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441.
J. Lipkowski A. Grabowska J. Waluk G. Calestani B. A. Hess Jr. 《Journal of chemical crystallography》1992,22(5):563-572
The crystal structures of two molecules undergoing fast intramolecular excited state proton transfer (2,2-bipyridyl-3,3-diol, BP(OH)2, and 2,2-bipyridyl-3-ol, BP(OH)) are reported and compared with the results ofab initio and molecular mechanics calculations. Strong intramolecular hydrogen bonding is observed in both cases. The pyridyl rings are coplanar in BP(OH)2, whereas in BP(OH) they form an angle of 3.7°.Dedicated to Prof. R. Zahradnik on the occasion of his birthday. 相似文献
442.
J. W. Mietelski P. Szwałko E. Tomankiewicz P. Gaca S. Małek J. Barszcz S. Grabowska 《Journal of Radioanalytical and Nuclear Chemistry》2005,262(3):645-660
Results for 137Cs, 40K, 90Sr, 238,239+240Pu, 241Am and 243+244Cm measurements in plant, insects and forest litter samples collected at three sites in Poland are presented. New results are compared with some existing data for locations examined during previous studies. Insect samples were introduced now for the first time. Relatively high activities of 90Sr were noticed for spruce bark beetle (Ips typographus) and those for 137Cs, plutonium and 241Am for forest dung beetle (Anoplotrupes stercorosus). Faster than caused by physical decay decrease of radiocesium activity was noticed for the majority of plant and litter samples. The results for 239+240Pu for litter were similar to previous results, but the activities of 238Pu were smaller. The activity ratio between 241Am and 239+240Pu was found lower than expected for known proportions between global and Chernobyl fallout. Thus a kind of dynamic behavior of Pu and Am in the forest ecosystem is concluded. Transfer factors and aggregation coefficients were estimated and discussed for both plants and insects as well as between insects and the part of plants (or litter) they feed on. Many of them were never presented before.The authors are thankful to the Polish State Committee for Scientific Research for financial support of this investigation, Grant No. PG04 07520. 相似文献
443.
13C cross-polarization magic angle spinning (CP/MAS) NMR data for 2,2,5,7,8-pentamethylchroman-6-ol (2), 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox c) (3) and its acetate (4), 2-methoxy-2,2,5,7,8-pentamethylchroman-6-ol (5), 2-hydroxy-2,2,5,7,8-pentamethylchroman-6-ol (6) and 2,2,5,7,8-pentamethylchroman (7) are reported. A deshielding of 7.7 ppm for the carboxylic carbon was observed in solid Trolox due to formation of intermolecular hydrogen bonds within cyclic dimers. Such crystal packing permits effective cross-polarization and fast relaxation (short T1rho(H)). The impact of the proton concentration on the CP dynamics is reflected by the longer T(CP) and T1rhoH for Trolox-d2 (deuterated at mobile proton sites). The calculated GIAO RHF shielding constants are sensitive to intramolecular effects: rotation around the C-6-O bond (changes of sigma up to 8 ppm) and conformation at C-2. 相似文献
444.
Bachorz RA Harańczyk M Dabkowska I Rak J Gutowski M 《The Journal of chemical physics》2005,122(20):204304
The neutral and anionic formic acid dimers have been studied at the second-order Moller-Plesset and coupled-cluster level of theory with single, double, and perturbative triple excitations with augmented, correlation-consistent basis sets of double- and triple-zeta quality. Scans of the potential-energy surface for the anion were performed at the density-functional level of theory with a hybrid B3LYP functional and a high-quality basis set. Our main finding is that the formic acid dimer is susceptible to intermolecular proton transfer upon an excess electron attachment. The unpaired electron occupies a pi(*) orbital, the molecular moiety that accommodates an excess electron "buckles," and a proton is transferred to the unit where the excess electron is localized. As a consequence of these geometrical transformations, the electron vertical detachment energy becomes substantial, 2.35 eV. The anion is barely adiabatically unstable with respect to the neutral at 0 K. However, at standard conditions and in terms of Gibbs free energy, the anion is more stable than the neutral by +37 meV. The neutral and anionic dimers display different IR characteristics. In summary, the formic acid dimer can exist in two quasidegenerate states (neutral and anionic), which can be viewed as "zero" and "one" in the binary system. These two states are switchable and distinguishable. 相似文献
445.
The catalytic conversion of acyclic and cyclic esters into ketones was studied. Based on an analysis of data on the yields of the products of ester conversion, the conclusion was drawn that ketones result from a reaction between two ester molecules with the intermediate formation of a -ketoester. This reaction is accompanied by the partial thermal decomposition of esters to aldehydes (reverse Tishchenko reaction) with the subsequent condensation. 相似文献
446.
H. Grabowska W. Mis´ta J. Trawczyn´ski J. Wrzyszcz M. Zawadzki 《Research on Chemical Intermediates》2001,27(3):305-313
Two samples (A and B) of zinc aluminate spinel were prepared and used as catalysts of phenol methylation. Both catalysts were synthesised at hydrothermal conditions from zinc acetate and from various aluminium precursors: aluminium isopropoxide (catalyst A) or basic aluminium nitrate (catalyst B). Catalyst A was pure ZnAl2O4 and B - besides ZnAl2O4 contained traces of -Al2O3. Reactions of phenol alkylation with methanol were carried out in the gas phase under atmospheric pressure in a standard flow reactor with fixed bed. Catalysts indicated different properties in dependence on the aluminium precursor used during preparation. In the presence of catalyst A higher selectivity of ortho-methylation of phenol was obtained. Catalyst B was active both in O-alkylation and C-alkylation. 相似文献
447.
