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121.
When explosives are present in natural aqueous media, their concentration is usually limited to trace levels. A preconcentration step able to remove matrix interferences and to enhance sensitivity is therefore necessary. In the present study, we evaluated solid-phase microextraction (SPME) technique for the recovery of nine explosives from aqueous samples using high-performance liquid chromatography with ultraviolet detection (HPLC-UV). Several parameters, including adsorption and desorption time, coating type, rate of stirring, salt addition, and pH, were optimized to obtain reproducible data with good accuracy. Carbowax coating was the only adsorbent found capable of adsorbing all explosives including nitramines. Method detection limits (MDL) were found to range from 1 to 10 microg/L, depending on the analyte. SPME/HPLC-UV coupling was then applied to the analysis of natural ocean and groundwater samples and compared to conventional solid-phase extraction (SPE/HPLC-UV). Excellent agreement was observed between both techniques, but with an analysis time around five times shorter, SPME/HPLC-UV was considered to be applicable for quantitative analysis of explosives.  相似文献   
122.
The molecular mechanism for the 1,3-dipolar cycloaddition of nitrone with sulfonylethene chlorides has been studied using ab initio and DFT methods at the HF, MP2 and B3LYP levels together with the 6-31G* basis set. Relative rates, stereo and regioselectivity, have been analysed and discussed. For this cycloaddition four reactive channels associated with the formation of two pairs of diastereoisomeric regioisomers have been characterized. Analysis of the geometries of the corresponding transition structures shows that the cycloaddition takes place along a concerted but asynchronous mechanism. Activation energies as asynchronicity are dependent on the computation level. Thus, while HF calculations gave large barriers, MP2 calculations tend to underestimate them. DFT calculations gave reasonable values. These 1,3-dipolar cycloadditions present an endo stereoselectivity while the meta regioselectivity depends on the computational level. Thus, while HF and DFT calculations predict meta path, in agreement with the experimental results, MP2 calculation predict ortho regioselectivity. The frontier molecular orbitals analysis shows that the reaction is controlled by the (HOMOdipole–LUMOdipolarophile) interaction in agreement with the charge transfer analysis carried out at the transition structures. Inclusion of diffuse functions at the B3LYP/6-31+G* level increases the energy barriers about 4 kcal/mol, giving a similar endo/meta selectivity. Solvent effects have been taken into account, by means of self-consistent reaction field.  相似文献   
123.
Sa'sa' S  Rashid A  Jalal I 《Talanta》1984,31(5):397-399
The method reported provides a fast, sensitive, accurate and reproducible reverse-phase HPLC assay for acetaminophen and dextropropoxyphene simultaneously. The total elution time is < 5 min. The method is stability-indicating since it can also determine p-aminophenol, a degradation product of acetaminophen, in a concentration as low as 0.005% of the acetaminophen concentration.  相似文献   
124.
An Agilent 3DCE capillary electrophoresis system using sulfobutylether-beta-cyclodextrin (SB-beta-CD)-ammonium acetate separation buffer pH 6.9 was coupled to a Bruker Esquire 3000+ quadrupole ion trap mass detector via a commercially available electrospray ionization interface with acetonitrile sheath flow. The CE-MS system was applied in negative ionization mode for the resolution and detection of nitroaromatic and polar cyclic or caged nitramine energetic materials including TNT [2,4,6-trinitrotoluene, formula mass (FW) 227.13], TNB (1,3,5-trinitrobenzene, FW 213.12), RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine, FW 222.26) HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, FW 296.16), and CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, FW 438.19). The CE-MS system conformed to the high-performance liquid chromatography with ultraviolet absorbance detection (HPLC-UV) and HPLC-MS reference methods for the identification of energetic contaminants and their degradation products in soil and marine sediment samples.  相似文献   
125.
