全文获取类型
收费全文 | 2543篇 |
免费 | 102篇 |
国内免费 | 11篇 |
专业分类
化学 | 2068篇 |
晶体学 | 26篇 |
力学 | 34篇 |
数学 | 127篇 |
物理学 | 401篇 |
出版年
2022年 | 9篇 |
2021年 | 25篇 |
2020年 | 52篇 |
2019年 | 37篇 |
2018年 | 26篇 |
2017年 | 17篇 |
2016年 | 48篇 |
2015年 | 45篇 |
2014年 | 65篇 |
2013年 | 138篇 |
2012年 | 143篇 |
2011年 | 191篇 |
2010年 | 85篇 |
2009年 | 93篇 |
2008年 | 148篇 |
2007年 | 147篇 |
2006年 | 153篇 |
2005年 | 141篇 |
2004年 | 122篇 |
2003年 | 111篇 |
2002年 | 97篇 |
2001年 | 55篇 |
2000年 | 34篇 |
1999年 | 32篇 |
1998年 | 34篇 |
1997年 | 39篇 |
1996年 | 29篇 |
1995年 | 24篇 |
1994年 | 25篇 |
1993年 | 26篇 |
1992年 | 24篇 |
1991年 | 15篇 |
1990年 | 28篇 |
1989年 | 28篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 19篇 |
1985年 | 36篇 |
1984年 | 28篇 |
1983年 | 18篇 |
1982年 | 30篇 |
1981年 | 34篇 |
1980年 | 36篇 |
1979年 | 20篇 |
1978年 | 19篇 |
1977年 | 13篇 |
1976年 | 12篇 |
1975年 | 19篇 |
1974年 | 11篇 |
1973年 | 19篇 |
排序方式: 共有2656条查询结果,搜索用时 203 毫秒
81.
[reaction: see text] A concise and enantioselective synthesis of (S)-2-cyclohexyl-2-phenylglycolic acid as a key intermediate for (S)-oxybutynin is reported. The crucial asymmetric tetrasubstituted carbon center was constructed with excellent stereoselectivity through the proline-catalyzed direct asymmetric aldol reaction between cyclohexanone and ethyl phenylglyoxylate under mild conditions. 相似文献
82.
Akhtaruzzaman M Kamata N Nishida J Ando S Tada H Tomura M Yamashita Y 《Chemical communications (Cambridge, England)》2005,(25):3183-3185
Dithiazolylbenzothiadiazoles easily obtained have high electron affinity and the FET device of a trifluoromethylphenyl derivative exhibited a good n-type performance with high electron mobility. 相似文献
83.
Tadatomi Nishikubo Atsushi Kameyama Jun Yamashita Takahiro Fukumitsu Chika Maejima Masao Tomoi 《Journal of polymer science. Part A, Polymer chemistry》1995,33(7):1011-1017
The addition reaction of oxiranes ( 15a-d ) with carbon dioxide (CO2) was carried out using 1 mol % of soluble polymer-supported quaternary onium salts as catalysts under atmospheric pressure. The reaction of 15a-d with CO2 proceeded very smoothly to give the corresponding five-membered cyclic carbonates ( 16a-d ) in high yields at 90-100°C. The catalytic activity of the soluble polymer-supported quaternary onium salts was strongly affected by the following factors: kind of reaction solvent, degree of introduction of the pendant onium salt residues in the polymer chain, and type of alkyl group on the onium salts due to the balance between lipophilicity and steric hindrance of the onium salt residue. Furthermore, these soluble polymer-supported quaternary onium salts were found ordinarily to have higher catalytic activity than low molecular weight quaternary onium salts under the same reaction conditions. It was also found that the rate of reaction was proportional both to catalyst concentration and to oxirane concentration. © 1995 John Wiley & Sons, Inc. 相似文献
84.
Novel thermally crosslinkable fluorine‐containing poly(arylene ether ketone)s comprised of 2,3,5, 6‐tetrafluoro‐1,4‐phenylene moiety were synthesized by the termination of polymer chain ends with propargyl ether groups in order to improve solvent resistance. Crosslinking reaction occurred over 250°C through the formation of both chromen ring and polyene structure. This structure change brought about not only the outstanding solvent resistance but also the increase in glass transition temperature (Tg). The cured films also exhibited excellent thermal stability, transparency and hydrophobicity derived from fluorine atoms. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
85.
Masami Sawada Yasuo Okumura Hitoshi Yamada Yoshio Takai Shigetoshi Takahashi Takahiro Kaneda Keiji Hirose Soichi Misumi 《Journal of mass spectrometry : JMS》1993,28(12):1525-1528
Using fast atom bombardment (FAB) mass spectrometry (MS), cross-chiral relationships were confirmed for the first time for the diasteromeric host-guest complexations between the chiral crown ether host (1) and the chiral organic ammonium ion guest (2) on the basis of the relative peak intensities (RPI). Both host–guest combinations (R, R, R, R) – 1, (R) – 2 and (S, S, S, S) – 1, (S) – 2 obviously provided larger RPI values than the combination of both (R, R, R, R) – 1, (S) – 2 and (S, S, S, S) – 1, (R) – 2 by a factor of 1.6 as an averaged value: 1.87 (n = 4)/1.16 (n = 4) = 1.6. These results are consistent with the expected stabilities of the host-guest complexations by CPK model examinations. Successfully observed cross-chiral examinations strongly suggest a potentially useful FABMS/RPI methodology for rapidly searching newly designed and synthesized crown ether-like host compounds with a higher degree of enantioselectivity. 相似文献
86.
