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991.
Abstract

Butadiene-1, 3 and acrylonitrile were copolymerized by alkylaluminum halides and ferric chloride systems. The efficiency of the aluminum components of these systems appears to decrease in the following order: Et2AlCl < EtAlCl2 < Et3Al. Effective catalysts were not necessarily dependent on the mixing orders of the catalyst and monomers. It was found that different feed ratios between two monomers, which were varied over a wide range, always resulted in the formation of 1:1 copolymer. In order to investigate the initiation and propagation reactions, electron paramagnetic resonance was measured and a polarographic method was used on the reduction reaction of ferric chloride with organoaluminum halides in acetonitrile.  相似文献   
992.
993.
A highly practical reductive cross-aldol reaction of alpha-bromoaldehydes with various aldehydes has been developed using Ge(II)Cl 2 to produce aldehyde germanium(IV) aldolates, which were directly transformed to various multifunctionalized compounds. A remarkable change in stereoselectivity depended on the alpha-bromoaldehydes employed; secondary alpha-bromoaldehydes gave syn selectivities, while tertiary alpha-bromoaldehydes accomplished the synthesis of anti-selective aldol products with a quaternary carbon center. NMR studies and X-ray analysis strongly suggested the formation of germanium enolate in the reaction of alpha-bromoaldehyde 2h with GeCl 2-dioxane. Detailed mechanistic studies, including NMR analysis and ab initio calculations, revealed the generation of stable germanium aldolates, which was due to the remarkably low Lewis acidity of the germanium(IV).  相似文献   
994.
Several azulene-substituted thioketones, 1-thiobenzoylazulene (1a) and di(1-azulenyl) thioketone (2a) and their derivatives (1b and 2b-d) with alkyl substituents on each azulene ring, were prepared and their intramolecular pericyclization reaction was examined. The thioketones with a 3-alkyl substituent on each azulene ring exhibited the presumed pericyclization reaction under thermal and acid-catalyzed conditions, although the cases of the 1-azulenyl thioketones without the 3-alkyl substituents afforded a complex mixture under similar conditions. The intramolecular reaction following the intramolecular hydrogen transfer afforded the products 13b, 14b, and 14c. The products 13b and 14b were converted into the corresponding cations 18(+) and 19(+), which have structural similarity with that of the phenalenyl cation. These cations exhibited the expected two-step reduction waves upon CV, although the ESR analysis revealed that the neutral radical state did not have the presumed high stability.  相似文献   
995.
A concise and stereoselective formation of azepines is achieved through the conjugate addition of formamides to nitroalkenes and the subsequent intramolecular nitrile oxide cycloaddition (INOC) reaction. High cis-selectivity was observed. The one-pot modification of the two reactions provides direct preparation of azepines from nitroalkenes and formamides in moderate yields. The formyl group was readily removed by an acidic treatment without significant epimerization.  相似文献   
996.
The photoinduced electron transfer of a series of meta- and para-linked triphenylamine-naphthalimide dyads, N-{3- and 4-[bis(4-R-substituted phenyl)amino]phenyl}-1,8-naphthalimide, 1m,p (R = H), 2m,p (R = Me), 3m,p (R = OMe), and 4m,p (R = NMe2) was investigated in toluene and DMF. The singlet charge-transfer (CT) states were observed in all cases. The decay rates were found to be faster in DMF (tau = 6.5 ps to 100 ps) than those in toluene (tau = 190 ps to 7 ns). The long-lived triplet CT states were observed in toluene for 3 (ca. 10% contribution, tau = 670 ns for 3m, 240 ns for 3p). No long-lived species were detected in DMF. The decay rates were somewhat faster in the para-isomers than in the meta-isomers in most cases. The photolysis of 5 (p-phenylene extended analogue of 3, R = OMe) gave a singlet CT state and a locally excited triplet state on the naphthalimide chromophore.  相似文献   
997.
Repair glycosylases locate and excise damaged bases from DNA, playing central roles in preservation of the genome and prevention of disease. Two key glycosylases, Fpg and hOGG1, function to remove the mutagenic oxidized base 8-oxoG (OG) from DNA. To investigate the relative contributions of conformational preferences, leaving group ability, enzyme-base hydrogen bonding, and nucleobase shape on damage recognition by these glycosylases, a series of four substituted indole nucleosides, based on the parent OG nonpolar isostere 2Cl-4F-indole, were tested as possible direct substrates of these enzymes in the context of 30 base pair duplexes paired with C. Surprisingly, single-turnover experiments revealed that Fpg-catalyzed base removal activity of two of the nonpolar analogs was superior to the native OG substrate. The hOGG1 glycosylase was also found to catalyze removal of three of the nonpolar analogs, albeit considerably less efficiently than removal of OG. Of note, the analog that was completely resistant to hOGG1-catalyzed excision has a chloro-substituent at the position of NH7 of OG, implicating the importance of recognition of this position in catalysis. Both hOGG1 and Fpg retained high affinity for the duplexes containing the nonpolar isosteres. These studies show that hydrogen bonds between base and enzyme are not needed for efficient damage recognition and repair by Fpg and underscore the importance of facile extrusion from the helix in its damaged base selection. In contrast, damage removal by hOGG1 is sensitive to both hydrogen bonding groups and nucleobase shape. The relative rates of excision of the analogs with the two glycosylases highlight key differences in their mechanisms of damaged base recognition and removal.  相似文献   
998.
Optically active ent-calystegine B4 was prepared in 13 steps from commercially available chiral L-dimethyl tartrate. The synthesis was achieved by the Michael addition and the aldol reaction of nitromethane to form cycloheptanone in a stereoselective manner. Reduction of the nitro group in the presence of Boc(2)O accomplished an efficient conversion to amino cycloheptanone, which readily afforded the desired ent-calystegine B4.  相似文献   
999.
Stable hexanuclear lithium phenolate bearing a cage-shaped tripodal ligand was isolated, which had a hexagonal-prismatic Li(6)O(6) core at room temperature, because of the hard mobility of the ligand and its reduction of the problematic steric repulsion. The properties of the lithium phenolates were analyzed by X-ray crystallography and NMR spectroscopy.  相似文献   
1000.
An indium triiodide catalyst promoted the direct transformation from esters to β-hydroxycarbonyl compounds using hydrosilanes and silyl enolates by a one-stage process. Various esters were applicable, and the high chemoselectivity of this system brings compatibility to many functional groups: alkenyl, alkynyl, chloro, and hydroxy.  相似文献   
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