全文获取类型
收费全文 | 1676篇 |
免费 | 44篇 |
国内免费 | 10篇 |
专业分类
化学 | 1222篇 |
晶体学 | 27篇 |
力学 | 15篇 |
数学 | 116篇 |
物理学 | 350篇 |
出版年
2023年 | 11篇 |
2022年 | 9篇 |
2021年 | 14篇 |
2020年 | 17篇 |
2019年 | 32篇 |
2018年 | 34篇 |
2017年 | 13篇 |
2016年 | 26篇 |
2015年 | 21篇 |
2014年 | 57篇 |
2013年 | 76篇 |
2012年 | 76篇 |
2011年 | 121篇 |
2010年 | 70篇 |
2009年 | 59篇 |
2008年 | 106篇 |
2007年 | 94篇 |
2006年 | 114篇 |
2005年 | 97篇 |
2004年 | 73篇 |
2003年 | 90篇 |
2002年 | 84篇 |
2001年 | 50篇 |
2000年 | 21篇 |
1999年 | 15篇 |
1998年 | 16篇 |
1997年 | 11篇 |
1996年 | 8篇 |
1995年 | 14篇 |
1994年 | 18篇 |
1993年 | 14篇 |
1992年 | 17篇 |
1991年 | 13篇 |
1990年 | 14篇 |
1989年 | 11篇 |
1988年 | 8篇 |
1987年 | 12篇 |
1986年 | 20篇 |
1985年 | 19篇 |
1984年 | 14篇 |
1983年 | 12篇 |
1982年 | 12篇 |
1981年 | 15篇 |
1980年 | 15篇 |
1979年 | 7篇 |
1978年 | 11篇 |
1977年 | 16篇 |
1976年 | 8篇 |
1974年 | 7篇 |
1973年 | 9篇 |
排序方式: 共有1730条查询结果,搜索用时 15 毫秒
51.
Improved synthesis of four stereoisomeric chiral 1,3-dimethyl-1,2,3,4-tetrahydroisoquinolines (1a, b, ent-1a, b) was achieved via the super acid-induced cyclization of chiral N-[1-methyl-2-(phenylsulfinyl)ethyl]-N-(1-phenylethyl)formamides (4a, b, ent-4a, b) using the Pummerer-type cyclization reaction as a key step. The cyclization leading to the isoquinoline ring proceeded in a quantitative manner when trifluoromethane sulfonic acid (TFSA) was used as the super acid, although Friedel-Crafts-type alkylation of 4-phenylsulfanyl TIQ derivatives (5) with benzene used as the solvent accompanied cyclization to yield the 4-phenyl-TIQs (7). The byproduct (7) was exclusively formed when a large excess amount of TFSA was used. 相似文献
52.
Shuji YasuikeSatoru Okajima Kentaro YamaguchiHiroko Seki Jyoji Kurita 《Tetrahedron》2003,59(27):4959-4966
Racemic 2,2′-bis[diarylstibano]-1,1′-binaphthyls [(±)-BINASbs] and 2,2′-bis[di(p-tolyl)bismuthano]-1,1′-binaphthyl [(±)-BINABi], which are the antimony and bismuth congeners of BINAP, have been prepared from 2,2′-dibromo-1,1′-binaphthyl (DBBN) via 2,2′-dilithio-1,1′-binaphthyl intermediate by treatment with the appropriate metal halides [(p-Tol)2SbBr, Ph2SbBr and (p-Tol)2BiCl]. The optical resolution of the (±)-BINASbs could be achieved via the separation of a mixture of the diastereomeric Pd-complexes derived from the reaction of (±)-BINASbs with di-μ-chlorobis{(S)-2-[1-(dimethylamino)-ethyl]phenyl-C1,N}dipalladium(II). Optically active (R)-BINASb and (R)-BINABi could be also obtained from optically active (R)-DBBN by the same procedure. The enantiopure BINASbs have been shown to be effective chiral ligands for the rhodium-catalyzed asymmetric hydrosilylation of ketones. 相似文献
53.
