首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   973篇
  免费   23篇
  国内免费   6篇
化学   790篇
晶体学   9篇
力学   10篇
数学   44篇
物理学   149篇
  2022年   11篇
  2021年   7篇
  2020年   11篇
  2019年   8篇
  2018年   17篇
  2017年   7篇
  2016年   13篇
  2015年   12篇
  2014年   18篇
  2013年   52篇
  2012年   49篇
  2011年   52篇
  2010年   19篇
  2009年   28篇
  2008年   55篇
  2007年   58篇
  2006年   55篇
  2005年   69篇
  2004年   45篇
  2003年   45篇
  2002年   47篇
  2001年   16篇
  2000年   11篇
  1999年   8篇
  1998年   10篇
  1997年   6篇
  1996年   10篇
  1995年   8篇
  1994年   13篇
  1993年   8篇
  1992年   16篇
  1991年   11篇
  1990年   12篇
  1989年   11篇
  1988年   14篇
  1987年   16篇
  1986年   9篇
  1985年   10篇
  1984年   19篇
  1983年   10篇
  1982年   16篇
  1980年   12篇
  1979年   10篇
  1978年   5篇
  1977年   10篇
  1976年   6篇
  1975年   9篇
  1974年   6篇
  1973年   4篇
  1971年   7篇
排序方式: 共有1002条查询结果,搜索用时 31 毫秒
911.
The chemical synthesis of 13C-labeled d-galactose as useful molecular probes for studying the conformation of oligosaccharides attached to proteins was performed. The method for synthesizing the title labeled compounds was newly developed via the corresponding 1-ene and 5-ene compounds derived from 1,2:5,6-di-O-isoproppylidene-α-d-galactofuranose by considering the efficient introduction of the atom. All protons of galactose from H-1 to H-6 were observed by the HMQC-HOHAHA technique using 1:1 mixtures of methyl [1-13C]- and [6-13C]-β-d-galactopyranoside, which were prepared from the title compounds.  相似文献   
912.
Polymerizations of butadiene, penta-1,3-diene, and isoprene with n-butyl titanate–triethylaluminum catalyst are examined by ESR measurements on the polymerization state. At Al/Ti molar ratios greater than 2.9 where the conjugated dienes are polymerized, the polymerization system of butadiene always gives an ESR signal with a g value of 1.983 and with a hyperfine structure of about 19 components. This signal does not appear at all, even in the presence of the monomer, at Al/Ti molar ratios smaller than two where butadiene is not polymerized. The absorption intensity of the signal coincides fairly well with the concentration of polymer chain calculated from polymer yield and the molecular weight. On the basis of these facts, the signal is assigned to the growing end of polybutadiene with this catalyst. The structure of the growing end is proposed to have both two substituted π-allyl groups and an alkoxy group in coordination to titanium (III), by analysis of the hyperfine structure. The polymerization system of penta-1,3-diene and that of isoprene respectively, give a new signal with a g value of 1.983, although the signal for the former monomer has a hyperfine structure of 11 components and that for the latter monomer has no hyperfine structure. A structure for the growing end in the polymerization of each of these two monomers analogous to that of the growing end of polybutadiene is proposed.  相似文献   
913.
Sodium and potassium ions in waters are determined by flow-injection extraction-spectrophotometry. The ion-association complexes formed between the metal/crown ether cations and the tetrabromophenolphthalein ethyl ester anion (TBPE-) are extracted into chlorobenzene/benzene (1:3) and the absorbance of the organic phase is measured after phase separation with a porous membrane. Sodium and potassium are separated on-line with a column (1 mm i.d.×30 cm) packed with silica gel (100-200 mesh). The manifold comprises four streams, each at 0.8 ml min?1. The sample is injected into a water stream and mixed with a reagent stream containing lithium acetate and benzo-18-crown-6 before entering the silica gel column; after the separation, the stream is mixed with EDTA (trilithium salt) and lithium hydroxide, and then with the extraction solution containing TBPE.H. Extraction proceeds in a 2-m coil; the absorbance of the organic phase is measured at 620 nm. CAlibration graphs are linear inthe ranges 0–×10?3 M sodium and 0–2×10?4 M potassium. The sample throughput is 15 h?1. The procedure is applicable to river and tap waters.  相似文献   
914.
The energies of the S0 and S1 states of polychlorinated dibenzofurans (PCDFs) were calculated using the Hartree–Fock (HF) and configuration interaction-singles (CIS) methods. We can obtain the (0–0) transition energies of PCDFs with good accuracy if the energies calculated using the HF and CIS methods are adjusted to take the electron correlation energy into account. The correlation energy of the S0 state was calculated using the Møller–Plesset correlation correction truncated at the second order (MP2), and that of the S1 state was determined using experimental data. The correlation energies for both S0 and S1 states were expressed as the sum of the contributions arising from dibenzofuran (DF) and substituted chlorine atoms. The energy of the ground state calculated using the additivity approximation was in good agreement with the energy given directly by the MP2 method. The (0–0) S1←S0 transition energies corrected for electron correlation energy agreed well with the available experimental data. The approach proposed in this paper may be useful for the estimation of the electronic transition energy for large aromatic molecules.  相似文献   
915.
