全文获取类型
收费全文 | 959篇 |
免费 | 17篇 |
国内免费 | 2篇 |
专业分类
化学 | 776篇 |
晶体学 | 27篇 |
力学 | 11篇 |
数学 | 40篇 |
物理学 | 124篇 |
出版年
2021年 | 7篇 |
2020年 | 5篇 |
2019年 | 4篇 |
2018年 | 7篇 |
2017年 | 4篇 |
2016年 | 5篇 |
2015年 | 12篇 |
2014年 | 15篇 |
2013年 | 44篇 |
2012年 | 42篇 |
2011年 | 45篇 |
2010年 | 37篇 |
2009年 | 17篇 |
2008年 | 48篇 |
2007年 | 74篇 |
2006年 | 63篇 |
2005年 | 58篇 |
2004年 | 46篇 |
2003年 | 45篇 |
2002年 | 53篇 |
2001年 | 12篇 |
2000年 | 13篇 |
1999年 | 11篇 |
1998年 | 11篇 |
1997年 | 25篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 9篇 |
1993年 | 9篇 |
1992年 | 5篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1986年 | 5篇 |
1985年 | 17篇 |
1984年 | 15篇 |
1983年 | 11篇 |
1982年 | 10篇 |
1981年 | 15篇 |
1980年 | 16篇 |
1979年 | 17篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1976年 | 19篇 |
1975年 | 16篇 |
1974年 | 10篇 |
1973年 | 6篇 |
1968年 | 9篇 |
1967年 | 4篇 |
排序方式: 共有978条查询结果,搜索用时 172 毫秒
31.
Baba T Abe Y Nomoto K Inazu K Echizen T Ishikawa A Murai K 《The journal of physical chemistry. B》2005,109(9):4263-4268
Methane is shown to react with ethene over In-loaded ZSM-5 to higher hydrocarbons such as propene and toluene at around 673 K. Such methane conversion is not catalyzed by proton-exchanged ZSM-5 (H-ZSM-5) under the same conditions, only C2H4 being converted to higher hydrocarbons. By using 13C-labeled methane (13CH4) as a reactant, the reaction paths for the formation of propene, benzene and toluene were examined. 13C-labeled propene (13CC2H6) is formed by the reaction of 13CH4 with C2H4. The lack of 13C-labeled benzene revealed that propene is not transformed to benzene, which instead originates entirely from C2H4. The 13C atom is inserted both into the methyl group and benzene ring in the toluene formed. This indicates that toluene is formed by two reaction paths; the reaction of 13CC2H6 with butenes formed by the dimerization of C2H4 and the reaction of benzene with 13CH4. The existence of the latter path was proved by the direct reaction of 13CH4 with benzene. The reaction of methane with benzene was also carried out in a continuous flow system over In-loaded ZSM-5. The reaction afforded 7.6% and 0.9% yields of toluene and xylenes, respectively, at 623 K. 相似文献
32.
Tozuka Z Kaneko H Shiraga T Mitani Y Beppu M Terashita S Kawamura A Kagayama A 《Journal of mass spectrometry : JMS》2003,38(8):793-808
Triple-stage quadrupole (TSQ) electrospray ionization (ESI) tandem mass spectrometry (MS/MS) and ion trap ESI-MS/MS can be used to cleave protonated molecules to produce carbocations and neutral molecules in the positive ion mode. Dissociation products which correspond to protonated forms of neutral fragment molecules can also be trapped and detected. These protonated molecules in turn can cleave via carbocation cleavage, ipso cleavage, onium cleavage or McLafferty or related rearrangements. One can elucidate the structures of metabolites from the differences in m/z ratios of the fragments arising from the original drug compound and its metabolite. This strategy for structural elucidation is further facilitated by estimates of the reactivity of drugs with oxygen diradicals involved in cytochrome P-450 cycles. 相似文献
33.
Motohisa Kaneko Akio Matsuyama Chikayoshi Nagata 《Photochemistry and photobiology》1978,27(3):359-361
Abstract. Photodimerization of thymine in aqueous solution in the presence of tyrosine was studied with monochromatic UV irradiation. The total dimer formation was sensitized in the presence of tyrosine. The action spectrum of sensitized total dimer formation has a peak near 280 nm corresponding to the absorption maximum of tyrosine. Triplet quenchers reduced the sensitization substantially. It seems probable that tyrosine-sensitized photodimerization of thymine occurred via triplet-triplet energy transfer from tyrosine to thymine. 相似文献
34.
35.
Akio Hosoya 《Nuclear Physics B》1980,165(1):34-44
The variational calculation of the path-ordered phase factor is streamlined. We propose that the wave functional for a string state is approximately of the form .The real factor K is shown to be proportional to the square root of the dielectric permeability in the Kogut-Susskind-'t Hooft model of colour flux collimation. 相似文献
36.