Kentaro Ageishi Takeshi Endo Makoto Okawara 《Journal of polymer science. Part A, Polymer chemistry》1981,19(5):1085-1090
The behavior of viologen polymer (P-V2+) as an electron transfer catalyst in the reaction of hydrogen generation was studied. In the photoirradiation system, which contains triethanolamine (TEA), Ru(bpy)3+3, and P-V2+, the amount of hydrogen evolution was less than methyl viologen (MV2+); P-V2+, however, was more effective in sodium dithionite as the electron donor and showed higher initial rates than MV2+. 相似文献
54.
Nakanishi I Miyazaki K Shimada T Iizuka Y Inami K Mochizuki M Urano S Okuda H Ozawa T Fukuzumi S Ikota N Fukuhara K 《Organic & biomolecular chemistry》2003,1(22):4085-4088
Electron-transfer reduction of molecular oxygen (O2) by the phenolate anion (1-) of a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), occurred to produce superoxide anion, which could be directly detected by a low-temperature EPR measurement. The rate of electron transfer from 1- to O2 was relatively slow, since this process is energetically unfavourable. The one-electron oxidation potential of 1- determined by cyclic voltammetric measurements is sufficiently negative to reduce 2,2-bis(4-tert-octylphenyl)-1-picrylhydrazyl radical (DOPPH*) to the corresponding one-electron reduced anion, DOPPH-, suggesting that 1- can also act as an efficient radical scavenger. 相似文献
55.
Takagi R Miyanaga W Tamura Y Ohkata K 《Chemical communications (Cambridge, England)》2002,(18):2096-2097
The Diels-Alder reaction of spirolactones with cyclopentadiene afforded the adduct with high pi-facial selectivity; a hydrophilic analogue of scyphostatin was synthesized from the Diels-Alder adduct. 相似文献
56.
A method to characterize the class of all generalized inverses of any given matrix A is considered. Given a matrix A and a nonsingular bordered matrix T of A, the submatrix, corresponding to A, of T-1 is a generalized inverse of A, and conversely, any generalized inverse of A is obtainable by this method. There are different definitions of a generalized inverse, and the arguments are developed with the least restrictive definition. The characterization of the Moore-Penrose inverse, the most restrictive definition, is also considered. 相似文献
57.
Kaname Takagi Tomoji Aotsuka And Hikari Morita Yoshihisa Okamoto 《Journal of heterocyclic chemistry》1986,23(5):1443-1449
Reactions of 4-ethoxycarbonylamino-1H-1,5-benzodiazepine-3-carbonitrile (2) with aliphatic primary amines gave 1-substituted 4-(2-aminoanilino)pyrimidin-2(1H)-one-5-carbonitriles 3. Analogous reactions of 2 with aromatic primary amines afforded 2-(2′-anilino-1′-cyanovinyl)benzimidazoles 5 and 6. Upon treatment with triethylamine, 3 underwent intramolecular cyclization to give 3-substituted 5-aminopyrimidino[4,5-b]-[1,5]benzodiazepin-2(3H,11H)-ones 8 . Heating of 3 with p-toluenesulfonic acid in ethanol gave 2-substituted pyrimidino[1,6-a]benzimidazol-1(2H)one-4-carbonitriles 9 . Reactions of 2 with hydrazines were also described. Mechanistic pathways are proposed to account for the products. 相似文献
58.
Tadahiro Kato Motoyuki Tanaka Shin‐suke Takagi Keita Nakanishi Masahiro Hoshikawa 《Helvetica chimica acta》2004,87(1):197-214
The synthesis of kempa‐6,8‐dien‐3β‐ol ( 4a ), as a synthetic leading model of the natural product 4b , was carried out starting from intermediate 12 , the synthetic route of which has been developed previously (Scheme 1). The conversion of 12 to the model compound 4a involved the elaboration of three structure modifications by three processes, Tasks A, B, and C (see Scheme 2). Task A was achieved by epoxy‐ring opening of 41 with Me3SiCl (Scheme 9), and Task B being performed by oxidation at the 13‐position, followed by hydrogenation, and then epimerization (Schemes 4 and 5). The removal of the 2‐OH group from 12 (Task C) was achieved via 30b according to Scheme 6, whereby 30b was formed exclusively from 30a / 31a 1 : 1 (Scheme 7). In addition, some useful reactions from the synthetic viewpoint were developed during the course of the present experiments. 相似文献
59.
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
60.
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