Richard S. Herrick Iwona Wrona Glenn Jones Janet Shaw 《Journal of organometallic chemistry》2004,689(25):4848-4855
The title compounds were prepared in good yield by heating ester protected amino acids (H-l-Ala-OEt, H-β-Ala-OEt, H-L-Val-OMe, H-GABA-OMe, H-L-Asp(OMe)-OMe, H-L-Met-OMe) with Re(CO)5Cl in the presence of pyridine-2-carboxaldehyde. The resulting novel complexes contain a bound, conjugated diimine ligand with a pendant ester group. All compounds were characterized by 1H and 13C NMR, IR and UV-Vis spectroscopy. Compounds prepared from chiral amino esters give diastereomers because of the presence of a stereogenic metal center. Syntheses using Re(CO)5Br as starting material were discontinued because of metathesis involving the chloride in the amino ester · HCl salts and the metal bromide. Photophysical studies on Re(CO)3Cl(pyca-β-Ala-OEt), 2, show that it is luminescent in organic solvents in air at room temperature. The crystal structures of 2 and Re(CO)3Cl(pyca-l-Asp(OMe)-OMe), 5, were determined. 相似文献
448.
J. W. Mietelski L. Kitowski P. Gaca S. Grabowska E. Tomankiewicz P. Szwałko 《Journal of Radioanalytical and Nuclear Chemistry》2006,270(1):131-135
Summary The present study focused on analyzing samples of bones, livers and kidneys of European white-tailed eagles (Halia?tus albicilla) and lesser-spotted eagle (Aquila pomarina). Bone samples were collected for both species, from 7 and 2 individuals, respectively, whereas liver and kidney samples
for white-tailed eagle species only, 2 and 1 individuals, respectively. The samples were analyzed for the presence of gamma-emitters
and then for 90Sr, 238Pu, 239+240Pu and 241Am. The applied radiochemical method is presented. Activity concentration in ashen bones (600 °C) for 90Sr ranged from 4.6±1.2 to 31.0±2.5 Bq/kg, for 239+240Pu from <1.7 to 21±4 mBq/kg, for 238Pu from <2.7 to 6.5±1.3 mBq/kg and for 241Am from <1.2 to 6.5±4.4 mBq/kg. Relatively high 239+240Pu activity concentration of 78±9 mBq/kg (for fresh weight) was recorded in a single kidney sample. The liver samples showed
activities of magnitude at least one order lower. No clear correlations were found between the activities of different radionuclides. 相似文献
449.
J. W. Mietelski P. Szwako E. Tomankiewicz P. Gaca S. Maek J. Barszcz S. Grabowska 《Journal of Radioanalytical and Nuclear Chemistry》2004,262(3):645-660
Results for 137Cs, 40K, 90Sr, 238,239+240Pu, 241Am and 243+244Cm measurements in plant, insects and forest litter samples collected at three sites in Poland are presented. New results are compared with some existing data for locations examined during previous studies. Insect samples were introduced now for the first time. Relatively high activities of 90Sr were noticed for spruce bark beetle (Ips typographus) and those for 137Cs, plutonium and 241Am for forest dung beetle (Anoplotrupes stercorosus). Faster than caused by physical decay decrease of radiocesium activity was noticed for the majority of plant and litter samples. The results for 239+240Pu for litter were similar to previous results, but the activities of 238Pu were smaller. The activity ratio between 241Am and 239+240Pu was found lower than expected for known proportions between global and Chernobyl fallout. Thus a kind of dynamic behavior of Pu and Am in the forest ecosystem is concluded. Transfer factors and aggregation coefficients were estimated and discussed for both plants and insects as well as between insects and the part of plants (or litter) they feed on. Many of them were never presented before.The authors are thankful to the Polish State Committee for Scientific Research for financial support of this investigation, Grant No. PG04 07520. 相似文献
450.
Anusiewicz I Berdys-Kochanska J Czaplewski C Sobczyk M Daranowski EM Skurski P Simons J 《The journal of physical chemistry. A》2005,109(1):240-249
In an attempt to shed light on the mechanism by which gaseous samples of negatively charged oligonucleotides undergo extremely slow (i.e., over 1-1000 s) charge loss, we have carried out molecular dynamics simulations on an oligonucleotide anion, T(5)(3-), containing five thymine, deoxyribose, and phosphate units in which the first, third, and fifth phosphates are negatively charged. The study is aimed at determining the rate at which an electron is detached from such a trianion by way of an internal Coulomb repulsion induced event. In this process, the intrinsic 5.0-5.1 eV electron binding strength of each phosphate site is reduced by the repulsive Coulomb potentials of the other two negative sites. As geometrical fluctuations cause the distances among the three negative phosphate sites to change, this causes the Coulomb repulsion energies at these sites to fluctuate. Once the Coulomb potential at any phosphate site exceeds ca. 5 eV, the electron on that site is able to undergo autodetachment. Although such an electron must tunnel through a barrier to escape, it is shown that the tunneling rate is not the rate-limiting step in electron loss; instead, it is the rate at which geometrical fluctuations cause the Coulomb potentials to exceed 5 eV that determines the rate of electron loss. Because these rates are extremely slow, special techniques had to be introduced to allow results of dynamics simulations on more flexible models of T(5)(3-) to be extrapolated to predict the behavior of the actual T(5)(3-). 相似文献