全文获取类型
收费全文 | 4434篇 |
免费 | 67篇 |
国内免费 | 22篇 |
专业分类
化学 | 3044篇 |
晶体学 | 52篇 |
力学 | 63篇 |
数学 | 258篇 |
物理学 | 1106篇 |
出版年
2022年 | 28篇 |
2021年 | 25篇 |
2020年 | 44篇 |
2019年 | 51篇 |
2018年 | 36篇 |
2017年 | 33篇 |
2016年 | 59篇 |
2015年 | 72篇 |
2014年 | 74篇 |
2013年 | 237篇 |
2012年 | 200篇 |
2011年 | 240篇 |
2010年 | 125篇 |
2009年 | 149篇 |
2008年 | 238篇 |
2007年 | 252篇 |
2006年 | 244篇 |
2005年 | 207篇 |
2004年 | 164篇 |
2003年 | 174篇 |
2002年 | 175篇 |
2001年 | 108篇 |
2000年 | 113篇 |
1999年 | 80篇 |
1998年 | 53篇 |
1997年 | 46篇 |
1996年 | 73篇 |
1995年 | 47篇 |
1994年 | 35篇 |
1993年 | 46篇 |
1992年 | 69篇 |
1991年 | 49篇 |
1990年 | 45篇 |
1989年 | 46篇 |
1988年 | 65篇 |
1987年 | 52篇 |
1986年 | 56篇 |
1985年 | 74篇 |
1984年 | 67篇 |
1983年 | 31篇 |
1982年 | 47篇 |
1981年 | 48篇 |
1980年 | 49篇 |
1979年 | 50篇 |
1978年 | 47篇 |
1977年 | 54篇 |
1976年 | 31篇 |
1975年 | 43篇 |
1974年 | 34篇 |
1973年 | 30篇 |
排序方式: 共有4523条查询结果,搜索用时 31 毫秒
91.
A low-capacity cation-exchange HPLC method for the determination of UV-absorbing organic cations such as amino acids, histidine dipeptides, and creatinine was developed. A commercially available reversed-phase column was dynamically coated with hexadecylsulfonate, and was successfully used for the cation-exchange separation with ethylenediammonium eluting ion at pH 2.5. The coated column was enough stable for the specific use with a completely aqueous mobile phase at low and constant pH; and the day-to-day reproducibility for retention time was 0.9-1.7% of RSD (relative standard deviation). The linear relation between concentrations and detector responses (area) by using a photodiode-array UV detection at 210 nm ranged from 0.2 to 1000 microM (sample size 50 microl) for 1-methylhistidine, 3-methylhistidine, histidine, creatinine, anserine, carnosine, and homocarnosine, and from 0.5 to 2000 microM for creatine, tyrosine, and phenylalanine, with less than 5% of RSD. The UV spectrum (190-300 nm) obtained during chromatography was very indicative for each analyte. Overall recoveries were 97-104%. The developed HPLC method in conjunction with preliminary fractionation technique could be applied to the analysis of urine of patient with metabolic disorder such as phenylketonuria. 相似文献
92.
A convenient synthetic approach is established to prepare a new class of 1-l-α-amino acid derivatives of phospholene oxides by amination of (±)-1-chloro-2-phospholene-1-oxides with several optically pure l-α-amino acid esters. All compounds obtained as a diastereomeric mixture in good to high yields. The two diastereomers were successfully separated by column chromatography and structurally identified by their spectral analyses. 相似文献
93.
A novel enantiopure 1,2-diamine (5) having two phenolic hydroxy groups was attached into chloromethylated polystyrene through benzyl ether linkage, which was used as a chiral ligand of the catalyst in asymmetric hydrogenation of acetophenone. 相似文献
94.
Enzymatic transformations into cyclic oligomers were carried out with the objective of developing chemical recycling of poly(lactic acid)s, such as poly(D,L-lactic acid) (PDLLA), poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA), which are typical biodegradable polymers. They were degraded by lipase in an organic solvent to produce the corresponding cyclic oligomer with a molecular weight of several hundreds. PDLLA (with a Mw of 84,000) was quantitatively transformed into cyclic oligomers by lipase RM (Lipozyme RM IM) in chloroform/hexane at 60 degrees C. PLLA (with a Mw of 120,000) was transformed into cyclic oligomer by lipase CA (Novozym 435) at a higher temperature of 100 degrees C in o-xylene. The oligomer structure was identified by 1H and 13C NMR spectroscopy and MALDI-TOF (matrix assisted laser desorption/ionization-time-of-flight) mass spectrometry. 相似文献
95.
