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排序方式: 共有103条查询结果,搜索用时 15 毫秒
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Wegrzyńska J Para G Chlebicki J Warszyński P Wilk KA 《Langmuir : the ACS journal of surfaces and colloids》2008,24(7):3171-3180
The interfacial behavior of aqueous solutions of newly synthesized bis- and tris-ammonium salts (i.e., bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]alkylamine dichlorides and bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]dialkylammmonium trichlorides, respectively) was analyzed, both experimentally and theoretically. The dynamic and equilibrium surface tension of multiple ammonium salt solutions was measured by using a pendant drop shape analysis method. The determined surface tension isotherms indicated the lack of significant differences in surface activity between bis- and tris-ammonium salts, contrary to the expectations for divalent and trivalent surfactant ions. That effect was explained by assuming the formation of multiple surfactant ion-counterion associates. Taking into account the association process, a good correlation between experimental data and theoretical predictions was obtained by means of the "surface quasi two-dimensional electrolyte" (STDE) model of ionic surfactant adsorption. The degree of association necessary to explain the lack of difference in surface activity between bis- and tris-ammonium salts was in quantitative agreement with the results of measurements of the concentration of free chloride anions in the surfactant solution. 相似文献
44.
Orzeł L Fiedor L Wolak M Kania A van Eldik R Stochel G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(30):9419-9430
The mechanism of acetate-assisted transmetalation of tetrapyrroles was investigated in a model system consisting of chlorophyll a and copper(II) acetate in organic solvents by using a spectroscopic and kinetic approach. Surprisingly, acetate ions bind to the central Mg in chlorophyll much more strongly than do acetonitrile, methanol and even pyridine, one of the best ligands in chlorophyllic systems. This exceptionally strong non-symmetrical axial ligation of the central Mg by acetate causes its out-of-plane displacement and deformation of the tetrapyrrole ring, thus facilitating the interaction with an incoming CuII complex. This mechanism is controlled by a keto-enol tautomerism of the chlorophyll isocyclic ring. Additionally, depending on solvent, acetate activates the incoming metal ions. These new insights allow to suggest a mechanism for the acetate method of metal exchange in tetrapyrrolic macrocycles, which resembles biological insertion of metal ions into porphyrins. It also provides a guideline for the design of more efficient methods for the metalation of porphyrins and related macrocycles. 相似文献
45.
Electron ionization mass spectra of 12 derivatives of 2-thioorotic acid have been discussed and general fragmentation routes of their molecular ions have been proposed. The compounds under discussion were three groups of four chemical species (2-alkylthioorotic acid, methyl 2-alkylthioorotate, ethyl 2-alkylthioorotate and 2- alkylthioorotic hydrazide) each with the same relative molecular mass. The comparison of selected ions relative abundances and their correlation with the abundance of molecular ions enable differentiation between isomeric or isobaric species in this class of compounds. 相似文献
46.
We present ab-initio density functional theory studies on the interactions of small biologically active molecules, namely NO, CO, O(2), H(2)O, and NO(2) (-) with the full-size heme group. Our results show that the small molecule-iron bond is the strongest in carbonyl and the weakest in nitrite system. Trans influence induced by NO binding to the five-coordinate heme complex is shown. Nitric oxide in the resulting complex might be described as NO(-). The differences among the small ligands of XO type (CO, NO, O(2)), and their distant chemical behavior from H(2)O and NO(2) (-) ligands in binding to the Fe(II) ion, are shown. Moreover, the role of the heme ring as a reservoir of electrons in the studied complexes is invoked. The analysis of the parameters defining the iron-histidine bond indicates that this bond is longer and weaker in nitrosyl and carbonyl complexes than in the other systems. Our findings support the proposed mechanism of soluble guanylate cyclase (sGC) activation and suggest that the first step of sGC activation by CO may be the same as during the activation by NO. Obtained results are then compared with the data concerning smaller model of the heme, the porphyrin complexes, available in the literature. 相似文献
47.
