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排序方式: 共有843条查询结果,搜索用时 15 毫秒
831.
Arrieta-Baez D Cruz-Carrillo M Gómez-Patiño MB Zepeda-Vallejo LG 《Molecules (Basel, Switzerland)》2011,16(6):4923-4936
The main monomer of tomato cuticle, 10,16-dihydroxyhexadecanoic acid (or 10,16-dihydroxypalmitic acid; 10,16-DHPA), was isolated and used to efficiently synthesize two different monomers (16-hydroxy-10-oxo-hexadecanoic and 7-oxohexa-decanedioic acids) in addition to a dimer and linear and branched trimers. These compounds were fully characterized using NMR and MS techniques and could be used as starting materials for the synthesis of a wide range of chemicals and bio-polyesters, particularly the latter due to their physical properties, non-toxicity, and relative abundance among raw materials. 相似文献
832.
The fragmentation mechanisms of progesterone have been studied by triple quadrupole tandem mass spectrometry (MSMS) and density functional theory (DFT). Mechanisms leading to major product ions are proposed. The data suggest that progesterone fragments preferentially via hydrogen and other rearrangements lead to neutral losses. These fragmentations are quite complex and are preceded by σ-bond cleavages in most cases. Four major pathways for progesterone fragmentation are proposed involving: (1) cleavage of ring B at C9-C10, (2) cleavage of C6-C7 bond in ring B through m/z 191, (3) two types of cleavages of ring D, and (4) ketene elimination in ring A. Pathways (1)-(3) proceed via charge-remote fragmentations while pathway (4) proceeds via charge-site initiated mechanism. The geometry of product ions in these pathways were optimized using DFT at the B3LYP/6-311G(d,p) level of theory from which the free energies of the pathways were calculated. The effect that the choice of basis sets and density functionals has on the results was tested by performing additional calculations using B3LYP/6-31G(d) and B3PW91/6-311G(d,p). 相似文献
833.
Parajón-Costa BS Baran EJ 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,78(1):133-135
The FTIR and FT-Raman spectra of the oxovanadium(IV) complex of 3-hydroxy-2-methyl-4-pyrone (maltol) bis(maltolato)oxovanadium(IV) were recorded and briefly discussed by comparison with the spectra of uncoordinated maltol and with some related species. 相似文献
834.
Miquel Barceló-Oliver Antonio Bauzá Beatriz A. Baquero Angel García-Raso Angel Terrón Elies Molins Antonio Frontera 《Tetrahedron letters》2013
We report the synthesis and X-ray characterization of N1-hexylcytosine (1) and N1-hexylcytosinium nitrate (2). N1-hexylcytosine (1) does not follow the same behaviour previously described for N1-hexylthymine and N1-hexyluracil in the solid state. This different behavior has been analyzed by means of density functional theory (DFT) calculations including the latest available correction for dispersion (D3). In addition hydrophobic and anion–π noncovalent interactions play a key role in stabilizing the 3D architectures of the compounds, which have been energetically studied using theoretical calculations and compared with similar structures retrieved from the Cambridge Structural Database (CSD). The anion–π–anion binding mode observed for nitrate in compound 2 is also observed in several crystal structures involving cytosine ring coordinated to transition metals. 相似文献
835.
Rosa Ana Pérez Beatriz Albero Consuelo Sánchez-Brunete José Luis Tadeo 《International journal of environmental analytical chemistry》2013,93(14):1666-1678
A method for the analysis of chlorotoluenes (CTs) in soil has been developed based on ultrasonic assisted extraction with a low volume of organic solvent and determination by gas chromatography-tandem mass spectrometry (GC–MS/MS). A simultaneous clean-up on an alumina–anhydrous sodium sulphate mixture was carried out to remove soil interferences. However, an additional clean-up with graphitised carbon was needed for some very dirty samples. Several solvents were assayed and a mixture of ethyl acetate:hexane (80?:?20, v/v) was selected to carry out soil extractions. Recovery studies were performed at 0.2, 0.1, 0.05 and 0.02?ng?g?1 fortification levels, and recoveries obtained for all the compounds and concentrations were higher than 81% with standard deviations fulfilling the requirements of the IUPAC. LODs from 0.7 to 5.2?ng?kg?1 and LOQs from 2.2 to 17.5?ng?kg?1 were achieved for the analysed compounds, being pentachlorotoluene the compound with the highest limits, followed by the monochlorinated toluenes. The proposed analytical method was applied to determine CT levels in agricultural and industrial soils. These compounds were found in all the industrial soils analysed and some CTs were present in agricultural soils at lower levels. 相似文献
836.