Taichi Sato Takato Nakamura Toru Takahashi 《Journal of Thermal Analysis and Calorimetry》1985,30(1):97-106
The complexes (R3NHVO(OH)Cl2)2 and (R3RNVO(OH)Cl2)2 prepared by drying in vacuo the organic solutions from the extraction into benzene of aqueous vanadyl chloride solution with trioctylamine (TOA, R3N) and trioctylmethylammonium chloride (TOMAC, R3RNCl), were studied by thermogravimetry and differential thermal analysis under an atmosphere of nitrogen, and the products of their thermal decomposition, such as volatile matter and residues, by gas chromatography. X-ray diffraction and infrared spectroscopy. It was found that the complexes with TOA and TOMAC thermally decompose to V2O3 at 200–300° by cracking of the compounds R3NHVO(OH)Cl2 and R3RNVO(OH)Cl2 formed via dissociation of the complexes at about 150°.
We wish to thank the Koei Chemical Co. Ltd. for samples of TOA and TOMAC. 相似文献
Zusammenfassung Die Komplexe (R3NHVO(OH)Cl2)2 und (R3RNVO(OH)Cl2)2 wurden hergestellt, indem benzolische und wässrige Vanadylchlorid-Lösungen mit Trioctylamin (TOA, R3N) und Trioctylmethylammoniumchlorid (TOMAC, R3RNCl) extrahiert und die erhaltenen Lösungen im Vakuum eingedampft wurden. Die Komplexen wurden in Stickstoffatmosphäre mittels TG und DTA untersucht und die thermischen Zersetzungsprodukte, und zwar sowohl die flüchtigen Verbindungen als auch die Rückstände, gaschromatographisch bzw. röntgendiffraktometrisch und IR-spektroskopisch bestimmt. Es wurde festgestellt, daß die Komplexe mit TOA und TOMAC bei 150° in die Verbindungen R3NHVO(OH)Cl2 und R3RNVO(OH)Cl2 dissoziieren, die bei 200–300° thermisch zu V2O3 zersetzt werden.
(R3NHVO(OH)Cl2)2 (R3RNVO(OH)Cl2)2 (R3,) ( , R3RNCl). , , . , 200–300° V23 R3NV()l2 R3RNVO(OH)Cl2, 150°.
We wish to thank the Koei Chemical Co. Ltd. for samples of TOA and TOMAC. 相似文献
96.
Total synthesis of (-)-tuberostemonine 总被引:1,自引:0,他引:1
The first total synthesis of the complex pentacyclic Stemona alkaloid tuberostemonine was accomplished in 24 steps and in 1.4% overall yield from a hydroindole intermediate which is readily obtained in three steps from Cbz-l-tyrosine. An innovative synthetic strategy was applied that relays the single stereocenter of the amino acid precursor into nine of the ten stereogenic carbons of the target molecule. Among the highlights of the synthetic methodology are the 3-fold use of ruthenium catalysis, first in an azepine ring-closing metathesis and then in an alkene isomerization followed by a cross-metathesis propenyl-vinyl exchange, as well as the stereoselective attachment of the gamma-butyrolactone moiety to the core tetracycle by use of a lithiated ortho ester. 相似文献
97.
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
98.
A method for the quantitation of polycyclic aromatic hydrocarbons (PAHs) in indoor and outdoor air by high-performance liquid chromatography (HPLC) with a spectrofluorometric detection and programmed excitation and emission wavelength pairs is proposed. The mobile phase is a linear gradient of methanol-water. The relative standard deviations (n = 5) are in the range 0.38-1.7% at concentration levels of 0.69-11.40 ng ml(-1). The determination limits (S N = 10 ) are 0.5-15.9 pg. The proposed method was successfully applied to quantitate 12 PAHs in gas phase and particulates in indoor and outdoor air. The recoveries of PAHs from gas phase and particulates were 95.7-117.5 and 94.8-112.4%, respectively. This highly sensitive automatic HPLC analysis for PAHs both in gas phase and particulates can be applied to indoor and outdoor survey. 相似文献
99.
Metallic Na formation in/on NaCl crystals with irradiation by electron or vacuum ultraviolet photons
Shigehiro Owaki Shigeko Koyama Masao Takahashi Masao Kamada Ryouichi Suzuki 《Radiation Physics and Chemistry》1997,49(6):609-615
Metallic Na was formed in/on NaCl single crystals by irradiating them with a variety of radiation sources, namely, 21 MeV electron pulses, an electron beam of 30 keV and photon fluxes in the VUV region. The physical states were analysed using several methods, optical absorption, lifetime measurement of positron annihilation, Auger electron spectroscopy and UV photoelectron spectroscopy. Metallic Na was obtained in different physical states; clusters were formed in bulk, thin layers (islands) and thick layers on the surface. 相似文献
100.
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