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51.
The differences between the molecular structures of the PCP-pincer complex [RuCl{C6H3(CH2P(C6H5)2)2-2,6}(PPh3)] ([RuCl(PCPH)(PPh3)], 1) and its tetrakis-pentafluorophenyl substituted analogue [RuCl{C6H3(CH2P(C6F5)2)2-2,6}(PPh3)] ([RuCl(PCPF20)(PPh3)], 2) have been rationalised by performing calculations on the cations [Ru(PCPH)(PPh3)]+ (1cat) and [Ru(PCPF20)(PPh3)]+ (2cat). The molecular interactions between the chloride ligand and the axial rings, as found in 1 and 2, respectively, have been studied computationally in the model systems [(C6X5PH2)2Cl−] (X = H, F). The calculations on 2cat show that in 2 it is most likely the attractive electrostatic interaction between the chloride ligand and the fluorinated phenyl rings that forces the Cipso atom to occupy an axial position rather than an equatorial one in the observed (X-ray of 2) square pyramidal arrangement. In 1, however, repulsive steric hindrance forces the PPh3 ligand to take the apical position. The applicability of the TD-DFT method for the calculation of the electronic spectra of the PCP-pincer compounds 1 and 2 has been tested. The results indicate that the excitation energies calculated for both complexes are in a reasonable agreement with the experimental absorption maxima. However, for 1, all the calculated transition energies are underestimated. 相似文献
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54.
Dr. Mauro Carraro Dr. Giulia Fiorani Dr. Lorenzo Mognon Dr. Francesca Caneva Dr. Martino Gardan Dr. Chiara Maccato Dr. Marcella Bonchio 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(41):13195-13202
Anchoring terminal octenyl tails on molecular polyoxotungstates yield polymerizable organic–inorganic monomers with formula [{CH2?CH(CH2)6Si}xOySiWwOz]4? [x=2, w=11, y=1, z=39 ( 1 ); x=2, w=10, y=1, z=36 ( 2 ); and x=4, w=9, y=3, z=34 ( 3 )]. These molecular hybrids can use aqueous hydrogen peroxide to catalyze the selective oxidation of organic sulfides in CH3CN. Copolymerization of 1 – 3 with methyl methacrylate and ethylene glycol dimethacrylate leads to porous materials with a homogeneous distribution of the functional monomers, as indicated by converging evidence from FTIR spectroscopy and electronic microscopy. The catalytic polymers activate hydrogen peroxide for oxygen transfer, as demonstrated by the quantitative and selective oxidation of methyl p‐tolyl sulfide, which was screened as model substrate. The hybrid material containing monomer 2 was also tested in n‐octane to evaluate its potential for the oxidation and removal of dibenzothiophene, a well‐known gasoline contaminant. 相似文献
55.
Marcella Niglio Cosimo Damiano Vitale 《Methodology and Computing in Applied Probability》2012,14(1):17-34
In this paper the (strict and weak) stationarity of threshold autoregressive moving average models is discussed. After examining
the strict stationarity, mainly based on the random coefficient autoregressive representation of the model, we provide sufficient
conditions for its weak stationarity that allow to obtain a wider stationarity region with respect to some previous results
given in the literature. These conditions are discussed to distinguish between global and local stationarity, whose relation has been considered in detail. The threshold process has been further evaluated to face the
problem related to the so called existence of a threshold structure in the data generating process that is strictly related to the stationarity and has significant
relevance when the parameters of the model have to be estimated. 相似文献
56.
Marcella Pani Angelo Mugnoli Enzo Sottofattori 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(7):840-842
In the crystal structure of C12H11ClO, the (planar) molecules give rise to a parallel packing. A model crystal obtained by semi‐empirical and packing‐energy calculations is consistent with the observed structure. 相似文献
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58.