Takashi Kajiwara Kensuke Katagiri Shinya Takaishi Masahiro Yamashita Nobuhiko Iki 《化学:亚洲杂志》2006,1(3):349-351
A dodecaholmium wheel of [Ho12(L)6(mal)4(AcO)4(H2O)14] ( 1 ; mal=malonate) was synthesized by using p‐tert‐butylsulfonylcalix[4]arene (H4L) as a cluster‐forming ligand. The wheel consists of three fragments of mononuclear A3? ([Ho(L)(mal)(H2O)]3?), trinuclear B3? ([Ho(H2O)2(mal)(Ho(L)(AcO))2]3?), and C3+ ([Ho(H2O)2]3+), and an alternate arrangement of these fragments (A3?? C3+? B3?? C3+? A3?? C3+? B3?? C3+? ) results in a wheel structure. The longest and shortest diameters of the core were estimated to be 17.7562(16) and 13.6810(13) Å, respectively, and the saddle‐shaped molecule possesses a pocketlike cavity inside. 相似文献
87.
We are reporting on a new method of constructing dearomatized compounds from α-substituted aryltriazenes. Deprotonation occurs at C atom α to N3. Nucleophilic attack of generated anion at the ortho-position of aryl group forms a new carbon-carbon bond. A stereoselective reaction was observed when the substituents on the C α to N3 are tied together in either a pyrrolidine or a piperidine. The product of this reaction possessed an interesting dearomatized tetrahydrobenzotriazine framework. 相似文献
88.
Single nucleotide polymorphisms (SNPs) of cancer repression gene p53 were analyzed electrochemically with ferrocenyl naphthalene diimide (1) as a hybridization indicator. The SNPs studied were the transition to A from G in the codon for amino acid at positions 175, 248 or 273 and the transversion to C from G in the codon for the amino acid at position 72. Thus, 20-meric oligonucleotides carrying the SNP site were used both as a sample and a probe with the latter immobilized on an electrode. Even one base difference on the p53 gene resulted in a significant difference in the current response of 1 and the magnitude of the response correlated with the amount of the DNA hybrid on the electrode. Moreover, when PCR products of exon 4, on which the P72/R72 SNP resides, of the p53 gene were analyzed by this method, the heterozygote and homozygotes were discriminated with modest precision. 相似文献
89.
Moribe K Tsuchiya M Tozuka Y Yamaguchi K Oguchi T Yamamoto K 《Chemical & pharmaceutical bulletin》2004,52(5):524-529
We prepared and characterized a grinding-induced equimolar complex of thiourea with ethenzamide. When thiourea and ethenzamide were co-ground at a molar ratio of 3 : 1, new powder X-ray diffraction (PXRD) peaks were observed in addition to PXRD peaks of thiourea crystals. The optimum stoichiometry of the new structure was confirmed as 1 : 1 mol/mol. Effect of grinding time on the thiourea-ethenzamide equimolar complex formation was investigated by using PXRD, differential scanning calorimetry and Fourier transform infrared spectroscopy. The equimolar crystal structure was confirmed by X-ray diffraction measurements of the single crystal which was recrystallized from ethanol. It was found that the intermolecular hydrogen bond formations between thiourea and ethenzamide molecules contributed to the equimolar complex formation. The complex formation was not observed in the cases where benzamide, salicylamide or 3-ethoxybenzamide was co-ground with thiourea. 2-Alcoxyl benzamide structures should be required for the grinding-induced equimolar complex formation with thiourea. 相似文献
90.
Intercrystalline migration and a migration-assisted chemical reaction of adsorbed aromatic species between zeolite particles in physical contact were visualized by fluorescence microscopy coupled with a particle manipulation technique. The luminescence color characteristics of particular zeolite particles originating from the specific photochemistry of adsorbed species was exploited to follow the migration of the molecules. Two examples are shown that are relevant to the visualization of the time-dependent migration process: A one guest-two sets of zeolite crystals system: chrysene (Chry)-loaded zeolite Na+ -X (the sodium form of zeolite X) crystals were placed in contact with unloaded Tl+ -X (thallium-exchanged X) crystals and allowed to stand at room temperature. Initially, the blue fluorescence of Chry was detected only from the Na+ -X particles, but later, the development of green phosphorescence emission was discernible from the Tl+ -X which suggests that Chry migrated from the Na+ -X to the Tl+ -X crystals. A two guest-species systems: Electron-donating Chry-loaded Na+ -X crystals were placed in contact with electron-accepting 1,2,4,5-tetracyanobenzene (TCNB)-loaded Na+ -X or Na+ -Y crystals. With time, the former system (Chry/Na+ -X and TCNB/Na+ -X) gave rise to the emission of Chry-TCNB charge-transfer complexes resulting mainly from the migration of Chry while the latter system (Chry/Na+ -X and TCNB/Na+ -Y) afforded the same emission resulting largely from the migration of TCNB. The present investigation reveals that there is a certain direction for guest migration depending on the zeolite host and the nature of host-guest or guest-guest interaction. 相似文献