Kimiaki Imafuku Akio Yamane Hisashi Matsumura 《Journal of heterocyclic chemistry》1980,17(6):1293-1296
3-Acetyltropolone ( 1 ) reacted with phenylhydrazine to give 3-acetyltropolone phenylhydrazone ( 3 ) and 3-methyl-1-phenyl-1,8-dihydrocycloheptapyrazol-8-one ( 4 ). The former ( 3 ) cyclized to afford the latter ( 4 ). The reaction of 3-acetyl-2-methoxytropone ( 2a ) with phenylhydrazine gave 4 , 3-methyl-2-phenyl-2,8-dihydrocyclo-heptapyrazol-8-one ( 5 ), and 3-methyl-2-phenyl-2,8-dihydrocycloheptapyrazol-8-one phenylhydrazone ( 6 ). The compound ( 5 ) reacted with phenylhydrazine to afford 6 . The reaction of 7-acetyl-2-methoxytropone ( 2b ) with phenylhydrazone gave 7-acetyl-2-methoxytropone phenylhydrazone ( 7 ), 7-acetyl-2-(N′-phenylhydrazino)-tropone phenylhydrazone ( 8 ), 3-methyl-1-phenyl-1,8-dihydrocycloheptapyrazol-8-one phenylhydrazone ( 9 ), and 6 . The compound ( 7 ) was heated to afford 4 and reacted with phenylhydrazine to afford 8 and 9 . The compound ( 8 ) was also refluxed to give 9 . 相似文献
37.
Tetsuro Kato Ryosuke Nemoto Hisashi Mori Ryoetsu Abe Katsuo Unno Akio Goto Hideyuki Murota Masaoki Harada Motofumi Homma 《Applied biochemistry and biotechnology》1984,10(1-3):199-211
To achieve targeted distribution of anticancer drugs with sustained activity, ferromagnetic ethylcellulose microcapsules containing
an anticancer drug, mitomycin C (FM-MMC-mc), were prepared by a method based on phase separation principles. Two prototypes
of FM-MMC-mc were made: one with the drug as the core and zinc ferrite on its capsular surface (outer type); the other with
both the drug and zinc ferrite as the core (inner type). Both preparations provided a sustained-release property and a sensitive
response to conventional magnetic force, although certain differences in the release rate of drug, magnetic responsiveness,
and particle size were found between the two dosage forms. Animal studies showed that the magnetic microcapsules could be
magnetically controlled in the artery and urinary bladder. VX2 tumors in the rabbit hind limb and urinary bladder were successfully
treated with magnetic control of FM-MMC-mc. Pharmacokinetic study revealed that the targeting of the microcapsules markedly
enhanced the drug absorption into the surrounding tissues for a prolonged period of time. The results indicate the feasibility
and effectiveness of the magnetic microcapsules as a targeted drug delivery system. 相似文献
38.
[reaction: see text] We describe an efficient template-directed photoligation of oligodeoxynucleotides (ODNs) using alpha-5-cyanovinyldeoxyuridine (alpha(C)U). An efficient photoligation was produced by photoirradiation of an ODN containing alpha(C)U at the 3' end with an ODN containing thymine at the 5' end in the presence of a template ODN. This photoligation method is a new and efficient way to synthesize branched ODNs. 相似文献
39.
Nishizawa M Kohno M Nishimura M Kitagawa A Niwano Y 《Chemical & pharmaceutical bulletin》2005,53(7):796-799
Using a new method having been developed for the purpose of quantitative determination for peroxyradicals, the presence of peroxyradicals was proved in cigarette smoke. In brief, peroxyradicals in cigarette smoke were measured by ESR spectrometry coupled to non-reductive scavenging of 1,1-diphenyl-2-picrylhydrazyl (DPPH). As a result, peroxyradicals were found to be major reactive oxygen species (ROS) since the concentration of peroxyradicals recovered from cigarette smoke was much higher than that of any of other ROS (superoxide and hydroxyl radical) and nitric oxide. Furthermore, several antioxidants (ascorbic acid, reduced glutathione, epigallocatechin gallate, shikonin) were examined for scavenging activity against peroxyradicals in the cigarette smoke. Among them shikonin alone exerted the scavenging activity, suggesting that shikonin is promising antioxidant for cigarette filters because of its effectiveness against broad range of ROS including peroxyradicals, heat resistance, nonvolatility and high affinity to the filter. 相似文献
40.
Kadota I Ohno A Matsuda K Yamamoto Y 《Journal of the American Chemical Society》2002,124(14):3562-3566
The Lewis acid mediated reaction of alpha-acetoxy ethers 15-22 gave the corresponding cyclized products 23, 25, 27, 29, 31, 32, 34, and 36 in good yields with high stereoselectivities. Those cyclized products were subjected to ring-closing metathesis to afford the polycyclic ethers 38-42, 44, and 45 in good yields. The usefulness of the present methodology was demonstrated by the convergent synthesis of the CDEF ring system of brevetoxin B (1) and the CDEFG ring system of gambierol (2). 相似文献