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991.
Paulsen H Trautwein AX Wegner P Schmidt C Chumakov AI Schünemann V 《Chemphyschem》2011,12(17):3434-3441
Nuclear resonant vibrational spectra of the reduced and oxidized form of a mutant of rubredoxin from Pyrococcus abyssii were measured and are compared with simulated spectra that were calculated by a combined quantum mechanics (QM) and molecular mechanics (MM) method. Density functional theory was used for the QM level. Calculations were performed for different models of rubredoxin. Realistic spectra were simulated with reduced models that include at least the iron center, the four cysteins coordinating it, and the residues connected to the cysteins together with a QM layer that comprises the first two coordination shells of the iron center. Larger QM layers did not lead to significant changes of the simulated spectra. 相似文献
992.
Schmidt SW Kersch A Beyer MK Clausen-Schaumann H 《Physical chemistry chemical physics : PCCP》2011,13(13):5994-5999
We have used temperature-dependent single molecule force spectroscopy to stretch covalently anchored carboxymethylated amylose (CMA) polymers attached to an amino-functionalized AFM cantilever. Using an Arrhenius kinetics model based on a Morse potential as a one-dimensional representation of covalent bonds, we have extracted kinetic and structural parameters of the bond rupture process. With 35.5 kJ mol(-1), we found a significantly smaller dissociation energy and with 9.0 × 10(2) s(-1) to 3.6 × 10(3) s(-1) also smaller Arrhenius pre-factors than expected for homolytic bond scission. One possible explanation for the severely reduced dissociation energy and Arrhenius pre-factors is the mechanically activated hydrolysis of covalent bonds. Both the carboxylic acid amide and the siloxane bond in the amino-silane surface linker are in principle prone to bond hydrolysis. Scattering, slope and curvature of the scattered data plots indicate that in fact two competing rupture mechanisms are observed. 相似文献
993.
Thomas Rosenau Antje Potthast Karin Krainz Yuko Yoneda Thomas Dietz Zenaida Peralta-Inga Shields Alfred D. French 《Cellulose (London, England)》2011,18(6):1623-1633
In the present work, aged cotton linters have been analyzed for their chromophore content according to the CRI (“chromophore
release & identification”) method. Despite the very low contents in the ppb range, nine chromophores have been unambiguously
identified, which makes this account the first one on defined chromophoric structures isolated from cotton. A common feature
of the chromophores are 2-hydroxy-[1,4]benzoquinone, 2-hydroxyacetophenone and 5,8-dihydroxynaphthoquinone moieties, which
resemble chromophoric structures found in other cellulosic substrates, such as bleached pulps or fibers. The finding of these
compounds in lignin-free cotton linters confirms the previous hypothesis that those chromophores are formed from (oxidized)
carbohydrate structures rather than from lignin fragments. 相似文献
994.
Theoretical and experimental gas-phase studies of carbohydrates show that their hydroxyl groups are located in homodromic
partial rings that resemble cooperative hydrogen bonds, albeit with long H…O distances and small O–H…O angles. On the other
hand, anecdotal experience with disaccharide crystal structures suggested that these clockwise ‘c’ or counter-clockwise (reverse
‘r’) sequences are not prevalent in the crystalline state. The situation was clarified with quantum mechanics calculations
in vacuum and in continuum solvation, as well as Atoms-In-Molecules analyses. From the experimental side, the Cambridge Structural
Database was searched. Geometric criteria for these sequences were developed. A criterion based on 120° ranges of hydroxyl
orientations accepted 4% of sequences as having ‘c,c’ or ‘r,r’ orientations instead of the 7% expected based on chance. Criteria
based on an O–H…O angle > 90° and a 90° lower limit of the absolute value of the H–O–C…H improper torsion accepted 7.0% of
the 358 sequences as ‘c,c’ or ‘r,r’. Highly variable orientation of the hydroxyl groups in crystals was seen to depend mostly
on strong inter-residue or intermolecular hydrogen bonds. That lack of specific orientation in general for the crystal structures
was supported by the solvated calculations that showed very little variation in the energy when one of the hydroxyl groups
in 1,2-dihydroxycyclohexane was rotated. The vacuum calculations found the energy to vary with rotation by more than 4 kcal/mol,
confirming the gas-phase experiments and calculations on more complicated molecules. Molecules examined in some detail include
scyllo inositol and native and methylated cellobiose. 相似文献
995.
Miles WC Huffstetler PP Goff JD Chen AY Riffle JS Davis RM 《Langmuir : the ACS journal of surfaces and colloids》2011,27(9):5456-5463
The colloidal stability of polymer-stabilized nanoparticles is critical for therapeutic use. However, phosphates in physiological media can induce polymer desorption and consequently flocculation. Colloidal characteristics of PEO-magnetite nanoparticles with different anchors for attaching PEO to magnetite were examined in PBS. The effects of the number of anchors, PEO molecular weight, and chain density were examined. It was observed that ammonium phosphonates anchored PEO to magnetite effectively in phosphate-containing solutions because of interactions between the phosphonates and magnetite. Additionally, a method to estimate the magnetite surface coverage was developed and was found to be critical to the prediction of colloidal stability. This is key to understanding how functionalized surfaces interact with their environment. 相似文献
996.