The sensitive and selective method reported is based on the reaction of boric acid with 2,6-dihydroxybenzoic acid in acidic aqueous solution to form a 1:2 complex anion which can be extracted into chloroform as an ion-pair with 4-(4-diethylaminophenylazo)-N-methylpyridinium ion. The ion-pair formed by the excess of reagents and co-extracted into chloroform, is removed by washing the organic phase with phosphate buffer solution (pH 7.0) and 0.025 M sulfuric acid solution. The absorbance of azo dye in chloroform is measured at 570 nm. Calibration graphs are linear in the range 0–1.5 × 10-5 M of boron. The molar absorptivity is 6.6 × 104 l mol-1 cm-1 and the absorbance of the reagent blank is 0.030. The method was applied to the determination of boron at the 0.001% level in steels.  相似文献   
916.
Wei Y  Oshima M  Simon J  Moskvin LN  Motomizu S 《Talanta》2002,58(6):1343-1355
The concentration distribution of an analyte in a chromatomembrane cell (CMC) was examined by using various air samples of different air pollutant (NO2) concentrations and volumes, and the results obtained could be explained by a proposed principle of the concentration distribution of the analyte in the CMC. This principle was for the first time proved experimentally in the present study. On-line preconcentration and continuous determination of the air pollutant (NO2) in air samples were realized by coupling a three-hole CMC with a flow injection analysis (FIA) system, where a triethanolamine (TEA) aqueous solution (2 g l−1) was used as an absorbing solution for NO2 in the air samples. A calibration method with standard nitrite aqueous solutions was developed for the determination of NO2 in the air samples. Concentrations of NO2 in indoor air and its diluted air samples were determined by the proposed CMC/FIA method. The volume of air sample necessary for the measurement was decreased to only 5 ml. The measuring time for one sample was about 5–6 min even when a 20 ml air sample was used.  相似文献   
917.
Lee KH  Oshima M  Motomizu S 《The Analyst》2002,127(6):769-774
A new on-line flow injection (FI) pre-treatment system using a disk-type chelating resin (5 mm diameter, 0.5 mm thickness) was developed for the simultaneous multi-element determination of trace metals in sea-water samples by inductively coupled plasma mass spectrometry (ICP-MS). A chelating resin possessing an iminodiacetate (IDA) functional group was used for the collection of trace elements and the elimination of alkali and alkaline earth metals in highly concentrated salt solution. A 1 ml volume of a sea-water sample (pH 5.5) was applied to the chelating resin disk. Considering the removal efficiency for Ca, 50 mM ammonium acetate buffer solution (pH 5.5) was chosen as a sample carrier. The enriched trace metals were eluted with 0.1 M nitric acid and the eluate flowed into the ICP-MS system. The processing time for one sample was < 6 min (350 s). One of the important observations is the possibility of working with a low recovery, even lower than 50%. For example, several elements such as Mn, Cr, As, Mo, Ba and U, the recovery of which was < 50% in a batch-wise method, showed good linearity and reproducibility. The proposed method was evaluated by analyzing two kinds of sea-water certified reference materials, CASS-4 and NASS-5. Analytical data for eight heavy metals, V, Mn, Co, Ni, Cu, Mo, Cd and U, obtained from the present study agreed well with the certified values.  相似文献   
918.
[structure; see text] An iterative synthetic route to organic-soluble ureido-naphthyridine oligomers has been developed. Use of this protocol allowed synthesis of a short ureido-naphthyridine oligomer, which presents a self-complementary DDAADDAA hydrogen bonding array (D = hydrogen bond donor, A = hydrogen bond acceptor). Strong self-association via eight hydrogen bonds was observed in organic solution.  相似文献   
919.
Gramicidin S (GS) analogs, (D-Ser(Bzl)4,4′]-GS (17), [D-Ser4,4′]-GS (18), and [L-Orn(Boc)2,2′,D-Ser4,4′]-GS (19) were synthesized and 17 showed high antibacterial activity. One residue of D-Ser in 19 was converted asymmetrically to D-Ala via α,β-dehydroalanine.  相似文献   
920.
The applicability of the matrix-free laser desorption/ionization on silicon mass spectrometry (DIOS-MS) to measuring serum drug levels was examined by analyzing serum salicylic acid. The optimized and simple solid-phase extraction (SPE) allowed good recovery, 88.9 +/- 5.8%, for 1.4 mM (200 mg/L) of salicylic acid in serum. The negative ion MS allowed measurements of deprotonated molecules without interference from other signals. Using a deuterium-labeled internal standard, good linearity was obtained in the 0.14 to 4.2 mM (20-600 mg/L) range, which was sufficient for monitoring the therapeutic anti-inflammatory dose. SPE followed by DIOS-MS is anticipated to be a method of measuring drug levels in blood and may allow high throughput analysis.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号