domino Wittig reaction and reverse aromatic Cope rearrangement took place to give alpha-allyl-3-indole acetate derivatives in good yields. The aromatization as a new driving force in the Cope rearrangement is preferable to the conjugation with the carbonyl and cyano groups and also to the alkyl substitution pattern, which are well-known driving forces. 相似文献
66.
The solvent extraction of technetium from urine with TBP has been investigated. The distribution ratio of technetium was determined as a function of HCl concentration and reaction time. The distribution ratio in the HCl-TBP system containing urine is consistently lower than that without urine. The chemical forms of technetium in urine, analyzed by paper chromatography, indicated that pertechnetate was reduced in the presence of HCl and that the reduction of pertechnetate was enhanced by urine. The observed decrease in the distribution ratio was attributed to the enhanced reduction of pertechnetate by urine. 相似文献
67.
Masayoshi Watanabe Tadashi Iida Kohei Sanui Naoya Ogata Tadahiko Kobayashi Zentaro Ohtaki 《Journal of polymer science. Part A, Polymer chemistry》1984,22(6):1299-1307
Novel polyamides that contain tetrathiafulvalene (TTF) moieties were synthesized by direct polycondensation of Δ2,2′ -bi-1,3-dithiole-4,4′(5′)-dicarboxylic acid with aromatic diamines in the presence of triphenylphosphine, hexachloroethane, and pyridine. Complexes of these polyamides with bromine were TTF cation radical () salts which resulted from the charge transfer of TTF moieties to bromine. The electrical conductivity of undoped polyamides increased with a decrease in diamine length. By doping the bromine the conductivity of the polyamides was enhanced by 3–5 orders of magnitude and reached 10?5–;10?9S cm?1. Maximum conductivity was observed at the doping ratio of [Br]/[repeat unit] < 1. The electrical conduction in the bromine complexes may be attributed to the hopping of odd electrons of between TTF moieties. 相似文献
68.
Fumiyo Saito Ryo Takeuchi Tomoyuki Kamino Kouji Kuramochi Kengo Sakaguchi Susumu Kobayashi Masashi Tsuda 《Tetrahedron letters》2004,45(43):8069-8071
A total synthesis of the proposed structure of plakevulin A was accomplished. However, the NMR spectral data of the synthetic plakevulin A were not identical of those of the reported compound. We next converted the synthetic plakevulin A into 1-dihydrountenone A. The 1H and 13C NMR spectral data of 1-dihydrountenone A were identical with those of reported plakevulin A except for the peaks derived from levulinic acid. Thus, we repurified sample of the natural product and confirmed that the natural sample contained 1-dihydrountenone A and levulinic acid in the ratio of one to one. We also found that not plakevulin A but 1-dihydountenone A possessed the inhibitory activity against mammalian DNA polymerases α and β. 相似文献
69.
Tomoyuki Yoshimura 《Tetrahedron letters》2004,45(50):9241-9244
Diastereoselective construction of the 19-epi-C18-C25 segment of (−)-lasonolide A was achieved using a 5-exo-trigonal mode of radical cyclization for the creation of the contiguous quaternary and tertiary stereogenic centers at C22 and C23 as the key reaction step. During the dehydration stage, it was found that an unusual inversion of configuration took place. 相似文献
70.
Radiation-induced copolymerizations of perfluorovinyl acetic acid and its methyl ester with α-olefin
Mitsuo Kawaguchi Terutaka Watanabe Osamu Matsuda Yoneho Tabata Sueo Machi 《Journal of polymer science. Part A, Polymer chemistry》1981,19(6):1525-1531
Homopolymerizations and copolymerizations of perfluorovinyl acetic acid (FVA) and its methyl ester (MFVA) were carried out by γ radiation at a temperature of 25°C, a dose rate of 1 × 106 rad/hr, and FVA/α-olefin and MFVA/α-olefin ratios of 10/90-90/10 in the monomer mixture. FVA and MFVA gave small quantities of brown and greasy low-molecular-weight homopolymers. The polymerization rates of both FVA and MFVA were extremely small, as shown by the maximum G value of monomer consumption of 12. FVA and MFVA reacted with α-olefin to form waxlike copolymers. The copolymerization rates of both FVA and MFVA with α-olefin were remarkably larger than those of the homopolymerizations, particularly with ethylene. The polymer compositions of FVA/ethylene or MFVA/ethylene was nearly 1/2 over a wide range of the monomer compositions. The Mayo–Lewis method gave negative r1 (FVA) and r1 (MFVA). The polymer composition curves could be well interpreted by introducing the penultimate model. 相似文献