The interaction between carboxylic acid-stabilised gold nanoparticles (AuNP) and pH-responsive microgels is shown. The microgel particles are a copolymer of N-[3-(dimethylamino)propyl]methacrylamide (DMAPMA) and N-isopropylacrylamide (NIPAM). The microgel properties are presented by their hydrodynamic diameter and electrophoretic mobility in response to pH. These microgel particles are pH-responsive under neutral conditions decreasing in diameter beyond pH 7. The dispersion characteristics of AuNP adsorbed onto the microgel network are shown with respect to adsorbed amount and the pH-responsive properties of the AuNP. This data is presented between pH 3 and 6 where the microgel properties remain constant. Asymmetric adsorption of AuNP onto poly(DMAPMA-co-NIPAM) microgels is achieved by adsorption of nanoparticles, from the aqueous phase, onto microgel-stabilised oil-in-water emulsions. These asymmetrically modified microgels display very different dispersion behaviour, in response to pH, due to their dipolar nature. 相似文献
837.
838.
Santos J Illescas BM Martín N Adrio J Carretero JC Viruela R Ortí E Spänig F Guldi DM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(10):2957-2964
The synthesis of the first fully conjugated tetrathiafulvalene–tetracyano‐p‐quinodimethane ((TTF)–TCNQ)‐type system has been carried out by means of a Julia–Kocienski olefination reaction. In particular, a tetracyanoanthraquinodimethane (TCAQ) formyl derivative and two new sulfonylmethyl‐exTTFs (exTTF=2‐[9‐(1,3‐dithiol‐2‐ylidene)anthracen‐10(9H)‐ylidene]‐1,3‐dithiole)—prepared as new building blocks—were linked. A variety of experimental conditions reveal that the use of sodium hexamethyldisilazane (NaHMDS) as base in THF afforded the E olefins with excellent stereoselectivity. Theoretical calculations at the B3LYP/6‐31G** level point to highly distorted exTTF and TCAQ that form an almost planar stilbene unit between them. Although calculations predicted appreciable electronic communication between the donor and the acceptor, cyclic voltammetric studies did not substantiate this effect. It was only in photophysical assays that the electronic communication emerged in the form of a charge‐transfer (CT) absorption and emission. Once photoexcited (i.e., the locally excited state or excited charge‐transfer state), an ultrafast, subpicosecond charge separation leads to a radical ion pair state in which the spectroscopic features of the radical cation of exTTF as well as the radical anion of TCAQ are discernable. The radical ion pair is metastable and undergoes a fast ((1.0±0.2) ps) charge recombination to reconstitute the electronic ground state. Such ultrafast charge separation and recombination processes come as a consequence of the very short vinyl linkage between the two electroactive units. 相似文献
839.
María José Castro-Castro Dolors Dot-Bach Beatriz Candás-Estébanez Ruth Cano-Corres Xavier Fuentes-Arderiu 《Accreditation and quality assurance》2011,16(12):643-647
In the clinical laboratory, one of the most objective ways to perform the final review of patients’ measured values is the
use of computerized plausibility control (i.e., set of procedures used to decide whether a patient’s measured value is valid according to established clinical and
biological criteria). This study is focused on the estimation of alert and change limits to be applied to detect doubtful
patients’ measured values. These limits are useful to improve the final review of patients’ measured values since these limits
are produced objectively and are selected according to the clinical laboratory needs, letting the clinical laboratory professional
staff to save time and effort. 相似文献
840.
Beatriz Luna 《Physica D: Nonlinear Phenomena》2008,237(21):2685-2695
Discrete-time regulatory networks are dynamical systems on directed graphs with a structure that is inspired on natural systems of interacting units. Using a notion of determination between vertices, we define sets of dominant vertices, and we prove that in the asymptotic regime, the projection of the dynamics on a dominant set allows us to determine the state of the whole system at all times. We provide an algorithm to find sets of dominant vertices, and we test its accuracy on several examples. We also explore the possibility of using the dominant set characteristics as a measure of the structural complexity of networks. 相似文献