全文获取类型
收费全文 | 3854篇 |
免费 | 110篇 |
国内免费 | 18篇 |
专业分类
化学 | 2803篇 |
晶体学 | 38篇 |
力学 | 36篇 |
数学 | 189篇 |
物理学 | 916篇 |
出版年
2022年 | 18篇 |
2021年 | 20篇 |
2020年 | 36篇 |
2019年 | 54篇 |
2018年 | 28篇 |
2017年 | 31篇 |
2016年 | 65篇 |
2015年 | 66篇 |
2014年 | 77篇 |
2013年 | 165篇 |
2012年 | 183篇 |
2011年 | 207篇 |
2010年 | 125篇 |
2009年 | 119篇 |
2008年 | 199篇 |
2007年 | 200篇 |
2006年 | 221篇 |
2005年 | 161篇 |
2004年 | 191篇 |
2003年 | 162篇 |
2002年 | 143篇 |
2001年 | 92篇 |
2000年 | 78篇 |
1999年 | 51篇 |
1998年 | 38篇 |
1997年 | 37篇 |
1996年 | 63篇 |
1995年 | 50篇 |
1994年 | 46篇 |
1993年 | 56篇 |
1992年 | 65篇 |
1991年 | 47篇 |
1990年 | 32篇 |
1989年 | 38篇 |
1988年 | 47篇 |
1987年 | 40篇 |
1986年 | 29篇 |
1985年 | 84篇 |
1984年 | 71篇 |
1983年 | 43篇 |
1982年 | 55篇 |
1981年 | 51篇 |
1980年 | 44篇 |
1979年 | 44篇 |
1978年 | 47篇 |
1977年 | 40篇 |
1976年 | 35篇 |
1975年 | 31篇 |
1974年 | 18篇 |
1973年 | 33篇 |
排序方式: 共有3982条查询结果,搜索用时 31 毫秒
141.
Tohru Takabatake Yumiko Hasegawa Minoru Hasegawa 《Journal of heterocyclic chemistry》1993,30(6):1477-1479
The cyclocondensations of benzofuroxan 1a with carbonyl compounds were smoothly and efficiently carried out by the adsorption of the components on the surface of silica gel or a molecular sieve to form a 2,3-disubstituted quinoxaline 1,4-dioxide. When the reactions using a molecular sieve 3A (powder) were carried out at 90°, the actual reaction times were reduced to 0.5-2 hours. Although Duerckheimer has reported the isolation of only the 7-substituted quinoxaline 1,4-dioxide when 5-methoxybenzofuroxan 1e was allowed to react with ethyl acetoacetate 2j , it produced only the 6-methoxy isomer as the reaction product by our method. 5-Carboxybenzofuroxan 1f did not react with the carbonyl compound. 相似文献
142.
Relative bone mineral content in the lumbar spine was measured by phantom B-MAs with the apparatus of computerized tomography in order to study the relationships between bone mineral content with age and parathyroid hormone in sera of randomized out- and in-patients and the hemodialysis patients. There exists a significant difference between male and female in the loss of bone mineral content of three groups. 相似文献
143.
Kobayashi K Yamada Y Yamanaka M Sei Y Yamaguchi K 《Journal of the American Chemical Society》2004,126(43):13896-13897
Selective formation of a homo- or hetero-cavitand cage via metal-coordination, by using tetra(4-pyridyl)-cavitand (1), tetrakis(4-pyridylethynyl)-cavitand (2), or tetrakis(4-cyanophenyl)-cavitand (3) as deep cavitand ligands and Pd(dppp)(OTf)2 (4) as a connector, has been investigated by 1H NMR and CSI-MS. When the cavitand and 4 were mixed in CDCl3 in a 2:4 molar ratio, 1 gave a complicated mixture, whereas 2 or 3 formed a homo-cavitand cage {2(2).4[Pd(dppp)]}8+.8(TfO-) (5) or {2(3).4[Pd(dppp)]}8+.8(TfO-) (6), respectively, as a single species. In a 1:1:4 mixture of 2, 3, and 4, homo-cavitand cages 5 and 6 were observed in a 1:1 ratio. In marked contrast, a mixture of 1, 3, and 4 in a 1:1:4 ratio was exclusively self-assembled into a hetero-cavitand cage {1.3.4[Pd(dppp)]}8+.8(TfO-) (7). The selectivity for the self-assembly of the homo- or hetero-cavitand cage via metal coordination would arise from a combination of factors such as coordination ability and steric demand of cavitand ligands. 相似文献
144.
