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81.
Monica L. Parentis Norberto A. Bonini Elio E. Gonzo 《Reaction Kinetics and Catalysis Letters》2001,72(2):303-308
Reaction rates of primary and secondary alcohols over chromium on silica catalysts prepared by the ion exchange method, in the presence and absence of oxygen, were measured. The effect of temperature and feed composition on activity and selectivity has been examined. 相似文献
82.
Stark CB Lopes NP Fonseca T Gates PJ 《Chemical communications (Cambridge, England)》2003,(21):2732-2733
We report the use of RuCl3 as an "alkali metal sponge". This is a general and highly efficient method for generating protonated parent ions for a variety of compounds that usually do not show this ion in electrospray mass spectrometry. This technique is demonstrated to be highly useful in "cleaning up" spectra from multiply metallated ions, thereby substantially improving the signal-to-noise ratio. 相似文献
83.
Furtado NA Vessecchi R Tomaz JC Galembeck SE Bastos JK Lopes NP Crotti AE 《Journal of mass spectrometry : JMS》2007,42(10):1279-1286
Diketopiperazines (DKPs) corresponding to cyclic dipeptides have been reported to exhibit antimicrobial, antitumor, antimutagenic and antiviral properties. These compounds are commonly isolated from microorganisms and sponges and from a variety of tissues and body fluids. In this work, we used electrospray ionization tandem mass spectrometry (ESI-MS/MS) to investigate the fragmentation of a series of DKPs previously isolated from Aspergillus fumigatus, which exhibit the same structural core. Loss of CO directly from the protonated molecule was found to be a fragmentation process common to all the compounds analyzed. However, our results revealed a series of ions that are diagnostic for the substituents at C(4) and C(9). In order to rationalize the differences in the fragmentation pathways of substituted and nonsubstituted DKPs, the relative Gibbs energies (DeltaG) of the product ions and intermediate ions were estimated using the B3LYP/6-31 + + G(d,p) model. The data reported here can be used for the structural elucidation of DKPs from low sample amounts, as an alternative to NMR. 相似文献
84.
A mechanism involving a six-membered cyclic transition state where the hydrogen of the hydroxyl group interacts with the oxygen of the carbonyl group has been proposed previously to describe the thermolysis of many beta-hydroxyl compounds. In this paper, the proposed mechanism is studied for a series of beta-hydroxyl aldehydes. Rate constants and activation energies are reported as well as a study of the influence of tunneling on the reaction rates. The electron density at the ring critical points, population analyses by the theory of atoms in molecules (AIM) and the natural bond orbital (NBO) method, as well as atomic energy analyses are used to gain insight into this interesting mechanism and into the effects of substituents. 相似文献
85.
Alessia Aprea Valentina Colombo Simona Galli Norberto Masciocchi Angelo Maspero Giovanni Palmisano 《Solid State Sciences》2010,12(5):795-802
Thiazolo[5,4-d]thiazole-2,5-dicarboxylic acid, C6H2N2O4S2, was isolated as a polycrystalline material, and its crystal structure was determined by ab-initio X-ray powder diffraction (XRPD) methods. This species, upon deprotonation, was subsequently used in preparing the new coordination polymers Ag2(C6N2O4S2), Mn(C6N2O4S2)(H2O)2, Co(C6N2O4S2)(H2O)2, Cu(C6N2O4S2)(H2O) and Zn(C6N2O4S2)(H2O)2, fully characterized by analytical, thermal and XRPD structural methods – including in situ thermodiffractometry and simultaneous TGA and DSC. In the first-row transition metal derivatives, the [C6N2O4S2]2? anion systematically prefers the N,O-chelating, vs. the expected O,O′-bridging, coordination mode, not allowing the formation of porous 3D frameworks. Indeed, these species are dense 1D coordination polymers. At variance, the silver derivative possesses a complex, dense 3D framework, due to the presence of μ6-[C6N2O4S2]2? ligands showing two μ2-bridging carboxylates and two monohapto N-donor sites. When dehydration is viable, materials of En(C6N2O4S2) formulation are irreversibly recovered (n = 1 for E = Mn, Co, Zn, Cu; n = 2, for E = H). 相似文献
86.
A theoretical study of the H2O and O2 adsorption on an illuminated TiO2 anatase surface is presented. The electronic structure and the spin distribution were examined by employing the DFT formalism and the BHandHLYP functional. The adsorbates geometries were fully optimized, including the cluster relaxation. Our results show the dissociative adsorption of the H2O molecule on the photoactivated TiO2 (0 0 1) surface. This reaction produces one hydroxyl group with radical character (OH) and an unpaired electron localized in the 5c-Ti atom. In case of the O2 molecule, the non-dissociative chemisorption was obtained. This molecule shows one unpaired electron and a negative charge. In these sense, the adsorbed O2 acts as a superoxide species (O2−). 相似文献
87.