We have computationally studied the bimolecular nucleophilic substitution (SN2) reactions of MnNH2(n−1) + CH3Cl (M+ = Li+, Na+, K+, and MgCl+; n = 0, 1) in the gas phase and in tetrahydrofuran solution at OLYP/6-31++G(d,p) using polarizable continuum model implicit solvation. We wish to explore and understand the effect of the metal counterion M+ and of solvation on the reaction profile and the stereochemical preference, that is, backside (SN2-b) versus frontside attack (SN2-f). The results were compared to the corresponding ion-pair SN2 reactions involving F and OH nucleophiles. Our analyses with an extended activation strain model of chemical reactivity uncover and explain various trends in SN2 reactivity along the nucleophiles F, OH, and , including solvent and counterion effects. © 2019 Wiley Periodicals, Inc.  相似文献   
126.
A simple sample pre-treatment method for rare earth elements enrichment from environmental water samples prior to optical emission spectrometry determination with inductively coupled plasma (ICP OES) is proposed based on solid phase extraction with octadecyl silica gel modified with 1,2,5,8-tetrahydroxyanthraquinone. Optimal experimental conditions including pH of sample solution, sample volume, type, concentration and volume of eluent were investigated and established. Rare earth elements ions were quantitatively adsorbed from aqueous solution onto octadecyl silica gel modified with Quinalizarin at pH 7.0. The adsorbed ions were eluted with 1.0 mL of 2 M HNO3 and simultaneously determined by ICP OES. Under obtained optimum conditions the calibration curves were linear with the coefficient of variation better than 0.99. The limit of detection of the method for the studied elements was in the range of 0.0004–0.01 ng/mL. The proposed method has a pre-concentration factor of 320–450 in water samples, which results in high sensitivity detection of trace ions. The developed method gave recoveries better than 80% and RSDs less than 15%.  相似文献   
127.
The redox activity of a ferrocenyl monolayer grafted on an n-type Si111 substrate was investigated by scanning electrochemical microscopy (SECM) in conditions where the substrate plays the role of an insulator. This approach permits the differentiation between the different possible electron-transfer and mass-transport pathways occurring at the interface. As an exciting result, the thin ferrocenyl monolayer behaves like a purely conducting material, highlighting very fast electron communication between immobilized ferrocenyl headgroups in a 2D-like charge-transport mechanism.  相似文献   
128.
Cyclocondensation reaction of ethyl 7,8-diamino-1-cyclopropyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylate with 1-methylisatin produced a separable mixture of the corresponding indolo[3,2-b]- and [2,3-b]pyrido[2,3-f]quinoxaline-3-carboxylates, of which the latter isomer predominates. On the other hand, interaction with 1H-isatin or 5-chloroisatin gave the respective indolo[2,3-b]pyrido[2,3-f]quinoxaline-3-carboxylates as the sole regiospecific products. The structures of these new pentacyclic derivatives are based on microanalytical, spectral (IR, MS, and NMR) and X-ray crystal structure data.  相似文献   
129.
A microporous metal-organic framework 1, Cu(R-GLA-Me)(4,4'-Bipy) 0.5.0.55H 2O (R-GLA-Me = R-2-methylglutarate, 4,4'-Bipy = 4,4'-bipyridine), with a primitive cubic net was synthesized and characterized. With pores of about 2.8 x 3.6 A, the activated 1a exhibits exclusive adsorption of water over methanol in a binary water-methanol (1:1) liquid mixture.  相似文献   
130.
Cerpegin (1) was synthesized through a ‘one-pot’ reaction in 71% overall yield. Lithiation of commercially available 2-methoxynicotinic acid (2) as its lithium salt using LTMP, followed by addition of acetone at low temperature and a specific acidic treatment of the intermediate 3 thus obtained, gave the 1,1-dimethyl-3,4-dioxo-1,3,4,5-tetrahydrofuro[3,4-c]pyridine (4). The latter was finally selectively alkylated using methyl iodide and caesium carbonate to afford cerpegin (1).  相似文献   
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