Aqueous Titanate Sols from Ti Alkoxide-α-Hydroxycarboxylic Acid System and Preparation of Titania Films from the Sols 总被引:1,自引:0,他引:1
Ohya Tomokazu Ito Masamichi Yamada Keiko Ban Takayuki Ohya Yutaka Takahashi Yasutaka 《Journal of Sol-Gel Science and Technology》2004,30(2):71-81
Aqueous titanate sols were prepared by reactions of titanium tetraisopropoxide (TIP) with -hydroxycarboxylic acids in water. IR and Raman spectra, and elemental analyses of the precipitates obtained from the sols revealed that the carboxylates were chelated to titanium but the Ti species were polymerized to form a cluster that had a colloidal nature. Spin-coating of titania (TiO2) thin films from these sols was also examined. Interestingly, it was found that (004) preferentially oriented anatase films with refractive index of 2.54 were obtained from TIP-lactic acid-NH3 (1:1:1, molar ratio) aqueous sol. This crystallographic orientation was characteristic of the TIP-lactic acid-NH3 (1:1:1) system, and was not affected by the kinds of substrates used and the heating rate of the film. TEM observation indicated that small anatase grains had already formed at 200°C. Therefore, the crystallographic orientation might depend strongly on the structure of the chemical species of the precursor solutions. 相似文献
145.
Hiroto Nagaoka Kaoru Kobayashi Toshiaki Okamura Yasuji Yamada 《Tetrahedron letters》1987,28(52):6641-6644
The sequential Michael reaction of (S)-4,5-di-O-isopropylidenepent-2- enoate
and
with lithium enolate
afforded diastereo- and enantioselectively bicyclo[2.2.2]octane
and
(or
), respectively. The adducts were efficiently converted into both enantiomeric keto aldehydes (−)-
and (+)-
. 相似文献
146.
Morio Yashiro Sachiko Miyama Tohru Takarada Makoto Komiyama 《Journal of inclusion phenomena and macrocyclic chemistry》1994,17(4):393-397
Cyclodextrins form complexes with lanthanide ions in basic aqueous solutions. This complex formation in basic solution dramatically enhances the solubility of lanthanide ions, which are otherwise insoluble due to the formation of hydroxide gels. Solutions of the -cyclodextrin-Ce3+ complex effectively hydrolyze 2-deoxyadenosine-5-monophosphate to 2-deoxyadenosine. 相似文献
147.
Ooi T Takahashi M Yamada M Tayama E Omoto K Maruoka K 《Journal of the American Chemical Society》2004,126(4):1150-1160
A series of (2,7-disubstituted-1,8-biphenylenedioxy)bis(dimethylaluminum) (2) has been readily prepared in situ by treatment of the requisite 2,7-disubstituted-1,8-biphenylenediol (1) with Me3Al (2 equiv) in CH2Cl2 at room temperature; this primarily relies on the successful establishment of a new synthetic procedure of 1 starting from inexpensive m-anisidine. Evaluation of 2 as a bidentate organoaluminum Lewis acid has been performed by the reduction of ketonic substrates using Bu3SnH as a hydride source in comparison to the conventional monodentate Lewis acid dimethylaluminum 2,6-xylenoxide (11), uncovering the significantly high activation ability of 2 toward carbonyl. Particularly, (2,7-dimethyl-1,8-biphenylenedioxy)bis(dimethylaluminum) (2a) exerted the highest reactivity, which has also been emphasized in the Mukaiyama aldol reaction. The structure of the bidentate Lewis acid 2 was unambiguously determined by single-crystal X-ray diffraction analysis of 2g possessing a bulky 3,5-di-tert-butylphenyl substituent, revealing the rigid dimeric assembly in the solid state. The double electrophilic activation of carbonyl substrate by 2a has been supported by low-temperature 13C NMR analysis as well as theoretical study using the Gaussian 98 program. Moreover, unique stereoselectivity has been observed in the 2a-promoted Mukaiyama Michael addition, and highly chemoselective functionalization of carbonyl compounds in the presence of their acetal counterparts has been realized using 2a. Finally, the effectiveness of 2a for the activation of ether functionality has been demonstrated in the Claisen rearrangement of allyl vinyl ethers. 相似文献
148.
Performance of six quinolylazo compounds was studied as a pre-column derivatizing reagent for the separation of metal ions in the reversed-phase high-performance liquid chromatography. 2-(4-Methyl-2-quinolylazo)-5-diethylaminophenol (QADP) was suited for the determination of Co, Ni and Cu in the presence of a large amount of Fe. The detection limits under the optimized conditions [stationary phase: L-column ODS; mobile phase: acetone-water (62:38, v/v) containing 0.01 M NH(4)SCN and 0.01 M 2-morpholinoethanesulfonic acid-NaOH (pH 6); injection volume: 10 mul] were 0.24 ng for Co, 0.22 for Ni and 0.53 for Cu. This method was successfully applied to the determination of these metal ions in a standard steel sample. 相似文献
149.
Takashi Murashima Kazuyuki Hayata Jun Matsui Daisuke Miyoshi Takashi Yamada Naoki Sugimoto 《Tetrahedron letters》2007,48(48):8514-8517
The DNAs modified with tetraphenyl porphyrin at the center of 13mer oligonucleotide were synthesized using phosphoramidite chemistry and automated DNA synthesis. When the porphyrin modified oligonucleotide was annealed with its complementary strand, they formed a standard B-form duplex. The porphyrin moiety intercalated in the duplex, and moderately lowered the thermal stability. 相似文献
150.