We have developed a theory of the impurity band in doped semi-conductors, using a tight-binding basis and a self-consistent treatment to handle the configurational averages. New ingredients in the present calculation are: (1) overlap between different impurity orbitals; (2) simultaneous treatment of diagonal and off-diagonal disorder; (3) inclusion of electron—elctron interactions through the substitution of the bare Coulomb impurity potential by a Yukawa potential. The density of states and the position of the Fermi level (for T=0) are calculated for different impurity concentrations. 相似文献
88.
José María Rivera Susana Rincón Norberto Farfán Rosa Santillan 《Journal of organometallic chemistry》2011,696(11-12):2420-2428
Nine new boronates, six of them chiral, with five-six-membered ring heterobicycles were prepared by reaction of the Schiff bases and phenyl boronic acid. The boronates were fully characterized by spectroscopic techniques, NMR 1H, 13C, 11B, Infrared spectroscopy, mass spectrometry and elemental analysis. The reaction showed high diasteroselectivity, only in the case of compound 4c, containing a methyl substituent in the aliphatic moiety, the induction is low giving a 2:1 mixture of two diatereoisomers. The results showed that the preferred stereochemistry in the heterocycles is that where all substituents in the five membered ring and the phenyl group attached to boron atom are on the same side. 相似文献
89.
Michele Iafisco Ismaela Foltran Michele Di Foggia Sergio Bonora Norberto Roveri 《Journal of Thermal Analysis and Calorimetry》2011,103(1):41-47
Lactoferrin (LF), a non-heme iron-binding protein of blood plasma and milk with antioxidant, cariostatic, anticarcinogenic,
and anti-inflammatory properties, has been studied by differential scanning calorimetry (DSC) and Raman spectroscopy over
a wide pH range (4.0–9.0). Using these two techniques, the modifications in the quantity of iron bounded in the cow’s milk
LF and in the secondary structures, as a function of pH and heating, have been evaluated. DSC curves showed higher value of
denaturation temperatures and enthalpy changes when LF was saturated with iron (holo-form) than when it was in its unsaturated
form (apo-form). The denaturation curves of the protein solutions at pH ≥ 5.5 confirming that LF is a mix of apo- and holo-forms;
on the contrary at pH 4.0, the holo-form is practically absent. Spectroscopic investigation showed that, as a function of
pH, the content of α-helix increases up to pH 7.4, followed by a small decrease by further pH increase. The β-sheet percentage
exhibits the opposite behavior, while the random-coil and turn structures do not change noticeably. In contrast, after heat-induced
denaturation, strong variations were observed in the secondary structure, with an evident increase of β-sheet and decrease
of the α-helix percentage. Finally, both thermal and spectroscopic analysis pointed out that the structure of cow’s milk LF
is strictly sensible to pH variation and it has the highest thermal stability at physiological pH. 相似文献
90.
Dr. Pablo Labra-Vázquez Ricardo Flores-Cruz Aylin Galindo-Hernández Dr. Justo Cabrera-González Cristian Guzmán-Cedillo Dr. Arturo Jiménez-Sánchez Dr. Pascal G. Lacroix Dr. Rosa Santillan Dr. Norberto Farfán Dr. Rosario Núñez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(69):16530-16540
A set of BODIPY-carboranyl dyads synthesized by a Sonogashira cross-coupling reaction, where different C-substituted ortho- and meta-carboranyl fragments have been linked to a BODIPY fluorophore is described. Chemical, photophysical and physicochemical analyses are presented, including NMR and single XRD experiments, optical absorption/emission studies and partition coefficient (log P) measurements. These studies, supported by DFT computations (M06-2X/6-31G**), provide an explanation to the largely divergent cell income that these fluorescent carboranyl-based fluorophores display, for which a structural or physicochemical explanation remains elusive. By studying the cell uptake efficiency and subcellular localization for our set of dyads on living HeLa cells, we tracked the origins of these differences to significant variations in their static dipole moments and partition coefficients, which tune their ability to interact with lipophilic microenvironments in cells. Remarkably, m-carboranyl-BODIPY derivatives with a higher lipophilicity are much better internalised by cells than their homologous with o-carborane, suggesting that m-isomers are potentially better theranostic agents for in vitro bioimaging and boron carriers for boron neutron capture therapy. 相似文献