Jennifer M. Kashmirian Alfred Uhlherr Alan Dorin David G. Green 《Journal of computational chemistry》2012,33(15):1364-1373
Molecular simulation models are increasingly important tools in efforts to understand the role that water plays in biochemical processes. However, existing models of water have limited capacity to deal with the characteristics of hydrogen bond networks. This article proposes a new fluctuating network (FN) algorithm as an extension of the standard molecular dynamics algorithm. The new algorithm allows for the simulation of a molecular system based on an underlying network, such as the hydrogen bond network in water. This algorithm distinguishes strong from weak network connections, applying a potential that best describes the specific connection behavior. We model liquid water with this new technique using a single‐site, isotropic, short‐range potential. We successfully reproduce liquid water's signature molecular spacing (as represented by the radial distribution function) and characterize its dynamic properties including the exponential hydrogen bond lifetime distribution, diffusion rate, and average hydrogen bonds per molecule. The FN algorithm allows exploration of the behavior of networked systems where explicit coordination limits are required. As such it could also be used to model covalent interactions, reaction dynamics, and applied to simulation of cellular networks. © 2012 Wiley Periodicals, Inc. 相似文献
997.
Sengupta S Ebeling D Patwardhan S Zhang X von Berlepsch H Böttcher C Stepanenko V Uemura S Hentschel C Fuchs H Grozema FC Siebbeles LD Holzwarth AR Chi L Würthner F 《Angewandte Chemie (International ed. in English)》2012,51(26):6378-6382
Conductive tubes: Self-assembled nanotubes of a bacteriochlorophyll derivative are reminiscent of natural chlorosomal light-harvesting assemblies. After deposition on a substrate that consists of a non-conductive silicon oxide surface (see picture, brown) and contacting the chlorin nanowires to a conductive polymer (yellow), they show exceptional charge-transport properties. 相似文献
998.
Jian-Feng ZhaoYi Liu Jiayun Bethia SohYong-Xin Li Rakesh GangulyKai-Qi Ye Fengwei HuoLing Huang Alfred Iing Yoong TokJoachim Say Chye Loo Qichun Zhang 《Tetrahedron letters》2012,53(45):6044-6047
A novel diazaperinone 12H-pyrazino[2′,3′:3,4]pyrrolo[1,2-a]perimidin-12-one has been successfully synthesized through a one-step condensation. Single crystal X-ray analysis shows that the product is of planar structure with strong π?π interactions and H-bonding. Interestingly, the 12H-pyrazino[2′,3′:3,4]pyrrolo[1,2-a]perimidin-12-one can recognize three different metal cations Cu2+, Ag+ and Hg2+ via naked eye visualization, and shows turn-on fluorescence for Fe3+ and Ag+ among the 22 metal cations. 相似文献
999.
Poineau F Johnstone EV Weck PF Forster PM Kim E Czerwinski KR Sattelberger AP 《Inorganic chemistry》2012,51(9):4915-4917
A second polymorph of technetium trichloride, β-TcCl(3), has been identified from the reaction between Tc metal and Cl(2) gas. The structure of β-TcCl(3) consists of infinite layers of edge-sharing octahedra, similar to its MoCl(3) and RuCl(3) analogues. The Tc-Tc distance [2.861(3) ?] between adjacent octahedra is indicative of metal-metal bonding. Earlier theoretical work predicted that β-TcCl(3) is less stable than α-TcCl(3). In agreement with the prediction, β-TcCl(3) slowly transforms into α-TcCl(3) (Tc(3)Cl(9)) over 16 days at 280 °C. 相似文献
1000.
Werner Korsatko Christian Knopp Alfred Fuchsgruber Prof. Dr. Gustav Zigeuner 《Monatshefte für Chemie / Chemical Monthly》1976,107(3):745-755
The structures of tetrahydro-4-phenylspiro([1]benzopyran-2,4(1H)-pyrimidin)-2(3H)-ones and-thiones4 a, b resp., are proved by synthesis. 3-(2-methoxy-3,5-dimethylphenyl)-3-phenylpropionic acid11 b is prepared from 3,4-dihydro-6,8-dimethyl-4-phenylcoumarin10. The lithium salt of11 b reacts with isobutenyl-lithium to 1-(2-methoxy-3,5-dimethylphenyl)-5-methyl-1-phenyl-4-hexen-3-on12 a. 12 a is transferred with urea in acid medium and NH4CNS resp. in a mixture of dihydro-6-[2-(2-methoxy-3,5-dimethylphenyl)-2-phenyläthyl]-4,4-dimethyl-2(1H)-pyrimidinone and-thione13 a, b and tetrahydro-6-[2-(2-methoxy-3,5-dimethylphenyl)-2-phenyläthyliden]-4,4-dimethyl-2(1H)-pyrimidinone and-thione14 a, b resp.14 b leads to13 a, b with H2O2. Heating of13 a, 14 a and14 b resp. with pyridin-HCl leads to the spiro compounds4 a, b. 相似文献