Giuseppina Castronuovo Marcella Niccoli 《Journal of inclusion phenomena and macrocyclic chemistry》2005,53(1-2):69-76
The formation of complexes between β-cyclodextrin or methyl-β-cyclodextrin and monocarboxylic acids from C5 to C12 has been studied calorimetrically at 298 K in aqueous buffer phosphate, pH 11.3. When a complex forms, calorimetry enables
the calculation of both the enthalpy and the association constant, from which the Gibbs free energy and the entropy of the
process can be obtained. The forces involved in the association process are then discussed in the light of the signs and values
of the thermodynamic parameters obtained. For β-cyclodextrin, 1:1 inclusion complexes form, characterized by an irregular
variation in the values of enthalpies, entropies and association constants at increasing alkyl chain length. A model is proposed
to explain this unusual behaviour for acids longer than ten carbon atoms. The association involving methyl-β-cyclodextrin
shows, instead, a regular variation of the thermodynamic parameters up to the C12 term. An elongation of the cavity effect is discussed: the cavity behaves as it were deeper than that of the natural cyclodextrins.
Association is characterized prevailingly by hydrophobic interactions; for the longer terms, the high and almost invariant
entropic contribution determines the large association constants, notwithstanding the positive, unfavorable enthalpic contribution.
This is an indication that the relaxation of water molecules from the hydrophobic hydration shells of the external methyl
groups is the contribution that mainly determines the association process. 相似文献
59.
Giulio Jori Marcella Folin Giorgio Gennari Guido Galiazzo Orfeo Buso 《Photochemistry and photobiology》1974,19(6):419-433
Abstract— Rare-earth metal ions give 1:1 complexes with hen's egg-white lysozyme. Spectroscopic and enzymic activity measurements suggest that the binding site consists of the side chains of glutamic-35 and aspartic-52. The spatial conformation of these complexes is practically identical to that of native lysozyme, especially as concerns the environment of the tryptophyl side chains. Irradiation of La3+ -lysozyme by visible light, in the presence of proflavine as photosensitizer, causes the oxidative modification of all the tryptophyl and methionyl residues at almost the same rate as in uncomplexed protein. On the other hand, when lanthanide ions with nonvanishing magnetic moments were coordinated with lysozyme, at least some tryptophans and methionines were protected from photooxidative attack. The distance of the protected residues from the coordination site increased with increasing magnetic moment of the bound metal ion, which suggests that inhibition of the photoprocess was mainly due to perturbation of the lifetime of the electronically excited intermediate species. On the basis of the atomic coordinates of lysozyme in the crystal state, it is thus possible to define a "quenching radius" for the various lanthanide ions; these radii could in turn be used to evaluate intramolecular distances in proteins of unknown tertiary structure, by identifying the amino acid residues that are protected or photooxidized upon irradiation of complexes between the given protein and several different lanthanide ions. Our studieson lysozyme allow us to define five radii of protection, ranging from 6·7 Å for Sm3+ to over 17 Å for Dy3+ , Ho3+ , Er3+ and Tb3+ . Therefore, this technique opens the possibility of mapping appreciably large regions of a protein molecule. 相似文献
60.
The thermodynamics and kinetics of the binding of Ga(III) and In(III) to two hydroxamic acids, C6H5-C(O)N(OH)H (BHA) and C6H5-C(O)N(OH)C6H5 (PBHA), have been investigated in acidic media. Spectrophotometric titrations in the UV region reveal that, with excess metal, only the chelate ML forms, whereas the concentration of the protonated species, MHL, is negligible. The thermodynamic parameters indicate that the driving force for formation of ML from MOH2+ and HL is mainly enthalpic, with entropic contributions favoring InL2+ and disfavoring GaL2+ formation. The kinetic (stopped-flow) experiments are interpreted on the basis of two parallel reaction paths both involving reaction of the undissociated ligand (HL): (a) M + HL <==> MHL <==> ML + H where MHL is in a steady state and (b) MOH + HL <==> ML + H2O. Whereas gallium binding to BHA and PBHA proceeds mainly through path b, indium binding to PBHA proceeds through both a and b paths. The rates of both the a and b steps are ligand dependent. Two alternative mechanisms are proposed. The first is based on the electronic characteristics of the ligands and is of the Ia type. The second, of the Id type, assumes that a considerable fraction of the ligand is unreactive owing to intramolecular hydrogen bonding (possibly including a water molecule) which blocks the reaction site. The reasons for preferring the former mechanism are discussed. 相似文献