To elucidate the applicability and properties of ionic liquids (ILs) to serve as chemical reaction media for the activation of small molecules by transition-metal complexes, detailed kinetic and mechanistic studies were performed on the reversible binding of NO to FeCl(2) dissolved in the IL 1-ethyl-3-methylimidazolium dicyanamide ([emim][dca]) as a solvent. We report, for the first time, the application of laser flash photolysis at ambient and high pressure to study the kinetics of this reaction in an IL. The kinetic data and activation parameters for the "on" and "off" reactions suggest that both processes follow a limiting dissociative (D) ligand substitution mechanism, in contrast to that reported for the same reaction in aqueous solution, where this well-known "brown-ring" reaction follows an interchange dissociative (I(d)) ligand substitution mechanism. The observed difference apparently arises from the participation of the IL anion as a N-donor ligand, as evidenced by the formation of polymeric [Fe(dca)(3)Cl](x)[emim](2x) chains in the solid state and verified by X-ray crystallography. In addition, infrared (IR), Mo?ssbauer, and EPR spectra were recorded for the monomeric reaction product [Fe(dca)(5)NO](3-) formed in the IL, and the parameters closely resemble those of the {FeNO}(7) unit in other well-characterized nitrosyl complexes. It is concluded that its electronic structure is best described by the presence of a high-spin Fe(III) (S = 5/2) center antiferromagnetically coupled to NO(-) (S = 1), yielding the observed spin quartet ground state (S(t) = 3/2). 相似文献
48.
Michal Kawalec Henryk Janeczek Grazyna Adamus Piotr Kurcok Marek Kowalczuk Mariastella Scandola 《Macromolecular Symposia》2008,272(1):63-69
Summary: The carboxylate induced degradation of the poly[(R,S)-3-hydroxybutyrate] (PHB) has been investigated with non-isothermal measurements. The apparent activation energies for PHB degradation have been determined. Application of the Kissinger's and Flynn-Wall-Ozawa's method for TG and DSC derived data gave good correlation of the results proving applicability of the non-isothermal DSC measurements for the study. Moreover, dependence of the apparent activation energies on the activity of the carboxylate has been found. 相似文献
49.
Gorski A Köhler T Seidel D Lee JT Orzanowska G Sessler JL Waluk J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(14):4179-4184
Three recently obtained expanded porphyrins represent nice examples of compounds for which the electronic and spectral properties can be predicted from symmetry considerations alone. Perimeter-model-based theoretical analysis of the electronic structure of doubly protonated cyclo[6], cyclo[7], and cyclo[8]pyrrole leads to the anticipation of qualitatively the same electronic absorption and magnetic circular dichroism patterns for all three compounds. These predictions are fully confirmed by experiments, as well as DFT and INDO/S calculations. Due to a characteristic pattern of frontier molecular orbitals, a degenerate HOMO and a strongly split LUMO pair, the three cyclopyrroles show comparable absorption intensity in the Q and Soret regions. Magnetic circular dichroism spectra reveal both A and B Faraday terms, of which the signs and magnitudes are in remarkably good agreement with theoretical expectations. The values of the magnetic moments of the two lowest degenerate excited states have also been obtained. 相似文献
50.
Taras-Goślińiska K Wenska G Skalski B Maciejewski A Burdziński G 《Photochemistry and photobiology》2002,75(5):448-456
Intramolecular processes of deactivation of 1,3-dimethyl-4-thiouracil (DMTU) from the second excited singlet (S2) (pi, pi*) and the lowest excited triplet (T1) (pi, pi*) states have been studied using perfluoro-1,3-dimethylcyclohexane (PFDMCH) as a solvent. The spectral and photophysical (PP) properties of DMTU in CCl4, hexane and water have also been described. For the first time, the fluorescence from S2 state DMTU has been observed. The picosecond lifetime of DMTU in the S2 state (tau(S2)) in PFDMCH has been proposed to be determined by a very fast intramolecular reversible process of hydrogen abstraction from the ortho methyl group by the thiocarbonyl group. The shortening of tau(S2) in CCl4 is interpreted to be caused by the intermolecular interactions between DMTU (S2) and the solvent. Results of the phosphorescence decay as a function of DMTU concentration were analyzed using the Stern-Volmer formalism, which enabled determination of the intrinsic lifetime of the T1 state (tau0(T1)) and rate constants of self-quenching (k(sq)). The lifetimes, tau0(T1), of DMTU in PFDMCH and CCl4 are much longer than the values hitherto obtained in more reactive solvents. The PP properties of DMTU both in the S2 and T1 states have been shown to be determined by the thiocarbonyl group. 相似文献